Rupture directivity of the 2021 ML 2.2 Gwangyang, Korea, microearthquake; toward resolving high-resolution rupture process of a small earthquake

Moderate-to-large earthquakes (M>6) frequently show clear rupture directivity. Recent studies revealed that a substantial percentage of small-sized earthquakes (M< ∼5) display rupture directivity as well, owing to enhanced seismic monitoring. Is rupture directivity a common feature for earthquakes of all sizes? In this study, we investigated the rupture directivity of the 27 August 2021 ML 2.2 Gwangyang microearthquake that occurred at the southern tip of the Korean Peninsula by utilizing data from a recently expanded dense seismic network. The mainshock had two foreshocks and three aftershocks, which enabled us to define the fault plane from precise relative event locations and a well-constrained focal mechanism. Analysis of apparent source time functions obtained with the empirical Green's function technique reveals that the mainshock ruptured unilaterally toward the east. A detailed analysis of the source pulse leaving the focal sphere in 3D indicates that rupture directivity is consistent with the fault geometry and slip direction. Our study demonstrates that we can resolve rupture directivity of unilaterally rupturing microearthquakes (M< ∼3) given adequate azimuthal coverage, and that this will lead to an improved understanding of the seismogenic processes in regions of low seismicity worldwide.