The Characterization of Microearthquakes in Offshore Area of Northeastern South Korea from Multidisciplinary Data Set

Abstract:

A number of small earthquakes in offshore Gangneung-Sokcho-Goseong area of northeastern South Korea has been identified and the hypocenters of these events are relocated by double-difference algorithm using multidisciplinary data set. Korea Institute of Geoscience and Mineral Resources (KIGAM) lists 845 events for the period Jan. 2010 – Jun. 2015 with local magnitude up to 3.3. Peer review shows that the offshore events can be grouped into a number of small clusters, whereas onshore events are relatively few and unevenly distributed. It appears to have little relationship between the location of offshore event clusters and the direction of main onshore faults running NNE – SSW. In this study, we focus on the small events occurred in the area of N37°45´-38°35´ and E128°30´ – 129°20´. The Korea Integrated Seismic System (KISS) network operated by KIGAM and 5 temporary stations with Guralp CMG-40T short-period seismometers placed along the east coast of South Korea record the seismic energy generated from small events. The 137 events recorded from May to October 2014, ranging in local magnitude from 0.4 to 2.3, are already relocated using 1849 direct P-wave traveltimes identified from 29 stations. Relocation produced two major clusters in offshore Goseong (cluster 1) and Yangyang (cluster 2) province. The waveforms of relocated events in cluster 1 and 2 are similar to those from explosion in waters rather than earthquake, and spectrum analysis shows energy concentration on low frequencies below 10 Hz. We are combining the multidisciplinary data set from land based broadband and short-period seismometers, Guralp OBS system deployed in July this year, accelerometer and hydrophone array mounted on the offshore platforms of the underwater seismic observatory in the north of the study area. The integration of seismic and hydro-acoustic data in offshore supplements uneven azimuthal coverage and gives improved characterization of offshore seismic activity.