First P wave arrival-time data from local earthquakes recorded by a dense geophone array deployed on the Ilan Plain and by existing permanent stations were combined to invert for high-resolution P wave velocity structures under northeast Taiwan. With relatively high resolution, we were able to examine the structures in more detail and to investigate their significance and tectonic implications. We introduce two distinct groups of proposals for mechanisms of subduction polarity flipping in Taiwan, referred to as the "tear model" and "breakoff model." While the predicted Philippine Sea Plate boundaries differ between the two models, those of the breakoff model and the related subducting indenter model are geodynamically similar. The surface junction and the west edge of the imaged high V p anomalous Philippine Sea Plate comply with those predicted by the subducting indenter model and thus favor the breakoff model over the tear model. While the observed high V p anomalous region in the mantle wedge can be explained as eclogitization of previously subducted crust, eclogitization of the overriding continental crustal roots cannot be ruled out. Those beneath the Taipei Basin and the Tatun Volcano Group exhibit a pattern potentially connected to the low V p anomalies in the mantle wedge, suggesting the involvement of the Philippine Sea slab, either by asthenospheric upwelling due to extensional collapse or by fluid migration due to slab dehydration. Those beneath the Ilan Plain exhibit a low V p pattern extending to deeper origins in the eastern offshore region, suggesting a connection with the opening of the Okinawa Trough.

In this study, we compiled the Central Weather Bureau (CWB) data in order to study the Gutenberg-Richter magnitude-frequency slopes (i.e., b-values) and seismicity rates of significant earthquake sequences in the area of Hualien. A total of ten events between 1973 and 2018 were selected for analysis. Using time windows 72 h before and after the main shock, we first examined the existence of detectable foreshocks and then applied the Gutenberg-Richter law and Omori's law to determine the b-value and seismicity rate, respectively. The compiled results were used to assess the abnormalities and other characteristics of the 2018 Hualien earthquake for their forecast potential. We concluded that seismicity rates alone are not sufficient to forecast whether a greater main shock is forthcoming. The foreshock sequence of the 2018 Hualien earthquake was characterized by a low b-value and a high seismicity rate. Another earthquake with a prominent foreshock sequence occurred in 1990, but it showed a different relationship between the magnitude and the seismicity rate. For both the 1990 and the 2018 Hualien earthquakes, we found that the b-values of the foreshocks were lower than those of the respective aftershocks. The b-values for earthquake sequences are depressed relative to the background seismicity in the area. The mechanisms proposed for temporal variation in b-values are briefly reviewed to explain the observed b-value patterns. Finally, we established an empirical relationship with moment magnitude (M w ) in order to estimate the spatial range of aftershock distributions in the area of Hualien for shallow earthquakes (hypocenter depth ≤ 20 km) with M w ≥ 5.3.

The 2017 M w 5.9 Batangas earthquake occurred on a location abutting the Macolod Corridor (MC) to the NE and above the steeply dipping slab of subducted South China Sea lithosphere to the SW. While the spatial distribution of the 2017 Batangas earthquake sequence provides a constraint on the scope of MC's geothermal activity, the focal mechanisms are associated with the states of stress in MC. Here, we relocated the 2017 Batangas sequence using data from two seismic network which cover the region of the NW Mindoro and Southern Luzon. Upon employing the HypoDD algorithm, we retrieve a compact distribution within the Batangas Bay, which in turn suggests that the Balayan Bay to the north remains aseismic and is within the scope of geothermally impact area by MC. In addition, the 2017 Batangas earthquake and the 1994 Mindoro Earthquake share similar focal mechanisms that exhibit an E-W extension in the region of SW Luzon, which differs from a few leftlateral moderate earthquakes to the east at 121.5°E on the Sibuyan Verde Passage Fault. Together with the results of 3D numerical modeling on coeval subduction of oceanic and continental lithospheres and P-wave tomography in the region from previous studies, we incline to explain the regional E-W extension in SW Luzon as induced by the toroidal asthenospheric flow and the mechanisms of MC formation as originated by the overriding-plate stretching. Both are incipiently driven by the collision of the Palawan Continental Block into the Philippine Mobile Belt and the subsequent detachment of the South China Sea slab beneath central Mindoro.