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In this study, we detect the 410 km discontinuity (410) beneath the Hindu Kush-Pamir region using broadband records of SdP converted phases. Beneath Hindu Kush, the 410 appears at the depths of 367–384 km with an average depth of 375 km; while beneath Pamir, it appears at the depths of 380–414 km with an average depth of 395 km. Compared with the Earth model IASP91, the 410 shows larger uplifts of 26–43 km with an average of 35 km beneath Hindu Kush, and smaller uplifts of 7–30 km with an average of 15 km beneath Pamir. Based on seismic tomographic images, the S410P conversion points are all located inside Indian slab beneath Hindu Kush, and near Asian slab with some of them inside the slab beneath Pamir. The 410 shows the 7–22 km uplifts with an average of 11 km near Asian slab, and the 21–30 km uplifts with an average of 25 km inside Asian slab. If depth variations are caused by the thermal heterogeneity, the 35 km uplift inside Indian slab indicates a low-temperature anomaly of 422 K; whereas the 11 km uplift near Asian slab and 25 km uplift inside Asian slab imply low-temperature anomalies of 133 K and 302 K, respectively. We suggest that the uplifts of the 410 are likely caused by cold subducted slabs, and the uplift differences imply different thermal impacts of the slabs beneath Hindu Kush and Pamir.

The south-western edge of the Calabrian Arc in southern Italy has been investigated throughout a joint analysis of field, marine and geophysical data which provided constraints on the fault pattern and on the seismotectonic potential. The study was focused on a poorly known sector of a larger belt of seismically active faults slicing across the NE corner of Sicily, the so-called Tindari Fault System. Our data pointed out that the investigated area, including the mainland and the Ionian offshore, is deformed by oblique faulting with a general NW-SE tectonic trend. Earthquake distribution and seismic profiles pointed out active deformation in the offshore while the mainland is characterized by the occurrence of a NW-SE oriented, >20 km-long, structural belt. However, scarce seismicity has been recorded in the last 30 years alongside this tectonic structure, accounting for a possible silent segment of the larger fault system. Tomographic images revealed that the Moho discontinuity is deformed by a NE-dipping lithospheric tectonic structure which has been here retained the main mode of deformation and responsible for coseismic displacement in the area. As a whole, field and geophysical data agree with a general NW-SE trend segmented pattern of recent/active faults that have the potentiality of generating magnitude 6.5–7 earthquakes.

The Yarlung Zangbo Suture (YZS) is the collisional front between the Indian and Eurasian plates. The deep structure beneath the YZS has long been a focus of research. Its particular tectonic location and formation time provide a natural laboratory for the study of ongoing continental collisional processes. Lateral variations in the Moho discontinuity provide evidence of basic processes governing plate tectonic evolution. The depth and geometry of the Moho provides first-order information for restoration of complex geodynamic systems. This paper presents a studying result of the Moho, both adjacent to and beneath the YZS, obtained using five large dynamite shots along two profiles. These five large shots were combined to construct two single-fold profiles (YZS-A and YZS-B) and obtain high-resolution images of the Moho beneath the YZS. Both profiles show that continental crust thickness beneath the YZS is twice the global average of continental crust, and the depth of the Moho along profile YZS-B is deeper than YZS-A. YZS-A (81°E) indicates a relatively flat Moho without offset related to continent-continent convergence beneath the collision zone. Conversely, a Moho structure with offset and overlapping reflections was imaged beneath profile YZS-B (88°E). Significant lateral variations in the geometry of the Moho discontinuity along the YZS may indicate that the Indian lithosphere is thrusting beneath Tibet at a different angle from west to east.

This study revisits subduction processes at the Hellenic Subduction Zone (HSZ) including tearing, segmentation, and backstepping, by refining the geometry of the Nubian slab down to 150–180 km depth using well-located hypocentres from global and local seismicity catalogues. At the western termination of the HSZ, the Kefalonia Transform Fault marks the transition between oceanic and continental lithosphere subducting to the south and to the north of it, respectively. A discontinuity is suggested to exist between the two slabs at shallow depths. The Kefalonia Transform Fault is interpreted as an active Subduction-Transform-Edge-Propagator-fault formed as consequence of faster trench retreat induced by the subduction of oceanic lithosphere to the south of it. A model reconstructing the evolution of the subduction system in the area of Peloponnese since 34 Ma, involving the backstepping of the subduction to the back-side of Adria, provides seismological evidence that supports the single-slab model for the HSZ and suggests the correlation between the downdip limit of the seismicity to the amount of subducted oceanic lithosphere. In the area of Rhodes, earthquake hypocentres indicate the presence of a NW dipping subducting slab that rules out the presence of a NE-SW striking Subduction-Transform-Edge-Propagator-fault in the Pliny-Strabo trenches region. Earthquake hypocentres also allow refining the slab tear beneath southwestern Anatolia down to 150–180 km depth. Furthermore, the distribution of microseismicity shows a first-order slab segmentation in the region between Crete and Karpathos, with a less steep and laterally wider slab segment to the west and a steeper and narrower slab segment to the east. Thermal models indicate the presence of a colder slab beneath the southeastern Aegean that leads to deepening of the intermediate-depth seismicity. Slab segmentation affects the upper plate deformation that is stronger above the eastern slab segment and the seismicity along the interplate seismogenic zone.

We investigate the mantle of central-eastern Greenland by using recordings with data from 24 local broad-band seismograph stations. We apply P wave receiver function technique and evaluate the difference in the arrival times of seismic phases that are formed by P to SV mode conversion at the 410-km and 660-km seismic discontinuities. These boundaries mark the top and bottom of the mantle transition zone (MTZ). The difference in the arrival time of the phases from the 410-km and 660-km discontinuities is sensitive to the thickness of the MTZ and relatively insensitive to volumetric velocity anomalies above the 410-km discontinuity. Near the east coast of Greenland in the region of the Skaergaard basalt intrusions we find two regions where the differential time is reduced by more than 2 s. The 410-km discontinuity in these regions is depressed by more than 20 km. The depression may be explained by a temperature elevation of ~150 °C. We hypothesize that the basaltic intrusions and the temperature anomalies at a depth of ~400 km are, at least partly, effects of the passage of Greenland over the Iceland hotspot at about 55 Ma. This explanation is consistent with the concept of tectosphere and implies that the upper mantle to a depth of ~400 km translates coherently with the Greenland plate.

Deciphering the internal composition of large-scale fault zones helps to better understand the various geologic factors that govern their seismic rupture behavior. We used new detailed geologic, structural, and geomorphic map of an excellent natural laboratory in the Southern Yangsan Fault (SYF) in the southeastern part of the Korean Peninsula to elucidate the fault zone architecture and its controls on the rupture processes of neotectonic earthquakes. The Yangsan Fault is a major strike–slip fault, and a subsidiary fault within the fault zone ruptured during the 2016 ML 5.8 Gyeongju earthquake which is the largest instrumental record in the intraplate Korean Peninsula. The 12-km-long study area in the SYF shows along-strike variations in wall rock lithology and fault zone architecture. At the south of the study area, the fault zone surrounded by crystalline rocks consists of a single major fault core. In contrast, at the north, the fault zone is sandwiched between crystalline rocks and sedimentary rocks and exhibits multiple major fault cores bounding broken-up wall rock blocks. The width of the across-fault damage zone, which was estimated from the cumulative fracture frequency with the distance from the major fault core, widens northward, indicating the distributed deformation of the northern fault zone over a long history of fault evolution. The geometry of paleoearthquake surface ruptures traced by the geomorphic and stratigraphic evidence of deformed young deposits diverge to multiple strands in the northern fault zone but converge to a single strand in the southern fault zone, directly reflecting its internal architecture. We propose that the long-lived rupturing patterns have been primarily controlled by intrinsic discontinuity- and mineralogy-dependent deformation processes in terms of strain-hardening vs. strain-weakening, along with propagation arrest and rupture tip damage due to a fault-bend geometry (geometric barrier) and an abrupt change in wall rock lithology (relaxation barrier).

Understanding the complex heterogeneity of the continental lithosphere involves a wide variety of spatial scales and the synthesis of multiple classes of information. Seismic surface waves and multiply reflected body waves provide the main constraints on broad-scale structure, and bounds on the extent of the lithosphere-asthenosphere transition (LAT) can be found from the vertical gradients of S wavespeed. Information on finer-scale structures comes through body wave studies, including detailed seismic tomography and P-wave reflectivity extracted from stacked autocorrelograms of continuous component records. With the inclusion of deterministic large-scale structure and realistic medium-scale stochastic features fine-scale variations are subdued. The resulting multi-scale heterogeneity model for the Australian region gives a good representation of the character of observed seismograms and their geographic variations and matches the observations of P-wave reflectivity. P reflections in the 0.5–3.0 Hz band in the uppermost mantle suggest variations on vertical scales of a few hundred metres with amplitudes of the order of 1%. Interference of waves reflected or converted at sequences of such modest variations in physical properties produce relatively simple behaviour for lower frequencies, which can suggest simpler structures than are actually present. Vertical changes in the character of fine-scale heterogeneity can produce apparent discontinuities. In Central Australia a ‘mid-lithospheric discontinuity’ can be tracked via changes in frequency content of station reflectivity, with links to the broad-scale pattern of wavespeed gradients and, in particular, the gradients of radial anisotropy. Comparisons with xenolith results from southeastern Australia indicate a strong tie between geochemical stratification and P-wave reflectivity.

The axial fault-bounded depression of the South Kenya rift (SKR) locally displays anomalously wide sectors resulting from the presence of one (or many) elevated and offset block(s) on the flanks of the main trough. Very little attention has been paid so far to the nature of the driving mechanisms responsible for these atypical rift patterns. New insights are supplied by the Natron-Ol Doinyo Ogol rift segment at the southern extremity of the SKR, immediately north of the North Tanzanian Divergence (NTD). On the basis of interpreted SRTM-30 satellite imagery and Digital Elevation Models, our work allows us: i) to depict the highly-segmented arrangement of the ~7 Ma-lasted SKR system, ii) to establish a two-stage kinematic rift model that emphasizes the role of an inherited transverse discontinuity on the arrest, as well as lateral jump and off-axis development of anomalously-propagating rift structures, iii) to define the relative contribution of border vs inner fault networks to the total extension, which is estimated at 7–6 km (11.6–9.2%), and iv) to emphasize that inner faulting was not the dominant mode of strain accommodation during recent inward focussing of strain, and that no sharp transition exists from border fault- to intra-rift fault-dominated strain accommodation over time in the SKR immature rift system.

Do plumes exist? If so, do plume tails connect hotspots to the core-mantle boundary (CMB), or only to the 660-km discontinuity (between the upper and the lower mantle)? Are they continuous or fragmented? Here, we demonstrate quantitatively that the Hawaiian plume distorts 24-h-period magnetovariational data by providing a continuous electrical connection between the Earth's surface (via the ocean) and the 660-km mid-mantle discontinuity. We extend our analysis to predict how a plume ascending from the CMB should distort magnetic secular variation observed locally at the surface. Hence, the Hawaiian plume is demonstrated to be a continuous feature with a deep-seated (at least mid-mantle) source. Our analysis provides a novel technique for determining whether the Hawaiian plume originates at the mid-mantle transition zone or CMB subject to magnetic secular variation data being acquired locally, as well as allowing the depth of genesis of other mantle plumes to be constrained should further geomagnetic data become available in their vicinity.

The Mohorovicic discontinuity (Moho) is defined on the basis of an abrupt increase in seismic velocity in the lithosphere which has been observed using seismic refraction and receiver function analysis methods worldwide. Moho depth varies regionally and remains a fundamental parameter of crustal structure. We present a new method of mapping the Moho using a 3D seismic tomography model. Since the tomographic method cannot locate discontinuities, we treat the Moho as a zone of high velocity gradient. Maximum lower crust/minimum upper mantle P-wave velocities in Japan are known to be 7.0 km/s and 7.5 km/s, respectively. We map the residual between isovelocity surfaces of 7.0 km/s and 7.5 km/s to find areas where the residual is small, the separation between the surfaces is narrow, and the velocity gradient is high. The Moho is best constrained where the isovelocity surfaces are close together, and under much of Japan, they are 10 km apart. We chose an isovelocity surface of 7.2 km/s as a representative Moho ‘proxy’ in these areas. Our resulting ‘Moho’ map under Japan compares favorably with existing regional Moho models that were obtained from controlled-source seismic investigations. The ‘Moho’ varies from shallow (25–30 km) to deep (> 30 km), and this variability relates to the structural evolution of the Japanese islands: the opening of the Sea of Japan back-arc, ongoing arc-arc collisions at the Hidaka and Izu collision zones, ongoing back-arc extension in Kyushu, and a possible failed back-arc extensional event of Mesozoic age. It is apparent that the Moho is less well-constrained in areas where the crustal structure has been modified by magmatic activity or thickened due to arc-arc collision.

The North Patagonian Massif is an Argentinean plateau that has an average height of 1200 m and stands from 500 to 700 m above the neighboring areas. During Paleogene, it suffered a sudden uplift of more than 1200 m without noticeable internal deformation; thus, it could be related to isostatic disequilibrium. To shed light on the geodynamic development of the area it is necessary to characterize the present-day configuration of the crust. In this study, a lithospheric-scale 3D density model was developed by integrating all the available data of the area with the objective of assessing the depth of the crust–mantle discontinuity (Moho). During the construction of the initial density model, we tested different mantle density scenarios obtained using P- and S-wave velocities from tomographic models, converting them into densities and comparing the conversions with densities obtained from xenoliths. Below the North Patagonian Massif plateau, we have derived a Moho depth between 40 and 50 km which is from 2 to 7 km deeper than its surroundings. There is an evident correlation between high topography and deep Moho that would indicate isostatic equilibrium at present. The model results provide a new approach to the Moho depth in an area where there is no seismic constraining information about this discontinuity. In addition, we found a spatial correlation between the variation of the mean crustal density and the location of the Paleozoic terranes that were proposed to constitute the basement of Argentina., Facultad de Ciencias Astronómicas y Geofísicas, Facultad de Ciencias Naturales y Museo, Comisión de Investigaciones Científicas de la provincia de Buenos Aires

A temporary seismic array was in operation between October 2010 and March 2013 in the Gyeongju area of Korea. Teleseismic records of the seismic array appropriate for receiver function analysis were collected, and selected seismograms were split into five groups based on epicenters—the Banda-Molucca, Sumatra, Iran, Aleutian, and Vanuatu groups. 1D velocity structures beneath each seismic station were estimated by inverting the stacked receiver functions for possible groups. The inversion was done by applying a genetic algorithm, whereas surface wave dispersion data were used as constraints to avoid non-uniqueness in the inversion. The composite velocity structure was constructed by averaging the velocity structures weighted by the number of receiver functions used in stacking. The uncertainty analysis for the velocity structures showed that the average of 95% confidence intervals was ± 0.1 km/s. The 3D velocity structure was modeled through interpolation of 1D composite velocity structures. Moho depths were determined in each composite velocity structure based on the AK135-F S-wave velocity model, and the depths were similar to the H-κ analysis results. The deepest Moho depth in the study area was found to be 31.9 km, and the shallowest, was 25.9 km. The Moho discontinuity dips in a southwestward direction beneath the area. A low velocity layer was also detected between 4 and 14 km depth. Adakitic intrusions and/or a high geothermal gradient appear to be the causes of this low velocity layer. The 3D velocity structure can be used to reliably assess seismic hazards in this area.

Passive seismic recordings of teleseismic P wave arrivals and their immediate coda along a dense profile of stations can be used to image the reflection structure beneath the profile. The process exploits the autocorrelation of the seismic signals that extracts the reflection response from the transmitted signals recorded at the surface, which can be migrated to provide a depth image. This common reflection point (CRP) imaging exploits the same portion of the seismic record used in receiver function studies, but is much less influenced by multiple problems in the presence of sediments. This style of passive seismic imaging is applied to a dense line of 68 stations at 3.5 km spacing at the northern edge of the Gawler Craton in South Australia in the MAL experiment. With nine months of passive seismic monitoring it has been possible to image fine-scale structural variations throughout the lithosphere. There is a strong variation in crustal thickness as the MAL profile crosses the inferred location of the craton edge under sedimentary cover. A transitional zone between crust and mantle can be identified that extends beneath the receiver function Moho, pinching out gently at the craton edge. The CRP imaging reveals distinct reflectivity in the lithospheric mantle with horizontal scales of 10–30 km, and vertical scales of no more than a few kilometres. The character of this reflectivity appears to change across the craton edge. There are indications of a semi-continuous mid-lithosphere discontinuity at around 90 km depth.

In this paper, we study the spatial distribution and temporal evolution of the postseismic deformation of the 2010–2011 Rigan earthquake sequence which occurred at the southern termination of the East Lut fault system, southeast Iran. One-year GPS measurements after the Rigan earthquake sequence reveals right-lateral postseismic displacement along the East Chahqanbar fault and left-lateral postseismic displacement along the South Chahqanbar fault. To investigate the deformation variations in time and space, InSAR time-series of COSMO-SkyMed images is applied using the Small Baseline Subset algorithm. The results confirm a clear cumulative postseismic signal increasing to 8 mm during the first five months following the first mainshock in the direction of the coseismic displacement. The cumulative postseismic displacements are well correlated with the cumulative number of the aftershocks and their associated moment release. Considering this correlation and the observation of a sharp discontinuity along the coseismic fault in the displacement map, it is concluded that the after-slip mechanism is responsible for the observed postseismic deformation in the study region. This study is the first observation of a short-term postseismic motion in eastern Iran through geodetic data in contrast with long-lasting postseismic displacements following the earthquakes that occurred around Lut block. Modeling of the postseismic displacement results in a distributed slip pattern with a maximum slip of 0.8 m on the fault plane responsible for the 2010 Rigan coseismic deformation. This indicates that the postseismic deformation on barriers remained unbroken during the mainshock.

Recent studies evoke dispute whether the Greater Himalayan Crystalline Complex (GHC) was exhumed during more than one phase. This contribution investigates the timing of ductile shear along the South Tibetan Detachment (STD), Main Central Thrust (MCT), and hidden discontinuities within the GHC of the central Himalaya. New data from the Nyalam transect, southern Tibet, suggest that ductile shear along the STD was active from ~ 27–25 Ma to ~ 17–15 Ma, whereas ductile shear along the MCT was active from ~ 19–16 Ma to ~ 10–8 Ma. Pseudosection modeling results and published metamorphic ages indicate a P–T–t path discontinuity within the GHC and the upper GHC reached higher peak metamorphic temperatures (ΔT 100 °C), similar peak pressures and older metamorphic ages (~ 5–10 Myr), in favor of the presence of the Nyalam Discontinuity. Summarized results from literature suggest that timing of ductile shear along the STD was ~ 5–10 Myr earlier than that along the MCT across the Nepal, Sikkim and Bhutan Himalayas. In addition, the GHC in Nepal can be separated into two portions by a regional-scale (> 800 km) in-sequence ductile shear zone — the High Himalayan Discontinuity. This shear zone was active coevally with the STD during 25–16 Ma, but ~ 5–10 Myr earlier than the MCT. The High Himalayan Discontinuity could possibly extend to the Sikkim Himalayas, but ends in Bhutan. The identified new architecture suggests that both in-sequence and out-of-sequence tectonometamorphic discontinuities are important components that formed the duplexing structure of the GHC in the central and eastern Himalaya, thus calling for more sophisticated numerical simulation than the current models such as the channel flow and critical taper.

Seismic data from northeast (NE) China and the Sino-Korean craton were combined to image the upper mantle discontinuities at 410 and 660 km. Fine-scale topographic variations on these two discontinuities provide important clues for both delineating geometry of the subducting Pacific slab particularly at arc–arc junction and interpreting regional Cenozoic intraplate volcanism. We used over 90,000 receiver functions from 1916 teleseismic earthquakes recorded by 584 broadband seismic stations, primary those of temporary seismic arrays. We found the average depths of the two discontinuities to be 410 km and 672 km, respectively, beneath the study area. Results show that the 660-km discontinuity is strongly depressed by about 20–30 km in a narrow region beneath and around the Changbaishan volcano, consistent with the results of previous receiver function studies. In contrast, much of the Sino-Korean craton exhibits typical transition zone thickness (~ 260 km) and thus offers no evidence of a stagnated Pacific slab. Our results also reveal an elevated 660-km discontinuity and a thinner transition zone both to the west of the observed depression region and beneath the Kuril–Japan arc junction. This feature is most likely due to a tearing of the descending Pacific plate at both its leading and junction edges. An additional elevated 660-km discontinuity together with a thinner transition zone appears in the vicinity of the Dariganga lava field, supporting interpretations of a deep-rooted mantle plume. Our observations of an elevated 410-km discontinuity and a thicker transition zone correlate spatially with the diffuse distribution of volcanism around Hannuoba, Aershan and Wudalianchi. This correlation may suggest lithospheric removal as a mechanism for these magmatic activities.

The Lishan fault has been characterized alternately as a major discontinuity in stratigraphy, structures and metamorphism, a ductile shear zone, a tectonic suture or non-existent. In addition to being a geological boundary, it also marks transitions in subsurface structures. Thus, the seismicity to the west of the fault permeates through the upper and mid-crust while beneath the Central Range it is noticeably less and largely concentrated in the upper 12 km. A prominent west-dipping conductive zone extends upward to meet the Lishan fault. Also, the eastward increase of crust thickness from ~ 30 km in the Taiwan Strait quickens under the Lishan fault to form a root of over 50 km under the Central Range. In the past, the small magnitude seismicity along the Lishan fault has been noticed but is too diffuse for definitive association with the fault. Recent processing of aftershock records of the 1999 Mw 7.6 Chi-Chi earthquake using Central Weather Bureau data and, especially, data from three post-Chi-Chi deployments of seismic stations across central Taiwan yielded hypocenters that appear to link directly to the Lishan structure. The presence of a near 4-km-long vertical seismic zone directly under the surface trace of the Lishan fault indicates that it is an active structure from the surface down to about 35 km, and the variety of focal mechanisms indicates that the fault motion can be complex and depth-dependent.

This paper presents a new study on 3-D structure, temperature and water content estimation of the mantle transition zone (MTZ) by P-wave receiver functions common conversion point (CCP) method on broadband array with 267 temporary stations and 203 permanent stations in southeastern China. The results show different structural characteristics of the MTZ to the north and south of the 29°N line. In the northern part, the 660-km discontinuity is generally deeper than globe models, and there are two independent local depressions with characteristics of high velocity, low temperature, and water shortage. We interpret them as two previous subducting plates stagnated in MTZ, possibly related to two separate subduction events at different times. While in the southern part, the 410-km discontinuity is locally depressed, resulting in thinning of the MTZ. Particularly, in the Cathaysian Block south of the 29°N line, based on the MTZ structure and the estimation of temperature and water content, we conside dehydration of the MTZ as one of the important mechanisms of lithospheric thinning through interaction with the asthenosphere. We then propose a dynamic model that combines paleo-Pacific flat-slab subduction and upper mantle convection to explain the lithospheric extension and thinning, magmatic rock distribution, weak seismic activity and basin-and-range geomorphology. This model differs from that north of the 29°N line in the North China Block.

Numerical simulations have indicated that the Large low-Shear Velocity Provinces (LLSVPs) dynamically interact with subducted slabs in D″, and slabs in turn play an important role in influencing the shape and location of the LLSVPs. In this study, we find evidence for the existence of the D” discontinuity near the northeastern margin of the Perm Anomaly — a smaller LLSVP beneath Eurasia, by analyzing the Scd (the lower mantle triplication) phases generated from two deep events occurring in the northwestern Pacific subduction zone and recorded at stations in Europe and Turkey. The D″ discontinuity in the north is characterized with a +2% velocity increase occurring at a height of 300 km above the core-mantle boundary (CMB), whereas the discontinuity in the south is characterized with a +3% velocity increase occurring at a height of 260 km above the CMB. The D″ discontinuity is most likely attributed to a phase transition from perovskite to post-perovskite in conjunction with the presence of relic slab materials (likely related to the subducted Mongol-Okhotsk slab). Our observations– along with previous seismic anisotropic detections– provide a clue to dynamically link the subducted slab to deformation taking place near the edges of the LLSVP.

We have investigated the crustal architecture of the Borborema Province of NE Brazil by constructing common conversion point (CCP) receiver function stacks from teleseismic P-waveforms recorded at 64 seismic stations in the region. The Borborema Province represents the western portion of a larger Neoproterozoic mobile belt that experienced extension in the Mesozoic, leading to the formation of a number of intra-continental rift basins and, eventually, continental breakup. After continental breakup, episodes of uplift in the Province — perhaps related to coeval episodes of Cenozoic volcanism — helped shape the high topographies of the Borborema Plateau. Our receiver function CCP stacks image clear P-to-S conversions at the crust–mantle boundary and confirm independent evidence for a 36–38 km thick crust under the southern portion of the Plateau and a thinner 30–32 km thick crust in the surrounding regions, including the northern Plateau. The cross-sections also reveal the presence of an intra-crustal discontinuity at 9–18 km depth under the regions of thin crust that fades away under the thick southern Plateau. We argue that the thin crust in the Borborema Province is the result of Mesozoic crustal stretching and that the intra-crustal discontinuity represents a low-angle detachment zone that helped accommodate extension in the crust. The thick crust under the southern Plateau would then represent a rheologically stronger portion of the Borborema Province that resisted deformation by Mesozoic extension, while the thin crust under the northern Plateau would be a portion of formerly depressed thin crust that was uplifted during the Cenozoic.

Close to the western Upper Rhine Graben Main Fault, Alsace, a NNE–SSW-striking fault zone, crosscutting porous, thick bedded Lower Triassic Bunter sandstone was investigated in detail, including its architecture, discontinuity system, mechanical rock properties and implications on its permeability structure and fault zone type. Field observations indicate a three-part fault zone structure including core-, transition- and damage zone. The at least 14 m thick fault core is composed of various slip surfaces and deformation bands, which encompass fractured host rock lenses. When connected, host rock lenses may transport fluids through the core zone. Adjacent transition zones are highly fractured in R1-orientation, show folded beds and contain P1-oriented deformation bands. R1 and P1-fractures are synthetic shear fractures and project with an acute angle (10–20°) toward the fault plane. Only in the damage zone, fault-parallel striking fractures occur. Here, increasing fracture apertures and connectivity may increase the permeability toward the fault core. Mechanical rock properties from 12 rock samples (Young's modulus, uniaxial compressive strength, tensile strength) measured in all the parts of the fault zone, show highest values within the transition zone. In-situ measurements of rebound-hardnesses with a Schmidt-Hammer and analytical approaches, however, indicate that effective Young's moduli are two to sixteen times lower than the Young's moduli of intact rock. Values clearly decrease toward the fault core, even in the transition zone and are in average lower than effective Young's moduli in the damage zone. Although many fault zones in sandstone are sealing structures these field study show, that fault zones in porous sandstone may allow fluid flow.

In 1997, the Kagoshima earthquake doublet, consisting of two closely associated Mw ~ 6 strike-slip events, five km and 48 days apart, has occurred in southwest Japan. The location is where an E–W trending discontinuity along 32°N latitude on southern Kyushu Island is clearly defined in GPS velocities, indicating the presence of a highly active left-lateral shear zone. However, there have not been any obvious indications of active faulting at the surface prior to the earthquake doublet, which could be associated with this shear zone. Three-dimensional inversion of magnetotelluric sounding data obtained in the source region of the earthquake doublet reveals a near-vertical conductive zone with a width of 20 km, extending down to the base of the crust and perhaps into the upper mantle toward the Okinawa trough. The prominent conductor corresponds to the western part of the active shear zone. Elevated 3He/4He ratios in groundwaters sampled from hot spring and drinking water wells suggest the emission of mantle-derived helium from the seismic source region. The geophysical and geochemical observations are significant indications that the invasion of mantle fluids into the crust, driven by upwelling asthenosphere from the Okinawa trough, triggers the notable left-lateral shearing in the zone in the present-day subduction system. In addition, the existence of aqueous fluids in and below the seismogenic layer could change the strength of the zones, and alter the local stress regime, resulting in the occurrence of the 1997 earthquake doublet.

Non-planarity is a common and natural characteristic of seismic faults that strongly affects the rupture dynamics and the associated seismic waves. Geometrically uniform planar fault model is widely used in studies of kinematic source inversion, seismic hazard estimation and rupture dynamic simulation because of the simplicity and computational efficiency of this method. The influence of the geometric heterogeneity of faults on rupture dynamics and associated seismic waves may under some circumstances be reasonably approximated by an appropriate heterogeneous distribution of shear strength of planar faults, particularly for the case with uniform initial stress distribution. An outstanding issue is how to approximate the dynamic behavior of a non-planar fault by means of heterogeneous properties on a planar fault. Thus, we propose the construction of an equivalent planar fault model for a given stepover fault through the introduction of a heterogeneous distribution of static friction coefficient. Our study shows that the distribution of static friction coefficient on the equivalent planar fault model is mainly controlled by the geometric irregularities of the stepovers regardless of the initial shear stress, thereby providing a way to quantify the effect of fault geometry, particularly the step width effect in seismic hazard estimation. Moreover, our results indicate that the influence of geometric discontinuity of stepovers on rupture dynamics is equivalent to a barrier of the planar fault model.

The crustal velocity structure in a region of central Apennines of Italy at the hinge between the highly stretched portion of the Monte Argentario promontory and the magmatic province of the Tolfa Domes Complex (Northern Latium) is discussed in this study. S-wave velocities at depth have been constrained by the modeling of P-wave receiver functions (RF) from both temporary and permanent broadband seismic stations. The computer 3D Vs models show a thin crust (19–25 km) made of a shallow and thin sedimentary cover, a very high velocity and anisotropic layer related to a metamorphic basement, and a low Vs anisotropic layer in the middle-lower crust above a shallow Moho discontinuity modeled at about 20 km depth. The volcano-tectonic evolution of this portion of Tyrrhenian back-arc margin has been strongly influenced by its peculiar crustal architecture. The low-Vs layer acted as a shear zone in the middle-lower crust during the Tyrrhenian extension, also helping the development of Plio-Quaternary magmatism. Our findings potentially give new constraints on the evolution of the area and to the general comprehension of back-arc development in collisional regions.

A teleseismic data set from the POLARIS project is used to obtain 3-D images of southern Ontario using two imaging techniques: scattering tomography and common-conversion-point stacking. The resulting images reveal a layered crust, the layering being interrupted by discontinuities associated with major crustal-scale faulting. Breaks in crustal continuity and Moho deflections associated with the Ottawa-Bonnechere Graben indicate that the graben is associated with faulting on a whole crust scale. We also detect similar discontinuities across the Mississauga Domain, supporting the previous interpretation that the domain is bounded by crustal-scale faults. We locate discontinuous sub-lithospheric negative-polarity arrivals which indicate complex three-dimensional structures within the lithosphere and may be associated with subduction remnants or a mid-lithosphere discontinuity.

We report the finding of a regional sub-lithospheric low velocity layer through the analysis of P-receiver functions in the Canary Islands, a volcanic archipelago in the east central Atlantic. The Moho discontinuity deepens towards the east, varying in depth from 11.5 to 12.5 km beneath the western islands up to 20–30 km beneath the eastern islands. The low velocity layer underneath the lithospheric mantle is located about 45–65 km of depth. This layer produces a delay of the arrival times of the converted phases at 410- and 660-discontinuities relative to the standard earth models. Other than the delays induced by the shallow low velocity material, there is no evidence of thickness variations in the mantle transition zone, indicating no upper mantle thermal perturbations. Beneath the majority of the islands, the low velocity layer is characterized by V P /V S greater than 1.81, suggesting the ubiquitous presence of melt (higher than 3%) beneath the islands. Our results support geodynamic models for the Canaries region that include a low velocity layer in the upper mantle, without a thermal anomaly perturbing the mantle transition zone discontinuities (410 and 660 km).

This paper proposes a structural investigation of the Turtle Mountain anticline (Alberta, Canada) to better understand the role of the different tectonic features on the development of both local and large scale rock slope instabilities occurring in Turtle Mountain. The study area is investigated by combining remote methods with detailed field surveys. In particular, the benefit of Terrestrial Laser Scanning for ductile and brittle tectonic structure interpretations is illustrated. The proposed tectonic interpretation allows the characterization of the fracturing pattern, the fold geometry and the role of these tectonic features in rock slope instability development. Ten discontinuity sets are identified in the study area, their local variations permitting the differentiation of the study zone into 20 homogenous structural domains. The anticline is described as an eastern verging fold that displays considerable geometry differences along its axis and developed by both flexural slip and tangential longitudinal strain folding mechanisms. Moreover, the origins of the discontinuity sets are determined according to the tectonic phases affecting the region (pre-folding, folding, post-folding). The localization and interpretation of kinematics of the different instabilities revealed the importance of considering the discrete brittle planes of weakness, which largely control the kinematic release of the local instabilities, and also the rock mass damage induced by large tectonic structures (fold hinge, thrust).

Simulation of naturally fractured reservoirs offers significant challenges due to the lack of a methodology that can utilize field data. To date several methods have been proposed by authors to characterize naturally fractured reservoirs. Among them is the unfolding/folding method which offers some degree of accuracy in estimating the probability of the existence of fractures in a reservoir. Also there are statistical approaches which integrate all levels of field data to simulate the fracture network. This approach, however, is dependent on the availability of data sources, such as seismic attributes, core descriptions, well logs, etc. which often make it difficult to obtain field wide. In this study a hybrid tectono-stochastic simulation is proposed to characterize a naturally fractured reservoir. A finite element based model is used to simulate the tectonic event of folding and unfolding of a geological structure. A nested neuro-stochastic technique is used to develop the inter-relationship between the data and at the same time it utilizes the sequential Gaussian approach to analyze field data along with fracture probability data. This approach has the ability to overcome commonly experienced discontinuity of the data in both horizontal and vertical directions. This hybrid technique is used to generate a discrete fracture network of a specific Australian gas reservoir, Palm Valley in the Northern Territory. Results of this study have significant benefit in accurately describing fluid flow simulation and well placement for maximal hydrocarbon recovery.

The deep structure and evolution of the crust and upper mantle in the Pan-African terrain of the Lutzow-Holm Complex (LHC), Dronning Maud Land, East Antarctica were investigated using Deep Seismic Surveys (DSS). The DSS were conducted on the continental ice sheet of the LHC in the austral summers of 2000 and 2002 by the Structure and Evolution of the East Antarctic Lithosphere (SEAL) program. Processing of the DSS has produced enhanced reflection images of the crust–mantle boundary and of the internal crustal structure. Laminated layering around the crust–mantle boundary is well imaged using coherency enhancement processing after NMO corrections applied to far offset data. The repetitive crust–mantle transition zone imaged by the SEAL-2002 profile suggests the presence of compression stress in a NE–SW orientation during the Pan-African, which occurred during the last stage of formation of a broad mobile belt between the East and West Gondwana super-terrains. Successive break-up processes of the super-continent in mid-Mesozoic could account for the formation of the stretched reflection structure above the Moho discontinuity beneath the LHC as imaged on in the SEAL-2000 profile.

Seismic investigation of the lithosphere by means of active source experiments is mostly confined to the crust and the Moho. Structures in the upper mantle are more likely to be discovered by analyses of teleseismic data, although these methods are restricted in their resolution capabilities. The relatively rare evidence for upper mantle refractors or reflectors in active source data enables challenging and interesting studies of the lower and not so well known part of the lithosphere. We present such an example from the tectonically complex region between the Eastern Alps and the Western Carpathians. This area was covered by several extensive 3D wide-angle reflection/refraction experiments within the last decade, and their layout was designed to illuminate the crustal structure and in particular the Moho discontinuity. In some areas, reflections from below the Moho are also recorded. These reflections occur at recording offsets between 200 and 500 km, and they are particularly strong in cross line recordings. We derive a set of travel times from the data and perform a tomographic inversion for the depth and shape of the reflecting interface. The inversion makes use of an existing 3D crustal model which also includes the Moho topography. Since the upper mantle velocities are poorly constrained and the azimuthal distribution of the rays is biassed, several tests are applied to investigate the reliability of possible solutions. The results from the tomographic inversion indicate an overall horizontal and radially dipping reflector. The average depth of the reflector is 55 km, which is about 25 km below the crust–mantle transition, and amplitude modelling suggests that the reflecting interface represents a velocity increase. The investigated area is further characterised by deep sedimentary basins, high heat flow, high velocities in the lower crust, diffuse Moho signature and a strong positive Bouguer anomaly. Nearby xenolith outcrops exhibit a pronounced change in anisotropy and indicate the presence of two distinct layers in the lithospheric mantle, whereas the deeper layer is thought to present more juvenile lithosphere derived from thermal relaxation in the post-extension phase. Most likely the upper mantle reflector also represents this change in anisotropy, though other scenarios are also possible. We conclude that the entire lithosphere is significantly shaped by extensional processes which affect the area since the late Oligocene/early Miocene.

The Andean orogenic front between 38 and 39°S is formed by extensional structures delimiting the Loncopue trough. These structures are superimposed to Late Cretaceous–Late Miocene compressive structures that formed the Agrio fold and thrust belt. Gravity data, and limited borehole and seismic data complimented with previous field studies were used to construct two structural cross sections across the area of Neogene extensional deformation and remnants of the previous compressive structure. One of them was restituted for Late Cretaceous times previously to extensional relaxation of the fold and thrust belt and Late Oligocene–Early Miocene and Pliocene to Quaternary stages. Different modeling techniques and filters for gravity anomalies show that i) a series of deeper and larger depocenters, where Late Oligocene to Early Miocene sequences are buried, were located particularly at the Present trough boundaries; while ii) smaller and probably shallower depocenters were concentrated at the axial-eastern trough following structures at surface with young morphological expression. The former are interpreted as representative of the initial extensional stage when Late Cretaceous compressional basement structures were relaxated, while the latter could be the product of incipient-young extensional reactivation of the Agrio fold and thrust belt. General coincidence between both areas of extensional relaxation at the western Agrio fold and thrust belt implies that a common basement discontinuity is recurrently being reactivated through time, probably in coincidence with the eastern main Loncopue fault boundary.

P- (Vp) and S-wave (Vs) velocities of garnet-free (two-pyroxene granulite) and garnet-bearing (garnet granulite and garnet pyroxenite) lowermost crustal rocks collected from the Kohistan arc, northern Pakistan, were measured at 0.1–1.0 GPa and 25–400 °C. Garnet granulite had higher Vp (+ 0.31 km/s) and Vs (+ 0.27 km/s) than two-pyroxene granulite. Although Vp and Vs increased with increasing volume percent of garnet, plagioclase-free garnet pyroxenite showed significantly higher Vp and Vs than plagioclase-rich garnet granulite mainly due to the low Vp and Vs of plagioclase. In contrast, we observed two quasi-linear relationships between Vp (Vs) and SiO2 content for the garnet-bearing and garnet-free rocks. The garnet-bearing rocks had relatively higher Vp and Vs and stronger SiO2 dependences than the garnet-free rocks. The stronger SiO2 dependences of Vp and Vs in the garnet-bearing rocks suggest that the garnet formation in mafic to ultramafic rocks (e.g., pyroxenite and hornblendite), having relatively lower SiO2, leads to more pronounced increases in Vp and Vs than that of relatively felsic rocks (e.g., felsic-to-mafic granulite). Indeed, the Vp and Vs of the garnet pyroxenite were significantly higher than those of garnet granulite but comparable to those of dunite. The significantly high Vp and Vs of the garnet pyroxenite yielded high reflection coefficients between the garnet granulite and garnet pyroxenite of up to 0.13 for P-waves and 0.14 for S-waves, comparable to values expected for Moho reflection. Thus the lithological boundary between plagioclase-rich garnet granulite and plagioclase-free garnet pyroxenite in the lowermost crust of the Kohistan arc corresponds to the seismic Moho discontinuity.

Magnetotelluric data were obtained along a 450 km, almost west–east profile at approximately 31.5° S, which extends from La Rioja to Santa Fe provinces in central Argentina. The profile crosses two main crustal domains that were juxtaposed during the Early–Middle Cambrian Pampean Orogeny: the Pampean terrane to the west and the Rio de la Plata craton to the east. The electrical resistivity structure of the crustal domains together with their boundary is presented. Through dimensionality analysis of the data, it was demonstrated that regional-scale electrical structures are mainly two-dimensional with a strike direction oriented parallel to the surface geological strike. The resistivity model shows a subvertical limit approximately along the eastern border of the Sierra Chica de Cordoba. To the east, the shallower structure is the Chaco–Paranense basin extending to a depth of 6 km with resistivities between 1 and 30 Ohm-m, whereas below the basin the ca 2.1–2.3 Ga Rio de la Plata craton shows resistivities in a range of 300–10,000 Ohm-m. The Pampean terrane presents a 6 km layer with a resistivity higher than 10,000 Ohm-m whereas below this layer the resistivity values range from 50 to 200 Ohm-m. Based on both the geological information and the magnetotelluric results, the sharp lateral discontinuity observed in the resistivity model to the east of the Sierras de Cordoba is conjectured to represent the boundary between the Rio de la Plata craton and the Pampean terrane which may correspond to the Early Cambrian suture.

We developed a mechanical model for the nonlinear, coupled system of plate subduction and back-arc spreading on the basis of mathematical formulation for internal deformation due to a moment tensor in an elastic–viscoelastic layered half-space under gravity. In our modeling the plate subduction and the back-arc spreading are rationally represented by the increase of tangential displacement discontinuity at the plate interface and normal displacement discontinuity at the spreading center, respectively. Through 3-D numerical simulation with this model we obtained a possible scenario for the tectonic evolution of back-arc basins as follows. At the first stage, steady plate subduction gradually forms tensile stress fields in the back-arc region. When the accumulated tensile stress reaches a critical level, back-arc spreading starts at a structurally weak portion of the overriding plate. The back-arc spreading pushes out the frontal part of the overriding plate toward the plate boundary and leads to the increase of slip rates at the plate interface. The local increase of slip rates at the plate interface produces additional tensile stress in the back-arc region. The incremental tensile stress is canceled out by further back-arc spreading. Such a feedback mechanism is necessary to maintain steady back-arc spreading. The long duration of slip-rate excess due to back-arc spreading leads to the gradual protrusion of the plate boundary toward the descending plate (trench retreat). As the plate boundary moves away from the back-arc spreading center, the accumulation rate of tensile stress at the spreading center gradually decreases, and the slip-rate excess at the plate interface due to back-arc spreading also decreases. Then, the original back-arc spreading center becomes less effective in releasing the tectonic tensile stress, and new active back-arc spreading will start somewhere closer to the plate boundary. Such a qualitative scenario accords with the evolution history of back-arc basins in the Mariana region. We obtained the slip-rate excess of 7 mm/year at the plate interface and the 20 km trench retreat for the last 4.5 Myr. These simulation results are significantly smaller than observations in the Mariana region. The discrepancy between simulation results and observations can be ascribed to the effect of the negative buoyancy acting on cold descending slab, which advances spontaneous plate subduction and directly increases the slip-rate excess and the trench retreat in our model.

Shallow and deep sources generate a gravity low in the central Iberian Peninsula. Long-wavelength shallow sources are two continental sedimentary basins, the Duero and the Tajo Basins, separated by a narrow mountainous chain called the Spanish Central System. To investigate the crustal density structure, a multitaper spectral analysis of gravity data was applied. To minimise biases due to misleading shallow and deep anomaly sources of similar wavelength, first an estimation of gravity anomaly due to Cenozoic sedimentary infill was made. Power spectral analysis indicates two crustal discontinuities at mean depths of 31.1 ± 3.6 and 11.6 ± 0.2 km, respectively. Comparisons with seismic data reveal that the shallow density discontinuity is related to the upper crust lower limit and the deeper source corresponds to the Moho discontinuity. A 3D-depth model for the Moho was obtained by inverse modelling of regional gravity anomalies in the Fourier domain. The Moho depth varies between a mean depth of 31 km and 34 km. Maximum depth is located in a NW–SE trough. Gravity modelling points to lateral density variations in the upper crust. The Central System structure is described as a crustal block uplifted by NE–SW reverse faults. The formation of the system involves displacement along an intracrustal detachment in the middle crust. This detachment would split into several high-angle reverse faults verging both NW and SE. The direction of transport is northwards, the detachment probably being rooted at the Moho.

Seismic wide angle and receiver function results together with geological data have been used as constraints to build a gravity-based crustal model of the central High Atlas of Morocco. Integration of a newly acquired set of gravity values with public data allowed us to undertake 2–2.5D gravity modelling along two profiles that cross the entire mountain chain. Modelling suggests moderate crustal thickening, and a general state of Airy isostatic undercompensation. Localized thickening appears restricted to the vicinity of a north-dipping crustal-scale thrust fault, that offsets the Moho discontinuity and defines a small crustal root which accounts for the minimum Bouguer gravity anomaly values. Gravity modelling indicates that this root has a northeasterly strike, slightly oblique to the ENE general orientation of the High Atlas belt. A consequence of the obliquity between the High Atlas borders and its internal and deep structure is the lack of correlation between Bouguer gravity anomaly values and topography. Active buckling affecting the crust, a highly elevated asthenosphere, or a combination of both are addressed as side mechanisms that help to maintain the high elevations of the Atlas mountains.

In this study, we have used later phases reflected at the Moho discontinuity in addition to direct arrival time data to study the 3-D crustal velocity structure beneath southwest (SW) Japan. We have collected more than 6000 direct and Moho-reflected phases and used them to obtain P- and S-wave velocity structures in the study area. Our results revealed significant low-velocity anomalies beneath the active and Quaternary volcanoes in SW Japan. Results obtained using each data set (P, PmP, S, and SmS) are generally consistent with each other and all showed low-velocity anomalies beneath most of the active and Quaternary volcanoes, western Kii Peninsula and eastern Shikoku. Ray-path analysis showed that the data set we used has a good ray criss-crossing that is sufficient to accurately locate the velocity anomalies in the study area. In addition, results of the checkerboard resolution test indicate that these structures are generally well recovered and that adding later phases could significantly improve the resolution scale of the obtained tomographic images in comparison with those from first arrivals alone.

One in-line wide-angle seismic profile was conducted in 1990 in the course of the Southeastern China Continental Dynamics project aimed at the study of the contact between the Cathaysia block and the Yangtze block. This 380-km-long profile extended in NW–SE direction from Tunxi, Anhui Province, to Wenzhou, Zhejiang Province. Five in-line shots were fired and recorded at seismic stations with spacing of about 3 km along the recording line. We have used two-dimensional ray tracing to model P- and S-wave arrivals and provide constraints on the velocity structure of the upper crust, middle crust, lower crust, Moho discontinuity, and the top part of the lithospheric mantle. P-wave velocity, S-wave velocity and V P / V S ratio are mapped. The crust is 36-km thick on average, albeit it gradually thins from the northwest end to the southeast end (offshore) of the profile. The average crustal velocity is 6.26 km/s for P-waves but 3.6 km/s for S-waves. A relatively narrow low-velocity layer of about 4 km of thickness, with P- and S-wave velocities of 6.2 km/s and 3.5 km/s, respectively, marks the bottom of the middle crust at a depth of 23-km northwest and 17-km southeast. At the crust–mantle transition, the P- and S-wave velocity change quickly from 7.4 to 7.8 km/s (northwest) and 8.0 to 8.2 km/s (southeast) and from 3.9 to 4.2 km/s (northwest) and 3.9 to 4.5 km/s (southeast), respectively. This result implies a lateral contrast in the upper mantle velocity along the 140 km sampled by the profile approximately. The average V P / V S ratio ranges from 1.68–1.8 for the upper crust to 1.75 for the middle and 1.75–1.85 for lower crust. With the interpretation of the wide-angle seismic data, Jiangshan–Shaoxin fault is considered as the boundary between the Yangtze and the Cathaysia block.

We delineate shallow structures of the Mozumi–Sukenobu fault, central Japan, using fault zone waves generated by near-surface explosions and detected by a seismometer array. Two explosive sources, S1 and S2, were placed at a distance of about 2 km from the array, and the other two, S3 and S4, were at a distance of about 4 km. Fault zone head waves and fault zone trapped waves following direct P wave arrivals were clearly identified in the seismograms recorded by a linear seismometer array deployed across the fault in a research tunnel at a depth of 300 m. Synthetic waveforms generated by a 3-D finite-difference (3-D FD) method were compared with observed fault zone waves up to 25 Hz. The best fitting model indicates a 200-m-wide low-velocity zone extending at least to shot site S1 located 2 km east of the seismic array with a 20% decrease in the P wave velocity relative to the wall rock. The width of the low-velocity zone is consistent with the fault zone defined by direct geological observation in the research tunnel. However, the low-velocity zone should disappear just to the east of the site S1 to explain the observed fault zone waves for shot S3 and S4 located 4 km east of the seismometer array. Yet the observation and the simulation show notable trapped wave excitation even though shots S3 and S4 are outside the fault zone. These results indicate that (1) the effective waveguide for seismic waves along the fault does not exist east of source site S1 although the surface traces of the fault are observed in this region, and (2) considerable trapped waves can be excited by sources well outside the fault zone. These results highlight the along-strike variability in fault zone structure.

The driving force for the basin subsiding against isostatic balance in and around Lake Biwa in the Kinki district, Japan is discussed. The lake region is characterized by strong negative Bouguer anomalies, especially by a steep horizontal gradient zone of gravity anomaly running along the western margin of the lake. The large negative anomaly (>50 mgal) cannot be explained by low-density sediments beneath it. A down-warping structure extending to the Moho depth should be taken into account. This conjecture has been strongly supported by a short-period receiver function imaging, which shows a clear offset of about 8 km for the Moho discontinuity under the steep gravity gradient zone. A question arises as to what is the driving force to create such a large down-warping structure. We consider that the subduction of the shallow-dipping slab under the region (Philippine Sea Slab) may cause crustal deformation by dragging the viscous mantle downward. In order to verify this model, we simulated the induced mantle flow due to the subduction of the Philippine Sea Slab and the pressure distribution on the crust–mantle boundary. This numerical experiment showed that the induced flow makes a strong negative pressure zone under the lake region if the slab has a vertical offset along the direction of subduction. This offset of the slab is consistent with plate models deduced from hypocentral distributions and Sp phases of the deep-focus earthquakes.

This work is a part of the TOR1 project (1996–1997) and is devoted to determining the lithospheric structure across the Sorgenfrei–Tornquist Zone in Northern Europe. For the first time in Europe, a very dense seismic broadband array has offered the possibility of determining very sharp lateral variations in the structure of the lithosphere at small scales using surface wave analysis. We measure phase velocities for Rayleigh waves with periods ranging between 10 and 100 s, both within arrays with apertures of 40–50 km (small compared to the wavelength), and along long profiles of at least 100 km. Dispersion curves are then inverted and shear-wave velocity models down to the depth of 200 km are proposed. We show that the Sorgenfrei–Tornquist Zone is a major tectonic feature within the whole lithosphere. North–east of this feature, in Sweden beneath the Baltic Shield, no lithosphere–asthenosphere boundary is observed to exist to depths of 200 km. South–west of the Sorgenfrei–Tornquist Zone, beneath Denmark, we find a lithospheric thickness of 120±20 km. The transition across the Sorgenfrei–Tornquist Zone is sharp and determined to be very steeply dipping to the south–west. We also demonstrate the existence of a sharp discontinuity between the lithospheres beneath Denmark (120±20 km thick) and beneath Germany (characterized by thicknesses of 50±10 km in the northernmost part and 100±20 km in the southwest). This discontinuity is most likely related to the Trans-European Fault at the surface.

Tomographic travel time inversion of seismic compressional (P) and shear (S) wave data from the long-range deep seismic sounding experiment Fennoscandian Long Range (FENNOLORA) reveals the velocity structure of the crust and upper mantle in the Baltic Shield. Pronounced scattering and delay in travel times of seismic P- and S-wave phases together with strong attenuation of S-wave phases beyond ca. 800-km offset are attributed to a low-velocity zone (LVZ) below the 8° discontinuity at a depth of ca. 100 km. Travel time inversion of P- and S-wave first arrivals shows that the 8° discontinuity represents the top of a zone with negative or very small vertical velocity gradients and a Vp/Vs ratio of 1.74–1.77. We observe clear, linear refracted P-wave phases (i.e. the Lehmann refraction) at offsets beyond 1100–1300 km, which suggest that the base of the low-velocity zone is at ca. 150-km depth in the Baltic Shield. No refracted S-wave phases are observed beyond 1200-km offset, which we attribute to strong S-attenuation within the zone below the 8° discontinuity. These features are interpreted by the presence of small amounts of partial melts, almost molten rocks or possibly free fluids, in the 100–150-km depth interval. Local variations in the Vp/Vs ratio of the crust and uppermost mantle correlate with Proterozoic terranes, which collided and were amalgamated during the Precambrian plate tectonic events that led to the assemblage of the Baltic Shield.

Gravity and magnetic data were collected and used to study the crustal structure of Jordan. Three new geophysical maps of Jordan were created: a Moho discontinuity map, a crystalline basement surface map, and a map showing the lowest limit of magnetic blocks. Depths of the Curie Isotherm were also calculated. Results indicate that the depth to the Moho discontinuity in Jordan varies from 32 to 33 km in the northwest to 38 km in the southeast. The basement complex rocks outcrop on the surface in the southwest but lie at about 8 km in the northeast. The Curie Isotherm (585 °C) lies at a depth of about 10 km in the area east of the Dead Sea and dips southeastward towards the Al-Sirhan (Wadi Sirhan), southeast Jordan, where it is located at 35 km depth. Local isostasy of rock masses (blocks) in Jordan does not occur. Nevertheless, this does not rule out the possible existence of isostasy in a regional scale at greater depths within the mantle.

Structural and stratigraphic studies of the western Daqing Shan segment of the Yinshan belt have recognized an Early Jurassic extensional episode supported by several lines of evidence. First, normal faults cut the lowermost Jurassic sequence and are overlapped by younger Lower Jurassic rocks. Second, Lower Jurassic rocks include growth strata in small-scale graben at the base of the Jurassic basin. Third, rapid lateral facies changes are mapped from boulder conglomerates along the basin-bounding faults to lacustrine and meandering fluvial rocks in the basin center. Fourth, paleodrainage systems provided sediment input from three sides of the basin, two transverse and one axial. Finally, there is a strongly asymmetric distribution of coarse proximal and fine distal facies within the basin. The Early Jurassic extensional episode was responsible for formation of an east-trending half-graben basin in the western Daqing Shan in which at least 1800 m of syn-extensional nonmarine sediments accumulated and are preserved. Previous studies in the Triassic and Jurassic of other parts of northwest and north-central China have concluded that the early Mesozoic was a time of continental amalgamation and contractile deformation. The recognition of an Early Jurassic extensional episode along the northern margin of the North China Block is problematic in this context, at least superficially. We propose two possible explanations for the Early Jurassic extension: transtension associated with strike-slip tectonics, or gravitational collapse of a pre-existing Late Paleozoic–Early Mesozoic contractile orogenic belt. The first possibility, transtensional deformation, is problematic because specific strike-slip faults have not been identified that could control extension in the western Daqing Shan. However, several lines of evidence allow the possibility of such a driving mechanism, including: documented Early Jurassic transtensional systems in the southwestern North China Block, several candidate strike-slip faults along the China–Mongolia border region, structural discontinuity between the Daqing Shan and southern Mongolia, and along strike-changes in structural style within the Yinshan belt. The second possibility, orogenic collapse, is similarly difficult to establish because of the limited amount of data concerning the regional distribution and orientation of pre-Jurassic contractile structures and Early Jurassic extensional structures. However, documented Late Paleozoic–Triassic contractile deformation, as well as the close temporal and spatial association, and parallelism between Early Jurassic extensional structures and older contractile structures in the western Daqing Shan requires consideration of gravitational collapse as a driving mechanism for Early Jurassic extension in the Yinshan belt.

The NNE-striking Longitudinal Valley Fault (LVF) in eastern Taiwan is an extremely active high-angle thrust fault. It bounds the Coastal Range and the Longitudinal Valley, which is considered a collision boundary between the Philippine Sea and the Eurasian plates. Repeated GPS data in the Longitudinal Valley area from 1992 to 1999 are utilized to study the spatial variation of crustal motion along the LVF. With respect to Penghu in the Chinese continental margin, velocities for stations on the western side of the LVF (Longitudinal Valley and eastern Central Range) are 18–35 mm/yr in directions 283–311°, whereas those on the eastern side of the LVF, the Coastal Range, are 28–68 mm/yr in directions 303–324°. A major discontinuity of about 30 mm/yr on the rate of crustal motion across the Longitudinal Valley is attributed to the aseismic slip along the LVF as revealed by trilateration data previously. To the south of Fengping, the block motions of the Coastal Range are 31–40 mm/yr in 317–330° relative to the Central Range, while the near-fault motions are 13–33 mm/yr in 309–336°. Various partitions on the left-lateral strike-slip and convergent components along the LVF are found. In the southern Longitudinal Valley crustal motion is mainly accommodated on the LVF and the Luyeh Fault. In contrast, those in the central and northern Longitudinal Valley are partly taken up on the faults to the east of the LVF or result in the elastic deformation of the Coastal Range. The crustal motion in the northern Longitudinal Valley area is likely to be distributed in the several NE-striking thrusts in a horsetail pattern and obliquely cut the northern Coastal Range, with a small portion of fault-slips along the LVF. Data from dense-deployed GPS networks across the LVF can be employed to give better estimates of near-fault motions and delineate the surface traces of the LVF. Repeated GPS and leveling data from two stations on both ends of the Yuli Bridge that are 575 m apart clearly indicate that the surface trace of the LVF passes beneath the bridge with oblique horizontal motion of 23 mm/yr in 306° and uplift rate of 24 mm/yr.

Seismic velocity discontinuities are commonly found within the upper 100 km of the mantle lithosphere, with great variability in their depth, lateral extent, and the polarity of velocity jump. Among the more commonly observed is the ‘80 km’ or Hales discontinuity, identified in a variety of tectonic environments, and sometimes associated with seismic anisotropy. Teleseismic receiver functions for Global Seismographic Network station RAYN on the eastern edge of the Arabian Shield contain a strong converted-phase P H s that we believe arises from the Hales discontinuity at the depth of 70 km. On the basis of the polarization, azimuthal behavior and frequency dependence of P H s, we characterize the Hales discontinuity as marking an upper boundary of a zone of depth-dependent anisotropy within the upper mantle. The RAYN receiver functions indicate a similar zone at the crust-mantle transition at 40 km. Forward modeling in simple one-dimensional anisotropic models indicates that the receiver functions require, assuming hexagonal anisotropy, an inclined symmetry-axis with N–S strike. In our preferred model there are two anisotropic zones bounded, respectively, by the sharp Moho and the Hales discontinuities on the top. Smooth gradients of anisotropic properties extend beneath the discontinuities. Within both zones the anisotropic symmetry axes, indicative of systematic fabric in the olivine-dominated upper mantle material, are oriented along the N–S direction (±10°), and are tilted from the vertical 50° toward the south. We propose that these regions of coherent fabric within the upper mantle represent shear zones developed during late Proterozoic continent–continent collision, when the Ar-Rayn tectonic block underwent northward ‘escape’ along the northern flank of the Najd fault system.

The large-scale seismic experiment POLONAISE'97 investigated the seismic structure of the crust and the uppermost mantle in the Polish region of the Trans-European Suture Zone. This paper covers the interpretation of seismic data along the 300-km-long P3 profile, which is located in the Precambrian East European Craton (EEC) parallel to the Teisseyre–Tornquist Zone. The recordings were of high quality, with seismic energy detectable out to 300 km offsets. The crustal model developed by two-dimensional raytracing forward modelling and waveform analysis is characterized by a subhorizontal uniform structure, although the boundaries of several large basement features are crossed. The crystalline crust consists of three parts: the upper, middle and lower crust with P-wave velocities of 6.1–6.4 km/s, 6.55–6.7 km/s and 7.05–7.15 km/s respectively. The PmP wave can usually be correlated at distances beginning at about 100 km, but it has variable reflection character. Moho depth increases from 38 km at the NW end of the profile to 44 km at the SE end. The Pn velocity of 8.05–8.1 km/s is less than that previously found in neighbouring areas of the EEC. The VP/VS ratio was determined separately for the upper/middle and lower crust to be 1.67 and 1.77 respectively. Following the Pn wave, another phase with an apparent velocity of about 8.3 km/s is interpreted as a weak reflection from a low-contrast discontinuity in the uppermost mantle.

We present here results from an analogue model designed to test the relative influence of along-axis variations of the crustal thickness and of the thermal structure of the lithosphere on the geometry and on the faulting pattern of the axial rift of a slow-spreading ridge. Rheologically calculated layered models are employed, using quartz sand and silicone putty as analogues of the brittle and ductile components of the lithosphere, respectively. Two parameters have been analysed in detail: (1) the thickness of the brittle layer, and (2) the viscosity of the ductile layer. This study shows that the thickening of the brittle layer and the increase of the viscosity of the ductile layer bring about a widening of the axial valley associated with an increase of the number of faults. A decrease of the depth of the axial valley and of the vertical throw of the faults is observed with an increase of the viscosity of the ductile layer and a decrease of the thickness of the brittle layer. These results are consistent with observations along Mid-Atlantic Ridge segments. For segment ends, the fault pattern obtained in the models is similar to that described on both sides of the axis for segments bordered by a zero-offset discontinuity, and on outside corners for segments bordered by a lateral offset discontinuity. Our results suggest that the viscosity of the ductile layer plays a more important role in the fault pattern than the thickness of the brittle layer. The influence of segment length, offset length and temporal variation in thermal input could explain the more or less important along-axis variation in the deformation pattern observed along segments of slow-spreading ridges.

The fault-segmentation method is commonly used to estimate the potential earthquake size. Segment boundaries play an important role in arresting earthquake ruptures from event to event. In the Basin and Range Province, earthquake rupture terminations are commonly associated with structural discontinuities, but not all-structural discontinuities have the capability to terminate an earthquake rupture. The size of structural discontinuities with respect to the rupture length and displacement may play an important role in controlling rupture termination. Studies of the geometric pattern of seven well-documented historical earthquake ruptures in the Basin and Range Province reveal three important relationships between rupture termination and the size of structural discontinuities. Firstly, terminations of normal faulting earthquakes are often associated with structural discontinuities, at least for the five historical ruptures studied in this paper. Secondly, the sizes of structural discontinuities at the end of earthquake rupture zones are generally the largest among the structural discontinuities within the rupture zone. Thirdly, there appears to be a trend that larger earthquake ruptures are stopped by larger structural discontinuities. The size of structural discontinuities that stopped the earthquake seems to scale with the length and displacement of earthquake surface rupture.

Based on remote sensing analyses of two panchromatic Spot scenes that cover the western and eastern parts of Crimea, completed with the study of Landsat Thematic Mapper scenes and local aerial photographs, we determined the geometry of major structures (folds and fractures). The strikes of large fractures are quite homogeneous in the studied area: three important sets are common, N10, NE–SW and NW–SE. Two strikes are particularly developed in the western part of Crimea: N25 and N105. Along important faults, we recognised the sense of motion, which often corresponds to the latest or to the major stages of movement. We also determined stress tensors by inversion of microtectonic data sets collected in the field. Combining remote sensing analysis with mechanical studies provides new views on the structural evolution of Crimea. We thus characterised major tectonic events responsible for most of the deformation. In eastern Crimea, the structural patterns fit well with two major tectonic events. (1) The latest one is a Plio–Quaternary faulting event (normal slip along N10 faults and strike-slip motion along N10 and NW–SE faults) corresponding to a transtensional regime, with σ3 trending E–W. (2) Large fractures (NW–SE and NE–SW sets) and fold-and-thrust development reveal a transpressional regime, with σ1 trending NNE–SSW during the Berriasian. In western Crimea, we characterised a more complex pattern of four major tectonic events. (1) The Plio–Quaternary tectonic phase induced left-lateral displacement along NW–SE faults and normal slip along WSW–ENE faults and this deformation is in agreement with the strike-slip regime that we reconstructed with σ3 trending NNE–SSW. (2) As for eastern Crimea, the Berriasian phase was well expressed, with folds, thrusts and strike-slip faults, under a transpressional regime with σ1 trending N–S to NNW–SSE. The two other events well identified in western Crimea are: (3) a N–S-directed extension that developed large N105 faults, and (4) a NW–SE-directed extension (involving NE–SW normal faults) that we relate to the development of peripheral troughs in the Oligocene. Western Crimea suffered a more complicated structural evolution than eastern Crimea. From eastern to western Crimea, we also noticed the systematic deviation in the trends of the principal stress axes of the recent stress field and in the Berriasian stress field as well. These differences may be related to the presence of a major crustal discontinuity: the transverse Alushta–Simferopol fault.