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

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. Key points • The 2018 Hualien mainshock ruptured southward on two fault segments • Weak but fast initiation (3.0 km s-1) in the main west-dipping segment • Slow (2.0 km s-1) yet significant slip on shallow east-dipping segment

Several recent studies suggest that the Tatun Volcano Group (TVG) in the Taipei metropolis of Taiwan is still active with a mappable magma chamber beneath it. Here we report new seismic evidence from dense seismic arrays in northern Taiwan to refute the presence of a massive magma chamber. We investigated two near Taipei earthquakes with focal depths of ~ 140 km. We found that all the waveforms exhibited distinct S waves even when they traversed across the previously postulated magma chamber. Instead, the S-wave shadows found in the previous study may result from seismic waves traveling through a magma diapir above the subducting Philippine Sea Plate offshore northern Taiwan. Moreover, we found the P-wave delay increased with hypocentral distance when the seismic waves propagated through the footwall (west side) of the Shanchiao fault, regardless of whether they traversed across the postulated magma chamber. Our study results also indicate no abnormal attenuation when seismic rays traversed across the postulated magma chamber. Furthermore, the average Q P / Q S ratio around the TVG is less than 1, which implies that scattering attenuation is dominant. We conclude that a highly fractured rock body is beneath the TVG with a tiny fraction of magma instead of a massive magma chamber. Without sufficient magma supply, the TVG may stay dormant (except for small phreatic eruptions).

This study investigates the three-dimensional seismogenic structures associated with the 2025 M6.4 Dapu earthquake in western Taiwan and compares these results with the relocated instrumental earthquake catalog. Utilizing a deep-learning-empowered earthquake cataloging method, 6,805 seismic events were identified from January 20 to 31, 2025, achieving a completeness magnitude ( M c ) of 0.4. Notably, the M6.4 earthquake sequence involved the reactivation of a potentially existing fault system, characterized by an east-dipping plane at 27 ∘ between 5 and 10 km in depth and a west-dipping plane at 37 ∘ between 10 and 15 km depth. The distribution of aftershocks and triggered seismicity reveals a complex pattern of strain accumulation that influences nearby active faults. These findings underscore the necessity of continuous and detailed monitoring to better understand and potentially mitigate future seismic hazards in this region. Key Points This study catalogs 6,805 earthquake events with a completeness magnitude of 0.4 A potential existing east-dipping plane at 27 ∘ and west-dipping plane at 37 ∘ are identified The three-dimensional seismogenic structure of the 2025 M6.4 Dapu earthquake sequence is analyzed.

This study conducts annual microseismic surveys along major active faults and their surrounding areas in the southwestern Taiwan. Most fault activity in this region occurs within the upper crust at depths of approximately 10–15 km. According to previous studies, the study area is located in the area known as the Chishan Transform Fault Zone (CTFZ) which exhibits left-lateral strike-slip faulting in the northwest-southeast direction. Based on data of three recent moderate earthquakes in this area, the spatial distribution of aftershocks is more or less parallel to the CTFZ, but the overall trend has no correlation with previously mapped faults. Therefore, the primary goal of this study is to establish a microseismic monitoring network and collect seismic data to obtain a three-dimensional velocity model that can represent the seismogenic zone. Therefore, this study analyzes the distribution of micro-earthquakes and focal mechanisms to determine the tectonic characteristics of several active faults in southwestern Taiwan. The correlation between seismicity, local tectonic stress and the velocity structure is used to define the complete contours of the seismogenic structures. The results are integrated to explore crustal deformation and seismic structures along the frontal orogenic belt in southwestern Taiwan. The results also provide insights for earthquake hazard assessment.

Eastern Taiwan overlies a suture zone between the Eurasian Plate and the Philippine Sea Plate and is characterized by frequent earthquakes, often resulting in significant disasters. Notably, the region exhibits characteristics such as a high frequency of earthquakes and a short recurrence period for intense seismic events. While prior research has explored seismic b values across various periods in Taiwan, detailed investigations of the b value in the eastern region are lacking. This study employs the earthquake catalog compiled by the Taiwan Central Weather Administration to analyze spatial–temporal variations in b values in eastern Taiwan. The analysis encompasses seismic events occurring between January 1996 and June 2019. The seismic catalog is divided into three distinct time periods related to large seismic events: period I, 1996–2003 (the Chengkung earthquake); period II, 2003–2013 (the Ruisui earthquake); and period III, 2013–2019 (the Hualien earthquake). Our results indicate that most seismic events with a magnitude greater than 6 are associated with low b values. The overall b value increases during period II and then decreases substantially during period III. Although the estimated b values changed slightly, but the uncertainty in b values remained stable in this study. The epicenters of large earthquakes often overlap with areas with lower b values, especially in plate suture zones, which means that areas with lower b values usually have a higher probability of larger earthquakes. Given the extremely high potential for a catastrophic earthquake, mitigating measures should be adopted at all times. Graphical Abstract

This study analyzes seismic activity related to the 2025 ML 6.4 Dapu earthquake sequence in Chiayi County and Tainan City, Taiwan. By integrating a machine learning-based earthquake catalog with the Weighted Template Matching Algorithm (WTMA), over 40,000 microseismic events were detected, many of which were previously undetected due to waveform overlap with other seismic events. These microseismic detections enhance the understanding and interpretation of detailed aftershock distributions. Additionally, Centroid Moment Tensor (CMT) solutions were analyzed for further insights into the underlying seismogenic structures. In particular, the mainshock region exhibits complex structural features characterized by both east-dipping detachment faults and reactivated west-dipping basement faults occurring at varying depths. These findings highlight the intricate structural dynamics within Taiwan's actively deforming orogenic belt. The results also suggest that interactions within the fault system triggered progressive seismic activity, gradually propagating to adjacent areas. Such insights are critical for refining seismic hazard assessments and contribute to enhanced understanding of regional tectonic processes. Key points WTMA provides a more complete earthquake catalog, refining the spatial distribution of seismicity. The Dapu earthquake sequence reveals both east-dipping faults and reactivated west-dipping basement faults contributing to seismic activity. The delayed activation of seismicity in adjacent regions suggests complex post-seismic fault interactions.

We conducted a comprehensive seismotectonic study of the 2025 Dapu earthquake sequence in Taiwan, using densely distributed temporary seismic stations in a total number of 39, at the epicentral area. The M L 6.4 mainshock occurred in the foothills of southwestern Taiwan, an area characterized by a series of imbricate folds and thrust faults. Based on the relocated hypocenters, focal mechanisms, and stress inversions analyses, we identified two distinct aftershock distributions: one aligned with westward-dipping faults and another exhibiting an eastward-dipping distribution dominated by thrust faulting. Our finding suggests that the rupture process of the Dapu earthquake sequence can be explained by a conjugate fault rupture model and a triggering mechanism that accounts for the observed spatial characteristics. This interpretation is supported by detailed focal mechanism solutions that indicates complex fault interactions during the sequence. By evaluating the distribution of strong earthquakes in the areas surrounding this seismogenic zone, this study provides insights for refining regional seismic hazard assessment, ultimately contributing to improved pre-disaster planning and post-event response efforts. Highlights The seismogenic structure of the 2025 Dapu earthquake. Detecting seismic events from dense array. Complex fault system in the highly fractured zone.

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

Most earthquakes are considered to be caused by stress accumulating in and subsequently releasing from the crust. To extract non-linear and non-stationary earthquake-induced signals associated with stress accumulation, the Hilbert–Huang transform was utilized to filter long-term movements, short-term noise, and frequency-dependent (annual and semi-annual) variations from surface displacements measured by the global positioning system (GPS) in Taiwan. Earthquake-related surface displacements were expressed as horizontal directions (i.e., GPS azimuths) using the north–south and east–west components of residual GPS data to bypass influences resulted from the inhomogeneous nature of the crust. Analytical results showed that the relationships between earthquake occurrence and the aligned GPS azimuth passed the statistical test of the Molchan’s error diagram. Aligned GPS azimuths were in agreement with direction of earthquake-related P axes for 81% (26/32) studied events. Areas with the highest paralleling orientations of GPS azimuths appeared around epicenters several days to weeks before earthquake occurrence. Durations from aligned GPS azimuths to earthquake occurrence are roughly proportional to earthquake magnitude. Similar variations of the GPS azimuths were observed in GPS data containing or excluding co-seismic dislocation (i.e., one day before) in the temporal and spatial domain. These suggest that the aligned GPS azimuth could be a promising anomalous phenomenon for studying crustal deformation before earthquakes.