Source parameters of the intraplate earthquakes in the offshore Ulsan, the Korean Peninsula

An earthquake of ML 5.0 occurred 52 km offshore from Ulsan, Korea on July 5, 2016. It was followed by two aftershocks greater than magnitude two within the source region of the mainshock. It is the fifth-largest earthquake occurred in around the Korean Peninsula, an intraplate low-to-moderate seismicity area, and also the largest in the southeastern offshore of the peninsula since the commencement of instrumental recording in Korea in 1978. The area including this earthquake sequence shows slightly higher seismicity compared to the rest of the peninsula, and earthquake with magnitude greater than three has persistently occurred over once a year during the last 10 years. This study aims to understand any linkage between the seismicity and geological structure at the area. Although the area is in the offshore, major geological structures including the folded zone and thrust faults are well known from the previous investigations. A total of 66 events in the offshore Ulsan from 2005 to 2016 was analyzed. All of the events are in shallow depth and are classified into the intraplate earthquake. Source parameters of the events using P- and S-wave arrival times were estimated. The hypocenter depth was constrained by an iterative optimal solution technique which is proven to be effective under the poorly known structure. Focal mechanism solutions were obtained from the analysis of P-wave first-motion polarity, and additionally, whenever possible, informations on SH-wave polarity and SH/P amplitude ratio were complemented to constrain the numbers of solution further and to detour the limitation of coverage due to the one-sided onshore seismograph network. Carefully determined locations of the earthquake and their focal mechanisms were compared with the existing subsea investigation results. The comparison gives information on how the present-day seismicity is related to the subsurface faulting in the intraplate region regardless of the offset traces exposed on the sea bottom.