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Recent studies suggested that transient and long‐term stress changes caused by Earth's surface processes (e.g., extreme weather events, annual variations on groundwater storages) can affect earthquake activities in the subsurface. However, these studies may be limited by the completeness of standard earthquake catalogs, especially during or right after extreme weather events. Here we apply the template matching method to build a more complete earthquake catalog in Taiwan spanning seven months before and 12 months after 2009 typhoon Morakot, which brought the highest rainfall in southern Taiwan in the past 60 years and triggered numerous landslides. We then use the enhanced catalog to investigate possible influences of typhoon‐driven Earth's surface processes (atmospheric pressure, precipitation, and erosion) on local seismicity. We find that the seismicity rate of a 40‐day earthquake sequence in northeastern Taiwan was reduced significantly right after the passage of typhoon Morakot's eye center. In the typhoon‐triggered landslide zone in southern Taiwan, we find a slight increase in background seismicity rate in the next year after Morakot, matching the results of a recent study. However, we do not observe a clear change in the Gutenberg‐Richter b‐value in this zone, which is different from the recent study. Station outages during and right after Morakot prevents us from better understanding short‐term precipitation effect on local seismicity. Overall, except for a reduction in seismicity rate near the typhoon's low‐pressure eye center in northeastern Taiwan, we do not observe other clear seismicity changes that can be attributed to surface changes induced by typhoon Morakot. Plain Language Summary: It is important to study cascading nature hazards (typhoons/hurricanes, landslides, earthquakes, etc.) because of their threats to human life and property. 2009 typhoon Morakot is the deadliest typhoon in Taiwan, which brought the highest rainfall in the past 60 years and led to numerous landslides. To investigate the possible influences of Morakot on subsurface earthquake activities, we build a more complete earthquake catalog for Taiwan spanning seven months before and 12 months after 2009 typhoon Morakot. This enhanced earthquake catalog includes many small earthquakes that cannot be felt by local residents in their ordinary life and were not in the standard earthquake catalog. In northeastern Taiwan, our results show that a 40‐day earthquake sequence was apparently shut down right after the passage of typhoon Morakot. If this observation is not a pure coincidence, it may suggest that the low‐pressure system of typhoon Morakot was capable of modulating the subsurface seismicity behavior. In southern Taiwan, the heavy‐rainfall and landslide region, we do not observe clear seismicity changes that can be attributed to surface changes induced by typhoon Morakot. Key Points: We use a template matching method to rebuild an earthquake catalog for Taiwan 7 months before and 12 months after 2009 typhoon MorakotThe seismicity rate of a 40‐day earthquake sequence in northeastern Taiwan was reduced significantly after the passage of MorakotThe seismicity rate increased slightly but without clear b‐value change after Morakot in typhoon‐driven landslide zone in southern Taiwan [ABSTRACT FROM AUTHOR]

Taiwan is one of the leading developers of earthquake early warning (EEW) systems. The Central Weather Bureau has been the primary developer of the EEW system in Taiwan since 1993. In 2010, the National Taiwan University (NTU) developed an EEW system for research purposes using low-cost accelerometers. As of 2014, a total of 506 stations have been deployed and configured. The NTU system can provide earthquake information within 15 s of an earthquake occurrence. Thus, this system may provide early warnings for cities located more than 50 km from the epicenter. Additionally, the NTU system also has an onsite alert function that triggers a warning for incoming P-waves greater than a certain magnitude threshold, thus providing a 2-3 s lead time before peak ground acceleration for regions close to an epicenter. Detailed shaking maps are produced by the NTU system within one or two minutes after an earthquake. Regions of high shaking indicated by the shalking map can indicate locations of damage and casualties and help estimate the damage incurred. The direction of earthquake ruptures are also potentially identified based on detailed shaking maps and strong motion records of the NTU system. [ABSTRACT FROM AUTHOR]

From the literature, we found that PGV–PD3 regressions for on-site earthquake early warning (EEW) can be quite different depending on the presumption whether or not PGV–PD3 data from different regions should be “mixable” in regression analyses. As a result, this becomes a source of epistemic uncertainty in the selection of a PGV–PD3 empirical relationship for on-site EEW. This study is aimed at examining the influence of this epistemic uncertainty on EEW decision-making, and demonstrating it with an example on the use of PGV–PD3 models developed with data from Taiwan, Japan, and Southern California. The analysis shows that using the “global” PGV–PD3 relationship for Southern California would accompany a more conservative EEW decision-making (i.e., early warning is activated more frequently) than using the local empirical model developed with the PGV–PD3 data from Southern California only. However, the influence of this epistemic uncertainty on EEW is not that obvious for the cases of Taiwan and Japan. [ABSTRACT FROM AUTHOR]

We detected 12 landquakes in Taiwan with collapse areas between 0.27 and 2.48 km2 associated with rock collapse, rockslide and debris and/or rock avalanche during the passage of Typhoon Morakot in 2009. These events were recorded by seismic stations of the Broadband Array in Taiwan for Seismology. Their locations were determined by a cross-correlation technique that maximizes the coherency of horizontal envelope function among seismic stations with a mean location error of 1.92 km. We applied time-frequency analysis to estimate the bandwidth of seismic energy generated by the landquakes. The predominant frequency ranges from 0.5 to 5.0 Hz, with higher-frequency signals likely caused by block impact. We extracted signal duration (SD), peak ground velocity, rise time (TR), area of velocity envelope function (AE) from the closest station and estimated the collapse area (AC) and run-out distance (DR) by mapping satellite images. Based on aforementioned seismic and geometrical parameters, we defined the potential of initial impact (PI), the frequency of rock impact signal (fI) and the mean quasi-front velocity (Vf) as indicators of landquake types, especially for events with dam formation (dam-formation-type events). We also derived an empirical linear relationship between the envelope area (AE) and collapse area (AC) with a high correlation coefficient of 0.83. Our automatic approach is very effective for rapid determination of landquake centroid location and collapse area, and for identifying dam-formation event using records from existing real-time broad-band seismic networks, thus providing an important alternative for landquake hazard mitigation. [ABSTRACT FROM AUTHOR]

Abstract: Following the 4 March 2010 MW 6.4 Jiashian earthquake the seismicity rate in southern Taiwan was determined to be significantly higher than before the quake and aroused seismic hazard awareness. In this work, seismic hazards were investigated in terms of earthquake activity, the Coulomb stress change, the rate-and-state friction model, and short-term seismic hazard assessments. The significantly higher seismicity rate that followed the 2010 Jiashian earthquake was found to mainly be attributed to aftershock decay, in terms of the modified Omori formula. The results suggest that aftershock duration may continue until the end of 2012. The spatial migration of seismicity was modeled using the Coulomb stress changes of large earthquakes. Most of the consequent events were distributed in the vicinity of large earthquakes. The observations corresponded to a remarkable stress increase within the same area. Additionally, large events were located within regions with stress increases promoted by previous earthquakes. The results confirm interactive relationships between large events. By considering time-dependency, the seismicity rate evolution was estimated using the rate-and-state friction model. The results indicated that a high seismic rate will persist at least until the end of 2012. Short-term probabilistic seismic hazard assessments were also applied in terms of the probability of strong ground motion. Using this application, a sudden jump in seismic hazards in southern Taiwan was accompanied by each large earthquake. At the end of 2012 it is expected that hazards will return to a background level. Our results may be valuable in the future to decision-makers and public officials engaged in seismic hazard mitigation. [Copyright &y& Elsevier]

Abstract: We combine the arrival time data from eleven ocean bottom seismometers (OBS) and from permanent stations of the Central Weather Bureau Seismic Network (CWBSN) and the Taiwan Strong Motion Instrumentation Program (TSMIP) to improve the three-dimensional Vp and Vp/Vs structures in the Taiwan region, relocate hypocenters of earthquakes in southern Taiwan including the 2006 Pingtung Earthquake sequence, and determine the focal mechanisms of some of the larger events. Our tomography result suggests that in the region south of Taiwan the Eurasian continental lithosphere is being subducted beneath the oceanic Philippine Sea plate, leading to a region of low P-wave speed with seismicity on top of a relatively high P-wave speed zone. The hypocentral distribution of the Pingtung Earthquake sequence indicates that the first event in the initial two largest shocks was a normal-faulting quake caused by the bending of the subducting lithosphere. The second event, perhaps triggered by the first one, was a strike-slip earthquake. [Copyright &y& Elsevier]

Abstract: We investigate the variations in seismicity pattern in the Taiwan region before the 2003 Chengkung, Taiwan, earthquake (M w =6.8) by calculating the standard normal deviate of the Z-values and b-values from the Gutenberg–Richter relation. The Mogi donut-shaped variations in the seismicity can be identified in the Z-value map surrounding the Chengkung earthquake source region. Noticeable decreases in the b-values have also been found around the mainshock region before the Chengkung event. The relatively low-seismicity rate and the decrease in the b-values may be the precursory phenomena associated with the quiescence in overall seismicity and the activation of moderate-sized events occurred around the mainshock region before the Chengkung event. [Copyright &y& Elsevier]

We introduce an empirical method of near real-time, near-field magnitude determination for large (M>6.5) crustal earthquakes. Time integration over the strong shaking duration on the absolute values of the acceleration records is carried out for nearby stations surrounding many large earthquake sources in Taiwan. The integrated quantity, here denoted as total effective shaking, is used in a regression process to derive an empirical relationship for a quick Mw determination useful for a reliable real-time operation in earthquake rapid reporting and earthquake early warning systems (EWSs). [Copyright &y& Elsevier]

Shallow slow slip events accompanied by seismicity has been increasingly reported in subduction zones during the last decade; however, the relationship between shallow slip events and seismicity is unclear. We report episodic slow slip events and seismicity at the southernmost Ryukyu Trench. The inversion results from GPS‐derived cumulative displacements indicate that the slow slip events occur offshore in northeastern Taiwan on the shallow subduction interface, where high seismic VP/VS ratios applies. Seismicity is observed above the subduction interface for locations of significant variations in the VP/VS ratio and does not follow the migration of peak slip along the subduction interface. We calculated the temporal evolution between the seismicity and the propagation of slow slip events and determined that seismicity is not primarily driven by slip propagation. The spatiotemporal relationship, coupled with the VP/VS ratio, suggests that high‐pressure fluid activity may be crucial to episodes of slow slip events and seismicity. Plain Language Summary: Slow slip events are a member of the slow‐earthquake family observed by geodetic measurements in global subduction zones in the past 15 years. Shallow slow slip events are considered to be important for the megathrust earthquake cycle of the shallowest locked zone. Recently, shallow slow slip events accompanied by different seismic behaviors has been increasingly reported. This paper reports episodic, shallow slow slip events accompanied by overlying plate seismicity at the southernmost Ryukyu Trench and analyzes their relationship. We discover that the spatiotemporal relationship between the propagation of slow slip and seismicity is weak, which infers that they may not have a causal relationship. We suggest that the initiation of shallow slow slip events and subsequent seismicity was probably caused by high‐pressure fluid activity. The interpretation may reveal the cause of many shallow slow slip events as a broad phenomenon. Key Points: Relationship between shallow slow slip events and seismicity is reported at the southernmost Ryukyu Trench for the first timeSpatiotemporal seismicity does not adequately correlate with propagation of slow slip but is dominated by mainshock‐aftershock sequencesHigh‐pressure fluid activity likely has a key role in the initiation of episodic slow slip events and subsequent seismicity [ABSTRACT FROM AUTHOR]

The Longitudinal Valley in eastern Taiwan is generally considered as the suture of the collision between the Philippine Sea and the Eurasian plates, and has attracted numerous geologic and seismologic studies. In the northernmost part of the valley, however, constraints on how structures develop as the suture turns into the Ryukyu subduction system offshore are still very limited. Therefore, we analyzed relocated seismicity distributions and focal mechanisms of earthquake sequences, together with tectonic geomorphic investigations to further understand the seismotectonic characteristics of this area. In our seismologic observations, we found two previously unidentified reverse faults in the northernmost part of the Longitudinal Valley suture. One is an E–W striking, south-dipping reverse fault near the Liwu River fan delta, and the other is a N–S striking, east-dipping reverse fault near the eastern Central Range front. Both these structures connect with a detachment at ~ 10 km deep, and may connect with each other to form a curved structural system. The Meilun fault, a well-known active structure that ruptured in a M7.3 earthquake in October 1951, is not seismically active in the past two decades, and may just be part of a secondary branch of the major structural system. In the northernmost part of the Longitudinal Valley suture, we propose that as the Coastal Range bedrocks subduct northward beneath the Eurasian plate with the Philippine Sea plate, the shallow sediments of the Longitudinal Valley, being a buoyant block, do not subduct, but overthrust northward and westward instead. [ABSTRACT FROM AUTHOR]

On Mar. 27, 2013, a M L 6.2 earthquake occurred in the Nantou area of central Taiwan, which caused one death and nearly 90 injured. Two months later, another M L 6.3 earthquake struck the same region on June 2, 2013, the epicenter of which is close to the March M L 6.2 earthquake. Seismicity is a sensitive indicator of stress rate and inelastic deformation process in crust. Therefore, examination of temporal changes in seismicity is important to understand the preparatory processes of damaging inland earthquakes. In this study, we applied the Epidemic-Type Aftershock-Sequences model (ETAS model) to the earthquake data covering broader Taiwan region, which is maintained by the Central Weather Bureau (CWB) of Taiwan, to investigate precursory temporal changes in seismicity for the M L 6.2 Nantou earthquake. We regard the March M L 6.2 and June M L 6.3 earthquakes as an event sequence and especially focus on temporal changes in seismicity prior to the M L 6.2 event. Application of more than one model to an earthquake catalog would be informative in elucidating the relationships between seismicity precursors and the preparatory processes of large earthquakes. Based on this motivation, we further applied two different approaches: the pattern informatics (PI) method and the ZMAP method, which is a gridding technique based on the standard deviate (Z-value) test to the same earthquake data of CWB. As a result, we found that the epicenter of the 2013 M L 6.2 Nantou earthquake was surrounded by three main seismic quiescence regions prior to its occurrence. The assumption that this is due to precursory slip (stress drop) on fault plane or its deeper extent of the M L 6.2 Nantou earthquake is supported by previous researches based on seismicity data, geodedic data, and numerical simulations using rate- and state-dependent friction laws. [ABSTRACT FROM AUTHOR]

Abstract: We explore regional stress states in the vicinity of the rupture area of the 1999 Chi-Chi, Taiwan, earthquake by reconciling data from the Taiwan Chelungpu-Fault Drilling Project (TCDP) in-situ measurements and earthquake focal mechanisms. Given the background deviatoric stress in the range of 10–50MPa and the horizontal NW–SE directed maximum principal stress axis, the predicted fault types show strike–slip and normal faulting near the coseismic surface rupture and thrust and strike–slip faulting in central Taiwan. Such predictions are able to fit TCDP in-situ observations in a local scale and aftershock earthquake focal mechanisms in a regional scale. Additionally, the proposed stress state explains remarkable rotations of the maximum stress axes observed near the northern segment of the Chelungpu Fault. This result provides key information for forecasting of consequent earthquakes and evaluation of focal mechanisms after the occurrence of a large earthquake. [Copyright &y& Elsevier]

Abstract: The first attenuation relationships of peak ground acceleration (PGA) and peak ground velocity (PGV) for northern Vietnam are obtained in this study. Ground motion data are collected by a portable broadband seismic network in northern Vietnam as a part of cooperation between the Institute of Geophysics, Vietnamese Academy of Science and Technology, Vietnam and Institute of Earth Sciences, Academia Sinica, Taiwan. The database comprises a total of 330 amplitude records by 14 broadband stations from 53 shallow earthquakes, which were occurred in and around northern Vietnam in the period between 01/2006 and 12/2009. These earthquakes are of local magnitudes between 1.6 and 4.6, focal depths less than 30km, and epicentral distances less than 500km. The new attenuation relationships for PGA and PGV are: where PGA is in cm/s2, PGV is in cm/s, and R is the epicentral distance in kilometers. The site corrections are also derived in this study. These site corrections are very suitable with the station corrections for ML and imply the qualification of the resulting attenuation relationships. [Copyright &y& Elsevier]

Abstract: Research at plate boundaries generally focuses on the kinematics of plate motions, the genesis of large earthquake, and natural resources. However, often the present state of knowledge is still insufficient to reveal the details of such dynamic systems. To define in more detail the dynamics of, a major plate boundary fault the Coastal Range Fault, eastern Taiwan, we use seismologic and geodetic data to unravel its subsurface geometry and relevant fault behavior. By recomputing the archive triangulation datasets (1917–1978), and using a dislocation algorithm, we determine that it consists of three segments. Our model results are consistent with the observation of the 1951 M7.3 Hualien–Taitung earthquake sequence: that the southern two segments are both oblique-slip faults with significant thrust components, while the northern segment is a pure strike-slip fault. According to our model each segment has the potential to produce an earthquake larger than M7. However, based on GPS observations and seismic records, slip on the southern segment may take the form of aseismic creep and multiple moderate earthquakes, rather than large events with long recurrence interval as occur on the two more northerly segments. With regard to the central segment, the epicenter distribution of catalogued earthquakes occurring after 1951 reveals the presence of a seismic gap, indicating that slip on this segment is predominantly coseismic, with a correspondingly higher potential for large earthquakes in the future. [Copyright &y& Elsevier]

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 ( Mw 7.9–8.0) 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. [ABSTRACT FROM AUTHOR]

Abstract: Variations in the fractal dimension of earthquakes have been suggested to be a precursor of a large earthquake. However, the physical characteristics and seismicity are always different along a large fault system, and it is difficult to segment a large fault for further investigating. Therefore, the fractal dimension of earthquakes on a given fault only reflects the average seismic characteristics of the area and may be unrelated to precursory activity. In this paper, the evolution of the temporal generalized fractal dimension (Dq) of a seismic cluster within a small fault system associated with the Chiayi earthquake (ML =6.4), Taiwan, is investigated. The earthquakes of this sequence are confined in small source volume and reflect the behavior of the local fault system. Our results show that the Dq curve of the background stage is smooth with a low multifractal degree (0.36) and the seismicity is nearly monofractal in the Chiayi region. During the foreshock stage, the seismicity becomes active such that not only the Dq curve exhibits greater variability especially for the steep slope of Dq curve at q=0 but also the temporal fractal dimension changes from nearly monofractal to multifractal. In the aftershock stage, the fluctuation of Dq is large and the multifractal degree is up to 1.0. The temporal multifractal property becomes more pronounced as well. Finally, when the seismicity returns to the background stage, the Dq curve becomes smooth and nearly monofractal again. Therefore, the fluctuations in the Dq spectrum of an earthquake cluster associated with a large earthquake not only give a clear picture of the temporal alterations in the seismogenic regimes but also illustrate their variability through time for a large earthquake. In addition, the variation of the slope of Dq curve at q=0 in the foreshocks may be used as a precursor of a large earthquake. [Copyright &y& Elsevier]