High-resolution analysis on seismic source characteristics of earthquakes near Pohang enhanced geothermal system

Disturbance of the stress field by fluid injection into geological formations stimulates rock volumes and occasionally generates earthquakes over broad magnitude ranges. On 15 November 2017, an earthquake of M_W 5.5 occurred near an enhanced geothermal system (EGS) in the Pohang, South Korea. It was reported as the largest one among the events near EGS site and has been suspected to have anthropogenic origin since the last hydraulic stimulation experiment at the site were performed just two months ago. Here we conducted in-depth seismological study on the source characteristics of earthquakes in the Pohang area until the M_W 5.5 earthquake occurred, and compared the results with a series of hydraulic activities at the site. The 239 earthquakes near the site identified from a matched filter analysis are spatially and temporally correlated with five times of alternating hydraulic stimulations at two geothermal wells. Among the events relocated by double-difference technique, seismicity related to one of the geothermal wells seems to migrate to southwest direction by 1km on an approximating plane, leading to the location of the M_W 5.5 event and its immediate foreshocks. The estimated fault geometry is matched with the focal mechanisms of earthquake on the plane. The planar structure crosses the stimulated wellbore at a depth of ~3,800 m at which high-density drilling mud was lost and the breakage of the well casing were observed by an image logging device. Despite the observation that the M_W 5.5 earthquake is located on the plane with other induced events in hydraulic stimulation periods, however, the injected volume and hydraulic energy are far less than theoretical estimations if the M_W 5.5 earthquake was induced by injected fluid. Linking a series of seismic and hydraulic activities at the EGS site, we concluded that the rupture of the M_W 5.5 Pohang earthquake initiated on a critically stressed fault system related to the fluid injection and released a large amount of accumulated tectonic strain.