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Temporal fault‐zone observations are important to better understand the evolution of fault structure and stress configuration. However, long‐term monitoring in the fault zone is rare after a large earthquake. Here, we use seismic data in the fault zone at 1‐km depth from the Taiwan Chelungpu‐fault Drilling Project to study long‐term anisotropy after the 1999 Mw7.6 Chi‐Chi earthquake. The direct S‐wave splitting measurements resolve the overall weak anisotropy in the shallow crust. In order to resolve fault damage zone anisotropy, we perform coda cross‐correlation technique for 794 microearthquakes between 2007 and 2013. We estimate the temporal change in background shear‐wave velocity, fast shear‐wave polarization direction (FSP), and strength of anisotropy (Aani) in the fault damage zone. We show the average FSP direction is N93°E with a significant Aani of about 12%, likely due to the pervasive vertical microcracks created after the earthquake. Temporal variations of anisotropy exhibit seasonal variation with periodicity every 9–12 months that correlates with rainfall events. Furthermore, long‐term anisotropy shows a gradual rotation of FSP direction of about 15° during the first 4 years of observation. At the same time, the strength of anisotropy reduced from 17% to 10% and shear‐wave velocity increased, suggesting the fault healed after the earthquake. This study reports in‐situ evidence for two key observations: (a) long‐term, fault‐zone healing after a major earthquake and (b) modulation of 1‐km deep fault‐zone properties by surficial hydrologic processes. These observations may provide constraints on the response of the fault damage zone in the interseismic period. Plain Language Summary: Earthquakes are generated when rocks slip quickly along a fault. Damage produced by fault rupture is often observed in an area with enhanced fracture networks called the fault damage zone. Fractures inside the damage zone can open or close in response to the background stress. Therefore, observing fault fractures may provide useful information on how the background stresses evolve in the fault. Here, we observe the time‐dependent change of fractures in the fault damage zone through seismic anisotropy analysis. We measure the velocity difference of a seismic shear‐wave in different azimuths to determine the anisotropy. The azimuth of the fastest shear‐wave indicates the dominant fracture orientation inside the fault damage zone. We use borehole seismometers installed within the Chelungpu fault in Taiwan to study the long‐term anisotropy after a major earthquake, and found the fault‐zone is highly damaged with fractures aligning with the tectonic stress direction. Anisotropy varies seasonally due to fluid pressure changes in fault zone fractures during and after rainfall events. In addition, we observe a long‐term anisotropy change that might indicate fracture closure. This study presents a unique observational dataset which allows us to better understand fault zone evolution after a large earthquake. Key Points: A 7‐year (2007–2013) fault‐zone seismic anisotropy is presented from the Taiwan Chelungpu‐fault Drilling Project borehole seismic arrayAverage fast shear‐wave direction of N93°E and anisotropy strength of 12% are observed after the Chi‐Chi earthquakeAnisotropy varies seasonally with rainfalls. Long‐term anisotropy from 17% to 10% with increasing velocity suggests fault healing [ABSTRACT FROM AUTHOR]

• Finite-fault rupture models suggest a simple rupture process with a single and compact asperity, and a single triangular STF. • Ruptures often propagated unilaterally along down-dip directions. • Rupture speed, size, and stress drop of intermediate-depth earthquakes in southern Java were determined. • Directivity analysis suggests near-vertical and subhorizontal fault planes as the preferred rupture planes. The rupture behavior of intermediate-depth earthquakes in southern Java remains poorly understood despite their potential seismic hazard. In this study, we performed finite-fault inversions to investigate the rupture processes and source characteristics of five intermediate-depth earthquakes (60–300 km depth) with moment magnitudes (M W) ≥ 6.1 from 1998 to 2017 in the southern region of Java and its surrounding areas. Utilizing teleseismic body waves and surface waves, we employed a wavelet-based seismic inversion technique. Initially, we conducted preliminary inversions of the focal mechanisms (strike and dip) from the Global Centroid Moment Tensor (GCMT) database to determine the optimal fault plane orientation for slip distributions and source time functions (STFs). Our findings reveal that most of the earthquakes exhibited a simple rupture process characterized by a single and compact asperity with a single triangular STF, except for the 1998 earthquake. The results indicate that the ruptures primarily propagated unilaterally along the down-dip direction, except for the 2014 earthquake. We further analyzed the data incorporating directivity, which confirmed the rupture behavior. Three events suggested that the preferred rupture planes were near-vertical (down-dip), while two events exhibited subhorizontal orientations. Considering the challenges in determining the rupture plane associated with the subducting slab, the densely deployed national seismic networks in Java are expected to provide valuable insights into the dynamics of the subduction zone. By elucidating the source characteristics and rupture behavior of these intermediate-depth events, our study offers valuable insights for future seismic hazard assessments, particularly for densely populated regions of Java, Indonesia. [ABSTRACT FROM AUTHOR]

A decrease in Q values within the hanging wall of the ruptured Chelungpu fault two years following the 1999 Chi-Chi earthquake was revealed by Q tomography images and an analysis of single-path Q. The synthetic and sensitivity tests of the Q determination were carried out accordingly to justify the temporal variation. A Q value within the hanging wall above the hypocenter was determined to be 157 ± 18 two years following the Chi-Chi earthquake, which is significantly lower than the Q tomography values of 238 ± 17 and 289 ± 13 prior to and two years after the main shock, respectively. Similar values using a signal-path Q analysis from events within the ruptured fault zone to stations along the fault were obtained. The corresponding Q values were 247 ± 85 prior to the Chi-Chi earthquake. After the earthquake we obtained Q values of 158 ± 75 and 318 ± 80 for 2 years following and 2 years after the earthquake, respectively. Considering the two independent methods in determination of Q, the reduction in Q by 89 two years following the Chi-Chi earthquake in both methods is significant. Along with 1 % V reduction revealed by the analysis of repeating earthquakes our studies suggested possible reduction both in V and Q values within the fault zone after the Chi-Chi earthquake. Temporal changes in Q after the Chi-Chi earthquake imply variations of pore-fluid saturation in the ruptured fault zone. The reduction in Q two years following the Chi-Chi earthquake indicates high pore-fluid saturation within the fractured fault zone due to the postseismic fluid redistribution. [ABSTRACT FROM AUTHOR]

SUMMARY Microearthquakes with magnitude down to 0.3 were detected by the Taiwan Chelungpu-fault Drilling Project Borehole Seismometers (TCDPBHS). Despite the large coseismic slip of 12 m at the drill site during the 1999 Chi-Chi earthquake, our studies show very little seismicity near the TCDPBHS drill site 6 yr after the Chi-Chi main shock. The microearthquakes clustered at a depth of 9-12 km, where the Chelungpu thrust fault turns from a 30° dipping into the horizontal decollement of the Taiwan fold-and-thrust tectonic structure. Continuous GPS surveys did not observe post-slip deformation at the larger slip region and no seismicity was observed near the drill site. Therefore we suggest that the thrust belt above the decollement is locked during this interseismic period. We further investigated source parameters of 242 microearthquakes by fitting ω−2-shaped Brune source spectra to our observation data using a frequency-independent Q model. We find that the static stress drop increases significantly with increasing seismic moment. However, due to the intense debate on this topic of scaling-relations and the related self-similarity of earthquakes, we further improve the data analysis and correct for path and site effects using the Projected Landweber Deconvolution (PLD) method for events within some clusters. The PLD method analyses the source time functions of the larger and the smaller event by an iterative technique. As a result we received source dimensions and stress drops of larger events including path and site effect corrections. The results from the PLD method are less scattered and also show a positive relation between static stress drop and seismic moment. We find a similar positive trend for the apparent stress scaling with seismic moment. [ABSTRACT FROM AUTHOR]

SUMMARY Theoretical and observational evidence implies that variable rupture velocity may be associated with high fault slip and high moment release rates during earthquakes. We investigate this relationship for the 2008 Mw 7.9 Wenchuan China, Earthquake, which appears to have experienced a highly variable moment release rate. We used an empirical Green's function (EGF) deconvolution analysis of teleseismic waveforms to retrieve the primary rupture characters for this event. Based on field observations and the deconvolved source-time function from thrust and strike-slip EGFs, we divided the ruptured fault into three segments, allowing us to determine the spatial slip distribution with different average rupture velocities on each segment. We deployed a grid search analysis, in which we integrated the teleseismic waveform inversion and forward modelling of the regional surface wave to determine the optimum rupture speed in each fault segment. Our result shows that the 2008 Wenchuan Earthquake had slip amplitude, direction and rupture velocity that were highly spatially variable. The earthquake initially ruptured with nearly pure thrust motion and a slow rupture velocity of 1.7 km s-1. Then, the rupture speed increased up to 3.1-3.3 km s-1 and produced the largest slip on the second segment, where two parallel faults ruptured simultaneously. Rupture velocity then slowed down to 2.5-2.9 km s-1 on the final segment, which underwent primarily strike-slip motion. In total, the fault extended 300 km with an average rupture speed in the range of 2.6-2.9 km s-1. The high rupture speed (close to the shear wave velocity) in the second segment may be related to the simultaneous rupture of the two parallel faults on well-established pre-existing structures. The spatial distribution of rupture velocity and slip are likely related to the strongly rotational and 3-D deformation in the eastern Tibetan Plateau margin, which leads to the heterogeneous stress field of the Longmen Shan region. [ABSTRACT FROM AUTHOR]

Abstract: Continuous cores and a suit of geophysical measurements were collected in two scientific drill holes to understand physical mechanisms involved in the large displacements during the 1999 Chi-Chi earthquake. Physical properties obtained from wire-line logs including P- and S-wave sonic velocity, gamma ray, electrical resistivity, density and temperature, are primarily dependent on parameters such as lithology, depth and fault zones. The average dip of bedding, identified from both cores and FMI (or FMS) logs, is about 30° towards SE. Nevertheless, local azimuthal variations and increasing or decreasing bedding dips appear across fault zones. A prominent increase of structural dip to 60°–80° below 1856 m could be due to deformation associated with propagation of the Sanyi fault. A total of 12 fault zones identified in hole-A are located in the Plio-Pleistocene Cholan Formation, Pliocene Chinshui Shale and Miocene Kueichulin Formation. The shallowest fault zone occurs at 1111 m depth (FZ1111). It is a 1 m gouge zone including 12 cm of thick indurate black material. We interpreted this zone as the slip zone during Chi-Chi earthquake. FZ1111 is characterized by: 1) bedding-parallel thrust fault with 30-degree dip; 2) the lowest resistivity; 3) low density, V p and V s, 4) high V p/V s ratio and Poisson''s ratio; 5) low energy and velocity anisotropy, and low permeability within the homogeneous 1 m gouge zone; 6) increasing gas (CO2 and CH4) emissions, and 7) appearance of smectite within the primary slip zone. In situ stresses at the drill site were inferred from leak-off tests, borehole breakouts and drilling-induced tensile fractures from borehole FMS/FMI logs, and shear seismic wave anisotropy from DSI logs. The dominant fast shear-wave polarization direction is in good agreement with regional maximum horizontal stress axis, particularly within the strongly anisotropic Kueichulin Formation. A conjugate set of secondary directions are parallel to microcrack orientations. A drastic change of orientation of fast shear-wave polarization across the Sanyi thrust fault at the depth of 1712 m reflects the change of stratigraphy, physical properties and structural geometry. [Copyright &y& Elsevier]

Abstract: We investigated the fault geometry effects and the corresponding coseismic slip distribution using various proposed earthquake fault models for the Chi-Chi earthquake of 21 September 1999. The types of fault geometries are threefold: a simple planar fault plane, a two segmented planar fault plane and a three dimensional (3D) curved fault surface rupture propagation model. We derived the estimated spatial slip distribution from an inversion analysis of GPS coseismic displacement data and show that the 3D fault model is the preferred solution. The simple and segmented fault models lead to significant artificial slip distributions associated with the pre-defined fault geometry and the spatial distribution of GPS stations. The spatial distribution of coseismic slip deduced from the 3D fault model has three observable features: (1) the overall slip is concentrated at depth of less than 12 km, which may well correspond to a shallow-dipping detachment; (2) the maximum slip of about 10 m is located 45 km to the north of the epicenter; and (3) the slip vector is dominated by the dip-slip component. In addition, the results from the inversion of GPS data are consistent with those from the inversion analysis of teleseismic broadband data. A resolution analysis, further, demonstrates that the results are highly correlated with field GPS data studies when we used synthetic test data. The inversion of spatially distributed GPS data is highly sensitive to fault geometry. We conclude that the use of the 3D fault model is not only necessary but also certainly competent enough to well explain the inferred slip style and the observed static coseismic displacements. [Copyright &y& Elsevier]

We investigate the thermo-mechanical properties beneath the young orogenic belt of Taiwan by constructing a shear strength profile from a vertical stratified rheological structure. The stratified rheological structure is estimated based on the recently developed thermal structure and its likely composition. Subduction–collision in the young orogenic belts and the thick accretionary wedge make a significant contribution to the growth of sialic crust in the hinterland. The sialic bulk crust not only results in a low seismic velocity but also produces weak crust in the hinterland. The earthquake depth–frequency distribution in the foreland and hinterland correlates very well with the regimes of the brittle/ductile transition revealed in the strength profile. Our results show that the observed two-layer seismicity in the foreland is due to a moderate geotherm and an intermediate mafic bulk composition; while single-layer seismicity in the hinterland is due to its felsic bulk composition. In the foreland, the mechanically strong crust (MSC) and the mechanically strong lithosphere (MSL) coincide with frequent seismicity. The shallow MSC in the hinterland is consistent with the 20- to 25-km seismicity occurring there. The total lithospheric integrated strength (LIS) in the hinterland is only about half of that in the foreland, suggesting a weak lower crust and lithosphere mantle in the hinterland. The results confirm that the earthquake cutoff depth is a proxy for temperature. The calculated decrease of effective elastic thickness (EET) from the orogenic margin (foreland) to the center (hinterland) is consistent with the results of flexure modeling in most orogenic belts. Due to the weak LIS in the hinterland, crustal thinning and rifting may occur in the future. Our results, thus, suggest that the mechanical structure is also closely related to the composition and is not directly reflected in the thermal structure. [Copyright &y& Elsevier]

Simple kinematic modeling of particle motion along a curved fault similar to the ruptured Chelungpu fault, Taiwan, indicates a unique spatial slip pattern. Specifically, we find that the large convergent slip on the curve is a result of the minimum deformation scenario of the fault geometry and regional northwestern movement of the Philippine Sea Plate (PSP). The modeled deformation regime portrays an accumulation of deformation in the curved region, which coincides very well with a long-term observed NW-SE-trending seismogenic zone in the central Taiwan. This consistence suggests that the Chelungpu fault is a preexisting curved fault. This is further evidenced by geological and geophysical observations. Because the spatial slip pattern is locally and regionally tectonically controlled, it indicates that the rupture behavior of the Chi-Chi earthquake is repeatable. Better knowledge of the fault geometry and the regional plate motion may help us to predict the possible spatial slip distribution of large earthquakes. This discovery is important for avoiding large buildings and constructions near predicted large slip regions. [Copyright &y& Elsevier]

[Display omitted] • Constructed finite-fault models suggest non-overlap asperities area. • Ruptures often propagated along-strike or down-dip directions with low stress drops. • Events exhibited a similar initiation behavior to their dynamic rupture. • Cascading asperities link to the less mature feature of western Flores Thrust. • Asperities ruptured neighboring fault patches and were un-grown into a large event. The Flores Thrust is a southward-dipping, low-to-moderate angle submarine active fault in the eastern Sunda-Banda back-arc (Indonesia). Significant shallow-depth destructive earthquakes have been reported along this fault zone. From 2002 to 2009, one of its fault segments, called the Sumbawa segment, experienced five earthquakes with moment magnitude (M W) values of 6.2–6.6. In this study, we performed finite-fault rupture inversions for these earthquakes, constrained with teleseismic body and surface waveforms, to investigate the characteristics of earthquake ruptures along this fault zone. We obtained the source-time-functions and finite-fault rupture models for these five earthquakes. Results indicated that ruptures often propagated along-strike or down-dip. The ruptures were initiated from the middle crust (depth of approximately 12–17 km) and exhibited a comparable initiation behavior to their entire rupture. The rupture speeds and stress drops were approximately 2.0–2.5 km/s and 1.0–2.0 MPa, respectively. Five cascading asperities ruptured neighboring fault patches and did not overlap each other. The characteristics of earthquake source parameters and rupture processes obtained in this study are robust and helpful for future regional seismic hazard assessment and earthquake early warning studies. These cascading asperities might be related to the fault immaturity of the western Flores Thrust. Alternatively, these earthquakes may act as asperities located at the down-dip patches of the Sumbawa segment, and its shallower section still has a potential of ruptures with M W > 7.0. [ABSTRACT FROM AUTHOR]

Advances in observational, theoretical and computational technologies have made it possible for automatic, real-time solutions of the focal mechanisms of earthquake point sources. However, for earthquakes of moderate and greater magnitudes, the complexity of the source kinematic processes often requires additional characteristics on the source rupture in order to make seismotectonic inferences and to explain the observed directivity effects of the radiation of seismic energy. We develop an efficient and effective approach to determining the average finite-rupture models of moderate earthquakes by fitting synthetic and recorded broadband waveforms. A Green's tensor database is established using 3-D structural model with surface topography to enable rapid evaluations of accurate synthetic seismograms needed for source parameter inversions without the need for high-performance computing. We take a two-step strategy: In the first step, a point-source model is determined by a grid search for the best fault-plane solution. Then, taking the two nodal planes in the point-source model as candidates of the actual fault plane, a second grid search is carried out over a suite of simplified finite-rupture models to determine the optimal direction and speed of the integrated rupture of the finite source. We applied our method to four moderate events (MW ≈ 6) in southeastern Taiwan. Results show that our technique provides an effective choice in semi-automatic, near real-time determinations of finite-source parameters for earthquake hazard assessment and mitigation purposes. [ABSTRACT FROM AUTHOR]

SUMMARY The 1909 April 14 Taiwan earthquake caused significant damage in Taipei. Most of the information on this earthquake available until now is from the written reports on its macro-seismic effects and from seismic station bulletins. In view of the importance of this event for assessing the shaking hazard in the present-day Taipei, we collected historical seismograms and station bulletins of this event and investigated them in conjunction with other seismological data. We compared the observed seismograms with those from recent earthquakes in similar tectonic environments to characterize the 1909 earthquake. Despite the inevitably large uncertainties associated with old data, we conclude that the 1909 Taipei earthquake is a relatively deep (50-100 km) intraplate earthquake that occurred within the subducting Philippine Sea Plate beneath Taipei with an estimated MW of 7 ± 0.3. Some intraplate events elsewhere in the world are enriched in high-frequency energy and the resulting ground motions can be very strong. Thus, despite its relatively large depth and a moderately large magnitude, it would be prudent to review the safety of the existing structures in Taipei against large intraplate earthquakes like the 1909 Taipei earthquake. [ABSTRACT FROM AUTHOR]

Abstract: This paper presents a brief summary of several types of precursors in the lithosphere, atmosphere and ionosphere that have been positively identified in relation to the 20 September 1999 M w 7.6 Chi-Chi earthquake and its aftershocks under the iSTEP Program. [Copyright &y& Elsevier]

The 2018 February 6 M w 6.3 Hualien earthquake caused severe localized damage in Hualien City, located 20 km away from the epicentre. The damage was due to strong (>70 cm s−1) and sharp (duration ∼2.5 s) velocity pulses. The observed peak ground-motion velocity in Hualien City symmetrically decays with distance from the nearby Milun fault. Waveforms observed on the opposite sides of the fault show reversed polarity on the vertical and N–S components while the E–W component is almost identical. None of the published finite-fault slip models can explain the spatially highly localized large velocity pulses. In this study, we show that an M w 5.9 strike-slip subevent on the Milun fault at 2.5 km depth, rupturing from north to south at ∼0.9 V s speed, combined with site effects caused by surficial layers with low S-wave speed, can explain the velocity pulses observed at the dense strong-motion network stations. This subevent contributes only 25 per cent of the total moment of the 2018 Hualien earthquake, suggesting that a small local slip patch near a metropolis can dominate the local hazard. Our result strongly suggests that seismic hazard assessments should consider large ground-motion variabilities caused by directivity and site effects, as observed in the 2018 Hualien earthquake. [ABSTRACT FROM AUTHOR]

We have developed a real-time moment tensor monitoring system (RMT) which takes advantage of a grid-based moment tensor inversion technique and real-time broad-band seismic recordings to automatically monitor earthquake activities in the vicinity of Taiwan. The centroid moment tensor (CMT) inversion technique and a grid search scheme are applied to obtain the information of earthquake source parameters, including the event origin time, hypocentral location, moment magnitude and focal mechanism. All of these source parameters can be determined simultaneously within 117 s after the occurrence of an earthquake. The monitoring area involves the entire Taiwan Island and the offshore region, which covers the area of 119.3°E to 123.0°E and 21.0°N to 26.0°N, with a depth from 6 to 136 km. A 3-D grid system is implemented in the monitoring area with a uniform horizontal interval of 0.1° and a vertical interval of 10 km. The inversion procedure is based on a 1-D Green's function database calculated by the frequency–wavenumber (fk) method. We compare our results with the Central Weather Bureau (CWB) catalogue data for earthquakes occurred between 2010 and 2012. The average differences between event origin time and hypocentral location are less than 2 s and 10 km, respectively. The focal mechanisms determined by RMT are also comparable with the Broadband Array in Taiwan for Seismology (BATS) CMT solutions. These results indicate that the RMT system is realizable and efficient to monitor local seismic activities. In addition, the time needed to obtain all the point source parameters is reduced substantially compared to routine earthquake reports. By connecting RMT with a real-time online earthquake simulation (ROS) system, all the source parameters will be forwarded to the ROS to make the real-time earthquake simulation feasible. The RMT has operated offline (2010–2011) and online (since January 2012 to present) at the Institute of Earth Sciences (IES), Academia Sinica (http://rmt.earth.sinica.edu.tw). The long-term goal of this system is to provide real-time source information for rapid seismic hazard assessment during large earthquakes. [ABSTRACT FROM AUTHOR]

Abstract: The attenuation factor, Q, at a fault zone is an important parameter for understanding the physical properties. In this study, we investigated the Q value of the Chelungpu Fault, the main rupture of the Mw 7.6 Chi-Chi earthquake, using the 7-level TCDP borehole seismometer array (TCDPBHS). The TCDPBHS was deployed at depths from 945 to 1270m throughout the 1999 ruptured slip zone at 1111m. Three borehole seismometers (BHS1–BHS3) were placed in the hanging wall, and the remaining three (BHS5–BHS7) were placed in the foot wall, with BHS4 near the slip zone. The configuration allowed us to estimate the Q-structure of the recent ruptured fault zone. In this study, we estimated Q values between BHS1 and BHS4, Qs 1 (Qp 1) at the fault zone and between BHS4 to 2km in depth, Qs 4 (Qp 4) beneath the fault zone. We utilized two independent methods, the spectral ratio and spectral fitting analyses, for calculating the Q value of Qs 1 (Qp 1) in order to provide a reliability check. After analyzing 26 micro-events for Qs and 17 micro-events for Qp, we obtained consistent Q values from the two independent methods. The values of Qs 1 and Qp 1 were 21–22 and 27–35, respectively. The investigation for the value of Qs 4 was close to 45, and Qp 4 was 85. These Qp and Qs values are quiet consistent with observations obtained for the San Andreas Fault at the corresponding depth. A low Qs 1 value for the recent Chelungpu Fault zone suggests that this fault zone has been highly fractured. Qs values within the Chelungpu Fault, similar to those within the San Andreas Fault, suggest that the Q structure within the fault zone is sedimentary rock independent. However, the possible existence of fluids, fractures, and cracks dominates the attenuation feature in the fault zone. [Copyright &y& Elsevier]

Abstract: Pressure solution is one of the main deformation mechanisms for lithify and stress release in the rocks from shallow subduction zones. We observed temperature-dependent pressure solution development in naturally deformed shale in Shimanto accretionary complex in southwest Japan. The pressure solution develops with shear-dominated or co-axial-dominated deformation. We evaluated apparent activation energy by applying the constitutive equation of pressure solution creep to the temperature-dependent relations. The activation energy of each deformation type was estimated as 18 kJ mol− 1 for shear-dominated and 45 kJ mol− 1 for co-axial-dominated shale. The energies enable us to speculate rate-limiting processes of pressure solution i.e. dissolution, diffusion and precipitation, by comparing the energies obtained in this study with energies had been measured by experiments. The lower activation energy estimated here was similar with that of diffusion. The similarity indicates that possible rate-limiting process of shale deformation in shallow subduction zone would be diffusion. The difference of energy between deformation types can be explained by distinctive grain boundary structure. [Copyright &y& Elsevier]

Abstract: 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. [Copyright &y& Elsevier]

The tsunami in the event of 2018 Sulawesi earthquake and tsunami (SET) occurred after an earthquake of magnitude 7.5. However, it is very difficult for such an earthquake to trigger a tsunami with 10-m flood depth. The causes of this tsunami event remain unknown. A submarine landslide, an additional seabed vertical displacement, and seiche effect may play roles. To gain a deeper understanding of this tsunami event, finding the location of the tsunami source is especially important. We developed two new tsunami analysis methods, impact intensity analysis method (IIA) and tsunami arrival time analysis (TATA) method, to analyze the tsunami source in the SET event. The IIA method can quickly filter out the areas with low impacts of the tsunami sources and effectively reduce the number of tsunami scenarios. However, for a tsunami with a strong direction, it is less easy to grasp. The newly developed TATA method can overcome this weakness. In this study, we perform different scenarios by simulating the tsunamis generated by the earthquake, in-the-bay landslides, and seiche. The earthquake scenario results show that the wave height contributed by the earthquake is less than 0.3 m, which is far away from the field survey. The results from in-the-bay landslide and seiche are away from the waveform and arrival time at Pantoloan. The IIA is then used to reduce the number of the scenarios. From both of the IIA results of Palu and Pantoloan, the possible tsunami sources are located either inside the Palu Bay or around the bay mouth. Based on the tidal gauge data at Pantoloan and the tsunami arrival time from BMKG, the precise location of the tsunami source is spotted. We further created several scenarios which satisfied most of the data we collected, such as the Pantoloan gauge record, Palu inundation area, and flooding depth from the field survey along the Palu Bay. The detailed results will be presented in the full paper. [ABSTRACT FROM AUTHOR]

We present earthquake shaking scenarios by a physics-based ground-motion simulation scheme for seismic risk assessments. In fitting requirements of the application of loss estimations, scenarios can refer to various kinds of damage and losses to constructions (buildings, bridges, etc.), casualties, economic losses, social losses and so on. We quantitatively assess the ground motion effects by the physics-based full-waveform ground-motion simulation which is a powerful tool for earthquake-induced ground-motion prediction. The entire time history of the ground motion induced by an earthquake depends on a number of factors, including source-rupture process, subsurface velocity structure, topographic relief, and local site condition. Then, the further risk assessment can also be revealed by a combination with exposure data. As an example, we carried out simulations to model rupture scenario of the Shanchiao fault and the potential large subduction earthquake in northern Taiwan, with accounting for effects of (1) source slip distribution, (2) seismic wave propagation, (3) surface topography and (4) site amplification. Output ground-motion parameters, including peak ground acceleration (PGA), peak ground velocity (PGV), peak ground displacement (PGD), 0.3-s-period spectral acceleration (SA0.3), and 1.0-s-period spectral acceleration (SA1.0), can be adopted to further implement to loss estimation. Our results referred different shaking-level potentials of earthquake scenarios can benefit to risk management, especially for the further application of strategy making of emergency response and disaster prevention and preparations. [ABSTRACT FROM AUTHOR]