Diurnal and Semidiurnal P- and S-Wave Velocity Changes Measured Using an Airgun Source

Monitoring the subtle seismic velocity changes provides a promising tool to study the Earth's dynamic processes. However, the precise monitoring of seismic velocity with high temporal resolution is still challenging especially at large distances. Here, we continuously monitor P-wave and S-wave velocities with a precision of up to 10(-5) using an airgun source. During a 1-week experiment conducted in the Yunnan Province, Southwestern China, unambiguous diurnal and semidiurnal velocity changes with amplitudes of 10(-4)-10(-3) were observed for both P- and S-waves at distances up to 21 km. The amplitudes of P-wave velocity (Vp) change are 1.2 to 3.8 times the amplitudes of S-wave velocity (Vs) changes. The velocity variation decreases with the increase in distance and is larger in the basin region than that in the mountainous region. Seismic velocity change shows less correlation with volumetric tide strain but strong correlation with changes of air temperature and air pressure. We propose that the thermal strains from air temperature changes are the primary cause of the observed diurnal and semidiurnal seismic velocity changes. The stronger Vp than Vs change is explained by the higher Vp sensitivity to the saturation rate. The velocity changes in mountainous area are ascribed mainly to the crack density changes, while both crack density change and saturation change are required to account for the velocity changes in the basin.