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The Taiwan–Ryukyu arc–trench junction area is a seismic active region and has high potential to generate large earthquakes. However, the detailed deep structures are not fully revealed because of data resolution problems. The Taiwan Integrated Geodynamic Research (TAIGER) project collected two wide‐angle and multichannel seismic profiles across the Taiwan mountain belt (T5) and the western end of the Ryukyu arc–trench system (T6). In this study, we perform 2‐D gravity modelling for T5 and T6 by taking advantage of well‐constrained P‐wave velocity models, which helps to reduce the nonunique problem. To better fit the observed gravity anomaly, a block with relatively high‐density must exist beneath eastern Taiwan. This high‐density material could be serpentinised mantle, which may be resulted from the dehydration of the subducting Eurasian plate. Also, our result indicates that the subducting Gagua Ridge has a deep crustal root and extends northward to the Ryukyu forearc area. [ABSTRACT FROM AUTHOR]

A thorough understanding of the internal structures of the Longitudinal Valley is crucial for comprehending the architecture of the suture zone between the Eurasian and Philippine Sea plates. Here, we present the findings of eight new reflection seismic profiles spanning north to south of the Longitudinal Valley. Our primary objective is to investigate the internal sedimentary and tectonic features of the Longitudinal Valley Basin, the nature of the contact between it and the metamorphic basement rocks to the west, and the geometry of the Longitudinal Valley fault system along its eastern flank. A significant outcome of our research is that there is minimal internal deformation within the Longitudinal Valley Basin, with sedimentary features prevailing. The presence of inclined reflections suggests the presence of fan deltas and therefore a marine or lacustrine environment in the Longitudinal Valley. The sedimentary characteristics indicate that the source region for the sediment was predominately the Central Range, but that locally the Coastal Range also supplied sediment. Furthermore, at least in the shallow subsurface (1 to 2 km), a west-dipping Central Range fault does not exist along the western side of the Longitudinal Valley. The Longitudinal Valley fault system does not affect the middle of the basin, instead it is located along the eastern flank, adjoining the topography of the Coastal Range. • Eight new reflection seismic profiles across Taiwan's Longitudinal Valley Basin. • A west-dipping Central Range fault is not imaged along the western side of the basin. • The Longitudinal Valley fault is imaged along the eastern flank of the basin. • Minimal internal deformation within the basin, with sedimentary features prevailing. • Clinoforms suggest marine or lacustrine environments in the Longitudinal Valley. [ABSTRACT FROM AUTHOR]

The Tatun Volcanic Group (TVG) in northern Taiwan is the sole volcanic region on the island, with its last eruptive episode around 5,500 years ago. It has been suggested that the TVG retains the capacity for volcanic explosivity which has the potential to devastate the nearby Taipei metropolitan area of 7 million inhabitants. Though the probability of future eruption is low by probabilistic estimation, the vulnerability to volcanic hazards is high for Taipei given that it is the centre of population, industry, and government for the island. An assessment of any activity precursors is thus vital but also particularly challenging. Here, we perform an eruption-potential assessment based on the analysis of a sixteen-year Land Surface Temperature (LST) time series of data derived from satellite-retrieved thermal imagery. A Hilbert-Huang Transform (HHT) is applied to decompose oscillatory components of various timescales within the LST time series. The annual-period components are compared with those from two active volcanoes in the Philippines to assess the potential of eruption cycles in the TVG. Results from the Philippine volcanoes show that annual-period components of LST tend to lose their regularity following an eruption. By contrast, the regular annual period component of LST of the TVG Taiwan suggests a quiet and resting status with no sign of an imminent eruption. [ABSTRACT FROM AUTHOR]

In this paper we test whether or not structural and morphological features inherited from the Eurasian continental margin are affecting the contemporary stress and strain fields in south-central Taiwan. Principal stress directions (σ1, σ2 and σ3) are estimated from the inversion of clustered earthquake focal mechanisms and the direction of the maximum compressive horizontal stress (S H) is calculated throughout the study area. From these data the most likely fault plane orientations and their kinematics are inferred. The results of the stress inversion are then discussed together with the directions of displacement, compressional strain rate and maximum shear strain rate derived from GPS data. These data show that there is a marked contrast in the direction of S H from north to south across the study area, with the direction of S H remaining roughly subparallel to the relative plate motion vector in the north, whereas in the south it rotates nearly 45° counter-clockwise. The direction of the horizontal maximum compression strain rate (εH) and associated maximum shear planes, together with the displacement field display an overall similar pattern between them, although undergoing a less marked rotation. We interpret the southward change in the S H, εH and the dextral maximum shear plane directions, together with that of the horizontal displacement field to be related to the reactivation of east–northeast striking faults inherited from the rifted Eurasian margin and to the shelf/slope break. Inherited faults in the basement are typically reactivated as strike-slip faults, whereas newly formed faults in the fold-and-thrust belt are commonly thrusts or oblique thrusts. Eastwards, the stress inversions and strain data show that the western flank of the Central Range is undergoing extension in the upper crust. S H in the Central Range is roughly parallel to the relative plate convergence vector, but in southwestern Taiwan it undergoes a marked counter-clockwise rotation westwards across the Chaochou fault. Farther north, however, there is no significant change across the Lishan fault. This north to south difference is likely due to different margin structures, although local topographic effects may also play a role. [ABSTRACT FROM AUTHOR]

The 150km long Longitudinal Valley (LV) in eastern Taiwan is located at the suture zone of Eurasian and Philippine Sea Plate, connecting two subduction systems in northeastern and southern Taiwan. In this area, considerable seismicity as well as crustal deformation are accommodated by the Longitudinal Valley fault system (LVF), which is mainly composed of high angle east-dipping thrust faults. To better understand the subsurface structure and the fault geometry of the southern LV, we conducted shallow seismic reflection survey across the LVF from Donli to Luyeh in 2017. Totally, there are four 4-km-long seismic reflection profiles. We used two mini vibroseis trucks with sweeping frequencies from 35-200Hz as sources and the common-midpoint spacing is 2.5m with maximum 30 folds. A strong east-dipping reflector relative to the bedrock reflection down to 1.5km deep can be observed after noise suppression. Some low reflectivity zones in the east side of the profiles could be related to the muddy Lichi mélange. It is interesting to note that in the eastern Luyeh profile, the syncline with the east-dipping axis implies the Lichi segment of the LVF and the orientation of the tectonic deformation. As a result, the southern LV displays an asymmetric sedimentary basin pattern with a deeper bedrock to the east. [ABSTRACT FROM AUTHOR]

Picking seismic events is a time-consuming task by just relying on manpower, especially in high seismicity regions. For instance, Taiwan, on February 2018, over 4,000 aftershocks were detected within 12 days after the Hualien earthquake. Although there are lots of papers propose various deep learning models that are suitable for dealing the problem, there is still a gap between the picking routine and the AI research. Thus, we design a toolbox that can be easily integrated into the regular work flow. In this study, based on PhaseNet developed by Zhu et al. (2018), the masking arrival-time label with a Gaussian distribution is the essential part for the training methodology. In addition, we implement the neural network model UNet++, which re-designs the skip pathways for better performance comparing to the original U-Net that is used by the PhaseNet. Here, we provide an open-source framework that can be easily installed and operated. This automatic picking toolbox is built upon Obspy and Keras with Tensorflow backbone, by reading S-file catalogs from SEISAN for generating labeled datasets. Besides, a Docker image is available for fast server deployment. As a result, in our scenario, most of the temporary stations are only contained single channel and mainly focus on P arrivals. We reduce the training set from input: three-components – output: P/S/Noise categories into input: Z-component – output: P category. The testing data is successfully identified by the model trained with 90,000 traces. Moreover, minor events missed by manually picking are detected by the model. [ABSTRACT FROM AUTHOR]

Autocorrelation of the ambient noise field and the seismic wave coda theoretically provide the near zero-offset reflectivity responses of the structure beneath the recording stations. The strong reflectivity in the stacked result implies the existence of the strong impedance of the subsurface layers. In this study, we investigate the crustal reflection characteristics beneath Taiwan by autocorrelating the ambient noise data and the regional/teleseismic coda waves. The broadband data are from the permanent broadband network BATS and the temporary deployments Formosa Array which aims to provide dense spatial sampling at the northern part of Taiwan. The autocorrelations are calculated by the phase autocorrelation and the amplitude autocorrelation with an additional spectral whitening at the pre-processing stage. Then the resulting stacks are computed using the time-frequency phase weighted stacking method. Similar results at the same location but from different recording periods and instrument types indicate the consistency and robustness of the subsurface signal. For the results of permanent stations which have years of data, seasonal variations of the amplitude of the signal can be observed. The priori information from receiver functions and tomographically inferred theoretical PmP two way times can help us to identify the signal from Moho, and most of the results show good correspondence with it, although some of the stations appear no clear Moho signal that can be attributed to gradual structural transition. Besides of explaining the single station result, for the dense array dataset, we can further trace the variation of the reflectivity responses along the profile we interest, which provides more constraint for illustrating the subsurface images. [ABSTRACT FROM AUTHOR]

On 18 September 2022, the MW 6.9 Chihshang earthquake struck the south half of the Longitudinal Valley, Taiwan, and caused severe damage. A precise and rapid report for the distribution of aftershock sequence after a devastating earthquake provides key information for deciphering the seismogenic structure in the source region. The utilization of deep-learning methodologies for earthquake event detection offers a significant acceleration in data analysis. In this study, we use SeisBlue, a deep-learning platform/package, to extract the whole earthquake sequence from September to October 2022, including the MW 6.5 Guanshan foreshock, the MW 6.9 mainshock, over 14,000 aftershocks, and 866 foal mechanisms from two sets of broadband networks. After applying hypoDD for earthquakes, the distribution of aftershock sequence clearly depicts not only the Central Range Fault and the Longitudinal Valley Fault but also several local, shallow tectonic structures that have not been observed along the southern Longitudinal Valley. [ABSTRACT FROM AUTHOR]

Knowledge of the crustal structure is important for understanding the tectonic framework and geological evolution of southeastern China and adjacent areas. In this study, we integrated the datasets from the TAIGER (TAiwan Integrated GEodynamic Research) and ATSEE (Across Taiwan Strait Explosion Experiment) projects to resolve onshore-offshore deep crustal seismic profiles from the Wuyi-Yunkai orogen to the Taiwan orogen in southeastern China. Three seismic profiles were resolved, and the longest profile was 850 km. Unlike 2D and 3D first arrival travel-time tomography from previous studies, we used both refracted and reflected phases (Pg, Pn, PcP, and PmP) to model the crustal structures and the crustal reflectors. In total, data from 40 shots, 2 earthquakes, and approximately 1,950 stations were used; 15,612 arrivals were selected among three transects. Using these data, we determined the complex crustal evolution since the Paleozoic era , involving the closed Paleozoic rift basin in central Fujian, the Cenozoic extension due to the South China Sea opening beneath the coastline of southern Fujian, and the on-going collision of the Taiwan orogen. The shape of the Moho, which also reflects the crustal evolution, can be summarized as follows: ~ 30 km deep to the west of Fujian, deepening toward central Fujian (~ 35 km), becoming shallower toward the Taiwan Strait (~ 28 km), deepening again toward the mountain belt of Taiwan (~ 42 km), and becoming shallower toward the Pacific Ocean (~ 10 km). [ABSTRACT FROM AUTHOR]

Abstract: Here we tackle a tectonically important question – the upper mantle velocity structure beneath central Taiwan – with seismically interesting observations – receiver-side slab waveform effects. We use teleseismic P waveforms of the NS broadband array deployed by the TAIGER project to examine patterns of variation in arrival time, pulse width, and amplitude – measuring the first two by Gaussian fitting – and contrast measurements of earthquakes to the southeast (SE earthquakes) with those of one Sumatra earthquake in order to focus on upper mantle heterogeneities. Overall variation patterns as a function of earthquake are compatible with ray-tracing predictions. Relative reduced arrival times and amplitudes at central Taiwan stations suggest the existence of a deep aseismic slab below. From simulations of 2-D wave propagation, we conclude that lateral heterogeneity of crust and uppermost mantle primarily contributes to variations in arrival time and only secondarily to variations in amplitude and pulse width. Furthermore, discrepancies between source-side and receiver-side waveform effects, where the latter are not always amplitude-reduced, are explained by constructive interference between the fast and slow phase. Thus, the use of full waveform information can provide independent constraints to complement results of previous studies. A future extension will be to incorporate S waves and apply waveform inversion to yield quantitative constraints. [Copyright &y& Elsevier]

Abstract: Array observations indicate that the relative PKP travel-time residuals in the Taiwan region vary from −2.5 to+2.0s, a range greater than the largest known teleseismic travel-time residuals from deep Earth. PKP waves are generally employed to probe structures of the deep Earth; however, the contribution of shallow geological structures to PKP waves is generally undefined and the implications of such a contribution have rarely been discussed. Current travel-time tomographic models of Taiwan are able to predict less than 40% of the observed PKP residuals, and the PKP residuals are consistent with the pattern of station corrections used in determining local earthquakes. Detailed island-wide gravity measurements indicate that the distribution of Bouguer anomalies, which were previously proposed to be induced by thick sedimentary deposits in large tectonic basins, is consistent with the pattern of PKP travel-time residuals. In conclusion, the observed PKP residuals are strongly affected by shallow sediments. Therefore, the contribution of residuals from shallow crustal structures should be carefully considered when employing PKP observations for imaging structures of the deep Earth or the deep crust and upper mantle. [Copyright &y& Elsevier]

Proposed in the late 1980s, cumulative absolute velocity (CAV) is a new intensity measure for earthquake ground motion characterizations, followed by studies and applications such as CAV ground motion prediction equations (GMPEs). In this study, two new CAV GMPEs were developed with 24,667 strong-motion records from Taiwan, and the first CAV seismic hazard assessment for Taipei (the most important city in Taiwan) was then conducted using the local CAV models. It shows that the annual rate for the study area to encounter a ground motion with CAV >0.97 g-sec is 0.002 per year, corresponding to a 10% occurrence probability in 50 years. By contrast, the deterministic scenario-based analysis shows that the CAV seismic hazard is about 0.60 g-sec for the study area. Future studies are worth conducting to develop more sophisticated, local CAV GPMEs and to explore more applications of such CAV prediction models, such as the developments of PGA-CAV joint probability distributions for conducting PGA-CAV joint seismic hazard assessments. • First CAV GMPE for Taiwan areas. • More than 4,000 strong motion records used. • CAV-based seismic hazard analysis for Taipei. [ABSTRACT FROM AUTHOR]

Graphical abstract Highlights • Seismogenic characteristics near the NE rifted margin of the South China Sea are revealed. • NW-SE tensional stress controls the Taiwan collision-Manila subduction transition. • Intraslab pre-existing faults affect the seismogenic behavior of a convergent system. Abstract Based on the distribution of an earthquake swarm determined from an ocean bottom seismometer (OBS) network deployed from 20 August to 5 September 2015 in the northernmost part of the South China Sea (SCS) and data from inland seismic stations in Taiwan, we resolved a M L 4.1 earthquake occurring on 1 September 2015 as a NE-SW-trending left-lateral strike-slip event that ruptured along the pre-existing normal faults generated during the SCS opening phases. The direction of the T-axes derived from the M L 4.1 earthquake and the background seismicity off SW Taiwan present high consistency, indicating a stable dominant NW-SE tensional stress for the subducted Eurasian Plate (EUP). The distinct stress variations on the two sides of these reactivated NE-SW trending features suggest that the presence of pre-existing normal faults and other related processes may lessen the lateral resistance between the Taiwan collision and Manila subduction system, and facilitate the slab-pull process for the subducting portion, which may explain the NW-SE tensional stress environment near the transition boundary in the northernmost part of the SCS. [ABSTRACT FROM AUTHOR]

Earthquake early warning (EEW) has been implemented in several regions around the world. However, because of natural randomness and uncertainty, false alarm and missed alarm can be expected in EEW. The key scope of this study is to evaluate the reliability of an on-site EEW in Taiwan, by testing the system's algorithm with 17,836 earthquake data from 1999 to 2013. The analysis shows that the on-site EEW system, empirically speaking, should have a false-alarm probability of 2.5%, and a missed-alarm probability of 14.1%. Considering missed alarm should be more critical to EEW, a new algorithm that could reduce the system's missed-alarm occurrences to 6% is also discussed in this paper. [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]