Development of a 1-D Velocity Model and Determination of Aftershock Hypocenters Associated with the 2017 Pohang Earthquake

It is necessary to develop a 1-D velocity model for determining the accurate hypocenters around the Enhanced Geothermal System (EGS) site in order to analyze the relationship between fluid injection in the EGS site and the M_W 5.5 Pohang earthquake on November 15, 2017. The boundary between layers is set up based on the change of the geological profile for a 1-D velocity model. Despite the lithological changes, two adjacent layers with the same physical properties were set to one identical layer in the velocity model. This velocity model is composed of two parts: the shallow part is divided into six layers which are consistent with the lithological changes of the geological profile and the deeper part is divided into two layers and a half-space below the Moho from the regional velocity model of the Gyeongsang basin. The layer velocities are determined by using the reflection survey, PS time differences for an event, rock properties, the difference of phase arrival time, the measurement of the least arrival-time errors, logging data, and the regional velocity model. Finally, the 1-D velocity model representing the EGS site was developed by combining six layers and a half-space. Immediately after the Pohang earthquake, a dense temporary broadband seismograph network consisting of 26 stations was established for the purpose of monitoring aftershocks. Hypocenters of the aftershock sequences were determined by using the 1-D velocity model. Initial cataloguing of the aftershocks was performed with the employment of an automatic algorithm for detection and location of an event. About 5,200 aftershocks were listed in the initial aftershock catalog from November 2017 to April 2019. Based on this initial catalog, all the events were reviewed with manual picking of seismic phases and relocation of hypocenters. The major distribution of aftershocks is overall consistent with the mainshock fault plane of the Pohang earthquake. However, they can be categorized into at least three groups which seem to be associated with the faulting geometries different from each other. This implies that the Pohang earthquake sequence has been occurring on the complex fault system of the region.