Abstract

The 2016 Gyeongju earthquakes, including the ML 5.8 earthquake of 12 September 2016 (the largest earthquake recorded since instrumental recording began in South Korea in 1905), occurred around the Yangsan fault system, which is the most prominent set of lineaments on the Korean Peninsula. We analyzed the earthquake sequence to better understand the seismicity around this fault system. The moment tensor solutions of the eight major events (Mw>3) involved right‐lateral strike‐slip mechanisms with east‐northeast–west‐southwest compression, coincident with the tectonic stresses of southeastern Korea. Hypocenters, relocated by the double‐difference technique and dipping east‐southeast at 70° with depths of 11 and 16 km, followed the north‐northeast–south‐southwest strike of the moment tensor solution. The observed lineation in the hypocenter cluster, inclined gently toward the Yangsan fault, indicated the presence of branches developing from the fault. From the relocation results, we determined the expansion of the hypocentral zone of the Gyeongju earthquake sequence. The aftershock zone, with a radius of 2  km, was very near the rupture area of the mainshock as derived by spectral analysis. In addition, three groups of waveforms classified by hierarchical clustering using waveform similarity were identified as representing three zones on the hypocenter cluster, indicating that the fault plane has several zones locked in slightly different conditions.

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