Assessment of controlling factors on seasonal variations of crustal seismic velocity in Taiwan using ambient noise single-station cross-component analysis

Ambient noise interferometry is an effective approach to studying temporal crustal behaviors through continuously measuring crustal seismic velocity changes (dv/v). However, the interpretation of such dv/v variations is not straightforward because both internal (tectonic/magmatic) processes of the crust and external (environmental) factors could affect dv/v simultaneously. The relationship between these potential factors and crustal dv/v is complicated and varies from place to place globally. Understanding the interplay between the internal and external processes in dv/v variations becomes an essential step toward accurate crustal monitoring. As an active orogenic belt in the subtropical zone, Taiwan is one of few places in the world that features strong interactions of both environmental and tectonic processes, with distinct dry and wet seasons and high seismicity rates. To investigate the temporal evolution of crustal seismic velocities across Taiwan, we apply the ambient noise single-station cross-component (SC) method analyzing continuous seismic data of 15 broadband stations from 1998 to 2019. We construct the daily SC functions and compute dv/v variations in a frequency band of 0.1 to 0.9 Hz. Our dv/v results reveal not only co-seismic velocity drops associated with regional moderate-to-large earthquakes but strong seasonal variations. Through a series of spectral and time-series analyses with the weather data, we suggest that the rainfall-induced pore-pressure change plays a predominant role in driving the dv/v seasonality. The effect of other factors is relatively local or secondary. We also further demonstrate how correcting such rainfall effects can improve the detection capability and accuracy of internal processes related to crustal damage caused by earthquakes.