Detecting Fluid at The Subsurface Using S-wave Disappearance

Disappearance of S waves generally occurs in the circumstances where seismic waves pass through crustal fluids. Such observation is very rare in the shallowest crust because the detection strongly depends on the location of earthquakes, high-quality tomography and the distribution of seismic networks. We focus on the Chingshui geothermal region, Taiwan, where the seismicity occurs at the shallow depths frequently and a temporary seismic network was established for obtaining a local tomographic image. Seismicity located within the seismic network is used for a regional double-difference relocation. To identify the S-wave disappearance for small local earthquakes, we examined the amplitude of S waves during 1 second before and after the arrival times in the both time and frequency domains. We defined "no S wave" type as the ratio of amplitude is lower than 4 and 5 in the time and frequency domain, respectively, and the "weak S wave" type is defined as one of the ratio of amplitude is over the threshold. To know the spatial distribution of the both types, we used pseudobending method to calculate their ray paths and then projected them on the cross sections of V_P/V_S ratio derived from local tomographic image. Our results show that the "no S wave" type is obvious while the ray paths were heading northeastward to a place northeast boundary of the Chingshui region. The ray paths are correlated to two high V_P/V_S ratio zones: one is located in the northeast of Chingshui region at the depths less than 2.5 km and another is located right beneath the center of Chingshui region at the depths from 5.0~5.5 km. Preliminarily, we did not find any correlation between S-wave disappearance and focal mechanism, earthquake numbers, magnitudes, and precipitation.