Possible earth-tide modulations of early aftershocks in southern Taiwan

Earth-tide stresses often have been considered to be incapable of directly triggering large earthquakes, but they do play a role in earthquake modulation. Despite its relatively minor role, tidal stress analysis is valuable for understanding earthquake generation. In this study, we used the matched-filter technique to detect approximately three times more early aftershocks (EAs) following the 2010 M_L 6.4 Jiashian and 2012 M_L 6.4 Wutai mainshocks than are listed in the Central Weather Bureau catalog in southern Taiwan. We examined the influence of tidal stresses on the occurrence of EAs and identified that small EAs frequently occurred near or at negative Coulomb and shear stress changes induced by Earth tide. Statistical tests indicated a very low likelihood for the EAs to be randomly distributed across a single day. It is likely that Earth-tide stresses can modulate these small EA occurrences. We proposed that tidal stresses can affect fluid diffusion and pore pressure of crustal cracks around the mainshock source regions when the regions are under a critically stressed circumstance. Our results were inconclusive regarding a causal relationship between tidal stresses and EA occurrences in the first few hours after the mainshocks.