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Large, destructive historical earthquakes off the coast of China's Fujian Province point to important tectonic activity in the western Taiwan Strait that, until recently, has received little attention. We present newly acquired reflection seismic data that is used to study the shallow crustal structure of the western Taiwan Strait. With these data we map the location of the Benhai fault for the first time, describe its upper crustal geometry and, in combination with seismicity and earthquake focal mechanisms, interpret its kinematics. These new data demonstrate that there is wide spread evidence of faulting that reaches the sea floor in the western Taiwan Strait, clearly indicating that the Benhai fault is active. Faults that cut up section from steep basin sidewalls to form flower structures or terraced sidewall fault zones, together with the fault and basin map pattern, are consistent with this fault zone being in the early developmental stages of a dextral strike-slip system. Earthquake focal mechanisms, although not definitive, support the model of an active dextral strike-slip fault system in the western Taiwan Strait. [ABSTRACT FROM AUTHOR]

The Taiwan Strait, along with the southeastern continental margin of the Eurasian plate, Fujian in SE China, is not far from the convergent boundary between the Eurasian plate and the Philippine Sea plate. Although this region is considered tectonically relatively inactive, many small earthquakes still occur, and normal faults are well developed in the strait. To better understand the geological processes in this region, we use 2 years of ambient noise data from more than 100 stations in Fujian and Taiwan to obtain a 3‐D crustal shear‐wave velocity model using a direct surface‐wave inversion method. Our results show that the low‐velocity zone beneath the Zhenghe‐Dapu suture zone plays an important role in the tectonic evolution of the Fujian area. The relatively high velocity in the eastern part of the suture zone and low velocity in the west correspond to the Mesozoic magmatic zone and the Wuyi‐Yunkai orogenic belt in Fujian, respectively. The coastline of Fujian presents a high‐velocity anomaly in the upper crust, which is related to the Mesozoic Pingtan‐Dongshan metamorphic belt. The long strip‐like high‐velocity zone through the rift basins in the strait is interpreted as igneous rocks due to extension of the lithosphere in the Cenozoic. Two‐stage extension with different extensive centers in the strait may be the reason for the high‐ and low‐velocity anomalies in the middle to lower crust and uppermost mantle of the strait. Key Points: A 3‐D crustal shear‐wave velocity model of Taiwan Strait and Fujian, SE China, was obtained from ambient noise tomographyGeological units were influenced by older to younger tectonic events from northwest to southeast in the study regionTwo‐stage extension with different centers occurring in the early and late Cenozoic may account for the structural pattern in the strait [ABSTRACT FROM AUTHOR]

Fujian, Southeastern China, has experienced multistage tectonic activities since the Neoproterozoic Era and is currently influenced by collision between the Eurasian and Philippine Sea plates. Topography, fault zones, and patterns of seismicity are the imprints of tectonic evolution. Historically, there have been several catastrophic earthquakes in the southeastern part of Fujian. To understand the crustal structure related to the fault zones, we performed Vp, Vs, and Vp/Vs travel-time tomography using joint inversion of active and passive sources. A total of 75,827 and 31,044 arrivals of P and S waves, respectively, from 33 explosions and 2543 earthquakes are used in our study. As a result, seismicity has indicated that two NE strike seismogenic zones, the Zhenghe–Dapu and Changle–Zhaoan fault zones, are currently active. Low Vp/Vs ratios in inland Fujian imply that the crust is mainly composed of felsic rocks as part of the Eurasian continental crust, which is consistent with geological observations at the surface. Based on Vp tomography, the thickness of the crust along the coastline is shallower than that on land, which is related to higher heat flow and the Bouguer anomaly. This shallow crust phenomenon near the coastline could be related to the regional extensional stress: the remaining structure of the back-arc extension that stretched the continental crust during the Mesozoic Era or/and the Cenozoic extension due to South China sea opening in Taiwan Strait. [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]

The Manila subduction zone is located at the plate boundary where the Philippine Sea plate (PSP) moves northwestward toward the Eurasian plate (EU) with a high convergence rate. However, historically, no large earthquakes greater than Mw7 have been observed across the northern Manila subduction zone. The poorly understood plate interaction between these two plates in this region creates significant issues for evaluating the seismic hazard. Therefore, the variation of mantle lithospheric buoyancy is calculated to evaluate the plate coupling status across the northern Manila subduction zone, based on recently published forward gravity modeling constrained by the results of the P-wave seismic crustal structure of the TAIGER (Taiwan Integrated Geodynamic Research) project. The results indicate weak plate coupling between the PSP and EU, which could be related to the release of the overriding PSP from the descending EU’s dragging force, which was deduced from the higher elevation of the Luzon arc and the fore-arc basin northward toward the Taiwan orogen. Moreover, serpentinized peridotite is present above the plate boundary and is distributed more widely and thickly closer to offshore southern Taiwan orogen. We suggest that low plate coupling may facilitate the uplifting of serpentinized mantle material up to the plate boundary. [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]

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]