Seismicity of microearthquakes around Gyeongju, Korea

Abstract:

The Korean Peninsula which is located in an intraplate area is known as seismically inactive region. However there were unusual several offshore earthquakes occurred around the southwestern Korean Peninsula in 2013. In addition, a small earthquake (Ml 3.5) occurred on 23 September 2014 around Gyeongju close to a nuclear power plant and many people felt a shock of the earthquake. Concerns over the safety of nuclear power plants were rising and therefore seismicity in this region was investigated. There were 61 earthquakes with local magnitude (Ml) greater than 1.7 occurred around Gyeongju since 1994 by KIGAM (Korea Institute of Geoscience and Mineral Resources) catalog. These events were relocated and calculated focal mechanisms. Also, I analyzed using continuous waveform data within 7 days before and after the mainshock recorded at HDB station with the epicentral distance of about 9 km, to find smaller events of the 2014 Gyeongju earthquake. 84 foreshocks and 108 aftershocks were identified and I located events with a minimum of 5 phases (4 P and 1 S). According to the relocated results, I may classify seismicity roughly into two groups. One group is aligned along right-hand side of the Ulsan Fault and the other is located near by the Bomunho. 13 quakes occurred from 1999 to 2003 close by artificial lake called Bomunho created in the late 1970s may be triggered microearthquakes by artificial water reservoirs but further research is needed to find causes of the events.