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To enhance our comprehension of the dynamic processes associated with complex plate subduction, volcanic magmatism, and lithosphere deformation beneath the Ryukyu–Taiwan–Philippines region, we utilized nonuniform inversion grids for Pn velocity and anisotropy tomography, and obtained the uppermost mantle structure of this area. The results demonstrate remarkable characteristics: cold oceanic subducting plates display high Pn velocities, whereas volcanic arcs, the extinct mid‐ocean ridge in the South China Sea, and the Palawan region exhibit low Pn velocities. The extinct mid‐ocean ridge also displays a low‐velocity anomaly, indicating that residual heat persists even after approximately 15 million years since seafloor spreading. The occurrence of a discontinuous low‐velocity beneath the Ryukyu arc supports the presence of a slab window at approximately 123°E. The volcanic arcs of all subduction zones within the study area displayed trench‐parallel Pn anisotropy. The observed Pn fast directions beneath the Taiwan orogenic belt are consistent with crustal anisotropy, providing evidence for the crust–mantle coupled deformation. Moreover, our results shed light on the deep structural characteristics of the complex subduction zones beneath the Philippines region. Plate subduction causes partial melting due to dehydration; then, the melt ascends and accumulates at the uppermost mantle, showing low Pn velocities. However, the low‐velocity anomaly is not widely distributed but corresponds to a narrow band. In particular, the east–west bidirectional subduction of the Philippines and Negros formed separate low‐velocity anomalies. Low Pn velocities and trench‐parallel anisotropy indicate the location of different subduction zones and finely characterize their impact on the uppermost mantle structure. Plain Language Summary: The Ryukyu–Taiwan–Philippines region serves as a convergence zone for multiple subduction systems and has undergone a complex tectonic evolution. In this study, we utilized an improved uppermost mantle seismic tomography method to investigate the precise Pn velocity and anisotropy structure of this area. The results reveal distinct patterns, wherein high Pn velocities are observed beneath the Eurasian plate, the South China Sea plate, and the Philippine Sea plate. In addition to the volcanic arcs, an extinct mid‐ocean ridge also shows low Pn velocity. The identification of discontinuous low velocities at approximately 123°E beneath the Ryukyu arc supports the existence of a slab tear. Pn fast directions, which align with the orogenic belt's trend, are observed beneath Taiwan Island. This observation concurs with surface and crustal deformation, suggesting that the deformation of the Taiwan orogenic belt is at a lithospheric scale. Low Pn velocities observed beneath the multiple subduction zones in the Philippines region indicate that the magma generated by this process is confined to a narrow band parallel to the subduction zone, instead of being widely distributed. The study precisely depicts the impact of plate subduction on the uppermost mantle structure of the Philippines region. Key Points: We employed a nonuniform grid Pn anisotropic tomography to obtain the uppermost mantle structure in the Ryukyu–Taiwan–Philippines regionThe consistency of Pn and the crustal anisotropy beneath the Taiwan orogenic belt supports crust–mantle coupled deformationThe results beneath the Philippines region indicate the influence of subduction in different directions on the uppermost mantle structure [ABSTRACT FROM AUTHOR]

Due to the lack of recognized creep on dip‐slip faults, the nature of fault creep and its role in the generation of large earthquakes is largely unexplored. The Chihshang fault in Taiwan serves as one of the best examples of a thrust creeping fault in the world, exhibiting a fast creep rate of 2–3 cm/year and the capability of producing magnitude six earthquakes. Land‐based geodetic measurements are less sensitive to fault slip with depth, especially for the deeper portion of the seaward dipping fault. Taking advantage of slip rates inferred from repeating earthquake sequences (RESs) at greater depths (up to 30 km), we present a method that embeds RES‐derived slip rates into the neighboring fault patch for geodetic data inversion. Using the geodetic and seismological data from 2007 to 2011, we achieve a higher resolution of inter‐seismic slip rate distribution below a depth of 10 km. The inferred low coupling ratio establishes an extensive creeping area that coincides with a location of abundant RES and earthquake swarms. The inferred high coupling ratio, on the other hand, delineates a locked area corresponding to the two co‐seismic slip zones of the 2003 Mw 6.8 Chengkung and 2022 Mw 6.7 Yuli earthquakes. The transition zone marking the gradual change from fully locked to shallow creeping areas appears to coincide with the post‐seismic slip area of the 2003 Chengkung event. Using the fault coupling model, the moment rate deficit measured in the two locked areas is capable of generating a Mw 6.8 event every 78 years to the south and a Mw 6.7 event every 28 years to the north. This moment rate deficit is decreased by 10% when the closely located Central Range fault is considered in a two‐fault model, leading to the slightly longer recurrence interval on the Chihshang fault. The two‐fault model potentially contributes to a better understanding of the regional inter‐seismic deformation, which is recommended to be adopted in the future studies of seismic hazard assessment. Plain Language Summary: Large earthquakes can occur on the creeping faults that slowly slip at the surface but accumulate energy at greater depth. How much different portions of the fault slip annually is the key question for seismic hazard assessment. However, for a seaward‐dipping thrust fault like the Chihshang fault in Taiwan, finding the answer is difficult because the loss of resolution at the greater depth (>10 km) due to a lack of offshore data. Taking advantage of the slip rates inferred from repeating earthquakes that mainly occurred at 10–30 km depth, we present a method that embeds repeating events data into the geodetic inversion for higher resolution of inter‐seismic slip model. We found that the areas slipped faster during the inter‐seismic period (higher inter‐seismic slip rate) coincides with the location of abundant repeating earthquakes and swarm events; The areas with lower inter‐seismic slip rate on the other hand, correspond to the zones that slipped during the 2003 Mw 6.8 Chengkung and 2022 Mw 6.7 Yuli earthquakes. By calculating how much and where the seismic energy was released through inter‐seismic slip rate, we are able to identify two separate zones on the Chihshang fault that is capable of generating a ∼Mw 6 event every 28 to the north and 78 years to the south of the fault, respectively. Key Points: We present a method that embeds repeaters‐derived slip rates into the neighboring fault patch for geodetic data inversionWe achieve a higher resolution of inter‐seismic slip rate distribution below a depth of 10 km, on a seaward‐dipping thrust faultThe inferred low coupling ratio establishes an extensive creeping area, coinciding with the location of active repeaters and swarm events [ABSTRACT FROM AUTHOR]

An analytically based method is proposed to simulate the acoustic-gravity waves in the horizontally stratified atmosphere-Earth structure generated by a point force on the Earth's surface. The method solves the linear momentum, continuity and adiabaticity equations in the atmosphere and elastodynamic equations in the solid Earth in the frequency–wavenumber domain. The time-domain waveforms are obtained by wavenumber integration and fast Fourier transform with respect to the frequency. Numerical simulations are conducted to investigate the properties of the acoustic-gravity waves, including both the high-frequency acoustic-mode waves and low-frequency gravity-mode waves. Simulations of the high-frequency responses show that disturbances in the atmosphere with three apparent horizontal velocities can be identified. They are, namely, the direct acoustic-mode wave generated by source travelling with the sound speed, the head wave generated by the seismic P -wave travelling with apparent horizontal speed identical to the P velocity, and the head wave generated by the Rayleigh wave with a horizontal speed same to the Rayleigh wave velocity. Simulations of the low-frequency responses show that the gravity-mode wave and Lamb wave can be identified. The gravity-mode wave travels with a speed lower than the sound seed and does not reach everywhere, especially the area directly above the source. The Lamb wave travels along the Earth surface with a speed of about 310 m s–1 and its energy decays with the altitude. We also apply our method to explaining the Doppler sounding data observed in Taiwan area during the 2011 Tohoku M 9 earthquake, and find good agreement between the predicted signals and observed data in the arrival time and wave envelope associated with the Rayleigh wave. [ABSTRACT FROM AUTHOR]

The strong Hualien earthquake (Mw 6.1) occurred along the suture zone of the Eurasian Plate and the Philippine Sea Plate, which struck the Hualien city in eastern Taiwan on April 18, 2019. The focal mechanism of this earthquake shows that it is caused by a rupture within a thrust. In the present study, the rupture plane responsible for this earthquake has been modeled using the modified semi-empirical technique (MSET). The whole rupture plane is assumed to be composed of strong motion generation areas (SMGAs) along which the slip occurs with large velocities. The spatiotemporal distribution of aftershocks of this earthquake within identified rupture plane suggests that there are two SMGAs within the rupture plane. The source displacement spectra (SDS) obtained from the observed records have been used to compute the source parameters of these two SMGAs. The MSET efficiently simulates strong ground motion (SGM) at the rock site. The shallow subsurface shear wave velocity profile at various stations has been used as an input to SHAKE91 algorithm for converting records at the surface to that at the rock site. The simulated records are compared with the observed records based on root-mean-square error (RMSE) in peak ground acceleration (PGA) of horizontal components. Various parameters of the rupture plane have been selected using an iterative forward modeling scheme. The accelerograms have been simulated for all the stations that lie within an epicentral distance ranging from 5 to 100 km using the final rupture plane parameters. The comparison of observed and synthetic records validates the effectiveness of the simulation technique and suggests that the Hualien earthquake consists of two SMGAs responsible for high-frequency SGM. [ABSTRACT FROM AUTHOR]

Supercritical flow bedforms are important elements of sedimentary environments, but their internal three‐dimensional structure has been elusive due to seismic imaging limitations. This article presents high‐resolution three‐dimensional seismic reflection data from Formosa Ridge – a ridge between two canyons that incise into the north‐eastern South China Sea margin. The ridge consists of 300 m thick submarine deposits including sediment waves that are manifested as crescentic depressions surrounded by elevated walls on the palaeo‐seafloor. Cross‐sectional profiles display scour fills that turn into step‐like sediment waves further downstream. These bedforms are 470 to 1370 m long and 30 to 140 m high. The three‐dimensional seismic data clearly show the step‐like bedforms that may be misinterpreted as faults or slumps on data with lower resolution. Despite exhibiting negative palaeo‐seafloor relief, they are overall depositional structures and have constructed at least part of the Formosa Ridge. The bedforms' morphology and upslope migration suggest that they are the continuum of partially depositional to fully depositional cyclic steps formed by bottom currents travelling, based on a series of simple calculations, at least 2 m s−1. These currents are able to transport sediments with grain size up to coarse sand, and such dynamic processes might impact seafloor infrastructure safety, oil and gas reservoir systems, and the functioning of benthic ecosystems in similar settings. [ABSTRACT FROM AUTHOR]

Carried by the Philippine Sea Plate converging with the Eurasian Plate, the Luzon Arc is obliquely colliding with and accreting to the Eurasian continental margin to build the Taiwan orogen. Northward along the collision suture, the Luzon Arc extends offshore and is being subducted under the Ryukyu Trench. How the northern segment of the Luzon Arc responds to both collision and subduction remains poorly understood. At present, seismic tomography and conventional stress analyses cannot fully capture images of this arc or discern how it deforms following subduction. We employed a three‐dimensional spatial clustering approach in eastern Taiwan to reveal how stress varies along the collision suture. Our results show that the maximum horizontal compression orientation, SH, is sub‐parallel to the plate motion at all depths across the lithosphere, except at shallow depths for the northern segment, where SH exhibits a clockwise rotation from convergence‐parallel by 10°–30°. Accompanying the SH rotation in the northern crustal segment is a tilt of the maximum compressive P‐axes by approximately 20°. We interpret this systematic variation along the suture and with depth as resulting from the rotation of the stress regime in the crust of the Luzon Arc forced by the oblique subduction, and likely augmented by the bending of the Central Range crustal block. The location of rotation/tilting and its disparity with depth may signify a decoupling of the volcanic arc from the mantle lithosphere during early subduction. Plain Language Summary: How a volcanic arc deforms following subduction remains poorly understood despite the frequent intrusion of such structures into oceanic trenches. A subducted arc is difficult to image using seismic tomography; however, its presence and behavior are sometimes revealed via indirect evidence. The collision‐subduction zone of Taiwan is an ideal region to monitor the deformation of the Luzon Arc during subduction because of its unique tectonic setting and high abundance of earthquakes, which allow the stress field to be mapped in three dimensions. We demonstrate that the variation in the stress orientations along the subducting Luzon Arc reveals its fate. We propose that the three‐dimensional rotation of the stress field associated with the Luzon Arc represents its response to the squeezing of the Philippine Sea Plate into the collision‐subduction corner and to the bending of the Central Range–Ryukyu Arc lithospheric sliver. We infer that the Luzon Arc has likely decoupled from the subducting lithosphere. This study provides an example of how to reveal interactions between an oceanic ridge and an overriding plate. Key Points: Along the Taiwan collision suture zone, the stress field shows a three‐dimensional rotation only in its northern segment at shallow depthsThe rotation is interpreted to be the response of the Luzon Arc crust subducted under the clockwise‐bent northern Central RangeThe Luzon Arc may have been partially decoupled from the Philippine Sea Plate [ABSTRACT FROM AUTHOR]

Geodetic evidence for slow slip recurrence changed by stress perturbations was rare, especially from afterslip following a nearby large earthquake. The first observed slow slip events in the southernmost Ryukyu subduction had occurred in 2005, 2009, and 2015 following the nearby 2002 March 31 Mw 7.1 Hualien offshore earthquake. To investigate whether the Mw 7.1 earthquake had influenced the occurrence times of the slow slip, we calculated the coseismic slip distribution and afterslip distribution using the surface displacements from onshore Global Navigation Satellite System observations. The stress perturbation on the slow-slip regions caused by the coseismic slip was quantified using the Coulomb failure criteria. We also examined the aftershock distribution and the evolution with time to clarify the stress perturbations from the afterslip offshore. Our results show that the primary afterslip distribution may have overlapped the 2009 and 2015 slow-slip patch at the downdip of the earthquake. The coseismic stress perturbation may have influenced the SSEs area directly by a Coulomb stress increase of probably 0.10 MPa. However, the 2005 SSE patch in the updip depths had only a little coseismic slip and afterslip with the Coulomb stress increase of approximately 0.06 MPa. We find that most of the aftershocks had occurred in the 2009 and 2015 slow-slip region that evolved into a typical aftershock sequence at least 2.5 years after the earthquake. The surface geodetic displacements reveal that the afterslip may have lasted longer than 4.5 years after the earthquake. The evidence for the afterslip proves that the 2009 and 2015 slow-slip patch was influenced by the afterslip directly for years. We suggest that the ongoing afterslip may have modulated the coseismic stress perturbation. It may explain the delay of the 2009 slow slip occurrence compared with the recurrence interval between the 2009 and 2015 slow slip events. [ABSTRACT FROM AUTHOR]

[Display omitted] • The sediment provenance of North Manila Trench is mainly from Taiwan Island. • Turbidity currents carry biogenic debris and fresh organic carbon into deep ocean. • Turbidity currents bury organic carbon as much as millennium sedimentation. Turbidity currents play a significant role in sediment transport in deep ocean, profoundly impacting sedimentation in the abyssal sea. This study focused on a 375 cm long sediment core collected from the Manila Trench, revealing a distinctive 15 cm turbidite layer characterized by coarse sediment, an upward fining pattern, and a basal erosion surface. Analysis of rare earth elements and Nd isotope indicates that this turbidite layer originated from Taiwan Island. Initiated in the estuarine shelf region of southwestern Taiwan Island, the turbidity current transported substantial biogenic debris and marine fresh organic carbon into the Manila Trench. Consequently, the turbidite layer exhibited an elevated CaO content and more positive δ13C values. Despite lower TOC content in the turbidite layer due to coarse particle size, the mixture of marine fresh organic matter significantly lowered the carbon age of this layer compared to the strata both below and above it. The rapid sedimentation of the turbidite deposit led to an organic carbon burial flux comparable to the rate at which normal deep-sea sediment buries organic carbon over seven hundred years. Notably, finer sediment overlaying the turbidite layer acted as a cap, enhancing carbon sequestration within the trench. In contrast to nearby core GEO6, which documented over 11 turbidity currents, core S02 recorded only one event. Detailed bathymetric analysis highlights the crucial role of local topography in detecting and preserving turbidity currents. The insights gained from this study would advance our understanding of how turbidity currents shape and influence the deep ocean sedimentation. [ABSTRACT FROM AUTHOR]

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]

An examination of repeating earthquakes in eastern Taiwan revealed previously unrecognized quasiperiodic repetition of aseismic creep along two reverse faults in an active suture zone. Using 202 ML 2.0 to 4.6 repeating earthquake sequences (RES) during the period from 2000 to 2011, we studied where and how certain faults creep. The RES were found to be highly concentrated in the southern segment of the Longitudinal Valley fault (LVF) and in the northern segment of the Central Range fault (CRF). They are mainly located at a depth of 10–25 km and show strong regional differences in creep behavior. Using the moment release rate of RES and geodetically derived long‐term slip rate, we re‐estimate the empirical relationship between deep creep and seismic moment for creeping sections in eastern Taiwan. For the 30‐km‐long LVF, the creep rate increased dramatically from 1.5 to 12.3 cm/year under the influence of the ML 6.4 Chengkung earthquake of 2003. For the 80‐km‐long CRF, the high creep rate of 4.3 cm/year appears to have been stable over time and is descriptive of a previously unrecognized deep structure underneath the eastern flank of the Central Range. The quasiperiodic pulsing of the deep slip rate has a predominant interval of 1 year for both segments. After the ML 6.4 event, the predominant interval for the creeping LVF halved in duration. The time‐dependent aseismic slip showed a strong correlation with the creepmeter data, suggesting that the positing of a common mechanism is needed to connect the surface and deep creep variation. Key Points: Using 202 ML 2.0 to 4.6 repeating earthquake sequences from 2000 to 2011, we studied where and how faults creep in an active suture zoneAn unexpected high creep rate of 4.3 cm/year is found to occur along the 80‐km‐long Central Range faultThe time‐dependent aseismic slip showed a strong correlation with the creepmeter data, suggesting a common mechanism [ABSTRACT FROM AUTHOR]

Some researchers have suggested that significant electromagnetic changes that can be detected prior to strong earthquakes are associated with the tectonics and subsurface structures beneath monitoring stations. Numerical simulation seems to support this phenomenon. However, to date, few field measurement data are available to address this issue. In this work, we used the shifting correlation method to analyze 2-month recordings of 20 geoelectric stations in Taiwan and found at least two stations had recorded geoelectric signals significantly correlated with known seismic events. These two stations are installed on large reverse faults, which are probably characterized by high stress concentrations before earthquakes. In addition, they share the same resistivity variation pattern, namely, a conductor layer sandwiched between two high-resistivity layers. Moreover, both stations are above the electrical resistivity boundaries, which means they have an increased probability of recording the abnormal disturbance once EM signals have been emitted from EQs. Our analysis results support the viewpoint that lateral and vertical electrical resistivity variation and conductors in the subsurface may be able to amplify seismic electric signals. [ABSTRACT FROM AUTHOR]

Strong ocean current influences a marine seismic survey and forces the streamer off-course from the survey line. The sideway drift of the streamer results in that the reflection data are no longer distributed in common midpoint gathers along the survey line but become swath distribution on one side of the ship track. This effect is known as "streamer feathering" which degrades the profile image of the 2D processed seismic data. However, if we have long streamer or closely spaced parallel 2D seismic survey lines, we may turn this deleterious effect into a good opportunity to generate 3D seismic volumes with swath distributed reflection data. We present two case studies in which 2D seismic data were collected offshore eastern Taiwan where the strong Kuroshio Current heavily influenced the ship speed and caused large streamer feathering. The first case is a large-offset 2D seismic profiling data collected using a 6-km long streamer. We processed the swath part of the reflection data in 3D that not only avoids the inappropriate smearing effect in 2D data processing but also generates a 3D seismic volume to help the seismic interpretation. In the second case, we adjusted our 2D survey strategy when realizing that strong Kuroshio Current was causing significant streamer feathering, and collected a set of closely spaced parallel 2D seismic lines. This multi-swath dataset covers a broad area which enables us to generate a 3D seismic volume. Since our datasets are not real 3D seismic data, we have tailored our processing flows to deal with different data configurations and limitations of each dataset. Our results show that not only we have enhanced 2D seismic images of the originally-interested survey lines, but also provide information on 3D geometry of the geological features imaged. The benefits and limitations of utilizing the streamer feathering effect to generate 3D seismic volumes from 2D seismic profile data are reported. Overall, this approach is a considerable way to handle 2D seismic data with large streamer feathering for both avoiding unreliable 2D seismic images and obtaining information on 3D geometry of the geological features imaged. [ABSTRACT FROM AUTHOR]

Coseismic surface ruptures during desctructive earthquakes (1999 Kocaeli–Düzce, Turkey and 1999 Chi-Chi, Taiwan) have caused devastating effects on buildings and infrastructures. Surface rupture remains a complicated phenomenon involving variable movements along near surface traces of both primary and secondary faults. The surface rupture patterns observed in nature, the rupture zone width and the magnitude of the surface rupture displacements, depend on the type of faulting, the earthquake magnitude, the complexities of fault geometry, as well as on the thickness and nature of the materials above bedrock. Surface rupture hazard assessment for determining the width of the surface rupture and rupture displacements magnitudes for civil engineering design needs to be site specific and incorporate various geological and geotechnical investigations. The current research on laboratory and numerical simulations to evaluate the impact of surface rupture on structure foundations is promising. However, it may be misleading to conclude that such models are sufficient to simulate the surface rupture complexities as observed in nature. [ABSTRACT FROM AUTHOR]

To explore a new discovering hydrothermal field Geolin Mounds in the Southern Okinawa Trough (SOT), a multi-scale geophysical investigation cruise including single-beam echo sounder, sub-bottom profiler (SBP) and multi-channel seismic (MCS) surveys were conducted in 2018 in a new discovering hydrothermal field we named Geolin Mounds. Taking advantage of streamer feathering caused by strong Kuroshio Current during MCS data acquisition, we performed a pseudo-3D processing and produced a pseudo-3D seismic cube. In addition to flare features in water column, “rock grove” like morphological feature above seafloor, widely-distributed amplitude anomalies including blanking zone and high-amplitude reflectors are observed around the Geolin Mounds hydrothermal field in our 2D MCS data. Pseudo- 3D seismic cube, on the other hand, provide estimation of the areas of blanking zone on selected time slice; furthermore, better characterize the fault structures in the hydrothermal field. The Geolin Mounds is the first disclosed site in the SOT where a hydrothermal field is without underlying submarine volcanos. We thus suggest that the Geolin Mounds hydrothermal field is in its embryo stage of evolution and is constantly supported by vigorous subsurface hydrothermal circulation. Consequently, the Geolin Mounds hydrothermal field may grow sustainably and serves as a good observatory for development of the seafloor edifice and ore mineralization associated with hydrothermal circulation activities. [ABSTRACT FROM AUTHOR]

The distributions and characteristics of coseismic surface ruptures are the key to understand fault behaviors. However, traditional field investigations could be time consuming while toppled buildings and infrastructures may be subject to repairs due to rescue missions and transportation recovery within a few days after the earthquake, leading to urgent needs for rapid, extensive surveys before surface breakages are modified. Therefore, we take advantage of UAS photogrammetry to collect aerial images and to map surface fractures for the Mw 6.4 Hualien, eastern Taiwan, earthquake on 6 February 2018. The earthquake struck Hualien City and generated dense surface breakages over a distance of ~8 km. Together with control surveys for ground control points by using RTK-GPS, UAS photogrammetry uses the methods of the Structure from Motion and Multi-View Stereo to produce high-resolution orthoimages and digital surface models. Our mapping results show that surface ruptures follow the trace of the Milun fault and northern Linding fault. Mapped surface ruptures are usually in en échelon arrays or distributed fractures rather than a through-going fault. Estimated sinistral offset based on orthoimages is more than 60 cm in Chihsingtan and decreases toward south. The fault behavior of the 2018 Hualien earthquake shows along-strike variations and is somewhat different from the October 1951 earthquake. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

A deadly Mw 6.4 earthquake occurred in the Hualien area of eastern Taiwan on 6 February 2018. It caused severe damage to infrastructure and creating surface ruptures in several areas mostly near the Milun Fault in Hualien City. In this study, we investigated the distribution of co-seismic surface ruptures by measuring the orientations of the ruptures, classifying the fracture patterns, and measuring the fracture geometries to calculate the principal displacement zone (PDZ) and the regional stress directions. As a result, local PDZ is observed to rotate anti-clockwise along the Milun Fault from north to south. Considering the deformation behaviors of the fractures and their relative positions along the Milun Fault, the shear zone in Qixingtan area is a horsetail structure derived from the right side of the end of the left-lateral strike-slip fault. In addition, the 170° trending fault splay in the middle segment and the fault branch corresponding to the Beipu Fault are elements of the Riedel shear structure associated with the left-lateral moving Milun Fault. Our results show that Riedel shear structures are common within co-seismic surface rupture zones in this area, and the variations in the orientations of Riedel structures reflect the influence of the pre-existing Milun Fault. We can also determine the most recent geometry, kinematics, and displacement characteristics of the Milun Fault through co-seismic surface ruptures and macro-scale Riedel shear structure analysis. This study provides a good example of understanding the relationship between the outcrop scale and the macroscale of the Riedel shear model. [ABSTRACT FROM AUTHOR]

Collision of the Luzon Volcanic Arc is the primary causative factor behind the creation of Taiwan Island. Additionally, the indention of the Peikang basement highs in the Taiwan Strait and the back arc spreading of the Okinawa Trough are responsible for the characteristic tectonics in Taiwan. Identification of active tectonic boundaries is important for understanding neotectonics in Taiwan. We analyzed Global Positioning System (GPS) horizontal velocity data in Taiwan with a hierarchical cluster analysis method. We also developed a statistical scheme introducing information entropy to quantitatively assess the fuzziness of the obtained cluster boundaries, which enables us to distinguish a trustworthy discontinuity. The hierarchical representation, which is expressed by a dendrogram, suggests the relative importance of tectonic sources in this region. The major clusters, which appear at higher hierarchy, are likely to represent major tectonic features such as the Longitudinal Valley fault and lateral extrusion in southwest Taiwan. Those at lower hierarchy suggest velocity discontinuities associated with local geological structures such as active faults. For example, boundaries in east offshore Taiwan and southern Taiwan are identified without using geological information by this analysis. In addition to the identification of these clear tectonic boundaries, our statistical cluster assessment shows the existence of fuzzy cluster boundaries along the Coastal Plain and the Western Foothills, reflecting continuum deformation in west Taiwan. Key Points: Hierarchical cluster analysis of GPS data in Taiwan enables extraction of locations of cluster boundaries and hierarchical structures of crustal deformationObtained cluster boundaries reflect regional and local tectonics in TaiwanIntroduction of information entropy into stability assessment of clustering enables quantitative estimation of the stability of cluster boundaries [ABSTRACT FROM AUTHOR]

An unusual foreland-dipping seismicity band has been identified at a depth between 7 and 25 km beneath the Coastal Plain, a foreland basin, in southwestern Taiwan. This seismogenic zone is closely associated with abrupt changes in Vp/Vs based on our new high-resolution seismic tomography. Rocks with high Vp/Vs ratios of up to 1.84 were found directly above the inclined seismicity band. Below it, the Vp/Vs ratio dramatically drops to about 1.72–1.78. We found that this change may result from illite-bearing slates transforming to muscovite-bearing phyllite/mica-schist. Previous studies suggested that the frictional instability regime for illite–quartz gouge and muscovite–quartz gouge falls in the range of 250–500 °C. We calculated the isotherms based on regional heat flow data and found that the foreland-dipping seismicity band can well be bounded between the 250 °C and 500 °C isotherms. In addition, in the same temperature range wet illite/hydromuscovite would transform to muscovite and release water. This dehydration may elevate pore pressure and facilitate the formation of an incipient decollement at the base of the seismicity band in front of the mountain belt. The hypothesis that such a decollement exists is supported by the existence of a corresponding zone of low resistivity and also by the geometry of the earthquake of October 22, 1999. The latter clearly shows a fault branched off the basal decollement with an angle about 45°. This finding implies that the effective friction coefficient of the basal decollement is at least 23% less than that of the overlying rocks. [ABSTRACT FROM AUTHOR]

In-situ 40Ar/39Ar laser microprobe dating was carried out on the Hoping pseudotachylite from a mylonite-fault zone in the metamorphosed basement complex of the active Taiwan Mountain Belt to determine the timing of the responsible earthquake( s). The dating results, distributed from 3.2 to 1.6 Ma with errors ranging from 0.2 to 1.1 Ma, were derived from a combination of two Ar isotopic system endmembers with inverse isochron ages of 1.55 ± 0.05 and 2.87 ± 0.07 Ma, respectively. Petrographical observations reveal fault melt containing ultracataclasites, therefore the older inverse isochron end-member may be attributed to the relic wall rock Ar isotopic system contained in micro-breccia. Without significant Ar loss expected, the ~1.6 Ma for the young end-member defines the exact time of the pseudotachylite formation. Seismic faulting therefore occurred during basement rock exhumation in the Taiwanese hinterland. [ABSTRACT FROM AUTHOR]

This study analyzes both 2D and 3D seismic images around the Palm Ridge area offshore of southwestern Taiwan to understand how the deformation front shifted westward and how tectonic activities interact with submarine canyon paths in the transition area between the active and passive margins. Palm Ridge is a submarine ridge that developed on the passive China continental margin by down-dip erosion of several tributaries of Penghu Canyon; it extends eastward across the deformation front into the submarine Taiwan accretionary wedge. The presence of proto-thrusts that are located west of the frontal thrust implies that the compressional stress field has advanced westward due to the convergence of the Philippine Sea Plate and Eurasian Plate. Since the deformation front is defined as the location of the most frontal contractional structure, no significant contractional structure should appear west of it. We thus suggest moving the location of the previously mapped deformation front farther west to where the westernmost proto-thrust lies. High-resolution seismic and bathymetric data reveal that the directions of the paleo-submarine canyons run transverse to the present slope dip, while the present submarine canyons head down slope in the study area. We propose that this might be the result of the westward migration of the deformation front that changed the paleo-bathymetry and thus the canyon path directions. The interactions of down-slope processes and active tectonics control the canyon paths in our study area. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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]

Very-long-period (VLP) seismic events have been detected in the Tatun Volcano Group (TVG), located around the border between Taipei City and New Taipei City in northern Taiwan, in which has ~ 7 million residents. By using both analyses of particle motions and travel-time delay, one VLP volcanic earthquake's source is estimated to be at a shallow depth (~ 800 m) beneath Mt. Chihsin, which is the highest and youngest volcano in the TVG. The significant variation of seismic energy at different azimuths provides strong evidence to distinguish a crack source from other kinds of sources, such as a sphere, vertical pipe or even double-couple source. This is further confirmed by synthetic modeling of the seismograms recorded at two stations as well as the compressional first-motion at three seismic stations. Thus, a deeper plumping system with high-pressure fluid is required to generate the VLP signals and other volcanic earthquakes in the TVG. Combining this result with those of previous studies, we conclude that the TVG might not be totally extinct and some further investigations must be carried out to improve understanding of the volcanic characteristics of the TVG. [ABSTRACT FROM AUTHOR]

A temporary seismic network consisting of 295 land stations was deployed in 2009 in Taiwan to monitor airgun sources during an onshore-offshore experiment in Taiwan. We exploit the continuously recorded dataset to detect and map seismicity in the seismically very active Taiwan. By combining recent automatic detection and phase picking techniques, we successfully generate an initial earthquake catalog of over 8400 events. The hypoDD algorithm is used to relocate and filter these events. This network recorded smaller events than the permanent regional network because of the many stations around the high mountain ranges and the generally high station density along 6 lines. The results based on the 2009 data generally reproduce the dominant seismicity features from many years of earthquake monitoring in Taiwan. In addition, we map hitherto unknown dipping zones under the Foothills and the Central Range that may correspond to seismogenic structures. [ABSTRACT FROM AUTHOR]

The Taiwan area, where the Philippine Sea Plate collides with Eurasia, is one of the most seismically active areas in the world and has been consequently struck repeatedly by destructive earthquakes. To better constrain the occurrence of large earthquakes, we have conducted two cruises in 2012 and 2013 from which five piston, gravity and box-cores were retrieved from selected sites in targeted areas that contain continuous and accurate turbidite deposition offshore eastern Taiwan. Forty-seven turbidite layers deposited between 675 BC and 2012 AD have been described in the cores with facies varying from silty clay to coarse sand. We modeled each turbidite layer using the sedimentation rate deduced from radiocarbon age measurements performed on planktonic foraminifers. Precise dating based on 210 Pb– 137 Cs chronology provided ages for the XXth century turbidites. Coring sites' locations, biotic association within turbidites beds and seismic calibration over the instrumental period, suggest that earthquakes are the most likely triggering mechanisms of turbidity currents over the last 2700 years. Calibration of correlations between turbidites and instrumental earthquakes allowed us to determine the minimum magnitude of recorded events (M w = 6.8) and the maximum distance between the epicenter and the slope failure. Turbidites' synchroneity between cores has been tested but results showed that this criterion cannot be used in our study area because of the high level of seismicity, i.e., a M > 6.8 earthquake recurrence interval much smaller than the uncertainty on radiocarbon ages. The excellent accuracy of dating allows correlating all the turbidites deposited since 1920 with instrumental earthquakes. For each core, we established a return time ranging between 112 and 147 years for the period pre-1900 and ~ 27 and 34 years for the period post-1900. The discrepancy between pre-1900 and post-1900 recurrence either suggests, that slope stability varies, with higher slope instability after 1900 possibly due to increased sediment delivery by rivers. [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]

Earthquake early warning (EEW) is an effective approach to mitigating earthquake damage. This is the first study evaluating borehole seismograph application to EEW in Taiwan. We selected inland and offshore earthquakes with ML larger than 4.0 occurring between 2012 and 2014 for this study. We investigated the Pd attenuation relationship as a function of the hypocentral distance (R) and magnitude (M). The new Pd attenuation relationship specific for the borehole records is expressed as: log(Pd) = 0.689ML - 0.741log(R) - 4.608 ± 0.248. Once the earthquake location is determined, this regression equation is used to quickly estimate Pd magnitude (MPd). According to the new regression equation formulated specifically for borehole observations in Taiwan, our result shows that the standard MPd deviation is about 0.21 relative to ML. This smaller standard deviation of 0.21 compared to that of the free-surface records might be attributed to the reduced influence of near-surface effects in the borehole records. We propose a new robust Pd regression equation for the Taiwan borehole seismic network. [ABSTRACT FROM AUTHOR]

The Taiwan orogenic belt results from convergence between the Philippine Sea plate and Eurasia plate since the late Cenozoic. An extremely high exhumation rate has been observed in the Backbone Range, which has motivated interpretive models that show underplating as the primary process in driving uplift and thickening. Here we integrate new (U-Th)/He and fission-track dates of detrital zircons with previously published thermochronology to document a significant out-of-sequence thrust in the core of Taiwan orogen. The thrust, informally named the Tayulin fault system, is identified by structural kinematics and offset metamorphic temperature trends, low-temperature thermochronometric dates, and seismic tomography. The OOST appears to be composed of three segments that crop out along the western flank of the Backbone Range. The apparent age-elevation profiles suggest a relatively slow exhumation rate, <1.0 mm/yr, in the early stages from 8 Ma to 2 Ma followed by a stage of significantly higher exhumation rates, ranging from 2.3 to 6.5 mm/yr after 2 Ma. The early slow exhumation stage is related to regional folding and foliation development. In contrast, we suggest the younger rapid exhumation stage is related to slip on the out-of-sequence thrust. • We detect a new Tayulin out-of-sequence thrust (OOST) in Taiwan orogeny. • The exhumation rate is less than 0.1 mm/yr from ca. 8 Ma to 2 Ma in Backbone Range. • Exhumation rate increases to 2.3 mm-6.5 mm/yr after 2 Ma in Backbone Range. • The slow exhumation could be related to the regional-scale folding process. • The OOST plays an important role in the rapid exhumation process after 2 Ma. [ABSTRACT FROM AUTHOR]

An unexpectedly large uplift of 91 mm was detected by geodetic observations during the rupture of 2016 M w 6.4 Meinong earthquake, Taiwan, which has been attributed as either a triggered anelastic and hydrologic related deformation from a proposed duplex/mud diapir or a triggered aseismic slip on a proposed backthrust. Here, using both high-rate GNSS and free-field strong motion data, we first estimated the coseismic source model. The locations of high PGV were inferred to explain the unexpected distribution of damaged buildings approximately 25–30 km west of the epicenter. Then, the aseismic surface displacements during the earthquake was differentiated using the coseismic source model. The aseismic extension of 3.8 μstrain is inferred at the unexpectedly large uplift region. Combining with local geology, residual gravity anomaly, seismic tomography, interseismic leveling vertical velocities, coseismic leveling uplifts and proposed Coulomb stress changes, the accelerated mud diapirism during the earthquake triggered by slip on the deep seismogenic fault was identified as the cause of unexpectedly large coseismic uplift. • High-rate GNSS and strong motion data were inverted to estimate coseismic slips. • Inferred PGV locations explained the unexpected distribution of damaged buildings. • Unexpectedly anelastic large uplift was caused by the accelerated mud diapirism. [ABSTRACT FROM AUTHOR]

The main goal of this study was to develop an updated and refined catalog of earthquakes in Taiwan (1900-2014) with homogenized M magnitudes that are compatible with the Harvard M. We hope that such a catalog of earthquakes will provide a fundamental database for definitive studies of the distribution of earthquakes in Taiwan as a function of space, time, and magnitude, as well as for realistic assessments of seismic hazards in Taiwan. In this study, for completeness and consistency, we start with a previously published catalog of earthquakes from 1900 to 2006 with homogenized M magnitudes. We update the earthquake data through 2014 and supplement the database with 188 additional events for the time period of 1900-1935 that were found in the literature. The additional data resulted in a lower magnitude from M 5.5-5.0. The broadband-based Harvard M, United States Geological Survey (USGS) M, and Broadband Array in Taiwan for Seismology (BATS) M are preferred in this study. Accordingly, we use empirical relationships with the Harvard M to transform our old converted M values to new converted M values and to transform the original BATS M values to converted BATS M values. For individual events, the adopted M is chosen in the following order: Harvard M > USGS M > converted BATS M > new converted M. Finally, we discover that use of the adopted M removes a data gap at magnitudes greater than or equal to 5.0 in the original catalog during 1985-1991. The new catalog is now complete for M ≥ 5.0 and significantly improves the quality of data for definitive study of seismicity patterns, as well as for realistic assessment of seismic hazards in Taiwan. [ABSTRACT FROM AUTHOR]

The Taiwan Earthquake Model (TEM) was established to assess the seismic hazard and risk for Taiwan by considering the social and economic impacts of various components from geology, seismology, and engineering. This paper gives the first version of TEM probabilistic seismic hazard analysis for Taiwan in these aspects. We named it TEM PSHA2015. The model adopts the source parameters of 38 seismogenic structures identified by TEM geologists. In addition to specific fault source-based categorization, seismic activities are categorized as shallow, subduction intraplate, and subduction interplate events. To evaluate the potential ground-shaking resulting from each seismic source, the corresponding ground-motion prediction equations for crustal and subduction earthquakes are adopted. The highest hazard probability is evaluated to be in Southwestern Taiwan and the Longitudinal Valley of Eastern Taiwan. Among the special municipalities in the highly populated Western Taiwan region, Taichung, Tainan, and New Taipei City are evaluated to have the highest hazard. Tainan has the highest seismic hazard for peak ground acceleration in the model based on TEM fault parameters. In terms of pseudo-spectral acceleration, Tainan has higher hazard over short spectral periods, whereas Taichung has higher hazard over long spectral periods. The analysis indicates the importance of earthquake-resistant designs for low-rise buildings in Tainan and high-rise buildings in Taichung. [ABSTRACT FROM AUTHOR]

Detecting anomalies before an earthquake has become an important task in seismic hazard prevention. The purpose of this research is to use the intrinsic attenuative dispersion of the P-wave, Qp, to serve as an anomaly before an earthquake. On November 5, 2009, an earthquake ( M L = 6.2; depth = 24 km) occurred in Ming-Jen, Nantou county, Taiwan. The vertical-component of the first-cycle of P-wave data recorded from the Central Weather Bureau Seismic Network from 2009/01 to 2010/01 was calculated to obtain the Qp parameter. Meanwhile, this study also utilizes global positioning system stations to examine changes in surface displacements that occurred during the Mingjen earthquake. Analysis results indicate that 2-months before the mainshock occurred; the Qp began to decrease for all stations, which implies that the crustal area became more fractured owing to a high stress accumulation. Such a lead time of ∼2 months is equal to the average lead time of Seismic Electric Signals (SES) activities being also compatible with a physical model suggested long ago for the SES generation. With stress accumulating a few days before the earthquake, an impeded region can be clearly identified by relatively-large difference values when the mainshock is going to occur. We conclude that the analysis of the Qp parameter and GPS index are very sensitive to changes in the stress state and may serve as a useful index in early warning of earthquake. [ABSTRACT FROM AUTHOR]

The northeastern convergence of the Philippine Sea plate toward the Eurasian plate causes the major western Philippine Sea plate boundary to subduct toward the northwest or west directions. However, this phenomenon is not clearly observed along the plate boundary between Luzon and Taiwan. Careful examination of deep seismicity in the southern Taiwan area from the earthquake catalog reported by the Central Weather Bureau shows two seismic zones dipping toward the opposing directions. The first dips toward the east from the surface down to 150 km in depth, while the second dips westward at depths between 150 and 200 km. These two seismic zones are confirmed further by seismogram observation and modeling results generated by two deep faults in the southern Taiwan area. The eastward seismic zone clearly results from the Eurasia plate subduction along the Manila trench, while a small section of the westward seismic zone might likely be a residual slab from the ancient subducted Philippine Sea plate. Based on the subduction speed obtained from GPS observations and the subducted Eurasian plate geometry, we can further estimate the eastward Eurasian plate subduction started at least 3.35 million years ago. This result is roughly consistent with the volcanic ages (3 - 4 Ma) observed in the arc between Luzon and Taiwan. [ABSTRACT FROM AUTHOR]

Seismic hazard evaluations require an estimate of the expected ground motion at the site of interest usually by using attenuation relationships. The mean shear-wave velocity over the top 30 m (Vs30) is incorporated in the ground motion attenuation relationships in this study. By comparing the standard deviations of the residuals between the observed and predicted values before and after incorporating the site effect term Vs30, the reduction in standard deviation for the peak ground velocity (PGV) is significantly reduced by about 11%. Clearly, the refined attenuation relationships will be more useful for engineering purposes. Analyzing the site effect term using the amplification factor (relative to a site with Vs30 = 760 m s-1), has revealed that the Changhua Plain, Chianan Plain, Pingtung Valley, Ilan Plain, and Taipei Basin have high values, implying large ground motion amplification. Following a disastrous earthquake, quick assessment and timely peak ground acceleration (PGA) and PGV map reporting will be critical for effective emergency response operations. After an earthquake we can combine the simple attenuation relationships, as determined from Model 1, to provide near real-time estimation and reporting of the PGA and PGV values for the Taiwan area. We can also use the relations between the intra-event site residual and the Vs30 to estimate the Vs30 for stations that have recorded strong motions, but do not yet have Vs30 information. Our approach including sites with estimated Vs30 has resulted in a refined Vs30 contour map that can be used for more realistic seismic hazard assessment for Taiwan. This approach is especially applicable to the foothill and mountain areas. [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]

Data from P-waves and from S-waves generated by P-S conversion on reflection from airgun shots recorded along four lines of ocean bottom seismometers were used to construct 2-D velocity sections near the Yuan-An Ridge, off southwest Taiwan. The locations of the ocean bottom seismometers were determined to high accuracy by an inversion based on the shot traveltimes. Traveltime inversion and forward modeling of multicomponent wide-angle seismic data result in detailed P-wave (Vp) and S-wave (Vs) velocities of hydrate-bearing sediment layers. The inversion indicates a relatively high P-wave velocity beneath topographic ridges which represent a series of thrust-cored anticlines develop in the accretionary wedge. S-wave velocities of the sediments over the entire section, down to 400m below seafloor, range from 320 to 570ms−1. We suggested the lateral variation in Vp/Vs profiles in the hydrate-affected zones may be related to the migration conduit of gas-rich fluid and a characteristic of hydrate content. We model Vp using equations based on a modification of Wood's equation to estimate the gas hydrate saturation. The hydrate saturation varies from 5% at the top ~200m below the seafloor to 10-15% of pore space close to the bottom simulating reflector (BSR) in the survey area. [ABSTRACT FROM AUTHOR]

The joint inversion of different geophysical datasets is an effective means to reduce the non-uniqueness and improve the reliability of geophysical inversion. In this study, seismic arrival time and Bouguer gravity datasets are jointly inverted to obtain an image of 3-D velocity structures in the Taiwan orogen. The model obtained from joint inversion fits the arrival time observations at least as well as when inverted individually, and the gravity observations are much better fit when included in the inversion, implying a reduction in ambiguity by simultaneously modeling the disparate datasets. Moho depth variations estimated by the 3-D P wave velocity model suggest a maximum Moho depth of 56 km located beneath the Backbone Central Range, and the trend of the Moho is largely consistent with the topography of the Central Range with eastward, asymmetric crustal thickening. The root beneath the Central Range appears to be smaller in lateral extent than previously imaged, and the velocity gradients into the uppermost mantle are significantly higher. The lack of evidence for a significant amount of Eurasian crust in the mantle supports geodynamical models of accreted, rather than consumed, continental crust in a collisional environment. [ABSTRACT FROM AUTHOR]

How deep the two opposite dipping subduction zones in the vicinity of Taiwan, northwestward-dipping Philippine Sea Plate (PSP) and eastward-dipping South China Sea Slab (SCS), penetrated down into mantle, are hotly on debate and insufficiently imaged by seismic tomographic inversions, either due to inadequate quantities of seismic data or limited vertical resolution of the techniques used by previous investigations. Here we employ an unprecedented number of 55,847 high-quality P-to-S radial receiver functions recorded by 304 broadband seismic stations located in SE China and adjacent areas to image the topography of the mantle transition zone (MTZ) discontinuities (d410 and d660) to delineate the configurations of the subducted slab segments and analyze the impacts of thermal and water content anomalies on the MTZ. The close-to-normal wavespeed-corrected d410 and a ~ 10 km depression of the corrected d660 to the east of northern Taiwan are revealed, indicating that the PSP breaks off above the d410 and the delaminated PSP drops into the lower MTZ along with slab dehydration. To the east of southern Taiwan, where the SCS begins to subduct eastward, the uplift of the apparent d410 is nearly twice as much as that of the d660. The positive wavespeed anomaly above the d410 and low temperature in the vicinity of the d410 are enough to contribute to the uplift of MTZ discontinuities, suggesting that the SCS just reaches the d410, rather than penetrating the entire MTZ. • The MTZ discontinuities in Taiwan and its adjacent regions are imaged using receiver functions of unprecedented resolution • The geometry of subducted slab segments in the vicinity of the MTZ is constrained • The break-off Philippine Sea Plate drops into the lower mantle transition zone along with dehydration • The eastward-dipping South China Sea Slab penetrates the d410 [ABSTRACT FROM AUTHOR]

The M ≥ 7 earthquakes that occurred in the Taiwan region during 1906-2006 are taken to study the possibility of memory effect existing in the sequence of those large earthquakes. Those events are all mainshocks. The fluctuation analysis technique is applied to analyze two sequences in terms of earthquake magnitude and inter-event time represented in the natural time domain. For both magnitude and inter-event time, the calculations are made for three data sets, i.e., the original order data, the reverse-order data, and that of the mean values. Calculated results show that the exponents of scaling law of fluctuation versus window length are less than 0.5 for the sequences of both magnitude and inter-event time data. In addition, the phase portraits of two sequent magnitudes and two sequent inter-event times are also applied to explore if large (or small) earthquakes are followed by large (or small) events. Results lead to a negative answer. Together with all types of information in study, we make a conclusion that the earthquake sequence in study is short-term corrected and thus the short-term memory effect would be operative. [ABSTRACT FROM AUTHOR]

Abstract: Direct observations of the physical structures of the seismogenic zones of active faults are rare, due to the difficulty in reaching the fault zone at depth. Current geological evidences, mostly from the surface, suggest that principal slip zone (PSZ) accommodated most shear displacement and was the place where physico-chemical processes occurred during an individual coseismic event and the thickness of PSZ is a few millimeter to tens of centimeter wide. However, the actual thickness of PSZ of a large earthquake, a key parameter of seismology in understanding energy dissipation and rupture processes, remains largely unknown. The Chelungpu fault that ruptured during the 1999 Mw 7.6 Chi-Chi earthquake (Taiwan) was drilled to a depth of 2003m providing a unique opportunity to sample an active fault that slipped in a recent large earthquake. The PSZ, corresponding to the 1999 Chi-Chi earthquake, was well characterized within cores at a borehole depth of 1111m from the Taiwan Chelungpu fault Drilling Project-A (TCDP-A). Here we determine the interval of clay anomaly that resulted from frictional melting/thermal decomposition process by state-of-art in-situ synchrotron XRD analysis providing very high spatial resolution for mineralogy. Combined with the interval of the presence of vesicles from microstructural observation, the thickness of Chi-Chi PSZ is estimated to be 1mm. Thus, the correlated contribution of surface fracture energy to earthquake breakdown work, at least in this locality, is quantified to be 1.9%. The huge remaining part of the breakdown work seems to be turned into heat associated with fault dynamic processes during the 1999 Chi-Chi earthquake. [Copyright &y& Elsevier]

One decade after the 1999 Chi-Chi earthquake, central Taiwan experienced more strong ground shaking [Central Weather Bureau (CWB), intensity VII] induced by a ML 6.2 earthquake on 5th November 2009. This earthquake occurred in the Mingjian Township of Nantou County, only 12 km southwest of the Chi-Chi earthquake epicenter. The broadband micro-earthquake monitoring network operated by the National Center for Research on Earthquake Engineering (NCREE) observed numerous aftershocks in the five days following the mainshock. The relocated aftershocks and the mainshock focal mechanism indicated a NE-SW striking fault dipping 60° toward the northwest. This fault plane is inside the pre-Miocene basement and the rupture extends from the lower crust to 10 km depth just beneath the basal décollement of the thin-skinned model that is generally used to explain the regional tectonics in Taiwan. The fault plane is vertically symmetrical with the Chelungpu fault by the basal décollement. The NW-SE compressive stress of plate collision in Taiwan, as well as the deep tectonic background, resulted in the seismogenic structure of the Mingjian earthquake at this location. [ABSTRACT FROM AUTHOR]

Predicting how large an earthquake can be, where and when it will strike remains an elusive goal in spite of the ever-increasing volume of data collected by earth scientists. In this paper, we introduce a universal model of fusion-fission processes that can be used to predict earthquakes starting from catalog data. We show how the equilibrium dynamics of this model very naturally explains the Gutenberg-Richter law. Using the high-resolution earthquake catalog of Taiwan between Jan 1994 and Feb 2009, we illustrate how out-of-equilibrium spatio-temporal signatures in the time interval between earthquakes and the integrated energy released by earthquakes can be used to reliably determine the times, magnitudes, and locations of large earthquakes, as well as the maximum numbers of large aftershocks that would follow. [ABSTRACT FROM AUTHOR]

Seismic hazard in the active collision zone of southeastern Taiwan has been poorly known. Although the area has experienced only a few magnitude 6 earthquakes during the modern seismic observation period, there are good geological and instrumental evidences that the area has repeatedly experienced magnitude 7 earthquakes, including a sequence in 1951. We estimated 3-D P- and S-wave velocity models by applying an earthquake tomography method to a large number of high-quality arrival-time data collected by a regional seismic network and a local seismic array. Earthquakes in the region that occurred between 1991 and 2011 were then relocated using the 3-D velocity models. The 3-D velocity models and seismicity depict the current deformation structures in the region. Numerous earthquakes have occurred along a narrow east-dipping seismic zone beneath the southern Coastal Range (CR) near Chengkung. The seismogenic structure extends from the surface eastward to depths greater than 20 km. It is characterized by low-Vp, low-Vs and low-Vp/Vs. Soft materials within the seismogenic structure may include fluids as well as cracks with high aspect ratios resulting from extended periods of tectonic loads. Soft sediments with high fluid contents between rigid materials are regarded as significant weak zones where stress concentration and nucleation of numerous earthquakes can easily occur. Beneath the western boundary of the Longitudinal Valley (LV), a vertical or steeply west-dipping strike-slip fault is identified. Apparently, significant slip partitioning is taking place to accommodate the oblique tectonic motions. In the eastern offshore, seismic velocities indicate that the materials are derived from deeper depth and that significant amounts of water have been introduced to form serpentines at shallow depth. The central LV and CR have been largely aseismic since a series of magnitude 7 earthquakes in 1951. A large amount of tectonic stress is stored in the region. Thus, the area has high potential to release seismic energy through future large earthquakes. [ABSTRACT FROM AUTHOR]

Signals from ten explosions were used to examine earthquake location uncertainty in Taiwan. Location errors for explosion sites determined using a relocation process were expressed in terms of statistical measurements for standard errors in the depth (ERZ), the epicenter (ERH), the root-mean-square of the travel time residuals (RMS), and the station coverage gap (GAP). In general, for this study, major factors in location errors resulted from the poor coverage of seismic stations (e.g., a large value of GAP). Using the relationship between location errors and parameters from the explosion location assessment, uncertainties for earthquake locations in the Taiwan region for a total of 384064 events from 1991 to 2011 were evaluated. Offshore regions in southwestern and northeastern Taiwan had larger location errors. For the inland locales, location errors in longitude, latitude, and depth were approximately 3.1 ± 2.7, 1.3 ± 1.6, and 4.6 ± 3.9 km, respectively. The uncertainties estimated from this study could offer a good reference for other related studies. [ABSTRACT FROM AUTHOR]

• A new detection scheme considering the consistency between three distinct declustering methods. • 153 M3-5.8 swarm sequences (4726 events) in Taiwan were identified. • Swarm events are active at locations with a high seismicity rate (> 0.2 yr − 1 km − 2). • 86% of the 1857 repeating events in Taiwan are closely located to swarm events. • Most swarm activities occur at the area experiencing high creep rate and episodic aseismic slip. Earthquake swarms can act as indicators of fluid activity and aseismic transients at depth. As a young and active collisional mountain belt with frequent earthquakes, Taiwan is a natural laboratory for studying the physical mechanisms controlling the characteristics of swarm activity in this complex tectonic environment. Where the earthquake swarms tend to occur and how they reflect the aseismic deformation in this young orogen? In this study, we propose a composite swarm detection method that relies on the correlation of mainshock density rates derived from three distinct declustering algorithms. Using earthquakes in Taiwan from 1990–2019 with magnitudes greater than or equal to three and at depths less than 50 km, 153 swarm sequences were identified as characterized by an event number greater than ten, a time span of less than fifteen days, and a maximum magnitude of 5.79. The swarm sequences were found to be active at locations with a high seismicity rate (> 0.2 yr − 1 km − 2) but not an anomalous b -value, suggesting that the environment with a high rate of earthquake occurrence is linked with the mechanism for swarm generation. They were found to largely overlap with the repeating earthquakes, as approximately 86% of the 1857 repeating events in Taiwan are closely located to swarm events. Such overlap is found to be largest in the southern Longitudinal Valley where a creeping fault is located and where a low coupling ratio was inferred. Consequently, it is proposed that the active collision boundary experiencing a high creep rate and the areas experiencing episodic aseismic slip are capable of producing active swarm sequences in Taiwan. [ABSTRACT FROM AUTHOR]

We propose an approach to imaging earthquake source rupture process by direct back-projection of local high-frequency (0.1–2.5 Hz) P-wave displacements from strong-motion records. A series of synthetic experiments are performed which demonstrate that our approach is capable of recovering the spatial-temporal distribution of the source slip with a good station coverage and a high average coherence value between the target and template waveforms. We demonstrate the effectiveness of our approach by applying it to image the slip distribution of an earthquake occurred on 2010 March 4, in Jiasian (Mw = 6.0 and ML = 6.4) in southern Taiwan. The resulting moment-rate amplitude images show that the source rupture initiated at the vicinity of the hypocentre, followed by a moderate moment-rate release to the southeast of the hypocentre and a subsequent upward propagation, and finally propagated in the northwest direction, in agreement with the distribution of aftershocks. The majority of the slip at 17–20 km depth occurred to the west of the hypocentre, in a general agreement with the slip distributions obtained from dislocation model and finite-fault inversions. Our modified back-projection approach relies on seismic waveforms with the considerations of a recent 3-D structure model, high average coherence value, station correction factor and simplified amplitude correction. It is computationally efficient and allows for near real-time determinations of source slip distributions after earthquakes using strong-motion records. A quick result for the rupture model can be used in the calculation of strong ground-motion, providing important, useful and timely information for seismic hazard mitigation. [ABSTRACT FROM AUTHOR]

We analyze in this study a new set of marine data including 3D local tomography, 1992–2008 relocated earthquakes and two recent multichannel seismic lines to characterize the deformation style in the collision area offshore east Taiwan. We have mapped in detail the Mohos of the converging plates as well as the subduction interface with a resolution never reached before. We show that the sharp continental subduction of the Eurasia plate, beneath the middle part of the Central Range, indents the Philippine Sea plate (PSP) as attested by intra-oceanic slicing and incipient subduction of the PSP beneath the east coast of Taiwan. The westernmost part of the PSP slab is probably experiencing a beginning of break-off as attested by NW-trending en-échelon shear zones beneath the southern slope of the southern Ryukyu arc (SRA). These en-échelon shear zones have a sinistral component favored by the “collision-free” subduction of the PSP north of 24°30′N. The down-faulting of the subduction interface forms ramps along which earthquakes clusterize. Three M7 subduction earthquakes occurred offshore Suao city along these ramps with a recurrence interval of about 40years: 1920 Mw7.7, 1963 Mw7.2 and 2002 Mw7.1 events. The 1966 Mw6.0–7.5 earthquakes sequence likely outlines a WNW-ESE left-lateral intra-slab shear zone. The SRA upper plate accommodates the complex geometry and deformation of the subducting PSP through seismic deformation. Shallow high velocities fringing the Luzon volcanic arc (LVA) beneath the Longitudinal Valley and north of the southernmost Ryukyu forearc basins are interpreted as relics of the LVA forearc basement squeezed in the collision zone. Based on the accommodation of a large part of the convergence through shortening within the PSP and the subsequent segmentation of the shallow subduction interface, we consider that the nucleation of a Mw ≥8 earthquake along the southernmost Ryukyu megathrust is unlikely. [ABSTRACT FROM AUTHOR]