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[S01-P-23] Mw 5.5 Gyeongju Earthquake of 12 September 2016 in Southeastern Korea: SCR Earthquake Sequence with Moderate Stress Drop
On 12 September 2016 a Mw 5.5 earthquake occurred about 10 km South-Southwest of the ancient capital town of Gyeongju on the southeastern Korean peninsula. The mainshock was preceded by a Mw 5.0 foreshock by about 50 minutes and was followed by over 580 earthquakes with magnitude greater than 1.5 within the first 5 months. The 2016 Gyeongju earthquake is the largest instrumentally recorded earthquake on Korean peninsula. Accurate relocation of the earthquake sequence revealed that earthquakes are aligned along NNE – SSW trending, steeply dipping fault plane (dip=74º) with depth range from 13 to 17 km. Focal mechanism of the foreshock, mainshock and the largest aftershock, vertical strike-slip fault at depth, suggests that the event represents reactivation of buried high-angle faults in the Precambrian basement by the contemporary E–ENE trending regional horizontal compressive stress. Strike of the fault plane(s) of the 2016 Gyeongju earthquake sequence is consistent with the clearly developed NNE–SSW trending lineaments in southeastern Korea, which were formed at the end of Tertiary during an orogeny involving opening of East Sea of Korea (Japan Sea) to the east. The mainshock shows a static stress drop of ~10 MPa, which is somewhat lower than other similar earthquakes around the area. The Mw 4.6 Odaesan earthquake in 2007 that occurred about 200 km north of the Gyeongju sequence yielded a stress drop of 20 MPa. Low stress drops of a few MPa are reported for the earthquakes in the Yellow Sea region about 400 km west of the Gyeongju sequence. We compare stress drops of earthquakes in southern Korea with those of other SCR regions such as Virginia and Oklahoma, USA.