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Delineation of physical factors that contribute to earthquake triggering is a challenging issue in seismology. We analyze hydrological modulation of seismicity in Taiwan using groundwater level data and GNSS time series. In western Taiwan, the seismicity rate reaches peak levels in February to April and drops to its lowest values in July to September, exhibiting a direct correlation with annual water unloading. The elastic hydrological load cycle may be the primary driving mechanism for the observed synchronized modulation of earthquakes, as also evidenced by deep earthquakes in eastern Taiwan. However, shallow earthquakes in eastern Taiwan (

Earthquake early warning systems assess the size of an earthquake based on initial ground motion and provide warnings before the arrival of large waves. We employ the iterative regression to simultaneously estimate the station correlations and the corresponding linear relations for magnitude. We report the standard error reductions of the magnitude estimation based on the initial peak P-wave acceleration (${P_a}$ P a ), velocity (${P_v}$ P v ), and displacement (${P_d}$ P d ) as $14.52\% $ 14.52 % , $7.63\% $ 7.63 % , and $7.58\% $ 7.58 % , respectively. Factor analysis of the stationary correction reveals its correlation with several factors. This study shows that station correction can help to improve the precision of magnitude estimation in the future. [ABSTRACT FROM AUTHOR]

Early warning systems can estimate the possible-shaking intensity of the earthquake based on the initial P-waves on-site. Researchers have used the following two indicators: the initial peak P-wave displacement ( P d ) and the ground motion characteristic period ( τ c ). In this study, we re-examine the performance of the two indicators by employing ground motion data from the P-Alert low-cost network in Taiwan. We have found that the signal-to-noise ratio of displacement ( S N R d ) is effective for data selection. In addition, we have observed the irregular behavior of τ c in the P-Alert dataset. [ABSTRACT FROM AUTHOR]

Recent studies in the earthquake magnitude‐frequency relation (b‐value) found that the b‐value can decrease with time before large earthquakes in the source area. Such a decrease even coincided with a burst of foreshocks immediately before the earthquakes implying a preslip migrating toward the nucleation zone. We systematically survey the b‐value variations in space and time in the source area of 17 earthquakes with magnitudes greater than ML 6.0 in Taiwan. We relocated the earthquakes released by the Central Weather Administration of Taiwan since 2012 as timing can provide a sufficiently low completeness magnitude (MC) of 1.5. We carefully determine the spatiotemporal search criteria on seismicity in the source area of all targeted earthquakes. We surveyed the temporal b‐value systematically using moving time windows with a given number of earthquake events with the magnitudes greater than MC. We found the b‐value decreased clearly for 2 days before the 2018 ML 6.3 Hualien earthquake, coinciding with a burst of ML 5.8 earthquake and its aftershocks nearby as foreshocks. The foreshocks migrated updip toward the hypocenter of the 2018 ML 6.3 Hualien earthquake and revealed a second decrease in the b‐value 10 hours before it occurred. The b‐value precursor primarily comes from a rapid reduction of small earthquakes in the foreshocks and agrees with recent key findings of preslip. However, there is no strong correlation between the b‐value precursor and coseismic slip. Apart from this, the b‐values did not change significantly in most cases of earthquakes in Taiwan before and after they occurred. Plain Language Summary: Direct evidence for the precursor of a large earthquake is rare and difficult to be quantified accurately. Earthquake magnitude‐frequency relation (b‐value) is a classic observation implying stress accumulation on a seismogenic fault. Scientists found that the b‐value can decrease within a few days before an impending large earthquake in the source area. Taiwan is one of the regions undergoing frequent M ≥ 6.0 earthquakes and can map the b‐value in high resolution based on the relocated earthquakes. This study systematically surveyed the b‐value in the source area of seventeen ML ≥ 6.0 earthquakes in Taiwan, probing into b‐value precursor in the 8‐year time series. We found that the b‐value had a robust decrease 2 days before the 2018 ML 6.3 Hualien earthquake, coinciding with the initiation of foreshocks nearby and at a downdip of the hypocenter of the 2018 ML 6.3 Hualien earthquake. The foreshocks showed clear evidence of precursor as they migrated updip toward the hypocenter with a second decrease in the b‐value 10 hours before the earthquake occurred. We found no evidence of a b‐value precursor in most of the targeted earthquakes that agrees with the knowledge of precursor as happening episodically and unexpectedly. Key Points: We found a b‐value precursor that coincides with foreshocks for 2 days before the 2018 ML 6.3 Hualien earthquake in the source areaThe b‐value precursor correlates with foreshock distribution in the source area and is likely uncorrelated to the high coseismic slipThere is no significant change in the b‐value before M ≥ 6.0 earthquakes in Taiwan in a majority of the cases that we analyzed [ABSTRACT FROM AUTHOR]