Investigation of microearthquakes and fault structures around the Seoul metropolitan area based on local and regional seismic arrays

The Seoul metropolitan area is the most populated county in the Korean Peninsula that is located in the eastern margin of the Eurasian plate. The instrumentally-recorded seismicity in the area is relatively low. Historical seismic damage records suggest possible presence of active faults in the region, raising public concern on seismic hazard potentials. It is crucial to identify the active faults for proper seismic hazard mitigation, which has been unsuccessful due to the difficulty in seismicity detection in high ambient noises. We search earthquakes based on dense local and regional seismic arrays that were recently deployed in and around the Seoul metropolitan area since 2018. The seismic network is composed of 60 stations equipped with three-component broadband seismometers. The inter-station distances are < 6 km in Seoul, and < 20 km in the surburb regions. The sampling rates of the seismometers are 200 Hz. A matched-filter technique is applied to find microearthquakes. The cross-correlation functions between the master event signals and continuous waveforms are calculated after bandpass filtering. The cross-correlation functions are stacked over multiple stations and components. Waveforms with high correlation coefficients are declared to be earthquake signals after manual checks. The hypocenters of earthquakes are precisely determined using a double-difference earthquake location algorithm (hypoDD) and template-matching-based grid search technique (match and locate method), which may suggest the fault structures. The observations may be useful for the assessment of potential seismic hazards in the Seoul metropolitan area.