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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.

We investigate the temporal and spatial seismicity patterns prior to eight M > 6 events nucleating in different regions of Taiwan through a region–time–length algorithm and an analysis of a self-organizing spinodal model. Our results show that the spatiotemporal seismicity variations during the preparation process of impending earthquakes display distinctive patterns corresponding to tectonic settings. Q-type events occur in southern Taiwan and experience a seismic quiescence stage prior to the mainshock. A seismicity decrease of 2.5 < M < 4.5 events occurs around the relatively high b-value southern Central Range, which contributes to the accumulation of tectonic stress for preparing for the occurrence of the Q-type event. On the other hand, A-type events occur in central Taiwan and experience a seismic activation stage prior to the mainshock, which nucleates on the edge of the seismic activation area. We should pay attention when accelerating seismicity of 3 < M < 5 events appears within the low b-value area, which could promote the nucleation process of the A-type event.

We investigate the temporal and spatial seismicity patterns prior to eight M > 6 events nucleating in different region of Taiwan through region-time-length algorithm and analysis of self-organizing spinodal model. Our results reveal that the spatiotemporial seismicity variations during the preparation process of impending earthquakes would display distinctive pattern corresponding to the tectonic setting. Q-type events occur in southern Taiwan and experience seismic quiescence stage prior to the mainshock. Seismicity decrease of 2.5 < M < 4.5 events around the high b-value southern Central Range, which contributes to accumulate the tectonic stress for preparing the occurrence of the Q-type event. On the other hand, A-type events occur in central Taiwan and experience seismic activation stage prior to the mainshock, which nucleated on the edge of the seismic activation area. We should pay attention when accelerating seismicity of 3 < M < 5 events appears within the low b-value area, which would promote the nucleation process of the A-type event.

The 2018 Mw 6.4 Hualien earthquake struck the eastern Taiwan and caused serious damage. We investigate the rupture properties of the 2018 Hualien earthquake by inverting teleseismic body wave and forward modeling GPS coseismic deformation. The rupture process and slip pattern of preferred model explain both the far-field (teleseismic data) and near-field (GPS) observations. The results show that the 2018 Hualien mainshock ruptured southward on two fault segments, with a weak but fast initiation (3.0 km s-1) in the main west-dipping segment F1 and slow (2.0 km s-1) yet significant slip on shallow east-dipping segment F2. In the past few years, several moderate-sized events, which struck eastern Taiwan and caused strong ground shaking and some seismic damage, are considered occurring on the west-dipping fault. Additional investigations are required to building up the knowledge of this not wellknown region.

The 2018 Hualien earthquake occurred at the junction of the deformed continental crust of Eurasian plate and the Heping sea basin, where a northeast trending seismic belt exists. In this study, a 3D seismic tomographic inversion is used to investigate the seismogenic structures of the 2018 Hualien earthquake sequence. An earthquake relocation procedure is performed and the focal mechanisms are analyzed to study the faulting behavior. Our results indicate that the source region of the 2018 Hualien earthquake consists of complex high-angle eastward and westward dipping reverse faulting. We also observe that most earthquakes that have occurred in the area exhibit considerable variation in Vp/Vs ratios. Fluid migration may have played an essential role in causing the Vp/Vs ratio variation. We suggest that the mainshock of the 2018 Hualien earthquake may be associated with a west-dipping fault, which is a blind fault tending toward the Heping sea basin and possibly belonging to the Central Range fault system.

We used near-field strong-motion data to investigate the complex combination of source effect and site response for two recent disastrous earthquakes in southwest Taiwan. We estimated strong-motion generation areas (SMGAs) of 2.8 km × 2.8 km and 6.0 km × 4.2 km in a frequency band of 0.4–10 Hz for the 2010 Jiashian and 2016 Meinong earthquakes, respectively. The high-stress drops of 26.2 and 17.0 MPa for these two buried events were potentially related to the small dimension and deep rupture. Our results revealed that both earthquakes exhibited westward rupture directivity, whereas the 2016 Meinong event exhibited a stronger directivity effect because of the consistency between the propagation and slip directions. The localized high peak ground velocity (PGV) patch and the nonlinear site response could be attributed to the soft sediment with high pore fluid pressure and low-velocity structure beneath this region. However, the greater seismic moment and closer faulting location to the thick-mudstone-layer region for the 2016 Meinong event reinforced the strong ground shaking and serious damage over the broad area. This implies that this thick-mudstone-layer region in southern Taiwan plays a crucial role in earthquake response, and an investigation of characteristic site effects should be conducted for seismic hazard mitigation.Graphic abstract

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.

The 2016 ML 6.6 Meinong earthquake struck southern Taiwan and caused serious damage due to the strong ground shaking. Anomalous seismicity spatiotemporal signature changes were identified prior to the 2016 Meinong event. We investigate the seismicity rate changes associated with the 2016 Meinong mainshock by applying the region-time-length (RTL) algorithm. The temporal result reveals a long obvious seismic quiescence stage soon after the 2012 Wutai event lasting until the 2016 Meinong mainshock. The spatial pattern of different background period lengths all exhibit that the 2016 Meinong event occurred near the abnormal seismic quiescence patch. Our study shows that the spatiotemporal information of the seismicity change could be a useful indication for potential seismic-hazard assessment.

Summary The coupling of pre-earthquake anomalous phenomena between long-term groundwater levels recorded at 42 monitoring stations and time-varying surface strain derived from 16 GPS stations was found in the Choshuichi Alluvial Fan before the 1999 M w 7.6 Chi-Chi earthquake in Taiwan. The noise-free groundwater-level anomalies consistently comprised by a sequence of decrease, rise and flat phases, which agree very well with changes in strain rates computed from the GPS stations. These coupling agreements show that in addition to compression, tension can be generated before a thrust earthquake occurrence as well. This case demonstrates that short-term surface deformation as signals against noise and accuracy of pre-earthquake anomalous phenomena can be simultaneously examined by using multiple-parameter crosscheck for significantly reducing the uncertainty of earthquake precursory evaluation.

In Taiwan, the Nantou area is a seismically active region where several moderate events have occurred, causing some disasters during the past century. Here, we applied the strong ground motion simulation with the empirical Green’s function method to investigate the source characteristics for the eight moderate blind-fault events that struck the Nantou area in 1999 and 2013. The results show that for these Nantou events, a high stress drop and focal depth dependence were noted, which might be related to the immature buried fault in this area. From the viewpoint of seismic hazard prevention and preparation, future earthquake scenarios that include high stress drop should be applied to more analyses, especially the moderate-to-large events originating from the immature blind faulting. Graphical abstract .

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.

The 2013 M_L 6.4 Ruisui earthquake struck a seismic gap around the northern part of the Longitudinal Valley where rare earthquakes have occurred for at least two decades. Seismic data with different frequency bands connote a diverse scale of source characteristics. The slip model derived by a previous study using long-period seismic data provided us with the preliminary earthquake source properties in low-frequency energy radiation. However, the high-frequency part of the seismic energy needs to be considered for the strong motions in the comprehensive frequency band. As a case study, we carried out broadband strong motion simulations through the hybrid scheme, which simulated waveforms combining the low-frequency motions (frequency-wavenumber integration method) and high-frequency motions (empirical Green's function method) to reveal the strong ground motion and source characteristics of the 2013 Ruisui earthquake. The results show that even though the local structure is complicated this hybrid simulation can effectively rebuild the overall broadband strong ground motion characteristics for the 2013 Ruisui earthquake. Additional study issues are required to better understand the nature of localized high peak ground acceleration and the related seismic hazards for future earthquakes.

[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).

Teleseicmic vertical P waves of an earthquake doublet occurring on 26 December 2006 offshore southwestern Taiwan are analyzed to recover spatial slip distribution. Ground-displacement records for period 0.01-0.5 Hz are inverted using linear inversion for finite fault analysis. Waveform inversion results show that a dual segment fault of different strike is necessary for the first event to explain teleseismic waveforms. The trend in fault geometry, thus, inferred was coincidentally consistent with the bathymetry in the fault rupture area, suggesting the possible tectonic structure controlling fault propagation. Both earthquakes show larger slip in the asperities near the hypocenter and there are two main asperities of similar moments. The first event ruptured along a high angle eastern dipping plane while the second event ruptured along a low-angle western dipping fault plane. The spatial distribution of the Pingtung earthquake doublet shows a compensative and conjugate relationship, and the seismic moments of the earthquake doublet were 2.7×10^26 and 1.8×10^26 dyne-cm, respectively. The stress drops corresponding to the asperities of the two earthquakes have values of 3 to 4 bars, individually, similar to the values observed in other subduction zones.