A new study has identified a strong correlation between changes in Taiwan's seismicity rate and its seasonal water cycle fluctuations, suggesting that many faults in this region are so stressed that even minor shifts in strain caused by changes in groundwater storage can trigger earthquakes. Ya-Ju Hsu and colleagues observed that western Taiwan's seismicity rate reaches its highest levels between February and April, when the crust rebounds as stress from the groundwater load decreases, and its seismicity reaches its lowest levels between July and September, at the tail end of monsoon season. However, the researchers also observed that shallow earthquakes in eastern Taiwan show the opposite correlation as the loading from stored water is relieved, a phenomenon that the authors suggest might be explained by another yet to be determined, time-dependent process. While previous research has shown that seismicity may fluctuate seasonally due to the shifting presence of water in areas of tectonic activity, it has remained challenging to isolate how various physical factors contribute to triggering earthquakes. To advance this research, Hsu et al. measured the relationship between seasonal fluctuations in the water cycle and earthquakes in western and eastern Taiwan, which they selected due to its frequent, damaging earthquakes and its wide fluctuations in precipitation and groundwater storage due to the monsoons and typhoons that hit the island from May to September. The researchers analyzed seismicity data from 2002 to 2018, continuous groundwater measurements from 40 monitoring stations during the same period, and Global Navigation Satellite System (GNSS) time-series data on seasonal water storage variation. They found that shallow earthquakes occurred more frequently in late winter and early spring in western Taiwan, when water loads exert less stress on the crust, while a more complex pattern emerged in eastern Taiwan, with the peak seismicity rate occurring in either winter or summer. Additionally, Hsu et al. studied the seasons between 1604 and 2018, during which 63 earthquake events of magnitude 6 or greater occurred, revealing historic trends in peak seismicity similar to those observed in the present day. These findings offer insights that may help improve regional hazard assessments, the authors say.
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