Gravity and magnetic field interpretation to detect deep buried paleobasinal fault lines contributing to intraplate earthquakes: a case study from Pohang Basin, SE Korea

Abstract

SUMMARY Potential field interpretation is a powerful method to locate deep buried tectonic fault lines that contribute to intraplate earthquakes. A magnitude 5.4 earthquake (2017, M5.4_PO) occurred in the Middle-Miocene Pohang Basin (PB), SE of the Korean Peninsula on 15 November 2017, in the area where no fault lines appear on geological and tectonic maps. To constrain fault locations, we calculate the gravity effect of the current basin fill with a gravity stripping method and used curvature analysis to identify former geological and tectonic structures, assumed formed in the Early-Miocene. The Early-Miocene PB is divided into two subregions (northern- and southern sub-basins) by a modelled NW–SE fault line, similar to the other Early-Miocene basins (e.g. Eoil basin). Fault line trends are NE–SW in the northern sub-basin, and NNE–SSW in the southern sub-basin. 2017M5.4_PO arose from a tectonic movement along the eastern boundary of the northern sub-basin, the cross-over area from the isolated high-magnetic/low-gravity region to low-magnetic/high-gravity region. The largest aftershock of the 2017M5.4_PO occurred along the NW–SE fault line bordering the northern- and southern sub-basin.