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Determining Small Earthquake Focal Mechanisms Using 360° S-wave Polarization

Description: 

Accurate earthquake focal mechanisms are essential for providing insights into subsurface faulting geometry and stress states. Conventionally, P-wave first-motion polarities and/or S-/P-wave amplitude ratios are used to determine the focal mechanisms; nonetheless, their accuracy, particularly for small (M<3) earthquakes, heavily depends on the station coverage. We introduce a novel approach for utilizing 360-degree S-wave polarizations to determine the focal mechanisms. Using data from the Gyeongju Hi-Density Broadband Seismic Network (GHBSN) in the southeastern Korean Peninsula, we assess the performance of the focal mechanism estimation with three different datasets: P-wave first-motion polarities, S/P amplitude ratios, and S-wave polarizations, incorporating different levels of measurement errors. The results indicate that utilizing the S-wave polarizations with the correction of S-wave splitting effectively lowers the minimum number of stations needed. For an M_L 2.9 event recorded by the GHBSN, with a 6-degree measurement error in S-wave polarization, the solution could be obtained using only two stations, with an average error of 12 degrees. With a 12-degree measurement error in S-wave polarization for smaller (M_L~1.0) events, an average error of 15 degrees in the focal mechanism solution is achieved for strike-slip events using three stations and for dip-slip events using four stations. A performance comparison for earthquakes with magnitudes between 0 and 3 using the HASH program, the most widely used program in the seismological community, shows that our method provides more accurate solutions as the earthquake magnitudes decrease. These findings highlight the robustness of our method in determining focal mechanisms for small earthquakes, even with fewer stations, providing a valuable tool for analyzing microseismicity.

Session: Advances in Reliable Earthquake Source Parameter Estimation - II

Type: Oral

Date: 4/16/2025

Presentation Time: 11:15 AM (local time)

Presenting Author: Sangwoo

Student Presenter: Yes

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