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The 2011 March 11 Tohoku-oki great (Mw 9.1) earthquake ruptured the plate boundary megathrust fault offshore of northern Honshu with estimates of shallow slip of 50 m and more near the trench. Non-uniform slip extended ~ 220 km across the width and ~ 400 km along strike of the subduction zone. Extensive data provided by regional networks of seismic and geodetic stations in Japan and global networks of broadband seismic stations, regional and global ocean bottom pressure sensors and sea level measurement stations, seafloor GPS/Acoustic displacement sites, repeated multi-channel reflection images, extensive coastal runup and inundation observations, and in situ sampling of the shallow fault zone materials and temperature perturbation, make the event the best-recorded and most extensively studied great earthquake to date. An effort is made here to identify the more robust attributes of the rupture as well as less well constrained, but likely features. Other issues involve the degree to which the rupture corresponded to geodetically-defined preceding slip-deficit regions, the influence of re-rupture of slip regions for large events in the past few centuries, and relationships of coseismic slip to precursory slow slip, foreshocks, aftershocks, afterslip, and relocking of the megathrust. Frictional properties associated with the slip heterogeneity and in situ measurements of frictional heating of the shallow fault zone support low stress during shallow sliding and near-total shear stress drop of ~ 10–30 MPa in large-slip regions in the shallow megathrust. The roles of fault morphology, sediments, fluids, and dynamical processes in the rupture behavior continue to be examined; consensus has not yet been achieved. The possibility of secondary sources of tsunami excitation such as inelastic deformation of the sedimentary wedge or submarine slumping remains undemonstrated; dislocation models in an elastic continuum appear to sufficiently account for most mainshock observations, although afterslip and viscoelastic processes remain contested.

The 28 October 1891 Nobi earthquake was one of the largest intraplate earthquakes in Japan, with an estimated magnitude of 8.0 determined by the Japan Meteorological Agency. It occurred in central Japan on the Eurasian Plate, beneath which the young Philippine Sea Plate is subducting. The geometry of this plate is not smooth; rather, beneath western Japan it appears distorted over a relatively short lateral distance. Beneath the Kinki region of central Japan, the subduction angle is approximately horizontal, and the Nobi earthquake occurred just above the horizontal section of the Philippine Sea slab. Temporary seismic stations were deployed in this area to investigate local seismic structure and its possible relationship to the cause of the earthquake. Crustal structure was investigated using receiver function analysis with a spatially dense temporary seismic network. A clear image of the subducted Philippine Sea slab was resolved, with ridge-like structures beneath the epicentral area of the Nobi earthquake. The ridge-like structure is in contact with the continental crust, and the fault plane of the Nobi earthquake is located at the edge of one such structure; i.e., in the contact area. The unique structure of the area is inferred to cause stress concentration and strain accumulation on local faults, which resulted in the large size of the Nobi earthquake.

On 22 November, 2014, an MS 6.3 earthquake occurred in Kangding County, China. Focal mechanism solution shows that the two nodal planes were 235°/82°/− 173° and 144°/83°/− 8° and the focal depth was 9 km. Seismic slip of the Kangding earthquake was bilateral with about 0.5 m maximum slip. The rupture zone was confined to depths ranging from 5 to 15 km and laterally extended along the slip and strike directions by about 10 and 12 km, respectively. Most of the seismic moment was released in the first 5 s of the rupture, resulting in an earthquake magnitude of MW 6.01. In contrast, a slip model obtained by interferometric synthetic aperture radar (InSAR) data indicates that the rupture zone was longer than that determined from the seismic data and the earthquake magnitude should be about MW 6.2. Although accounting for the contribution of the MS 5.8 aftershock and the other small aftershocks that occurred during the InSAR observations period, the total moment estimated based on the seismic slip model was significantly smaller than that obtained from the InSAR data. Based on our analysis, we found that the inconsistency between the results determined from the seismic data and the InSAR data may be caused by the decrease in the shear modulus at shallow depths, the noise in the InSAR data, and the occurrence of some afterslips in the northwest region of the fault zone. The seismic slip of this earthquake was too small to release the accumulated energy within the entire Xianshuihe fault. We also found that the Coulomb stress in the northwest zone of the Kangding–Daofu seismic gap increased as a result of the historical, 2008 MS 8.0 Wenchuan and the 2014 MS 6.3 Kangding earthquakes, suggesting that this area is expected to be a high seismic hazard region for the future.

The 2011 M9.0 Tohoku-Oki megathrust earthquake produced strong ground motions with a peak ground acceleration of 1.52 cm/s 2 in the Korean Peninsula, inducing large dynamic-stress changes in the crust. Sixty-one triggered earthquakes with magnitudes of 0.5–2.5 including 17 unusual, spatially-clustered events were identified from continuous seismic record sections of dense seismic networks for 18 h following the megathrust earthquake. The triggered earthquakes occurred in regions of high seismicity. The earthquake occurrence frequency increased after the megathrust earthquake, keeping the Gutenberg-Richter b value invariant. The seismic occurrence rates decreased with time following a modified Omori law. The focal mechanism solutions of triggered earthquakes are consistent with those observed before the megathrust earthquake. An unusual earthquake swarm displays apparent migration with a speed of 28.6 ± 4.4 m/h. The changes in pore fluid pressure induced by strong seismic waves may have caused the triggering of earthquakes. The duration and strength of dynamic triggering may be highly dependent on the magnitude and distance of megathrust earthquake.

The Talala region in Saurashtra, Western India is one of the seismically active intraplate regions on the Earth. In recent past, this region has been site of moderate magnitude earthquakes as well as swarm-type earthquake activities. To understand the processes of earthquake generation in this intraplate setting, we constrained the earthquake distribution pattern along with the crustal seismic P-wave velocity (Vp) and Vp/Vs variations, using local earthquakes data. We inverted 2470 P- and 2230 S-wave arrival times from 550 earthquakes which were recorded over 11 seismic stations during 2007 to 2012. The earthquakes distribution shows that the seismicity is following ~ NNE–SSW trend, extending for a distance of ~ 25 km and up to 15 km in depth. The seismic tomographic images show that the swarm-type earthquake activities at shallower depths are mostly in the zone of lower Vp and lower Vp/Vs. Whereas, the moderate magnitude earthquakes are occurring in a ~ NW trending zone of higher Vp and higher Vp/Vs, possibly indicating a zone of crystallized mafic magma, which was transported from deeper Earth. This zone represents a pronounced heterogeneity and provides locale for stress accumulation in this region. After 2001 Bhuj earthquake (Mw 7.7), due to stress perturbation the ~ NNE–SSW trending fault got activated and caused bigger earthquakes in this region. Moreover, the crystallized mafic magma is possibly feeding fluids at shallower depths for causing the swarm-type earthquake activities in this region.

Moderate-to-large damaging earthquakes in the peninsular part of the Indian plate do not support the long-standing belief of the seismic stability of this region. The historical record shows that about 15 damaging earthquakes with magnitudes from 5.5 to ~ 8.0 occurred in the Indian peninsula. Most of these events were associated with the old rift systems. Our analysis of the 2001 Bhuj earthquake and its 12-year aftershock sequence indicates a seismic zone bound by two linear trends (NNW and NNE) that intersect an E–W-trending graben. The Bouguer gravity values near the epicentre of the Bhuj earthquake are relatively low (~ 2 mgal). The gravity anomaly maps, the distribution of earthquake epicentres, and the crustal strain-rate patterns indicate that the 2001 Bhuj earthquake occurred along a fault within strain-hardened mid-crustal rocks. The collision resistance between the Indian plate and the Eurasian plate along the Himalayas and anticlockwise rotation of the Indian plate provide the far-field stresses that concentrate within a fault-bounded block close to the western margin of the Indian plate and is periodically released during earthquakes, such as the 2001 M W 7.7 Bhuj earthquake. We propose that the moderate-to-large magnitude earthquakes in the deeper crust in this area occur along faults associated with old rift systems that are reactivated in a strain-hardened environment.

Boosted by the launch of Sentinel-1A radar satellite from the European Space Agency (ESA), we now have the opportunity of fast, full and multiple coverage of the land based deformation field of earthquakes. Here we use the data to investigate a strong earthquake struck Pishan, western China on July 3, 2015. The earthquake fault is blind and no ground break features are found on-site, thus Synthetic Aperture Radar (SAR) data give full play to its technical advantage for the recovery of coseismic deformation field. By using the Sentinel-1A radar data in the Interferometric Wide Swath mode, we obtain 3 tracks of InSAR data over the struck region, and resolve the 3D ground deformation generated by the earthquake. Then the Line-of-Sight (LOS) InSAR data are inverted for the slip-distribution of the seismogenic fault. The final model shows that the earthquake is completely blind with pure-thrust motion. The maximum slip is ~ 0.48 m at a depth of ~ 7 km, consistent with the depth estimate from seismic reflection data. In particular, the inverted model is also compatible with a south-dipping fault ramp among a group of fault interfaces detected by the seismic reflection profile over the region. The seismic moment obtained equals to a Mw 6.4 earthquake. The Pishan earthquake ruptured the frontal part of the thrust ramps under the Slik anticline, and unloaded the coulomb stress of them. However, it may have loaded stress to the back-thrust above the thrust ramps by ~ 1–4 bar, and promoted it for future failure. Moreover, the stress loading on the west side of the earthquake fault is much larger than that on the east side, indicating a higher risk for failure to the west of the Zepu fault.

Field investigations reveal that the Mj 6.8 (Mw 6.2) Nagano (Japan) earthquake of 22 November 2014 produced a 9.3-km-long co-seismic surface rupture zone. The slip occurred on the pre-existing active Kamishiro Fault, which developed along the Itoigawa–Shizuoka tectonic line, and is inferred as the boundary between the Eurasian and North American plates. The surface-rupturing earthquake produced dominant thrusting and subordinate strike-slip displacement. Structures that developed during the co-seismic surface rupture include thrust faults, fault scarps, en-echelon tension cracks, folding structures such as mole tracks and flexural folds, and sand-boils. The surface displacements measured in the field range from several centimeters to 1.5 m in the vertical (typically, 0.4–1 m), accompanied by a strike-slip component that reached 0.6 m along NNE trending ruptures. These observations indicate a thrust-dominated displacement along the seismogenic fault. Our results show that (i) the pre-existing Kamishiro Fault, which strikes NNE–SSW, controlled the spatial distribution of co-seismic surface ruptures and displacements; and (ii) the style and magnitude of thrust displacements indicate that the present-day shortening strain on the Eurasian–North American plate boundary in the study area is released mainly by seismic thrust displacements along the active Kamishiro Fault.

One approach that can be used to evaluate the potential for an earthquake occurrence is the detection of the stress concentration at an earthquake fault. While the stress fields for pre- and post-seismic event stages differ, this change cannot provide information regarding the potential for an earthquake. Here, we propose a detection method for states of stress that uses focal mechanism data from microearthquakes. The state of stress can be defined both by the background stress and by a moment tensor equivalent to the stress concentration. We applied this method to actual focal mechanism data from the 2013 Awaji Island earthquake (M6.3), Japan, and the results showed the presence of stress concentration around the earthquake fault before the mainshock. In addition, the regional differential stress was shown to be about 13 MPa. The magnitude of the obtained stress concentration in the focal area and the high dip angle of the mainshock fault imply that the faulting occurred in the crust where it was overpressurized to a level near the lithostatic pressure.

Wide-band magnetotelluric surveys were performed at 44 sites in the inland earthquake zone on the western margin of the Kurile arc, where strain accumulation has been observed since a high-magnitude interplate earthquake in 2003 (Tokachi-oki earthquake, M 8.0). We estimated the three-dimensional (3-D) resistivity distribution based on magnetotelluric impedance tensors, which contained anomalously large impedance phases. Although anomalous impedance phases have previously been considered a major impediment for magnetotelluric studies, we found that the anomalous phase can be used to constrain the resistivity distribution. The 3-D resistivity model showed that the earthquake zone consists of resistive areas (> 300 Ω-m), whereas the fore-arc and back-arc areas consist of regional conductive layers (

Hidden faults often lead to misestimation of earthquake hazard. In western Taiwan where many blind faults are located, how the stress interactions correlate with potentially significant earthquakes and what is the role of blind faults in earthquake triggering are the important questions for earthquake hazard assessment. Given sequential rupture of active faults with an unexposed fault segment involved, the 1935 Mw 7.1 Hsinchu–Taichung earthquake sequence in central Taiwan provides an exceptional case to study. This destructive earthquake sequence took ~ 3000 lives and was composed of four M > 6 earthquakes occurred within three months. The Mw 7.1 mainshock and a subsequent Mw 6.8 event that occurred 12 s later caused surface ruptures on the Tuntzuchiao fault (TTCF) and Shihtan fault (STF) which are separated by 25 km. These two ruptures were characterized by different types of faulting: thrust-faulting on the STF and right-lateral faulting on the TTCF fault. About 24 min later, an Mw 6.0 event occurred ~ 45 km north of the mainshock. Three months later on July 17, the last event, Mw 6.2, occurred 30 km northwest of the mainshock. In this study we revisit the data available for the 1935 sequence in an effort to place constraints on the fault models. Using five different fault models, a series of Coulomb stress calculations are conducted to understand whether static stress transfer advances slip during the subsequent events. We propose that key features of the 1935 Hsinchu–Taichung earthquake triggering can be explained by the existence of an unexposed fault segment in between the segments of TTCF and STF. We also show that the 1935 earthquake sequence may play an important role in activating post-1935 earthquake activity along the Sanyi–Puli seismic zone, where the sequential ruptures of the 1935 events encourage ruptures of a NW–SE-trending seismic zone at later times.

Great earthquakes have occurred repeatedly along the Nankai Trough in southwestern Japan with recurrence intervals of 100–200 years. The magnitudes of Nankai earthquakes have historically varied, with many studies to date asserting that an event's magnitude is controlled only by the number of broken segments arranged along the trough. Recent evidence suggests that large tsunami with recurrence intervals of several hundred years have occurred along the Nankai Trough. Therefore, it is now essential to reconsider the scenario of the Nankai earthquakes, including the possibility of seismic slip on the shallow subduction interface. Recently, model calculations of interplate seismic cycles have been conducted using rate- and state-dependent friction. In these calculations, assuming heterogeneous distribution of fracture energy in the seismogenic zone and its shallower extension, both massive earthquakes with slips up to the trough and ordinary earthquakes in the seismogenic zone can occur. Here, we apply a heterogeneous distribution of fracture energy in the shallow plate interface of the Nankai Trough. In the model, we represent the difference in fracture energy by the difference in the characteristic slip distance L . To model the conventional seismogenic zone as a zone of low fracture energy, we set L to be 5 cm at depths of 10–20 km. Conversely, the shallow plate boundary near the trough is modeled as a zone of high fracture energy by setting L to 5 m. As a result, two types of earthquake with different moment magnitude occur alternately. The recurrence interval of the larger type is about 370 years, which is comparable to the recurrence of larger tsunami deduced from geological findings. Large coseismic slip extends to the trough during the larger types. In contrast, the smaller one, whose seismic slip is distributed along the seismogenic zone, occurs after ~ 200 years of the larger one.

The Bonin-Mariana subduction zone is the end-member example of a decoupled system, as described by Uyeda and Kanamori (1979), with no interplate thrust solutions of moments greater than 8 × 10 25 dyn cm known in the CMT catalog, although a number of earthquakes are reported with assigned magnitudes around or above 7, both during the WWSSN period and the historical pre-1962 era. We present a systematic study of these events, including relocation and inversion of moment tensors. We obtain 15 new moment tensor solutions, featuring a wide variety of focal mechanisms both in the fore-arc and the outer rise, and most importantly a shallow-dipping interplate thrust mechanism with a moment of 4 × 10 27 dyn cm for the event of 28 December 1940 at a location 175 km East of Pagan. Our results show that the modern CMT catalog still undersamples the seismicity of the Mariana arc, which is thus not immune to relatively large, albeit rare, interplate thrust events, with moments 40 times that of the largest Global-CMT solution. Frequency-magnitude relations would then suggest a return time of 320 years for a magnitude 8 interplate thrust faulting earthquake in the Bonin-Mariana system.

The great Tohoku-oki earthquake (Mw 9.0) on 11 March 2011 was accompanied by nearly 100 large foreshocks and aftershocks (M ≥ 6.0) in the Northeast Japan forearc. In this study we relocated 77 of these large events using three different velocity models. An updated three-dimensional velocity model significantly reduced the travel-time residuals in the earthquake relocation. We found that the large thrust earthquakes are generally located in or around high-velocity patches, suggesting that the generation of the large thrust earthquakes is controlled by structural heterogeneities in the megathrust zone. The high-velocity patches may represent asperities in the megathrust zone where the subducting Pacific plate and the overriding Okhotsk plate may be coupled strongly. Many non-thrust earthquakes occurred in the crust of the overriding Okhotsk plate, indicating that the stress field there has been changed by the Tohoku-oki mainshock. The present results may improve our understanding of the subduction dynamics and seismotectonics of the Northeast Japan arc.

The 2011 Tohoku-Oki earthquake ruptured a large source region, which included areas with a range of past M 7 to M 8 earthquakes, varying stress accumulation, and different structural features. This megathrust event was substantially larger than anticipated in this region, though there were indications that the sequences of events in recent times were insufficient to relieve the full accumulated strain in the relatively rapid subduction of the Pacific plate. The source process time of about 150 s included rupture of an area of very large slip (30 to 60 m) on the shallow portion of the megathrust, updip of the hypocenter. The area of large slip produced large amplitude low-frequency radiation, while the dominant high-frequency radiation was generated from deeper sources down-dip of the hypocenter. Real-time information systems in Japan were able to issue timely warnings of the strong shaking and tsunami, but the complicated pattern of rupture growth led to an underestimate in the initial estimates of magnitude and tsunami threat. Variations in the plate coupling and heterogeneities of physical properties in the megathrust zone characterize the different source areas that ruptured together during the earthquake. Tomographic images of shear-wave and bulk-sound speeds show subtle changes of physical properties that may be associated with coupling condition and present clues for understanding the rupture process of this Mw 9 earthquake, which combined the source areas of many past earthquakes.

We integrated a regional tomography model, 20-year seismicity and earthquake focal mechanisms of the 4 March 2010 M L 6.4 Jiashian earthquake source region to delineate the seismogenic structure and mechanics of a relatively rare damaging inland earthquake that occurred in the southwestern fold-and-thrust belt of Taiwan. The main shock of the Jiashian earthquake occurred at a depth of 23 km beneath the slate belt of the southern Central Range with a sinistral thrust mechanism. Based on seismic tomography and seismicity, the Jiashian earthquake sequence occurred in the Chishan transfer fault zone, which we interpret to be located near the transition zone between the subducted Eurasian plate to the south and the arc–continent Taiwan collision to the north. The distribution of the Jiashian aftershocks define a WNW-ESE striking fault plane that dips 30–40° NNE which is consistent with the optimal fault plane of the GPS-derived coseismic slip model. Focal mechanisms and inverted stress results of the earthquake sequence indicate both thrust and strike–slip motions with the maximum compressive stress at nearly 90° to the regional compressive stress and thus sub-parallel to the strike of the Taiwan orogen. We propose that the 2010 Jiashian earthquake resulted from rupturing of a buried oblique fault within the Chishan transfer fault zone at the subduction–collision transition zone in southern Taiwan. The orogen-parallel P-axis of the 2010 Jiashian earthquake represents collision-related lateral compression, although a local stress deviation is also possible.

article i nfo Article history: We estimated the source rupture of the 2008 Wenchuan Earthquake (Ms 8), China, based on a back- projection of seismic waves to their source plane, using data from regional broadband arrays in Taiwan and northern Vietnam. Observations from these arrays, located at different azimuths, were processed to eval- uate the spatio-temporal rupture behavior of the fault. The seismic energy spot converted from windowed array waveforms was back-projected to the rupture plane of the earthquake, to image the instantaneous slip on the fault plane. Rupture processes were reconstructed based on the imaged time-dependent seismic energy radiating from the earthquake fault plane. The high station density of both arrays enabled a detailed examination of the rupture processes. The results indicate that the 2008 Wenchuan Earthquake had a rupture duration of approximately 100 s, with major asperities radiating energy at 50 s after the initiation of rupture. The location of asperities is in good agreement with sections of the earthquake fault along which major dam- age occurred, such as along the Yingxiu-Beichuan Fault. The radiated seismic energy shows a complex spatial distribution on the fault plane. Our analysis indicates that the rupture initiated at the epicenter and extended to the northeast for approximately 280 km. The average rupture velocity of this earthquake was approxi- mately 2.8 km/s.

article The physical mechanism of intermediate-depth earthquakes is still under debate. In contrast to conditions in the crust and shallow lithosphere, at temperatures and pressures corresponding to depths >50 km, rocks ought to yield by creep or flow rather than brittle failure. Some physical process has to enable brittle or brittle-like failure for intermediate-depth earthquakes. The two leading candidates for that are dehydration embrittlement and ther- mal shear runaway. Given their great depth, intermediate-depth earthquake processes can't be observed direct- ly.Insteadwe must relyona combination ofseismologyand thestudy of laboratory analogs tounderstand them. Earthquake nests are regions of highly concentrated seismicity that are isolated from nearby activity. In this paper we focus on three intermediate-depth earthquake nests — Vrancea, Hindu Kush and Bucaramanga, and what they reveal about the mechanics of intermediate-depth earthquakes. We review published studies of tectonic setting, focal mechanisms, precise earthquake locations and earthquake source physics at these locations, with an emphasis on the Bucaramanga nest. All three nests are associated with subducting litho- sphere and at least two of the nests have consistently larger stress drops compared to shallow seismicity. In contrast, the Bucaramanga nest has a larger b-value, larger variability of focal mechanisms and shows no evidence of aftershock sequences unlike the other two. We also report for the first time finding a signifi- cant number of repeating earthquakes, some with reverse polarity.

The 2001 Bhuj earthquake took place in a region away from the active plate boundaries, which qualifies this earthquake to be a typical intraplate event that claimed a death toll of 20,000 people. The aftershock sequence of this earthquake is still continuing for the last eleven years, which makes this aftershock sequence as a unique intraplate earthquake sequence in the world. This sequence is being monitored by a close digital seismic network consisting of 5–12 three-component broadband seismographs and 10–20 accelerographs that led to a homogeneous earthquake catalog consisting of precise and accurate estimates of hypocentral and source parameters. This catalog enabled us to examine the evolution of this earthquake sequence. Our study reveals a few interconnected regularities in the rate of earthquake occurrence, b-values, fractal dimensions, stress drops, and the mean earthquake depth values. We observe that a slow decrease in background seismic activity and a number of bursts in seismic activity characterize the 2001 Bhuj earthquake sequence. We also notice that the background seismic activity is associated with a monotonic decrease in the mean earthquake depth values while the bursts in seismic activity are found to be associated with a decrease in fractal dimension, b-value and mean earthquake depth estimates, and an increase in stress drop values. We propose that these bursts in seismic activity are related to the episodes of break in deep fluid circulation in the direction of the Earth's surface. The effective background permeability of the mid-crust in the 2001 Bhuj earthquake region is evaluated to be about k ≈ 10 − 13 m 2 based on the estimates of changes in mean earthquake depth. Thus, we infer that seismogenesis of the 2001 Bhuj earthquake sequence is probably connected with active deep fluid circulation underlying the Kachchh seismic zone.

To investigate the generation mechanism of the 2011 great East Japan (GEJ) earthquake (M 9.0), we determined a three-dimensional (3-D) seismic model under the NE Japan forearc region using a large number of P and S wave arrival times from local earthquakes. Our results show that the GEJ main-shock occurred in a high-velocity (Vp, Vs) zone with higher Poisson's ratio (σ). The aftershocks relocated with a master-event location (MEL) procedure indicate that most of them are located at the corner of the mantle wedge along the upper boundary of the subducting Pacific slab, and their focal depths are slightly shallower than the background seismicity. The tomographic images and spatial distribution of the aftershocks imply strong interplate coupling (asperity) in the main-shock area and weak coupling in the surrounding areas of the megathrust zone. We think that fluids in the crust and uppermost mantle in the subduction zone could have affected the earthquake generation through the physical role of fluid pressure and a variety of chemical effects. We conclude that the 2011 GEJ earthquake initiation and the rupture processes of the aftershock sequence were influenced by fluid extrusion into the rupture zone due to the dehydration of the subducting Pacific slab. Fluid-bearing structural heterogeneities with thermal–petrologic variations in the megathrust zone played a key role in the initiation of the 2011 GEJ earthquake and its aftershocks.

We investigate spatial distribution of stress along the interplate boundary between Pacific and Okhotsk plates in Tohoku and Hokkaido regions, northern Japan from earthquake focal mechanisms compiled from the National Institute for Earth Science and Disaster Prevention (NIED) of Japan. The interplate boundary in this work is inferred as the surface defining the upper boundary of the double seismic zone (Benioff Zone). To investigate spatial variation in stress field we divide the interplate boundary into 20 km × 20 km subfaults and estimate stress field in each subfault of 10 km thickness. We find clear spatial variation in stress state along the subduction zone of the Pacific plate in northern Japan. Though most of the study area is characterized by trench normal uniaxial compression, in the Hidaka Collision Zone, the signature of collision between the Kurile and Japan arcs is evident in terms of radial compression. Distribution of the angle (ψ) between the maximum principal stress direction and the fault normal shows that the large subduction earthquakes have occurred in the region where this angle ψ varies between 30 and 45°. However in the case of recent M9.1, 2011 Tohoku-Oki earthquake, this correlation is not observed. We further investigate the frictional strength along the interplate boundary. Our results suggest: (1) The fault segments with ψ between 30 and 45 ° are frictionally strong (with friction coefficients between 0.6 and 0.8), (2) The fault segments with ψ between 45 and 62 ° are intermediately strong (with friction coefficients between 0.4 and 0.6), and (3) The fault segments with ψ above 62 ° are weak (with friction coefficients below 0.4).

Earthquakes along the shallow South American subduction zone exhibit heterogeneous rupture characteristics, going back several centuries of the earthquake record in this area. This heterogeneity is manifest in several ways, such as changes in rupture mode from magnitude > 8 events during one century followed by smaller ones in other time periods, as well as unusual tsunami events. There is also an apparent interaction between earthquake rupture and subducting plate complexity in this region. Significant complexity exists on the subducting Nazca plate, including fracture zones and ridges such as the large Nazca Ridge. Several large magnitude earthquakes have occurred in the region of ridge subduction, but no earthquake rupture has ruptured through these features into adjacent regions, suggesting that these subducting features act to segment the margin. Other features, such as fracture zones and variable sediment thickness on the Nazca Plate, appear to influence earthquake behavior over a wide range of magnitude scales. Upper plate features such as crustal faults also lead to heterogeneous earthquake behavior. Here I provide an overview of seismicity along the margin since 1850 in the context of the subduction zone structures. This includes great earthquakes such as the 1906 Ecuador and 1960 Chile events. I also present results showing increased rupture complexity in moderate magnitude earthquakes that can also be linked to certain Nazca Plate features.

A long-standing goal of subduction zone earthquake studies is to determine whether or not there are physical processes that control seismogenesis and the along-strike segmentation of the megathrust. Studies of individual earthquakes and global compilations of earthquakes find favorable comparison between coseismic interplate slip distributions and several different long-lived forearc characteristics, such as bathymetry, coastline morphology, crustal structure, and interplate frictional properties, but no single explanation seems to govern the location and slip distribution of all earthquakes. One possible reason for the lack of a unifying explanation is that the inferred earthquake parameters, most importantly the slip distribution, calculated in some areas were inaccurate, blurring correlation between earthquake and physical parameters. In this paper, we seek to test this possibility by comparing accurate slip distributions constrained by multiple datasets along several segments of a single subduction zone with the various physical properties that have been proposed to control or correlate with seismogenesis. We examine the rupture area and slip distribution of 6 recent and historical large (Mw > 7) earthquakes on the Peru–northern Chile subduction zone. This analysis includes a new slip distribution of the 14 November 2007 Mw = 7.7 earthquake offshore Tocopilla, Chile constrained by teleseismic body wave and InSAR data. In studying the 6 events, we find that no single mechanism can explain the location or extent of rupture of all earthquakes, but analysis of the forearc gravity field and its gradients shows correlation with many of the observed slip patterns, as suggested by previous studies. Additionally, large-scale morphological features including the Nazca Ridge, Arica Bend, Mejillones Peninsula, and transverse crustal fault systems serve as boundaries between distinct earthquake segments.

article i nfo A proper slip fault model and a reasonable dislocation theory are necessary to compute and investigate co- seismic deformations caused by the 2008 Wenchuan earthquake (Ms8.0). To find such a model, several comparisons are made of different dislocation theories, seismic fault models, and earth models. Results indicate that the fault model determined by combining GPS and seismic waveform data is the best. The spherical dislocation theory yields better results than that of a half-space theory, and the results obtained from a revised PREM earth model are better than those from PREM. According to results of earlier investigations, co-seismic deformations such as the displacement, geoid, gravity, and strain changes caused by the 2008 Wenchuan earthquake (Ms8.0) are computed, described, and discussed. Results show that the earthquake generated considerable co-seismic deformation in a large area around the epicenter. The modeled deformations are applicable as a reference for other researchers to study inter-structural inversion, crustal deformation, etc. The effects of the curvature and layer structure are great. Moreover, spherical dislocation theory is necessary for studying co-seismic deformations caused by a large event.

In historical records, no earthquake of magnitude comparable with that of the Wenchuan Mw 7.9 earthquake has ever been reported in Chengdu and Longmenshan regions. It is important, therefore, to assess if there is any evidence of prior large earthquakes in the recent geological record. In order to explore this problem, 3 trenches were excavated separately at 3 sites along the surface rupture zones in the middle segment of the Beichuan–Yingxiu Fault and Jiangyou–Guanxian Fault of the Longmenshan fault zone, and the offsets of geomorphic surfaces were measured in the trenches. The basic facts of recurrence of large earthquake on the seismogenic fault of the Wenchuan earthquake are demonstrated in this paper on the basis of the accumulated deformation amounts of old and young geomorphic surfaces as well as the paleoearthquake events revealed in the trenches. The document that at Leigu along the Beichuan–Yingxiu Fault, at Bailu along the Jiangyou–Guanxian Fault, and Xiaoyudong on a NW-trending fault, the height of fault scarp on the second terrace has a multiple relationship to that on the first terrace after the Wenchuan earthquake of 12, May. The elevation difference of the marker horizon (the yellow sandy clay bed) of the second terrace revealed in the trenches on both sides of the fault is nearly twice (or three times larger in Bailu) of that produced by the Wenchuan earthquake. These observations suggest the occurrences of paleoearthquake events with surface deformation comparable to that produced by the Wenchuan Mw 7.9 earthquake. According to the dating data and the fact that there is no such slip of comparable paleoearthquakes in the 2300-year-long historic literature in Longmenshan region, we suggest that the penultimate event occurred between 2300 a BP and 3300 a BP.

We have determined the location and focal mechanism for the 1924 Orotina earthquake using two different relocation algorithms developed for the analysis of historic earthquakes and forward waveform modeling of teleseismic body waves. Our relocation results suggest that the 1924 earthquake occurred ~ 50 km southeast of Orotina near the Pacific coast. The heavy damage reported in the Orotina region, coupled with a low population density to the southeast, had led previous researchers to believe that the earthquake occurred near Orotina, possibly on the Tarcoles or Bijagual faults. Our waveform modeling results indicate rupture on a thrust fault at a depth (25 ± 5 km) near the plate interface with Mw 6.4–6.7. These are inconsistent with slip on any mapped surface faults located near Orotina and with the mechanism of the nearby 2004 Mw 6.4 Damas earthquake that indicates transtensional deformation in the upper crust. This suggests that the 1924 event may be one of a sequence of events (1924, 1990, 1999) generated by subduction of the central Costa Rica seamount belt.

The structure of the Philippine Sea Plate in the offshore region adjacent to the Boso Peninsula in central Japan is resolved by comparison of recent oceanic seismic survey data with natural repeating earthquake data. This region is characterized by a range of seismic and aseismic phenomena associated with subduction motion of the Philippine Sea Plate, including slow slip events (SSEs), large backslip events, and repeating earthquakes. This offshore region is also the site of the largest aftershock of the 1923 Kanto earthquake (M7.9). The detailed structure of the Philippine Sea Plate is successfully resolved, and the upper boundary of plate is clearly traceable to depths of 20 km. The reflection intensity of the upper boundary varies spatially in accordance with the depth of the reflector. The upper boundary continues smoothly with a line of repeating earthquake hypocenters regarded as an indicator of subduction shear. A continuous model extending into the offshore region can thus be proposed. The variation in reflection intensity on the upper boundary of the Philippine Sea Plate also correlates well with variation in event types. A weak reflection zone overlaps the area of large backslip, while a moderately strong reflection zone encroaches into the large slip area of the Boso SSE. The reflectivity of the upper boundary is thus considered to be dependent on the physical properties and conditions of the reflector, which in turn affect the event types on the plate boundary. A source fault model is proposed for the largest aftershock of the 1923 Kanto earthquake. Based on geodetic data, the source fault is located eastward of the mainshock, with dimensions of 40 × 50 km and moment magnitude of 7.5. The source fault lies within the region of large backslip and the large slip area of the Boso SSE.

We compile a comprehensive list of hypocentral locations and source parameters for earthquakes occurred in the Taiwan region. The collected database is used to construct 3D patterns of seismic deformation that are associated with various tectonic processes such as subduction and collision between the Philippine Sea plate and Eurasia. Specifically, we divide the entire Taiwan region into a 3D grid of 10km interval and the amount of seismic moment released at each point is the sum of all individual events in the vicinity. In case when the physical dimension of an earthquake is larger than 10km, the corresponding seismic moment is distributed across multiple grid points. This quantitative approach gives a better depiction on many first-order features in the region. The most intriguing one is that a significant deficit in the total amount of released seismic moment can be clearly identified around the Chi-Chi source region before the big earthquake occurred in 1999. The deficit trough is then filled by the seismic moment of the Chi-Chi earthquake sequence. This deficit-then-fill pattern in a region's seismic moment distribution can be used as an indicator to pinpoint the locations of large earthquakes in the foreseeable future. Following this argument, we suggest that disastrous earthquakes with magnitudes comparable to that of the Chi-Chi earthquake are due for the two regions adjacent to the Chi-Chi source area (i.e., the Miaoli-Hsinchu domain to the north and the Chiayi domain to the south).

The 2003 Chengkung earthquake has been debated because of inconsistent fault slips determined by seismic and geodetic observations. A pure thrust fault plane solution has been estimated by seismic waveform inversions form the mainshock and its major aftershocks; however, thrust slips with obvious northward horizontal motions are necessary to interpret GPS co-seismic deformation. Based on individual observation, its implication can be dramatically different. In this study, near source dense seismic array and continuous GPS observations were employed to examine the spatial-temporal variation of fault slip of this earthquake. And average rupture properties of major aftershocks were determined by stress inversion of earthquake mechanisms. Based upon strong motion observations, the near source two-dimensional ground motions were reconstructed from 54 free-field strong motion records with epicenter distances of less than 60 km. The reconstructed two-dimensional snapshots provide the visualization for seismic wave propagations near the source region. Based on the reconstructed seismic wave-fields, numerical modeling results confirmed that the dominant seismic energy of mainshock was released from a thrust fault slip and the same as its major aftershocks from stress inversions. No obvious seismic energy was released by some strike–slip type events. Observations from creepmeter and continuous GPS showed limited co-seismic deformations and obvious after-slips near the earthquake fault. The integrated analysis provided unique information to understand the quasi-static fault slips of an earthquake sequence. In this study, a model of sudden thrust slip with a horizontal creeping movement fault system has been proposed to interpret the rupture behavior of the Chengkung earthquake. During the process, the seismic energy was released from a thrust type fault rupture and following some strike–slip type slow fault movements. If the thrust slip is the nature of faulting in the Longitudinal Valley during major earthquakes, it should be significant to seismic hazards, source physics and its tectonic implications. However, the detailed horizontal creeping cannot be resolved by seismometers and low sampling rate GPS observations, presently. To completely understand the quasi-static faulting behavior of an earthquake, the installations of a high sampling rate strainmeter and continuous GPS monitoring near a fault are necessary.

A shallow M6.4 inland earthquake occurred on 26 July 2003 in the northern part of Miyagi Prefecture, northeastern Japan. This earthquake was a typical inland thrust earthquake, a type that is common in NE Japan. We obtained a detailed seismic velocity structure in the focal area of this earthquake by the double-difference tomography method. Arrival-time data came from temporary seismic stations deployed above the mainshock fault plane. Both the P-wave and S-wave velocities in the hanging wall were lower than those in the footwall. Aftershocks were aligned along a zone where the seismic velocity changes rapidly. This is consistent with the interpretation that the 2003 northern Miyagi earthquake occurred along a fault that acted as a normal fault in the Miocene and has been reactivated as a reverse fault under the present compressional stress regime. The large slip area by the main shock rupture (asperity) corresponds to an area with relatively high P- and S-wave velocities. A zone with low Vp/Vs was detected along the aftershock area. One of the possible causes of this low-Vp/Vs zone is the existence of high-aspect-ratio pores that contain water. Hypocenters of the main shock, largest foreshock, and largest aftershock are also located within the low-Vp/Vs zone.

We found a characteristic space–time pattern of the tidal triggering effect on earthquake occurrence in the subducting Philippine Sea plate beneath the locked zone of the plate interface in the Tokai region, central Japan, where a large interplate earthquake may be impending. We measured the correlation between the Earth tide and earthquake occurrence using microearthquakes that took place in the Philippine Sea plate for about two decades. For each event, we assigned the tidal phase angle at the origin time by theoretically calculating the tidal shear stress on the fault plane. Based on the distribution of the tidal phase angles, we statistically tested whether they concentrate near some particular angle or not by using Schuster’s test. In this test, the result is evaluated by pvalue, which represents the significance level to reject the null hypothesis that earthquakes occur randomly irrespective of the tidal phase angle. As a result of analysis, no correlation was found for the data set including all the earthquakes. However, we found a systematic pattern in the temporal variation of the tidal effect; the p-value significantly decreased preceding the occurrence of Mz4.5 earthquakes, and it recovered a high level afterwards. We note that those Mz4.5 earthquakes were considerably larger than the normal background seismicity in the study area. The frequency distribution of tidal phase angles in the pre-event period exhibited a peak at the phase angle where the tidal shear stress is at its maximum to accelerate the fault slip. This indicates that the observed small p-value is a physical consequence of the tidal effect. We also found a distinctive feature in the spatial distribution of p-values. The small p-values appeared just beneath the strongly coupled portion of the plate interface, as inferred from the seismicity rate change in the past few years. D 2005 Elsevier B.V. All rights reserved.

The ground motion hazard for Sumatra and the Malaysian peninsula is calculated in a probabilistic framework, using procedures developed for the US National Seismic Hazard Maps. We constructed regional earthquake source models and used standard published and modified attenuation equations to calculate peak ground acceleration at 2% and 10% probability of exceedance in 50 years for rock site conditions. We developed or modified earthquake catalogs and declustered these catalogs to include only independent earthquakes. The resulting catalogs were used to define four source zones that characterize earthquakes in four tectonic environments: subduction zone interface earthquakes, subduction zone deep intraslab earthquakes, strike-slip transform earthquakes, and intraplate earthquakes. The recurrence rates and sizes of historical earthquakes on known faults and across zones were also determined from this modified catalog. In addition to the source zones, our seismic source model considers two major faults that are known historically to generate large earthquakes: the Sumatran subduction zone and the Sumatran transform fault. Several published studies were used to describe earthquakes along these faults during historical and pre-historical time, as well as to identify segmentation models of faults. Peak horizontal ground accelerations were calculated using ground motion prediction relations that were developed from seismic data obtained from the crustal interplate environment, crustal intraplate environment, along the subduction zone interface, and from deep intraslab earthquakes. Most of these relations, however, have not been developed for large distances that are needed for calculating the hazard across the Malaysian peninsula, and none were developed for earthquake ground motions generated in an interplate tectonic environment that are propagated into an intraplate tectonic environment. For the interplate and intraplate crustal earthquakes, we have applied ground-motion prediction relations that are consistent with California (interplate) and India (intraplate) strong motion data that we collected for distances beyond 200 km. For the subduction zone equations, we recognized that the published relationships at large distances were not consistent with global earthquake data that we collected and modified the relations to be compatible with the global subduction zone ground motions. In this analysis, we have used alternative source and attenuation models and weighted them to account for our uncertainty in which model is most appropriate for Sumatra or for the Malaysian peninsula. The resulting peak horizontal ground accelerations for 2% probability of exceedance in 50 years range from over 100% g to about 10% g across Sumatra and generally less than 20% g across most of the Malaysian peninsula. The ground motions at 10% probability of exceedance in 50 years are typically about 60% of the ground motions derived for a hazard level at 2% probability of exceedance in 50 years. The largest contributors to hazard are from the Sumatran faults.

The spatio-temporal variation of quasi-static (slow) slip on the plate boundary before and after three major earthquakes (Mj 7.1 in 1989, Mj 6.9 in 1992, and Mj 7.6 in 1994) off Sanriku, northeastern Japan, is estimated using small repeating earthquake data. The small repeating earthquake sequence employed is considered to correspond to repeated rupture of a small isolated asperity surrounded by a quasi-static slipping area, and the activity of the source reflects the quasi-static slip rate. Small repeating earthquakes accompanying moderate-sized events appear to have been activated prior to the major events under investigation, with activity starting 6 days, 2 days, and 8 months before the occurrence of the 1989, 1992, and 1994 events. These results indicate that the quasi-static slip near the hypocenters of these major earthquakes was accelerated prior to the main shocks, causing stress concentration at the asperities responsible for the main shocks. This idea is supported by the fact that both the epicenters (locations of initial breaks) and areas where quasi-static slip was accelerated were located on the same side of the main-shock asperities. Migration of quasi-static slip acceleration in the off-Sanriku region was also identified, apparent in the aftershock activity of the 1994 event and the earthquake swarms including the 1989 (Mj 7.1) and 1992 (Mj 6.9) events. The present results reveal that quasi-static slip may have played a crucial role in the occurrence of these large earthquakes, as well as contributing to aftershock and swarm activity on the plate boundary.

Along the Japan trench where some Mw8 class interplate earthquakes occurred in the past century such as the 1896 Sanriku tsunami earthquake (M6.8, Mt8.6, 12 £ 10 20 N m) and the 1968 Tokachi-oki earthquake (Mw8.2, 28 £ 10 20 N m), the Pacific plate is subducting under northeast Japan at a rate of around 8 cm/year. The seismic coupling coefficient in this region has been estimated to be 20‐40%. In the past decade, three ultra-slow earthquakes have occurred in the Sanriku-oki region (398N‐428N): the 1989 Sanriku-oki (Mw7.4), the 1992 Sanriku-oki (Mw6.9), and the 1994 Sanriku-oki (Mw7.7) earthquakes. Integrating their interplate moments released both seismically and aseismically, we have the following conclusions. (1) The sum of the seismic moments of the three ultra-slow earthquakes was (4.8‐6.6) £ 10 20 N m, which was 20‐35% of the accumulated moment (18.6‐23.0) £ 10 20 N m, in the region (398N‐40.68N, 1428E‐1448E) for the 21‐26 years since the 1968 Mw8.2 Tokachi-oki earthquake. This is consistent with the previous estimates of the seismic coupling coefficient of 20‐40%. On the other hand, the sum of the interplate moments including aseismic faulting is (11‐16) £ 10 20 N m, leading to a “seismogeodetic coupling coefficient” of 50‐85%, which is an extension of the seismic coupling coefficient to include slow events. (2) The time constants showed a large range from 1 min (,10 2 s) for the 1968 Tokachi-oki earthquake to 10‐20 min (,10 3 s) for the 1896 Sanriku tsunami earthquake, to one day (,10 5 s) for the 1992 Sanriku-oki ultra-slow earthquake, to on the order of one year (,10 7 s) for the 1994 Sanriku-oki ultra-slow earthquakes. (3) Based on the space‐time distribution, three “gaps of moment release,” (40.6 8N‐428N, 1428E‐1448E) 398N‐408N, 1428E‐1438E) and (398N‐408N, 1428E‐1448E), are identified, instead of the gaps of seismicity. q 2001 Elsevier Science B.V. All rights reserved.

In the continental forearcs of the Cascadia and Nankai subduction zones, geodetic strain measurements indicate crustal contraction in the direction of plate convergence (nearly ‘margin-normal’), but earthquake focal mechanisms and other stress indicators show maximum compression along strike (‘margin-parallel’). It is evident that the geodetic strain signals reflect the temporal fluctuations of elastic stress associated with subduction earthquake cycles, not the absolute stresses. The absolute margin-normal stress is not only much less than the margin-parallel stress but also no greater than the lithostatic value, indicating a very weak subduction thrust fault. Weak subduction faults are also consistent with geothermal data which require very low frictional heating along the faults. Great subduction earthquakes occur at very low shear stresses and cause small perturbations to the forearc stress regime. Because these small elastic stress perturbations are relatively fast, they give large strain rates that are detected by geodetic measurements. The present nearly margin-normal contraction in both places is due to the locking of the subduction fault and the consequent increase in elastic stress in the direction of plate convergence. With the margin-parallel compression being dominant, the small increase in margin-normal stress may change the forearc seismicity in Cascadia from a mixture of thrust and strike–slip types into mainly thrust before the next great subduction earthquake. In Nankai, the change should be from strike–slip to quiescence, as observed in the Nankai forearc prior to the 1944–1946 great subduction earthquakes.

International audience; Focal parameters of local earthquakes in the region of the Ionian Islands of western Greece are constrained with a temporary dense array of three-component seismographs operated jointly offshore and onshore. Seismic deformation is documented to be confined to the east of the N20°E-striking steep continental slope west of Cephalonia island, the right-lateral Cephalonia Transform Fault, CTF, inferred from large earthquakes. The pre-Apulian continental material appears to be only deforming east of the transform fault, where it is in upper plate position to the Hellenic subduction. East of the transform fault, the transmission velocity tomography from local earthquakes, compared in depth-section with a previous marine reflection profile, provides evidence in support of a shallow landward dipping boundary around 12 km deep under the Ionian Islands along which they may override the lower plate. On either side of this interface local earthquakes occur with different focal mechanisms, in support with its interpretation as the interplate. Under Cephalonia island, reverse-faulting deforms the upper plate along NW–SE structures, which may also be affected by left-lateral bookshelf-faulting. Small earthquakes show normal faulting along the western coast of Cephalonia and its extension 20 km SSW, the trace of the CTF as inferred from the occurrence of the large strike-slip earthquakes. Another group of normal-fault earthquakes locates in the lower plate from under Cephalonia to Zante, just outboard of a possible change of interplate dip suggested from reflection seismics landward under the islands. These normal-fault earthquakes appear to coincide in position with that of the load imposed by the upper plate transported over them, rather than occurring in an outer rise, outboard the plate boundary and trench, as observed in other subductions and attributed to the control by the flexural bending of the lower plate under the pull of the sinking slab. Interpretation has to consider several peculiar features of plate interaction in western Greece with respect to a steady-state model for major subduction zones, in particular: a fast deformation of the upper plate in front of an orogenically overthickened crust and of the southwestward push of extruding Anatolia; its transport, which is the cause of the migration of the plate boundary rather than the roll-back of a slab which has been proposed to be detached; possibly a flat and ramp shape of the interplate; the geometrical complexity of the shear limit across the CTF between subduction and collision, and the nearby variation of the nature of the foreland crust.

A significant correlation has been reported between the intraplate (inland) and the great interplate earthquakes in Southwest Japan, based on historical earthquake data since 868. The frequency of intraplate earthquakes is high for several decades before and up to a decade after great interplate earthquakes. To explain the correlation, we treated two types of interaction between the inland faults and the plate boundary: (a) in which the stress changes caused by great interplate earthquakes affect the occurrence of intraplate earthquakes; and (b) in which the stress changes caused by intraplate earthquakes trigger other intraplate earthquakes and great interplate earthquakes. To simulate the sequences of these earthquakes, we calculated the static stress changes resulting from large intraplate and great interplate earthquakes and also estimate the rate of increase in tectonic stress on the faults and the plate boundary. The estimated rates of increase here are smaller than those used in previous studies. Thus, in our models, stress changes could more strongly affect the seismicity than previously considered. We simulated the earthquake sequences for our models in four cases: (i) with no interaction among the inland faults in Southwest Japan and the plate boundary along the Nankai trough; (ii) with stress changes on the inland faults caused only by great interplate earthquakes; (iii) with stress changes on the plate boundary caused only by large intraplate earthquakes; and (iv) with stress changes both on the inland faults and on the plate boundary caused by large intraplate earthquakes. The simulated time series in case (i), of course, shows no correlation between intraplate and great interplate earthquakes. Those in cases (iii) and (iv) also show no significant correlation. Only the time series of intraplate earthquakes in case (ii) shows a temporal variation similar to that of the real intraplate earthquakes before and after the great interplate earthquakes. We examined statistically the similarity between the simulated time series and the real earthquake data. About 40% of the time series in case (ii) are significantly similar, although in the other three cases less than 5% of the time series are. Understanding of the effects of stress changes on the inland faults caused by the great interplate earthquakes along the Nankai trough is essential to explain the temporal variation in seismicity in Southwest Japan before and after the great interplate earthquakes.

One of the consequences of plate tectonics is that a spreading ridge will eventually approach a subduction zone. The problem whether the possible break-up of the approaching ridge will lead to the development of independent micro-plates, or not, is still unresolved. Some 4 million years ago the interaction between the Juan de Fuca Ridge and the Cascadia subduction zone resulted in ridge fragmentation in the Explorer region. There are two proposed post-Miocene kinematic models: one that proposes the presence of a micro-plate and the other that treats the region as a transform deformation zone, or so-called pseudo-plate. We use earthquake strain rates derived from 74 events since 1948 to estimate a long-term velocity field for the region. By comparing this result with the predicted velocity fields for both models we try to discriminate between the two. The earthquake strain rates indicate the presence of a transform deformation zone between the North American (NAM) and Pacific (PAC) plates. The velocity field derived from the inversion of the earthquake strain rates indicates that seismic activity takes up 50±30% (1σ) of the PAC–NAM relative motion (NUVEL-1A; DeMets et al., 1994. Effect of recent revisions to the geomagnetic reversal time scale on estimates of current plate motions. Geophys. Res. Lett. 21, 2191–2194.) within the seismogenic layer of this zone and is indistinguishable in direction from the NUVEL-1A PAC–NAM model. The presence of this Explorer transform zone is consistent with the strain rate and velocity field for the `pseudo-plate model' and indicates that seismicity defines a (new) plate boundary zone between the Pacific and North American plates. Earthquake-derived strain rates are low along the Nootka Transform, which accommodates relative motion between the Juan de Fuca and North American plates. The cause for the absence of significant seismic slip along this transform is unclear and may be closely linked to the fact that the adjacent Cascadia subduction zone is locked. The `micro-plate model', which we reject, predicts SW–NE convergence in the eastern Explorer region and this is inconsistent with the earthquake strain rates as well as with surface fault observations.

We have done a numerical simulation of tectonic loading at transform plate boundaries with a lithosphere-asthenosphere coupling model subject to steady relative plate motion. In this model a seismic fault is represented by a locked patch with a finite length on an infinitely long vertical dislocation surface cutting the entire lithosphere. Through the numerical simulation we have obtained the following results. Stress accumulation on the fault is partly due to viscous drag at the base of the lithosphere (base loading) and partly due to dislocation pile-ups at horizontal edges of the fault (edge loading). If the viscosity of the asthenosphere is less than 1018 Pa s, the base loading is not effective, since its rate exponentially decays very soon. If the viscosity of the asthenosphere is greater than 1020 Pa s, both types of loading are effective, and then the stress accumulation rate in the early stage of loading is formally written as dτdt = Vpl(a + bL), where Vpl is a rate of relative plate motion and L is a fault length. When the fault length L is small, the effect of edge loading (the second term) is dominant, and the stress accumulation rate is in proportion to the inverse of L. When the fault length L is large, the effect of base loading (the first term) becomes dominant, and the stress accumulation rate is independent of L. It is well-known that the general L-cubed dependence of seismic moment M0 breaks for large interplate earthquakes. This break in the moment-length relation can be ascribed to difference in loading mechanism between small and large earthquakes. From the results of numerical computation, if interplate earthquakes have the same stress drop regardless of their fault lengths, we may derive a L-squared dependence of M0 for large interplate earthquakes and a linear L-dependence of M0 for very large interplate earthquakes.

The 1932 deep Fiji earthquake is studied from the point of view of comparing it to the recent large events in the transition zone. Spectral analysis and body-wave modeling suggest a moment of 3.4 × 10 27 dyn cm (within a multiplicative or divisive factor of 1.5), which makes it comparable to, but not significantly larger than, the great 1994 earthquake, 770 km to the north. Its seismic moment remains well below that of the largest deep shocks (1994 in Bolivia or 1970 in Colombia). Relocation efforts show that the earthquake took place on the front edge of the Wadati-Benioff Zone, in an area where the latter features a complex geometry involving significant warping. The mechanism of the earthquake involves down-dip compression and can be readily explained in terms of the prevailing large-scale stresses in the slabs.

The Bhadrachalam earthquake of April 13, 1969 and the Broach earthquake of March 23, 1970 are two major intraplate earthquakes associated with paleorift zones in the stable continental region of peninsular India. These two events shook very large portions of the peninsula, and casualties were reported for the Broach earthquake. Ancient rifts have recently been identified as a major seismogenic tectonic feature for intraplate seismicity in stable continental regions. P-wave first motions and waveforms of long-period teleseismic P and SH waves have been modeled to constrain the focal mechanisms and to determine the source parameters for the earthquakes. Previous investigators proposed multiple fault-plane solutions for each of the events. In this paper, some of the early solutions are eliminated and the best source mechanism is derived for each earthquake. For the Bhadrachalam earthquake, the focal mechanism determined is predominantly strike-slip (strike = 245°, dip = 72°, rake = 358°). The strike direction of the NE-SW-trending nodal plane is in good agreement with the trend of the observed isoseismals and is probably the fault plane. The seismic moment is 4.59 × 1024 dyne-cm and the moment magnitude Mw is determined to be 5.7. The focal depth and the static stress drop are estimated to be 10 km and 17 bar, respectively. The strike-slip motion represents a reactivated fault motion under the present-day stress field. For the Broach earthquake, the focal mechanism determined is reverse faulting with a strike-slip component (strike = 273°, dip = 58°, rake = 130°). The direction of the E-W-striking nodal plane is in good agreement with the mapped Narmada fault, dislocation direction of fissures, and the trend of the isoseismals. The seismic moment and moment magnitude are determined to be 1.39 × 1024 dyne-cm and 5.4, respectivety. The focal depth and the stress drop are estimated to be 11 km and 9 bar, respectively. The reverse motion of this event represents an inverted slip or tectonic inversion within a rift zone under the present-day compressional stress field. Some rift-associated earthquakes are characterized by large multiple shocks with significant strike-slip motions. The two Indian earthquakes, however, occurred as regular main shock-aftershock sequences-and represent a different mode of strain release. These two earthquakes also represent events with low stress drops and relatively long recurrence intervals. The interpretation of this last observation is uncertain and needs more study in the future.

The White Mountains seismic gap (WMSG) is a broad area between the 1872 Owens Valley earthquake and the 1932 Cedar Mountain earthquake that has a complicated tectonic setting and seismicity patterns, and is considered to have the potential for a strong or larger-magnitude earthquake in the near future. We take the Sierra Nevada block as the western boundary of the WMSG, the Pancake Range lineament and the southernmost 1932 earthquake ruptures as its approximate northern boundary, a change in structure, tectonic rates and seismicity level as the eastern boundary, and the northernmost 1872 earthquake ruptures as the southern boundary. Seismicity within the WMSG, especially in the southern part, has been at a very high level since 1978. This activity has included six events of magnitude ⩾ 6 and their associated aftershocks. In addition many earthquake swarms have occurred throughout the WMSG, some of which are distinctly located at the ends of fault zones or near changes in structural orientation. Focal mechanisms show a predominance of strike-slip solutions for both small and large earthquakes, with NW-trending right-lateral and NE-trending left-lateral solutions for over half of the mechanisms. These are similar to the sense of displacements and orientations of the larger faults in the WMSG. Thus seismic strain is consistent with the faulting pattern in the WMSG and is accommodating contemporary deformation through a conjugate set of strike-slip and normal-oblique-slip faults. Examination and analysis of eighteen of the larger faults in the WMSG leads to the estimation of characteristic earthquakes ranging from magnitude 6.8 to 7.4. Given the size of the seismic gap (100 km) and the pronounced change in structure near the middle of the seismic gap, it seems likely that it will require two or more magnitude ~ M 7 + events to “fill” the gap.

Earthquake rate analysis explores the quantitative relationship between earthquake occurrence rates and geological deformation rates. One application is the estimation of tectonic deformation rates from seismicity, for comparison with tectonic models. Another application is the use of geological models to help quantify seismic hazards. Seismicity is described by a catalog of the largest earthquakes in the region. The contribution of small earthquakes towards causing geological deformation is insignificant, although the occurrence rate of small events may be predictable from the occurrence rate of large events by a b value relationship. The critical information about each earthquake is its seismic moment, or, better still, its seismic moment tensor. When moment is not available directly, it can be estimated from the earthquake magnitude, although this increases uncertainties considerably. The average annual moment from all the earthquakes in the catalog is the seismic moment rate. Uncertainties in seismic estimates of moment rate arise from the short time period of observations, from the magnitude-moment relationship, from the shape of the distribution function giving number of earthquakes as a function of magnitude and from uncertainties in the physical properties of the Earth near the source. Geological models for deformation rates are described by slip rates on individual faults or by regional strain rates. A regional strain rate can be derived as a limiting case where there are numerous individual faults. Such a strain rate is a particularly useful concept where information about each of the faults is limited. In the case of either faulting or regional strain, the average seismic moment rate is proportional to the deformation rate. Geological uncertainties arise from difficulties in estimating slip rates or strain rates, from fault creep and other aseismic deformation, from uncertainty in the part of the fault that fails during earthquakes and boundaries of the deforming region and from uncertainty about the Earth's physical properties. The seismic moment rate is the critical factor for relating the earthquake occurrence rate and the geological deformation rate. Using this intermediate parameter, one can infer earthquake occurrence rates from deformation rates, or infer deformation rates from earthquake occurrence rates. Generally, it is found that deformation rates and earthquake occurrence rates are consistent within uncertainties. Where both are well constrained, they are generally consistent within a factor of two or three.

Recent observations of strong earthquakes that occurred in the Aegean and surrounding areas during the last several years indicate rather low stress drop values. An interesting example is the 1978 Thessaloniki earthquake (northern Greece), with Ms = 6.5. Values between 6.6 and 12 bars have been determined for this earthquake by assuming unilateral or bilateral rupture propagation and circular or rectangular fault models. These values are considerably lower than the average values calculated not only for intraplate earthquakes (about 60 bars) but also for interplate earthquakes (about 30 bars). In this paper an attempt is made to investigate the cause of this fact by studying the dynamic rupture process of this earthquake. A three-dimensional, spontaneous, shear crack model on a dipping fault in a layered medium was assumed, and a finite-difference approach has been adopted for numerically solving the wave equations in a three-dimensional space. It was found that such a low stress drop may be obtained in cases of small differences between ambient stress and fault strength, and between tectonic shear stress and dynamic frictional stress. Such physical conditions in the focal region may result in an average fault displacement which is close to the observed one for this earthquake.

Ruff, L.J., 1992. Asperity distributions and large earthquake occurrence in subduction zones. In: T. Mikumo, K. Aki, M. Ohnaka, L.J. Ruff and P.K.P. Spudich (Editors), Earthquake Source Physics and Earthquake Precursors. Tectonophysics, 211: 61–83. Plate tectonics and the seismic gap hypothesis provide the framework for long-term earthquake forecasting of plate boundary earthquakes. Unfortunately, detailed examination reveals that earthquake recurrence times and rupture length vary between successive earthquake cycles in the same subduction zone. Furthermore, larger coseismic slip is commonly associated with larger rupture length. Hence, large earthquake occurrence in subduction zones is characterized by variability in: (1) recurrence times, (2) rupture length, and (3) coseismic slip. These facts, plus many other observations, indicate that there are significant spatial variations in the “strength” of the plate interface. One simple description of these variations and their role in the earthquake cycle is the asperity model, where the large strong regions of the plate interface are called asperities, and the large earthquakes occur when the large asperities break. The asperity model of earthquake occurrence is able to qualitatively explain several features of large plate boundary earthquakes. To go beyond general qualitative notions. I pose the following scientific test: are the observed asperity distributions and a simple model of their interaction self-consistent with the above three observed features of large earthquake occurrence? The distribution of the major asperities along plate boundary segments has now been determined for several subduction zones. Rupture process studies of adjacent large and great earthquakes have provided reliable estimates of the along-strike asperity lengths and separations for several adjacent asperities in the Kurile Islands, Colombia, and Peru subduction zones. The simplest mechanical model for asperity interaction is to idealize two adjacent asperities as frictional sliders that are connected by main springs to the upper plate, by a coupling spring to each other, and maintain frictional contact with a conveyer belt (the lower plate) that moves with a constant velocity. An “earthquake” occurs when the net force on the asperity frictional slider reaches some specified level. The failure force and spring constants are determined by the observed asperity distribution and simple models of elastic interaction. Two different macroscopic failure criteria are used. This simple mechanical model displays a remarkable range of behavior from simple to complex. When the two asperities are identical in all their properties, sequences of identical “earthquakes” are produced. For the more realistic case of non-identical asperities, “earthquake” sequences show great variety. Using system variables from the observed asperity distributions, the “earthquake” sequences typically display: (1) variable recurrence times, (2) variable rupture length, i.e. a combination of single-asperity and double-asperity failures, and for one of the failure criteria (3) larger coseismic slip for double-asperity failures. Statistical summaries of thousands of simulated “earthquake” sequences for asperity pairs in the Kuriles, Colombia, and Peru subduction zones are broadly consistent with the observed features of large earthquake occurrence in these subduction zones. The main conclusion is that the asperity model provides a self-consistent explanation for: fault zone heterogeneity, the rupture process, and recurrence times and rupture mode of large earthquake sequences via a simple model for adjacent asperity interaction. In addition, a conclusion independent of any particular model for fault zone heterogeneity is that simple deterministic models of fault zone interaction can explain complex patterns of large earthquake occurrence in subduction zones.

Umeda, Y., 1992. The bright spot of an earthquake. In: T. Mikumo, K. Aki, M. Ohnaka. L.J. Ruff and P.K.P. Spudich (Editors). Earthquake Source Physics and Earthquake Precursors. Tectonophysics, 211: 13–22. Field surveys in the source region for some large shallow earthquakes reveal that there is a special spot characterized by high acceleration, aftershock gap and parallel faults system. This spot is called an “earthquake bright spot” which occupies a relatively small region compared to the final rupture region. At least two kinds of phases, P1 and P2, which are responsible for the rupture initiation and the bright spot formation can be identified on the broad-band seismograms for large earthquakes. Duration times of preliminary waves (P1−P2) are proportional to the earthquake magnitudes. By using this evidence from the field and seismograms, a model having three rupture stages is proposed for a growth process of large earthquakes. The 1st stage is an initiation and propagation of rupture. The 2nd stage is formation of the bright spot which produces the high-frequency seismic waves. In the 3rd stage, smooth dislocations again propagate in bi-laterally and radiate the long-period seismic waves.

The spatial pattern of the epicentral distribution in the northwest Chubu district of Honshu Island, Japan changed drastically after the occurrence of the 1948 Fukui earthquake ( M = 7.1 ). The change can be explained by invoking that the WNW-ESE trending seismic belt which contains the focal region of the Fukui earthquake, was activated, and that the activity along another seismic belt trending NE-SW which had been conspicuous before the Fukui earthquake, disappeared. A notable feature is that large earthquakes occurred successively along the newly activated seismic belt migrating to the east. The 1984 Nagano-ken Seibu earthquake ( M = 6.8 ) occurred on the extension of that migrational activity. The focal region of the Nagano-ken Seibu earthquake is situated at a distinct inflection point in the seismic zone which was activated after the Fukui earthquake. The spatial pattern of the seismic zone is very similar to the distribution of Quaternary volcanoes in that region, and the same pattern is also recognized in the Bouguer anomaly. These features suggest that the seismic belt is a weak zone in the crust corresponding to the boundary of the tectonic blocks. The focal region of the Nagano-ken Seibu earthquake had been very quiet over a period of 10 years, with the seismic activity subsequently resuming several years before the Nagano-ken Seibu earthquake presenting two active centers in the seismic gap, near one of which a large destructive earthquake had occurred in 1906. It seems that the occurrence of the Nagano-ken Seibu earthquake could have been predicted if the characteristics of the seismic activity after the Fukui earthquake and the tectonic features in the region had been correctly interpreted.

The Lesser Antilles subduction zone is an extreme case of the subduction of old (~ 90 m.y.) lithosphere at a slow (~ 2 cm/y) convergence rate. Focal mechanisms of the largest earthquakes in the area have been obtained using body and surface wave data. During the time period (1950–1978) studied the subduction seismicity appears to represent primarily intraplate rather than interplate deformation. All three large (magnitude seven) earthquakes were from intraplate normal faults; no large thrust faulting earthquakes and few small ones occurred. These observations suggest that the plate boundary is largely decoupled, that subduction is at least partially aseismic, and that the downgoing slab is in a state of extension.

Characteristic patterns of intraplate large shallow earthquake occurrence in the Japanese Islands are investigated. It is shown that in each district there are seismic belts or seismotectonic active zones where large shallow intraplate earthquakes frequently occur. In some cases the activation of seismic belts seems to be related to the occurrence of large shallow earthquakes, such as the 1891 Nobi earthquake with a magnitude of 8.0 and the 1923 Kanto earthquake with a magnitude of 7.9. Also it is probably caused by stress increase due to interplate coupling, such as in the Chugoku district, southwest Japan, before the 1946 Nankaido earthquake with a magnitude of 8.1. In the Kyushu district, other unique tectonics seem to be involved in the intraplate seismic activity. Non-overlapping of active areas and the concentration of earthquake occurrence within a relatively short period are usually observed for each seismotectonic zone. Further, migration of seismic activity and synchronous activity as well as correlation of earthquake occurrences in distant parts along seismotectonic belts are recognized in most tectonic zones. It is pointed out that the same concept of seismic gap of the first kind, originally proposed for interplate large shallow earthquakes along the deep sea trench, may also be applicable to the occurrence of intraplate large earthquakes along seismic belts. These characteristic patterns are considered to give a promising clue to selecting the sites where large earthquakes are expected to occur in the near futue. Seismic belts or seismotectonic active zones probably play an important role in intraplate tectonics. They are a kind of boundary of blocks along which change in stress field can be transferred easily, though their tectonic origin may be different for each seismotectonic zone.

The most damaging earthquake to occur in New Zealand in 1982 was the Hawke's Bay earthquake of September 02 ( m b = 5.6). An aftershock study using portable microearthquake recorders has established that this event occurred some 20 km south of Hastings, at a depth of about 47 km. This depth places the event in the mantle of the subducted Pacific plate. This earthquake is a member of a recently recognized class of intraplate shocks occurring along the strike of the subducted plate near the east coast of the North Island. Such events are characterized by normal-faulting mechanisms. When taken together with activity in the crust of the subducted plate, this subcrustal activity resembles a double-planed seismic zone, restricted in the down-dip direction of the subducted plate. Several lines of evidence suggest that deformation in the mantle of the subducted plate is controlled by the state of coupling of the plate interface at shallow depth: 1. (1) the subcrustal activity occurs directly beneath the source region of major interplate earthquakes along the east coast of the North Island, and there appears to be a pairing of interplate and subcrustal events; 2. (2) stress directions determined for the subcrustal events are concordant with geodetic strain directions; and 3. (3) the amount of subcrustal activity appears to be related to the state of coupling of the plate interface.