Characteristics of Earthquake swarm around the Hyodong-ri, Southeastern part of Korea

A study region is located about 15 km east of the Gyeongju Earthquake(M 5.8) in 2016 and 35 km south of the 2017 Pohang Earthquake(M 5.4). According to the earthquake catalogue of KIGAM, a total of 204 small earthquakes have been intensively occurring since 2000 in the vicinity of Hyodong-ri, Gyeongju. There are 168 earthquakes with a magnitude of 1.0 or greater. The largest earthquake is the 3.9 earthquake that occurred on September 23, 2014. In this study, the seismic source location and magnitude are reanalyzed by the KEMS ARS (Analyst Review System) and it is relocated using the double-difference algorithm for a total of 24 earthquakes(Ml>2.0) in the area within the last 5 years. Earthquake swarm has occurred continuously from 2000 to 2019, in present. and It can be classified into 3 groups according to the earthquake occurrence time and area. Group A has a linear arrangement of about 9 km with an earthquake till 2011 which its located in around Yeonil tectonic line area. Group B is mainly located in the Waup basin which it distributes in the northern part of study area and its occurs in 2014 - 2015. Group C is located on the southern part an earthquake that occurred after 2016 and this area located between the Yeonil tectonic line and the Seokup fault. Its the hypocentral depth distribution estimates from 2 to 14 km and becomes deeper toward the east. Almost events occur in the southern part of Yeonil tectonic line and Waup basin region where 6 earthquakes occurred between Sep. and Dec. 2014. It occurring in the Waup basin is concentrated not only in the time of occurrence but also in the area and its focal mechanism measure thrust fault. In the Yeonil tectonic line, we analyzed strike-slip fault and main stress direction is East-West or ENE-WSW direction. This area is characterized by a very complex geological structure, relatively close to a large earthquakes occurrence area, and a large number of small earthquakes occurring over a long period of time.