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The Central Weather Bureau, Taiwan, has deployed a borehole seismic network of 62 stations nationwide in Taiwan since 2008. Each borehole station includes a force-balance accelerometer on the surface, and a combination of an accelerometer and a broadband seismometer in the deep hole. The noise analyses have shown significant improvement in the performance of the borehole instruments compared with the instruments deployed on the surface. We propose a method to obtain robust orientations, as well as the uncertainty for both the borehole accelerometer and broadband seismometer using long-period waves (>2 s) after the S-wave arrival time of three mid-large teleseismic events and one local event. Overall, we successfully calibrated the orientations of the borehole sensors and found that the significant azimuth change for some borehole seismometers was always due to sensor maintenance. Based on the results of this study, we conclude that the long-period waveforms generated by a teleseismic event, wavelength of which is four times longer than the borehole depth, are useful for borehole orientation calibration. The correlation coefficients of the three components for both surface and borehole sensors larger than 0.9 are appropriate. The magnitude and epicenter distance of the event and on-site noise level are the factors that affect the quality of calibration. In addition, we conclude that the method of orientation calibration we propose is helpful for data quality control, including data timing and sensor failure. With the supplement of this azimuth estimation information, we expect that these high-quality borehole data in Taiwan will improve seismological research and will benefit the society.

SUMMARYThe 2019 ML 6.3 Hualien earthquake struck the northern Longitudinal Valley (LV) and generated not only large strong motions (intensity of 7, as defined by the Central Weather Bureau, Taiwan) locally but also widespread strong shaking in metropolises in northern Taiwan. In this study, we analyse strong motion records from local seismic networks to understand the source properties of the 2019 event. We first obtain the centroid location of the 2019 event using the source-scanning algorithm (SSA) technique by applying the unfiltered records. The determined centroid location is 121.55°E, 24.10°N, with a depth of 22.5 km. This location is 5.5 km north–northwest of and 3.8 km deeper than the Central Weather Bureau hypocentre, suggesting that the 2019 event occurred on the high-angle west-dipping plane of the focal solution. The centroid time delay is 3.35 s. Then, we obtain strong motion generation areas (SMGAs) of the 2019 event using the empirical Green's function method by considering the broad-band waveforms (0.4–10 Hz). Unlike other moderate-sized earthquakes in Taiwan, which have one SMGA, we determine that there were two SMGAs in the 2019 event. SMGA1 initiated at the CWB hypocentre with a size of 4.00 km2, and SMGA2 initiated at the centroid location determined by the SSA approach with a size of 3.63 km2. Such small areas cause high stress drops of 13.7 and 27.4 MPa for SMGA1 and SMGA2, respectively. We infer that the localized high stress drop of SMGAs is one of the important factors responsible for high peak-ground accelerations (PGAs) in Taiwan in addition to a strong directivity effect coupled with the radiation pattern reported by the previous study. Furthermore, previous moderate-sized earthquakes on an active structure called the Xiulin segment revealed similar source properties with a high stress drop and generated large PGA locally as well as in the metropolises of northern Taiwan. Considering the stored moment deficit, the probability of a future large earthquake in the northern LV region remains high. It is essential to consider seismic hazard assessment and mitigation for this not-well-known but high-seismic-potential region.

The 2014 M L 5.9 Fanglin earthquake occurred at the northern end of the aftershock distribution of the 2013 M L 6.4 Ruisui event and caused strong ground shaking and some damage in the northern part of the Longitudinal Valley. We carried out the strong-motion simulation of the 2014 Fanglin event in the broadband frequency range (0.4–10 Hz) using the empirical Green’s function method and then integrated the source models to investigate the source characteristics of the 2013 Ruisui and 2014 Fanglin events. The results show that the dimension of strong motion generation area of the 2013 Ruisui event is smaller, whereas that of the 2014 Fanglin event is comparable with the empirical estimation of inland crustal earthquakes, which indicates the different faulting behaviors. Furthermore, the localized high PGV patch might be caused by the radiation energy amplified by the local low-velocity structure in the northern Longitudinal Valley. Additional study issues are required for building up the knowledge of the potential seismic hazard related to moderate-large events for various seismogenic areas in Taiwan.

We investigated broadband evidence of the rupture directivity effect and estimated stress drop for the 2013 Nantou blind‐thrust earthquakes (the 0327 and 0602 crustal events) from three individual analyses using empirical Green’s functions: deconvolution source time function analysis, source spectral ratio analysis, and broadband waveform modeling of a strong‐motion generation area. Although these two adjacent events have similar focal mechanisms, the 0327 event shows only up‐dip directivity to the west, with an estimated stress drop of 9.0 MPa, and the 0602 event displays significant rupture directivity to the southwest, with an estimated stress drop of 14.2 MPa. The obvious variance in the rupture behavior and stress drop of these two nearby events, which might be related to immature blind faulting, suggests that this region requires additional study of the stress heterogeneity to better assess the seismic hazard.

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.

From the end of August to early September 2011, 15 ocean‐bottom seismometers (OBSs) were deployed offshore northeastern Taiwan for approximately 20 days. During this period, the typhoon Nanmadol formed in the western Pacific, moved northwestward from the East Philippines, and made landfall on the island of Taiwan. In this study, we analyzed the seismic signals from the OBSs and the marine metrological data to investigate the influence of the typhoon on submarine seismic records. Our results show that the signals induced by the typhoon occurred mainly at approximately 0.15–0.5 Hz frequency. The magnitude of these signals depends substantially on water depth. Some exceptions, most likely generated by site effects, were observed. Also, a positive correlation exists between the signals energy and the local wave height, which suggests that the microseisms were affected by the pressure changes produced by the local wave activity as the typhoon passed over the stations. However, when an OBS was outside the typhoon periphery, any wave energy variations could only be caused by the elastic wave formed around the typhoon area, the energy of which is transmitted through the ocean bottom to the stations. Thus, no local waves were excited by the strong winds, and only a relatively small amount of energy was recorded.

An empirical Green’s function (EGF) deconvolution analysis was applied to study the source characteristics of the 2011 Mw 9.0 Tohoku and 2011 Mw 7.4 Sanriku-Oki earthquakes. For the 2011 Tohoku earthquake, we demonstrate that nucleation released weak but high-frequency energy and that the rupture propagated downward and sped toward the deep region after the up-dip slip extended to the trench. Moreover, the 2011 Sanriku-Oki earthquake results and a previous study on the 1994 Sanriku-Oki earthquake suggest that large earthquakes in the subduction zone around the Tohoku area prefer to rapidly rupture toward the deeper (down-dip) region.

The rupture process and tectonic surroundings of the 2008 Wenchuan, China, earthquake are both complex in a way that might be related to the heterogeneous stress field of the Longmen Shan region. In this study, we construct dynamic models with heterogeneous initial stress to reproduce a kinematic inversion result by Wen et al. (2012) and investigate the physical mechanisms of the variable slip pattern and rupture velocity in this event. The results show that, for the 2008 Wenchuan earthquake, (1) the rupture behavior and slip pattern can be explained by heterogeneities in initial stress and (2) the variations of rupture velocity and ground‐motion pattern are strongly affected by both stress drop and fault strength. Our preferred dynamic model agrees with the kinematic model that the 2008 Wenchuan earthquake initiated with a slow velocity in the southwest, sped up in the segment with the largest slip, and then slowed down for the remaining propagation to the northeast.

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.

SUMMARY We determine the finite-fault slip distribution of the 2001 Kunlun earthquake (Ms= 8.1) by inverting teleseismic waveforms, as constrained by geological and remote sensing field observations. The spatial slip distribution along the 400-km-long fault was divided into five segments in accordance with geological observations. Forward modelling of regional surface waves was performed to estimate the variation of the speed of rupture propagation during faulting. For our modelling, the regional 1-D velocity structure was carefully constructed for each of six regional seismic stations using three events with magnitudes of 5.1–5.4 distributed along the ruptured portion of the Kunlun fault. Our result shows that the average rupture velocity is about 3.6 km s−1, consistent with teleseismic long period wave modelling. The initial rupture was almost purely strike-slip with a rupture velocity of 1.9 km s−1, increasing to 3.5 km s−1 in the second fault segment, and reaching a rupture velocity of about 6 km s−1 in the third segment and the fourth segment, where the maximum surface offset, with a broad fault zone, was observed. The rupture velocity decelerated to a value of 3.3 km s−1 in the fifth and final segment. Coseismic slip on the fault was concentrated between the surface and a depth of about 10 km. We infer that significant variations in rupture velocity and the observed fault segmentation are indicative of variations in strength along the interface of the Kunlun fault, as well as variations in fault geometry.

[1] The integration of seismic and InSAR interferogram data allows us to have a comprehensive understanding of the earthquake fault geometry and distribution of asperities for a remote event, namely the 1997 Manyi, China earthquake. We first obtained the slip distribution from teleseismic stations for several fault models, and then made synthetic interferograms in order to determine the optimum fault geometry. Our results show that the 1997 Manyi earthquake ruptured bilaterally with two segments dipping north and south for the western and eastern segments, respectively. The south dip of the eastern segment is essential for a better interpretation of the stations near the nodal plane, and a reversal of dip between the west and east segment is necessary to explain the pattern of observed InSAR interferograms due to the broader distribution of asperities. The fault rupture extended 170 km with maximum displacement of 6.1 m, and a total seismic moment of 2.24*10 20 Nm (Mw = 7.5).