Depth-dependent seismicity and crustal heterogeneity in South Korea

Abstract

Depth-dependent seismicity has been suggested in the seismogenic layer, which is divided into upper and lower layers by its seismicity boundary. Since large earthquakes tend to occur in the lower layer, the structure of the seismogenic layer, which includes the seismicity boundary, provides important information for seismic hazard assessment. Using 107 land-based earthquakes, the structure of the seismogenic layer in South Korea was studied with focal depth determination by identifying the minimum residuals between the observations and several models. The same procedure was done for the depth relocation of 138 events associated with the Gyeongju earthquake (ML 5.8), the largest event in South Korea in the last two centuries. The geothermal temperature at the focal depth was also estimated for all earthquakes, including the Gyeongju earthquake cluster and the Pohang earthquake (ML 5.4), which caused more damage than did the Gyeongju earthquake. As observed in a large-scale averaging of mainland China, a 12-km seismicity boundary was suggested due to the frequent seismicity in the upper layer. At 12-km depth at the Gyeongju earthquake site, the geothermal temperature reached 300 °C, and the occurrence of aftershocks was limited below that depth. Based on the rigorously estimated depth, we separated the scattering attenuation, Qs−1, by a multiple lapse time window analysis using individual earthquakes and found higher and lower values for events in the upper and lower layers, respectively. This depth-dependent observation suggests differences in heterogeneity between the upper and lower seismogenic layers and supports the hypothesis that the seismicity of a depth-dependent seismogenic layer is related to cracks, which diminish with depth. Our results suggest that the upper limit of the focal depth can be predicted from the depth of the seismicity boundary for normal large earthquakes.