[S-IT20] Deep Earth Sciences

[SIT20-03] Shear wave splitting analysis of Thai Seismic Array recordings: Implications for the mantle flow beneath Thailand

*Tarudee Autaijaratrasmee¹, Satoru Tanaka², Kenji Kawai¹, Sutthipong Noisagool³ (1.University of Tokyo, 2.Japan Agency for Marine-Earth Science and Technology, 3.Mahidol University)

Keywords:Shear wave splitting, Transverse minimization technique, Fast polarization orientations

Many seismological studies have been conducted in and around Thailand to better understand the crust and mantle deformed by the collision between the Indian and Eurasian Plates and its associated slab rollback. One of the issues is the sudden change in fast polarization orientations from south-north around Myanmar to nearly west-east around the Indochina Peninsula, south of China, as suggested by shear-wave splitting (SWS) studies. The study in Thailand may serve as a crucial piece of the puzzle in understanding the geodynamic implications of this transition, partly due to the insufficient SWS measurements in Thailand. We apply the transverse minimization technique to extensively investigate upper mantle anisotropy under Thailand, utilizing significant data mostly from the Thai Seismic Array (TSAR) stations. A total of 406 SWS measurements from 37 stations of SKS and SKKS seismic phases were obtained. Among these, 224 splitting results indicate the upper mantle anisotropy beneath Thailand. We take the average at each station, and the average fast orientation from the north (phi_N) representing the Thailand area, is 61.1° from the north in a clockwise direction with a standard deviation of 34.0°. The average time difference (delta_t) is 1.3 ± 0.4 seconds. The direction of fast orientation from the north (phi_N) is predominantly either northeast to southwest or southwest to northeast, which is roughly consistent with previous research. The observed fast orientations (phi) are likely related to the mantle flow direction, supported by three hypotheses. Firstly, this can be interpreted as the eastward drifting of Thailand due to the collision between the Indian and Eurasian Plates, resulting in mantle flow from southwest to northeast. Secondly, our results align with the subduction direction of the Indo-Australian Plate and Sumatra, where the mantle might flow from northeast to southwest. Thirdly, our findings also support that the mantle could flow from the Hainan plume to the east of Thailand and extend from northeast to southwest. Furthermore, we found that the no-splitting measurements are significant at five stations in northern Thailand, three in northeastern Thailand, and three in other areas of Thailand. These no-splitting results could be interpreted through three models: mantle-upwelling induced by slab rollback of the subduction between the Indo-Australian Plate and Sumatra, past volcanic activities, and the initiation of asthenospheric upwelling.