Determining small earthquake focal mechanisms using 360° <i>S</i> -wave polarization: insights from dense seismic arrays

SUMMARY Determining earthquake focal mechanisms is a fundamental task in seismology, essential for understanding the fault structures and stress states in faulting regions. We present a new method for determining focal mechanisms of small earthquakes using 360° S-wave polarization alongside traditional P-wave polarity and S/P amplitude ratio. Ideally, measuring accurate 360° S-wave polarizations at near-source stations allows for a full recovery of the double-couple radiation patterns of direct body waves. By employing a process involving P–SV–SH transformation and correction for S-wave splitting, we show that S-wave polarizations for events with magnitudes less than 3 can be measured with average errors smaller than 7°. Our statistical analyses indicate that reliable focal mechanism solutions can be obtained with as few as two to three near-source stations. The method is particularly effective for strike-slip earthquakes, as their highly variable S-wave polarization patterns provide stronger constraints. We applied this method to the ML 2.8 and 2.9 sequences located in the centre of a dense seismic array in southeastern Korea, successfully determining focal mechanisms for events with magnitudes ranging from 2.9 down to −0.4. While the ML 2.8 sequence events display almost identical focal mechanisms along the main fault, those in the ML 2.9 sequence show variable mechanisms associated with off-fault microseismicity. We further validated the approach using the 2023 Mw 4.3 Parkfield and 2011 Mw 5.8 Mineral earthquake sequences, representing different tectonic settings. Despite using only 2–4 S-wave polarization measurements in Parkfield and 1–3 in Mineral, incorporating S-wave polarization significantly improved the accuracy of focal mechanisms in both cases. This research demonstrates that 360° S-wave polarization allows for robust determination of focal mechanisms in small earthquakes and offers a valuable tool for analysing microseismic activity.