Normal Faulting and Lower Stresses at Shallow Depth Increase Foreshock Activity in Taiwan

Foreshocks in laboratory experiments are directly linked to fault stress states and the relative position within the seismic cycle. However, foreshock sequences in nature are generally difficult to identify and characterize due to overall lower seismicity rates and sparse instrumentation. Here, we analyze foreshock activity in one of the seismically most active parts of the world, that is, Taiwan, where frequent large-magnitude earthquakes lead to pronounced space-time clustering of seismicity. We examine foreshock activity associated with ML≥5 mainshocks from 2012 to 2022 and explore the effect of depth, background seismicity rates, and mainshock focal mechanisms. We apply a high-resolution statistical method, that is the nearest-neighbor clustering method, to identify foreshock–mainshock sequences. The percentage of mainshocks with foreshocks in Taiwan varies systematically with focal depth and mainshock mechanisms so that lower differential stress at shallower depth and on normal faults leads to a higher percentage of mainshocks with foreshocks. The spatial extent of the foreshock area correlates with mainshock magnitude; however, this areal extent also correlates with the largest event within the foreshock sequence. Overall, only ∼25% of mainshocks in Taiwan are preceded by at least one foreshock, which is significantly lower than in many other regions, including California and Italy. Our observations support a global trend of lower background rates correlating with higher foreshock percentages. Taken together, these results suggest that there are significant regional variations in the percentage of mainshocks with foreshocks both at the scale of Taiwanese faults and at the larger scale of different tectonic areas. Differential stress and background seismicity rates are primary drivers of foreshock activity, with foreshocks being most likely to occur on shallow normal faults in areas of low background seismicity.