Earthquake properties and source mechanisms of micro to small earthquakes in Seoul

Numerous historical records present seismic damages in the Korean Peninsula that belongs to a stable intraplate region with a low frequency of earthquakes and long recurrence interval. The 2011 MW9.0 Tohoku-Oki megathrust earthquake affected the seismicity in Korean Peninsula. Recent devastating earthquakes including the 2016 ML5.8 Gyeongju earthquake and the 2017 ML5.4 Pohang earthquake raised public concerns greatly on possible seismic damages in the future. Quaternary faults including Chugaryeong fault, Wangsukcheon fault, and Pocheon fault run across the Seoul metropolitan area, leaving about half of Korean population exposed to seismic hazard potentials. In particular, the fault planes are hardly exposed on the surface, causing difficulty in seismic hazard assessment. However, studies on the fault structures and seismic activity of subsurface faults have been limitedly conducted. Temporal seismic stations were deployed densely in the Seoul metropolitan area since September 2018, allowing us to study the seismicity and subsurface structures in the area. We search micro to small earthquakes in Seoul by assessing the seismic evengy at the expected P and S arrival times for assumed locations and origin times. The seismic energy is assessed using a STA/LTA algorithm that is based on signal-to-noise average amplitude ratio. We detected 48 small-size earthquakes in Seoul in January 2019 to April 2021 using seismic records from 44 stations. We analyze the focal mechanism solutions of 10 earthquakes with magnitudes of ML2.5 in Seoul since 2004 using the seismic phase polarities and S/P amplitude ratios. The focal mechanism solutions represent the geometry of fault structure and motion senses. The dominant strikes of nodal planes range between N20E and N45E, which are subparallel to the fault traces on the surface. The seismicity and fault structures suggest possible activation of subsurface faults. This study may provide basic information on the faults and seismicity in the region for seismic hazard mitigation and seismic design.