Focal mechanism of the November 9, 2023, Mw 4.1  earthquake in northern Thailand determined  by full-waveform inversion

K. Saetang; W. Srisawat

doi:10.24028/gj.v47i6.333550

Authors

K. Saetang Nakhon Si Thammarat Rajabhat University, Nakhon Si Thammarat, Thailand, Thailand
W. Srisawat Муніципальна школа Ваттапхо, Накхонсітхаммарат, Таїланд , Thailand

DOI:

https://doi.org/10.24028/gj.v47i6.333550

Keywords:

focal mechanism, full-waveform inversion, minimal 1D velocity model, northern Thailand, seismotectonics

Abstract

A Mw 4.1 earthquake occurred in northern Thailand (19.498°N, 98.518°E) on 09 November 2023 (centroid time: 07:30:19.86 UTC), within the seismically active Shan-Thai terrane. Source parameter determination employed full-waveform inversion utilizing three-component seismic data from ten regional stations at distances of 73—108 km, operated by the Thai Meteorological Department’s seismic monitoring network, the Department of Mineral Resources network of Thailand, and the Myanmar National Seismic Network. The inversion methodology utilized the ISOLA software package, implementing iterative deconvolution based on the six-element moment tensor framework, and incorporated a minimal 1D velocity model with station corrections specifically calibrated for northern Thailand. Data preprocessing included instrumental response removal, baseline corrections, and band-pass filtering. Green’s functions were computed using the frequency-wave number integration technique within a 0.03—0.10 Hz frequency band. Moment tensor inversion yielded a predominantly strike-slip mechanism with two nodal planes: plane 1 (strike=1°, dip=59°, rake=–148°) and plane 2 (strike=253°, dip=63°, rake=–35°) with 75 % double-couple and –1.2 % CLVD components. Grid search analysis systematically explored source depths from 0—35 km and determined an optimal centroid depth of 2.5 km, with maximum correlation coefficients exceeding 0.4 within the 2—4 km depth range. Waveform analysis demonstrates variance reduction values ranging from 0.40 to 0.80 at near-regional stations (Δ<250 km), indicating robust source parameter determination. The focal mechanism and seismicity distribution indicate strain release along an NE-SW trending structure, located 17 km west of the N-S trending Wiang Haeng fault system.

References

Bouchon, M. (1981). A simple method to calculate Green’s functions for elastic layered media. Bulletin of the Seismological Society of America, 71(4), 959—971. https://doi.org/10.1785/BSSA0710040959.

Earthquake catalog for Thailand and adjacent regions (in Thai). (2024). Thai Meteorological Department. Retrieved from https://earthquake.tmd.go.th/inside.html.

Geological map of Thailand. (2007). Department of Mineral Resources. Retrieved from https://shop.geospatial.com/publication/24M1N7DW45KJ4E257BFCC4DR57/Thailand-1-to-250000-Scale-Geological-Maps.

Havskov, J., & Ottemöller, L. (1999). SEISAN earthquake analysis software. Seismological Research Letters, 70(5), 532—534. https://doi.org/10.1785/gssrl.70.5.532.

Kikuchi, M., & Kanamori, H. (1991). Inversion of complex body waves: III. Bulletin of the Seismological Society of America, 81(6), 2335—2350. https://doi.org/10.1785/BSSA0810062335.

Lienert, B.R., Berg, E., & Frazer, L.N. (1986). HYPOCENTER: An earthquake location method using centered, scaled, and adaptively damped least squares. Bulletin of the Seismological Society of America, 76(3), 771—783. https://doi.org/10.1785/BSSA0760030771.

Lienert, B.R., & Havskov, J. (1995). A computer program for locating earthquakes both locally and globally. Seismological Research Letters, 66(5), 26—36. https://doi.org/10.1785/gssrl.66.5.26.

Morley, C.K., Charusiri, P., & Watkinson, I.M. (2011). Structural geology of Thailand during the Cenozoic. In M.F. Ridd, A.J. Barber, M.J. Crow (Eds.), The geology of Thailand (pp. 237—334). Geological Society of London. https://doi.org/10.1144/GOTH.11.

Noisagool, S., Boonchaisuk, S., Pornsopin, P., & Siripunvaraporn, W. (2014). Thailand’s crustal properties from teleseismic receiver function studies. Tectonophysics, 632, 64—75. https://doi.org/10.1016/j.tecto.2014.06.014.

Saetang, K. (2017). Focal mechanisms of Mw 6.3 aftershocks from waveform inversions, Phayao Fault Zone, northern Thailand. International Journal of Geophysics. https://doi.org/ 10.1155/2017/9059825.

Saetang, K. (2022). Two-layer model of anisotropy beneath Myanmar and Thailand revealed by shear-wave splitting. Annals of Geophysics, 65(6), 1—13. https://doi.org/10.4401/ag-8769.

Saetang, K., & Duerrast, H. (2023). A minimum 1-D velocity model of northern Thailand. Journal of Seismology, 27, 493—504. https://doi.org/10.1007/s10950-023-10148-6.

Saetang, K., Srisawat, W., & Duerrast, H. (2018). Crustal structures, geothermal sources, and pathways beneath northern Thailand revealed by local earthquake tomography. Chiang Mai Journal of Science, 45(1), 565—575.

Simons, W.J.F., Socquet, A., Vigny, C., Ambrosius, B.A.C., Haji Abu, S., Promthong, C., Subarya, C., Sarsito, D.A., Matheussen, S., Morgan, P., & Spakman, W. (2007). A decade of GPS in Southeast Asia: Resolving Sundaland motion and boundaries. Journal of Geophysical Research: Solid Earth, 112, B06420. https://doi.org/10.1029/2005JB003868.

Sokos, E.N., & Zahradník, J. (2008). ISOLA: A Fortran code and a MATLAB GUI to perform multiple-point source inversion of seismic data. Computers & Geosciences, 34(8), 967—977. https://doi.org/10.1016/j.cageo.2007.07.005.

Seismic monitoring stations in Thailand (in Thai). (2024). Thai Meteorological Department. Retrieved from https://earthquake.tmd.go.th/stations.html.

Seismic station network (in Thai). (2024). Department of Mineral Resources. Retrieved from https://www.dmr.go.th/.

Tingay, M., Morley, C., King, R., Hillis, R., Coblentz, D., & Hall, R. (2010). Present-day stress field of Southeast Asia. Tectonophysics, 482(1—4), 92—104. https://doi.org/10.1016/j.tecto.2009.06.019.

Zahradník, J., & Plešinger, A. (2005). Long-period pulses in broadband records of near earthquakes. Bulletin of the Seismological Society of America, 95(5), 1928—1939. https://doi.org/10. 1785/0120040210.