Recent work shows erosion reduces vertical normal stress, triggering thrust earthquakes. Whether deposition will inhibit earthquake triggering, or favor different types of earthquakes is not clear. Here we address this question using earthquake catalogs, earthquake focal mechanisms, and a point-load stress transfer model to study induced seismicity at the Tsaoling impoundment after the 1999 Mw7.6 Chi-Chi earthquake. This impoundment formed when a river was blocked by the mainshock-induced landslide. Temporally, we found a marked increase and decrease in earthquake activities in this location at the beginning and end of this impoundment life cycle. Spatially, the number of earthquakes decreased away from the impoundment in a pattern correlated with surface point load models based on both the Boussinesq and Westergaard equations. Using first motions and SH to P amplitude ratios, we derived focal mechanisms, and found the point source on surface weakens the strike-slip and normal fault planes mostly occurred during our study period. Some of the Chi-Chi aftershocks not explained by dislocation stress transfer studies can be explained by such load-induced seismicity. Previous studies show earthquakes can affect surficial geological processes; this study suggests that surficial processes can create positive loading that feedbacks and triggers seismicity.
