Earthquake properties and fault plane solutions in the Seoul metropolitan area

Abstract:

Numerous historical records have warned of seismic damages in the Korean Peninsula that belongs to a stable intraplate regime with a low frequency of earthquakes and long recurrence interval. The 2011 M_L9.0 Tohoku-Oki megathrust earthquake affected the seismicity of the Korean Peninsula to increase. Public concerns on the earthquakes in the Korean Peninsula have risen greatly after the 2016 M_L5.8 Gyeongju earthquake and the 2017 M_L5.4 Pohang earthquake. A Quateranary fault, Chugaryeong fault, runs across the Seoul metropolitan area, leaving about half of South Korean population exposed to the seismic hazard potentials. However, studies on the fault structure and seismic activity of subsurface hidden faults in the Seoul metropolitan area have been made limitedly. In this study, we analyze the fault plane solutions of earthquakes with magnitudes up to M_L3.8, in 2009-2020. The fault plane solutions present the geometry of fault structure and motion senses. We determine the fault plane solutions based on the initial motion polarity of P and S waves, and S/P amplitude ratios. We detect micro to small earthquakes using densely-deployed seismic stations in the Seoul metropolitan area. The event detection is made by assessing the seismic energy at the expected P and S arrival times for assumed locations and origin times. Most fault plane solutions present nodal planes in NNE-SSW and N-S, which are nearly subparallel to the fault strikes observed on the surface. The seismicity and fault structure observation suggests possible presence of active subsurface faults. The study may provide useful information on the faults in the region for seismic hazard mitigation and seismic design.