The 14 December 2021 Mw 4.9 Offshore Jeju Island, Korea, Earthquake: Seismological Observation of an Intraplate Earthquake Provides Insight into Regional Seismotectonics

Abstract

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.

Resolving Multi‐Stage Rupture Process of the 2021 Mw 4.9 Offshore Jeju Island Earthquake From Relative Source Time Functions

“…We employed the EGF method to retrieve the RSTFs at stations situated at local distances (∆ < 100 km) (Figure 1a). The largest aftershock of M w 2.9 is the best EGF event among aftershocks, and its P-wave first motions at common stations with mainshock are all consistent, indicating a common focal mechanism (Kim et al, 2022). P wave on the vertical and S wave on the transverse records at nine stations, mostly broadband, on the island were filtered (0.8-8 Hz) and windowed 0.5 s before and 3 s after the phase arrivals.…”

“…where τ i (θ ij ) is the apparent time of the ith candidate location of subevent at the jth station measured on RSTFs, θ ij is the angle between the ray takeoff direction of the jth station and the ith candidate location vector from the EGF (Figure S2a in Supporting Information S1), ∆t i is the delay of the subevent origin time, D i is the distance of the ith candidate location from the EGF, and V P,S is the P-or S-wave velocities at the source depth, respectively. We used a 32 km thick two-layer crust with a 2 km thick surface low-velocity layer to calculate the ray takeoff angle at the source and for event location (Kim et al, 2022).…”

“…The locations were grid‐searched on the fault plane using least‐squares minimization of the apparent time of each subevent, using the following equation: $$${\tau }_{i}\left({\theta }_{ij}\right)={\increment}{t}_{i}-\frac{{D}_{i}\,\cos \left({\theta }_{ij}\right)}{{V}_{P,S}}$$$ where τ i ( θ ij ) is the apparent time of the i th candidate location of subevent at the j th station measured on RSTFs, θ ij is the angle between the ray takeoff direction of the j th station and the i th candidate location vector from the EGF (Figure S2a in Supporting Information ), ∆ t i is the delay of the subevent origin time, D i is the distance of the i th candidate location from the EGF, and V P , S is the P ‐ or S ‐wave velocities at the source depth, respectively. We used a 32 km thick two‐layer crust with a 2 km thick surface low‐velocity layer to calculate the ray takeoff angle at the source and for event location (Kim et al., 2022).…”

“…Horizontal and vertical bars indicate their uncertainties (25% misfit) along strike and dip directions. Aftershocks (black circle) and foreshocks (brown circle) are shown (Kim et al., 2022). The maximum beam power obtained from the back‐projection of local relative source time functions is shown in the background.…”

“…The P and S waves from the earthquake show complex waveforms that exhibit two distinct large source pulses with a time separation of ∼0.8 s. These source pulses are preceded by two small signals that are clearly discernible on local records (Figure 1c and Figure S1 in Supporting Information ), which can be nucleation phases that initiated the mainshock rupture (e.g., Ellsworth & Beroza, 1995, 1998). The earthquake was a vertical strike‐slip faulting at a depth of ∼12 km, and the fault strikes east‐west (275°) (Kim et al., 2022). The earthquake occurred in an intraplate region with low seismicity with the occurrence of small earthquakes (usually M < 4), and hence, it provides a rare window to understand the seismogenesis of moderate‐sized earthquakes in the vast continental shelf region of northeastern Asia.…”