Epicenter Relocation of 2013 Boryeong Earthquake Sequence in Korea, Using Travel-Time Double- Differences and Lg-Wave Cross- Correlation

We have analyzed Boryeong earthquake sequence from the Yellow Sea of Korea, expecting to provide basic information on offshore seismogenic structures of western Korea. The double-difference technique with Lg-wave cross- correlation enabled our computation of accurate epicenters for small earthquakes. Offshore of the city of Boryeong, more than 150 earthquakes occurred from July through August 2013, which is abnormal seismicity for Korea. We relocate 149 epicenters displaying high waveform similarity using 11 stations with an azimuthal gap exceeding 130˚. Relocated epicenters form a 700-m line with clear northeastward progression during the period, and correspond well with the northeast-southwest strike of the fault-plane solution for the largest event (ML3.8). From the sequence, we classified two spatiotemporally separated clusters by Lg-coda waveform similarity: Cluster A with 107 events occurred during entire Boryeong sequence , and Cluster B with 25 events occurred only from 21 to 25 June 2013. We used a dendrogram derived from the maximum correlation coefficients representing the similarity of 149 events among Lg coda. Relocated epicenters tightly clustered, and the longest epicenter line shrank in size by a factor of several tens. Spatial distribution of the relocated epicenters reflected locations of clusters classified according to the similarity of waveforms for Lg coda. In the Boryeong sequence, relocated epicenters showed clear northeastward migration during a 2-month period.