Shear wave velocity model by c1 and c2 combination using ambient seismic noise beneath Northern Thailand

Abstract

The ambient seismic noises (ASNs) in northern Thailand were used as the diffuse wavefield to reconstruct the empirical Green’s function (EGF) by cross-correlation between pair stations C1which is assumedly compatible with the impulse response of the Earth. However, in the case of inadequately synchronized station pair, the crosscorrelation of cross-correlation function (C2) is applied to enhance the quality of EGF by stacking C1 and C2. The objective of this study is to create a 1-D shear wave velocity and depth model for investigating the crustal structure beneath northern Thailand. The data were recorded from late 2019 to late 2020 from 22 stations of TM network operated by the Thai Meteorological Department (TMD), TH network operated by the Department of Mineral Resources (DMR), and IU network operated by the Incorporated Research Institutions for Seismology Network (IRIS). The vertical component was used for interstation ASNs cross-correlation to extract the Rayleigh waveform. Regarding EGFs under stationary phase assumption, the surface waveform can be significantly seen, and dispersion curves were extracted by the phase-matched filtering frequency-time analysis (phase-matched filtering FTAN) method with quality control based on signal to noise ratio criteria. The final dispersion curves from each station pairs are employed for estimating the group velocity tomography maps for each period. After that, the selected dispersion curves along the grid locations are inverted to obtain the 1-D S-wave velocity depth models and then combined for generating S-wave velocity and depth maps. The inverted models consist of 3 structural boundaries: sediment basin-basement rock, upper-lower crust, and crust-upper mantle boundaries. The solutions are presented as the S-wave velocity maps of upper and lower crust, and the boundary depth maps: 1) sediment basin-basement rock boundary, 2) upper-lower crust boundary (Conrad discontinuity), and 3) crust-mantle boundary (Mohorovičić discontinuity). The results indicate that the depth of sediment basin-basement rock boundary is deeper around the basins defined by previous studies which depth is up to 1.14 km at Chiang Mai Basin and around 0 at mountain ranges. The depth of upper-lower crust boundary is expected to be Conrad discontinuity depth beneath Northern Thailand and its shallowest depth is in the western province about 10.9 km, while its deepest depth locates beneath Phitsanulok Basin about 20.9 km, indicating the excessive sinking of this basin. The result of Mohorovičić discontinuity depth (Moho depth) are supporting that of previous studies which increase from the west (31.4 km) to the east (40.6 km) along with the tectonic blocks in Thailad: Sibumasu terrane (ST), Inthanon zone, Sukhothai terrane, and Indochina terrane (ICT).