A minimum 1-D velocity model of Northern Thailand

Standard seismological practices use a 1-D velocity model to calculate and determine earthquake hypocenters. For Northern Thailand, a minimum 1-D velocity model with station delays by applying the VELEST code is first presented here, which can be applied for earthquake location determinations as well as an initial model for 3-D seismic tomography studies. Altogether 614 P- and 689 S-wave travel time data from 145 events were manually picked from earthquake waveforms recorded by 13 seismic stations operated under the Thai Meteorological Department (TMD) from October 2009 through March 2021. A set of five velocity models with 5-km-layer thicknesses down to 40 km depth were tested with earthquake locations to obtain the best-fit velocity models. Results provided minimum travel-time differences between observed and calculated P and S first arrival times. After 13–20 iterations, a reduction of RMS (root-mean-square) values of the travel time residuals approaching a final minimum was observed. The vertical distribution of the hypocenters indicates that seismicity is concentrated in the upper 20 km depth range below northern Thailand. Only few events are found at deeper levels. The 1-D velocity model has slightly lower velocity values than the global velocity model (ak135 and iasp91). Station delays of P- and S-waves are in the range of −0.8 s and +0.7 s, indicating laterally varying geology correlating with near-surface geology. Positive delay times are related to softer sedimentary rocks and sediments, and negative delay times to igneous rock outcrops.