The Mw 4.9 Offshore Jeju Island, Korea, earthquake of 14 December 2021 is the most significant event that occurred close to the young intraplate volcano on the continental shelf south of the Korean Peninsula. About 185 small earthquakes occurred during nine days following the mainshock. We accurately located 39 events despite limited station azimuthal coverage. We found that the mainshock ruptured along a ∼1.4 km long fault striking east–west (275°) and triggered two distinct clusters in the north–northeast direction. The mainshock is pure strike-slip faulting on the vertical plane. Its horizontal P axis strikes 50°, which differs substantially from the average trend of ∼70° for earthquakes in the southern Korean Peninsula. However, it is consistent with the P-axes orientation of shallow earthquakes in the offshore western Kyushu and along Okinawa trough. The 2021 Offshore Jeju Island earthquake may be the first significant earthquake in southern Korea–East China Sea continental shelf to date, whose causative fault is well constrained by accurate aftershock locations and detailed space–time analysis of the earthquake sequence. This earthquake may represent intraplate deformation in response to the tectonic evolution of the Philippine Sea plate, which is manifested through volcanic activities.

Reliable information on the horizontal orientation of a seismometer is crucial to seismological research utilizing threecomponent seismograms. In this study, we provide misorientation angles of broadband seismometers in three permanent networks in South Korea from 2003 to 2021 by using two methods, denoted as P PCA and P minT , both utilizing P-wave polarization characteristics. Our estimates show that 36% of the sensors have been aligned within 5° from the geographic north during their operation periods, while 40% of the sensors have been rotated by more than 10° at least once. The estimates are highly consistent for both methods, with 95% of the total showing uncertainty less than 10°. Moreover, we identified a significant number of temporal changes in misorientation by taking automatic change point detection and visual inspection. The procedure described here and misorientation angles can be useful reference for seismic data preprocessing for research utilizing horizontal-component seismograms for earthquake sciences.

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.

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