A Study on the Focal Depth of the 2017 Pohang Earthquake using Moment Tensor Inversion

The 2017 Pohang earthquake (M 5.4) is the most devastating earthquake in South Korea since modern seismometers were installed. One of the reasons why the Pohang earthquake caused much damage is its shallow focal depth. The focal depth of the Pohang earthquake is 4.5 ~ 10 km according to recent studies. It is necessary to quantitatively measure the exact focal depth of the Pohang earthquake because it is closely related to whether the Pohang earthquake is induced by the nearby enhanced geothermal system (EGS) site or not. Kim et al. (2017) estimated the focal depth of the Pohang earthquake to be 4.5 km using travel times, which is similar depth to the depth of water injection beneath the nearby EGS site. On the other hand, Chung et al. (2018) proposed that the focal depth is 7 km based on travel time inversion and geothermal interpretation. In order to obtain a robust measurement of the focal depth with a quantitative uncertainty, we applied moment tensor inversion method to waveforms from the Pohang earthquake with 6 different crustal velocity models. Our results show stable estimations of 6 km depth regardless of any crustal models if we use shallow velocity structure from borehole logging, which is a little far from the water injection depth. However, the CLVD content reaches about 50 %, which may be related to fluid effects from the water injection.