Hydrologically-induced crustal stress changes and their association with seismicity rates in Taiwan

Studying crustal stress changes associated with hydrological cycles and their influence on seismicity rate illuminates the complex interplay between crustal stress conditions, faulting orientations, and earthquake nucleation. By analyzing GNSS position time series in 2006–2021 across Taiwan, we reveal a prevailing NW-SE trending seasonal contraction and expansion of the Earth's crust in response to hydrological loading and unloading in SW Taiwan, consistent with the maximum annual water storage change inferred from hydrological data. Inspection of seismicity rates in SW Taiwan indicates a positive correlation between excess seismicity rate and reduced NW-SE compression and/or decreasing vertical loading. Though hydrologically-induced contraction aligns with the tectonic compressive stress axis in the wet season, this alignment does not lead to more frequent earthquakes during peak water storage. Instead, seismicity peaks during the dry months, coinciding with maximum uplift and water unloading. This suggests that hydrologically-induced vertical stress or pressure changes play the dominant role in triggering earthquakes, as evidenced by vertical stress amplitudes 2∼4 times greater than the horizontal stress changes. The statistical correlation evaluating the timing of earthquakes and hydrologically-induced stress changes further affirms that the seismicity rate increases with reduced tectonic compression and enhanced vertical unloading in SW Taiwan. The observed relationship also implies the earthquake nucleation time is comparable to the stressing period of annual water cycles in SW Taiwan. Hydrologically-triggered earthquakes appear to be more sensitive to pressure variations than to shear stress changes, similar to tidally-modulated seismicity.