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Understanding the strength of a plate coupling is critical for assessing potential seismic and tsunamic hazards in subduction zones. The interaction between an overriding plate and the associated subducting plate can be used to evaluate the strength of plate coupling by examining the mantle lithospheric buoyancy. Here, we calculate the mantle lithosphere buoyancy across the northern portion of the southern Ryukyu subduction zone based on gravity modeling with the constraints from a newly derived P-wave seismic velocity model. The result indicates that the strength of the plate coupling in the study area is relatively strong, which is consistent with previous observations in the southernmost Ryukyu subduction zone. Because few large earthquakes (Mw > 7) have occurred in the southern Ryukyu subduction zone, a large amount of energy is locked and accumulated by plate coupling, that could be released in the near future.

Widely distributed Bottom-Simulating Reflectors (BSRs) have been observed in the area offshore of southwestern Taiwan where the active accretionary complex meets with the passive China continental margin. In order to clarify the link between seismic site response and sedimentary properties of submarine slope, we evaluate the response of seafloor sediments in regard to passive dynamic loads. The local site effect produced by shallow marine sediments was characterized by estimating the horizontal-to-vertical (H/V) spectral ratios of data recorded by the short-period Ocean Bottom Seismometers (OBSs). The results show that the maximal H/V ratios appeared in the range of 3.66 - 9.28 Hz, suggesting that the fundamental frequency is dominated by the effect related to the very shallow sediments. For most stations, the H/V ratios estimated based on the earthquakes and noise records were characterized by different patterns. Relatively broad H/V pattern was obtained when the signals were extracted from earthquakes. This phenomenon may be related to soil nonlinearity when a stronger motion applies. In comparison with the available geological structures and bulk density distribution obtained from coring experiments, we found a relatively higher fundamental frequency of about 8 - 9 Hz for the more rigid material, such as mud diapir and folding axes. For most of the area along the slope, the fundamental frequency shows a relatively low value, about 6 - 8 Hz. Finally, when a site is characterized by thick or lowest bulk density sedimentary layer, we observed a fundamental frequency lower than 5 Hz, which is the lowest in our assessment.

This paper presents the spatial variations in the crustal stress field for the Philippine region. Based on the stress configuration, we divided the stress regime in the area of the Philippines into four parts: (1) Trench-perpendicular compressive stress axes (σ1) with an intermediate plunge were observed along the eastern subduction systems of the Philippine Mobile Belt (PMB), which suggests that the subduction of the Philippine Sea Plate (PSP) is the primary factor controlling the stress distribution. (2) Characterized by systematically rotated σ1 with very shallow plunge, the central-western portion of the Philippines appears to be largely affected by the collision between the Palawan Block and the PMB. (3) The σ1 distribution in the northern part of Luzon Island (north of 14°N) is compatible with the plate motion of the PSP, whereas the extensional axes (σ3) has a fan-shaped stress signature with N-S direction at the south and a more spreading direction to the north, which implies its north escapement. (4) Bounded by the Scarborough Seamount, the northern and southern portions of the Manila Trench reveal a very distinct stress pattern with a trench-parallel σ3 to the north and a trench-perpendicular σ3 to the south. This observation may infer a potential effect caused by the presence of the oceanic bathymetric highs on the behavior of the subduction process. In addition to the identification of the four provinces with different stress regimes, we suggest that the Philippine Fault Zone may play an important role to alter the stress state in the region of the Philippines.

This paper presents the first whole Taiwan-scale spatial variation of the seismogenic zone using a high-quality crustal seismicity catalog. The seismicity onset and cutoff depths (i.e., seismogenic depths) are determined by the earthquake depth–moment distribution and used to define the upper and lower boundaries of the seismogenic zone, respectively. Together with the published fault geometries and fault area–moment magnitude relations, the depth difference in the onset and cutoff depths (i.e., seismogenic thickness) is used as the fault width to determine the moment magnitudes of potential earthquakes for the major seismogenic faults. Results show that the largest (Mw7.9–8.0) potential earthquake may occur along the Changhua fault in western Taiwan, where the seismic risk is relatively high and seismic hazard mitigation should be a matter of urgent concern. In addition, the first-motion focal mechanism catalog is used to examine the relation between the seismogenic depths and earthquake source parameters. For crustal earthquakes (≤ 50 km), the shallowest onset and cutoff depths are observed for normal and strike-slip events, respectively. This observation is different from the prediction of the conventional continental-rheology model, which states that thrust events have the shallowest cutoff depth. Thus, a more sophisticated rheology model is necessary to explain our observed dependence of the seismogenic depths on faulting types. Meanwhile, for intermediate to large crustal (Mw ≥ 4; depth ≤ 50 km) earthquakes, thrust events tend to occur at the bottom region of the seismogenic zone, but normal and strike-slip events distribute at a large depth range.

Summary The distribution of historic earthquakes in the Sumatra subduction zone reveals, in the fore-arc region, the intense seismic activity and frequent occurrences of Mw > 8 earthquakes throughout the whole area. In contrast, the neighboring region has less dense seismicity, and no large earthquake greater than Mw8 has been observed in the Java subduction zone. Such different seismic behaviors may be due to distinct degrees of the stress accumulation and release. In this study, the strength of plate coupling inferred from mantle lithosphere buoyancy (Hm) estimation is used to explain the seismogenic behavior in the Sunda-Andaman subduction zone. Strong and weak plate coupling status are obtained in the Sumatra and Java subduction zones, respectively. These results can explain the significant differences in seismogenic behaviors in the Sunda-Andaman subduction zone. In assessing the global implications of this finding, we observe that uplifted serpentinized fore-arc mantle peridotite is the critical phenomenon in weak plate coupling cases and leads to a limit on the width of the coupling zone. Strong plate coupling can cause a relatively low gravity anomaly as well as a negative trench-parallel gravity anomaly (TPGA) in the fore-arc regions and correlates well with the occurrence of large earthquakes, whereas weak plate coupling can cause a positive TPGA and constrain the potential occurrence of large earthquakes.

Located in the northern end of the Manila Trench, the deformation front (DF) off southwestern Taiwan is considered as a potential area for generating hazardous earthquakes and tsunamis. However, this area has been relatively seismically quiet, and the seismogenic structure has been unclear, particularly in the middle-to-lower crust. On February 10, 2017, an Mw5.3 earthquake occurred in the near-shore area of Tainan in SW Taiwan, which provides us with a good opportunity to study the seismogenic structure of this area. Five ocean bottom seismometers (OBSs) were deployed to cover the mainshock source area from February 13 to March 1, 2017, to record the aftershock data. All the OBSs were successfully recovered and, the P- and S-wave arrival were manually identified. In collaboration with arrival times from the Central Weather Bureau of Taiwan, we simultaneously determined the 1-D optimal Vp and Vs velocity models and hypocenter locations. The inversion results show that the upper crust (

Located at the arc-continental collision region between the Eurasian (EP) and Philippine Sea Plates (PSP), Taiwan is usually considered to have a complex tectonic environment, particularly along the eastern coast of the island. To gain a better understanding of the geological evolution of the east Taiwan area, the data from 8 Ocean Bottom Seismometers (OBS) acquired during the Across Taiwan Strait Explosion Experiment in 2012 and 14 inland seismic stations were used to determine a more detailed and accurate distribution of marine earthquakes. Based on the 333 relocated earthquakes and available geophysical data, we suggest two main tectonic boundaries for eastern Taiwan. South of 23.25°N, the homogeneous distribution of earthquakes in the crustal portion for both the inland and offshore areas suggests an ongoing collisional process. North of this location, between approximately 23.25°N and 23.8°N, the abrupt increasing of seismicity depth infers that the underthrusted arc/fore-arc material is deforming due to the collisional compression at depth. In this segment, the subsidence of the arc/fore-arc area determines the transition from collision to subduction. North of 23.8°N, the northwestern dipping PSP is well illustrated by the seismicity both onshore and offshore, indicating a dominant subduction process.

The Three Gorges Reservoir Region (TGRR) is one of the most sensitive areas of ecological environment in China. As vital backwater areas, the secondary anabranches of the TGRR were prone to eutrophication in Spring which would affect the distribution and transfer of organotins (OTs) among aquatic media. This study quantified the concentrations of butyltins (BTs) and phenyltins (PhTs) in water columns and aquatic media of two anabranches of the TGRR, the Daning River (DR) and the Xiaojiang River (XR) during the state of eutrophication. Our results showed that the average concentrations of BTs and PhTs in surface water are 43.91, 81.25 ng Sn L−1 in the DR, and 63.49, 69.21 ng Sn L−1 in the XR, respectively, and there were no obvious differences in the concentrations of BTs and PhTs across the water columns in the DR and XR. PhTs, especially monophenyltin (MPhT), are predominated in the dissolved phase, whereas BTs, especially dibutyltin (DBT), are predominated in both suspended particulate matter (SPM) and the sediment. Shipping and agricultural activity were likely the sources of OTs in both the DR and XR. High concentrations of tributyltin (TBT) and triphenyltin (TPhT) are still present in the aquatic media of the TGRR, and pose a significant risk to aquatic organisms due to the potential for bioaccumulation. Therefore, it is necessary to further monitor and assess OTs especially PhTs in surface water, and to continue to restrict the use of OTs to protect the aquatic environment of the TGRR.

In a complex geological environment, it is generally difficult to assimilate a complete understanding of the main stress regime without being distracted by various fault configurations. In this study, we examine the changes in radial seismic moment (Mrr) of earthquakes that occurred in the Sumatra area. This approach allows for simplification of the various focal mechanism solutions by compressive and extensional slip vectors. The results show that the epicenter of the 2004 mainshock is located near the area where the accumulated Mrr was highest during the inter-seismic period. Simultaneously, a pattern of negative accumulated Mrr observed at depths between 40 and 100 km suggests a down-dip extension mechanism caused by the slab pull effect, which could be strengthened by the locking procedure in shallow portions of the slab. Moreover, the right-lateral strike-slip Sumatra Fault and the left-lateral oceanic fracture zones exhibited positive and negative accumulated Mrr prior to and immediately after the 2004 mainshock. This transform of Mrr indicates stress release associated with the unlocking of the asperities during the earthquake. Therefore, the distinctly different Mrr patterns for the inter- and post-seismic period suggest that the occurrence of a substantial earthquake could change the intraplate stress state.

Kavitha and Raghukanth (Acta Geod Geophys 1–33, 2015) proposed an algorithm to forecast the earthquake energy release for the global seismogenic zones. They concluded that “the developed model is efficient in forecasting the annual earthquake energy release of most of the seismogenic zone”. However, for several representative case studies their predictions not only are significantly smaller than the observations but also have unreasonable uncertainty. This commentary discusses some of the problems associated with the earthquake data selection for the input of modeling, which may improve the accuracy of the earthquake energy prediction.

In this study, we establish the empirical relationships between the spatial dimensions of the aftershock zones and moment magnitudes ( M w) for the Taiwan region. The length ( l ) and width ( w ) of the aftershock zone of an earthquake is measured by the major and minor axes, respectively, of the ellipse of a two‐dimensional Gaussian distribution of one‐day aftershocks. Our data is composed of 649 mainshocks (depth≤70 km, M w 4.0–7.6) between 1990 and 2011. The relationships between aftershock zone dimensions and M w were obtained by least‐squares method with the corresponding uncertainties estimated by bootstrap. Our study confirms that aftershock zone dimensions are independent of faulting types and the seismic moment is proportional to l 3. The ratio ( w / l ) increases slightly with M w and is independent of faulting types. Together with previous study, our results suggest that earthquakes of both small ( M w=4.0) and large ( M w≥7.0) magnitudes have similar focal zone geometrical parameters. By using the M w– S relation, where the aftershock zone area S is estimated from l and w , we also provide an independent examination of the variations in the median stress drop. We find that the median stress drops of strike‐slip earthquakes are higher than those of thrust events. Moreover, the median stress drops are independent of the moment magnitudes for normal and strike‐slip events but decrease for large thrust events. These results are consistent with the latest global observations. Regardless of faulting types, the median stress drop decreases for larger (6≤ M w≤7.6) and relatively deep (depth∼60–70 km) earthquakes. Online Material: Effect of up‐dip rupture propagation on estimated maximum slip duration, details on teleseismic and geodetic stations, and figures of far‐field displacements, temporal change in slip, and snapshots of the semblance‐value distribution.

The main goal of this study is to provide moment tensor solutions for small and moderate earthquakes of the 2009 L’Aquila seismic sequence (central Italy). The analysis was performed by using data coming from the permanent Italian seismic network run by the Istituto Nazionale di Geofisica e Vulcanologia (INGV) and the “Cut And Paste” (CAP) method based on broadband waveform inversion. Focal mechanisms, source depths and moment magnitudes are determined through a grid search technique. By allowing time shifts between synthetics and observed data the CAP method reduces dependence of the solution on the assumed velocity model and on earthquake location. We computed seismic moment tensors for 312 earthquakes having local magnitude in the range between 2.7 and 5.9. The CAP method has made possible to considerably expand the database of focal mechanisms from waveform analysis in the lowest magnitude range (i.e. in the neighborhood of magnitude 3) without overlooking the reliability of results. The obtained focal mechanisms generally show NW–SE striking focal planes in agreement with mapped faults in the region. Comparisons with the already published solutions and with seismological and geological information available allowed us to proper interpret the moment tensor solutions in the frame of the seismic sequence evolution and also to furnish additional information about less energetic seismic phases. Focal data were inverted to obtain the seismogenic stress in the study area. Results are compatible with the major tectonic domain. We also obtained a relation between moment and local magnitude suitable for the area and for the available magnitude range.

The Philippine Fault Zone, a system of left-lateral strike-slip faults traversing the length of the Philippine Islands, is associated with the oblique convergence between the Philippine Sea Plate (PSP) and the Eurasian Plate (EP). Although it is a major deformational structure within the diffuse PSP–EP convergent boundary, some of its segments, particularly its marine extensions, are not well studied. To investigate the crustal deformation in the marine prolongation of the Philippine Fault Zone offshore Luzon Island, multi-channel seismic (MCS) data, gravity data and centroid moment tensor solutions were used in this study. Focal mechanism solutions from the Global CMT catalog were inverted to determine the average principal stress directions and consequently understand the prevailing stress regime in the study area. The stress inversion results indicate that the direction of maximum compression (σ1) is 321°N, which coincides with the PSP–EP convergence direction. From the MCS profiles, the study area was subdivided into deformation zone and a relatively stable zone. Thrust faulting, folding and general uplift are observed in the deformation zone. This zone is further subdivided into the active and inactive segments. In the active segment, uplift is occurring in the submarine ridge. This deformation pattern can be related to the ongoing uplift in some regions bisected by the PFZ. The inactive segment, characterized by intense folding of the sequences and faulting of the basement and overlying sequences, is suggested as the precursor of the Philippine Fault Zone. Deformation appears to be recently shifted to the east as delineated by an uplifted N-NW trending submarine ridge offshore NW Luzon Island.

The best estimate method (BEM), which shifts an object event towards the centroid of the events within its location uncertainty, was originally proposed to clarify the pattern of seismicity distribution. It was later widely applied to constrain the location and geometry of seismogenic structures. The goal of this study is to improve the efficiency, robustness and effectiveness of the BEM. We perform a series of synthetic experiments by generating a three-dimensional (3D) event dataset representing complex tectonic features and perturbing the presumed hypocenter locations. The synthetic experiments show that appropriate expression for the location uncertainty of the object event and restrictive thresholds of the event selection can significantly improve the collapsing seismicity image. We also demonstrate that the performance of our revised BEM (RBEM) is better than that of the original BEM. We then apply our RBEM to the seismicity catalog for the Taiwan region, collapsing the diffusive catalog hypocenters into sharp images of seismicity. Our RBEM result delineates a clearer low-seismicity zone in central Taiwan and a wider separation between the Wadati–Benioff double seismic layers in northeastern Taiwan than the corresponding features in either the catalog seismicity or the 3D relocation seismicity. Because of the simplicity and efficiency of the RBEM, it is perfectly suited to delineate the location and geometry of active faults in routine operations.

To understand the convergent characteristics of the westernmost plate boundary between the Philippine Sea Plate (PSP) and Eurasian Plate (EP), we have calculated the stress states of plate motion by focal mechanisms. Cataloged by the Harvard centroid moment tensor solutions (Harvard CMT) and the Broadband Array in Taiwan (BATS) moment tensor, 251 focal mechanisms are used to determine the azimuths of the principal stress axes. We first used all the data to derive the mean stress tensor of the study area. The inversion result shows that the stress regime has a maximum compression along the direction of azimuth N299°. This result is consistent with the general direction of the rigid plate motion between the PSP and EP in the study area. In order to understand the spatial variation of the regional stress pattern, we divided the study area into six sub-areas (blocks A to F) based on the feature of the free-air gravity anomaly. We compare the compressive directions obtained from the stress inversion with the plate motions calculated by the Euler pole and the Global Positioning System (GPS) analysis. As a result, the azimuth of the maximum stress axis, σ 1 , generally agrees with the directions of the theoretical plate motion and GPS velocity vectors except block C (Lanhsu region) and block F (Ilan plain region). The discrepancy of convergent direction near the Ilan plain region is probably caused by the rifting of the Okinawa Trough. The deviation of the σ 1 azimuth in the Lanhsu region could be attributed to a southwestward extrusion of the Luzon Arc (LA) block between 21°N and 22°N whose northern boundary may be associated with the right-lateral NE–SW trending fault (i.e. Huatung Fault, HF) along the Taitung Canyon. Comparing the σ 1 stress patterns between block C and block D, great strain energy along HF may not be completely released yet. Alternatively, the upper crust of block C may significantly have decoupled from its lower crust or uppermost mantle.

A comparison of the geological and geophysical environments between the Himalaya-Sumatra and Taiwan-Ryukyu collision-subduction systems revealed close tectonic similarities. Both regions are characterized by strongly oblique convergent processes and dominated by similar tectonic stress regimes. In the two areas, the intersections of the oceanic fracture zones with the subduction systems are characterized by trench-parallel high free-air gravity anomaly features in the fore-arcs and the epicenters of large earthquakes were located at the boundary between the positive and negative gravity anomalies. These event distributions and high-gravity anomalies indicate a strong coupling degree of the intersection area, which was probably induced by a strong resistance of the fracture features during the subduction. Moreover, the seismicity distribution in the Ryukyu area was very similar to the pre-seismic activity pattern of the 2004 Sumatra event. That is, thrust-type earthquakes with a trench-normal P-axis occurred frequently along the oceanward side of the mainshock, whereas only a few thrust earthquakes occurred along the continentward side. Therefore, the aseismic area located west of 128°E in the western Ryukyu subduction zone could have resulted from the strong plate locking effect beneath the high gravity anomaly zone. By analogy with the tectonic environment of the Sumatra subduction zone, the occurrence of a potential Sumatra-like earthquake in the south Ryukyu arc is highly likely and the rupture will mainly propagate continentward to fulfill the region of low seismicity (approximately 125° E to 129° E; 23° N to 26.5° N), which may generate a hazardous tsunami.

[1] We applied an improved stress inversion method to a comprehensive data set of earthquake focal mechanisms to depict the pattern of crustal stress along the western convergent boundary of the Philippine Sea plate. Our results indicate that the crustal stress along the Ryukyu fore arc is segmented with boundaries at or near the places of seamount subduction, including the Tokara channel. An extensional stress regime is observed along the entire Ryukyu back arc, implying that back-arc rifting may have extended northward to Kyushu. A triangular area near the southernmost terminus of the Ryukyu arc is characterized by a unique stress signature. The eastern boundary of this Ryukyu-Taiwan Stress Transition coincides with the 123°E meridian where the Gagua ridge intercepts the Ryukyu trench; whereas its western boundary agrees remarkably well with the border between the postcollision and waning-collision domains in northern Taiwan. The Taiwan collision zone is dominated by compression that rotates locally according to the structural configuration of the Lukang Magnetization High (LMH), suggesting that the LMH may be critical in controlling the local stress distribution. The stress signature of the Luzon arc–Taiwan collision reaches as far south as 19.5°N. The tectonic stress along the Manila trench–Luzon fore arc is dominated by a complex regime of extension that cannot be explained by simple plate bending or in-slab membrane stress. Since this extensional regime is observed only south of ∼22°N, it probably marks the northern limit of the contemporary boundary between the subduction along the Manila trench and the collision in Taiwan.

To understand the generation mechanism of the 2011 Tohoku-Oki earthquake, we analyzed the spatial and temporal distribution of the cumulative seismic moment (ΣM0) for all earthquakes along the Japan Trench listed in the global centroid moment tensor catalog between January 1976 and November 2011. Three areas with distinct characteristics of ΣM0 are identified: (1) in the 2011 Tohoku-Oki source area, the ΣM0 released in the down-dip portion (≥30 km) was greater than that in the up-dip portion (