1-D velocity model beneath Northern Thailand and its applications

Abstract

Northern Thailand lacks a region-specific 1-D velocity model, necessitating the use of global models that do not account for local crustal heterogeneities. Furthermore, station corrections for the regional seismic network have not been previously determined. A minimum 1-D velocity model for Northern Thailand, with specific application to focal mechanism determination in the Phayao Fault Zone. The minimum 1-D velocity model was derived using five different initial velocity models and local earthquake data recorded by 13 seismic stations from 2009 to 2021. Analysis of 614 P-wave and 689 S-wave travel times from 145 earthquakes reveals systematically increasing seismic velocities with depth, a consistent average Vp/Vs ratio of 1.70�0.07, and station delays ranging from -0.81 s to +0.64 s for P-waves and from -0.83 s to +0.67 s for S-waves that correlate strongly with near-surface geology. The vertical distribution of hypocenters indicates that seismicity is predominantly concentrated in the upper 20 km of the crust. Application of this locally-derived velocity model to the focal mechanism analyses of earthquakes in the Pan Segment of the Phayao Fault Zone, using waveform inversion techniques, reveals a predominance of right-lateral strike-slip motion. Notably, the locally-derived model demonstrates a significantly higher double-couple percentage (93.1%) compared to global models (61.2% for AK135, 50.7% for IASP91), establishing its superior suitability for seismotectonic studies in this specific geological setting. These results represent the first minimum 1-D velocity model and station corrections for Northern Thailand, providing improved constraints for earthquake location and focal mechanism determination in the region.

????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????? ?????????????????????????????????????????????????????????????????????????????????????????????????????????????????????? ????????????????????????????????????????????????????????? ???????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????? 13 ????????????? ?.?. 2552 ??? ?.?. 2564 (?.?. 2009-2021) ?????????????????????????????????????????????? 614 ????????????????????? (P-wave) ??? 689 ??????????????????????? (S-wave) ????????????????????? 145 ????????? ?????????????????????????????????????????????????????? ???????????????????????????????????????????????????????????? (Vp/Vs) ??????? 1.70�0.07 ????????????????????????????????????????? -0.81 ??? +0.64 ???????????????????????? ??? -0.83 ??? +0.67 ?????????????????????????? ??????????????????????????????????????????????????????? ?????????????????????????????????????????????????????????????????????????????????????????????????????????????????? 20 ??????????? ?????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????? ??????????????????????????????????????????????????????????????? (right-lateral strike-slip) ????????????????? ??????????????????????????????????????????????????????? (double-couple percentage) ?????? 93.1% ????????????????????????????????? (61.2% ?????? AK135 ??? 50.7% ?????? IASP91) ???????????????????????????????????????????????????????????????????????????????????????????????????? ?????????????????????????????????????????????????????????????????????????????????????????????????? ???????????????????????????????????????????????????????????????????????????????????????????