Analysis of the 2020 Haenam, Korea, earthquake sequence
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Geosciences Journal
GJ
Analysis of the 2020 Haenam, Korea, earthquake sequence Dong-Hoon Sheen* Department of Geological Environment, Faculty of Earth Systems and Environmental Sciences, Chonnam National University, Gwangju 61186, Republic of Korea
ABSTRACT: From 26 April to 8 May 2020, an earthquake sequence occurred in the Haenam region in southwestern Korea. The main shock with a moment magnitude of 3.2 occurred on 3 May. It was preceded by 38 foreshocks that occurred over a week, with magnitudes ranging from 0.8 to 2.7, and was followed for another week by 35 aftershocks with magnitudes ranging from 0.5 to 2.4. The P-wave first-motion indicates that the focal mechanism of the mainshock was a strike of 193° or 102°, a dip of 83° or 78°, and a rake of –168° or –7°. The stress orientation of the focal mechanism is consistent with the regional tectonic stress field. Spectral analysis of the mainshock indicates that it occurred on a fault with a radius of 216 ± 4.58 m and a stress drop of 31.4 ± 2.35 bar. The hypocenters of the sequence, determined using the double-difference method, were shown to cluster along a trend striking 100° and dipping 65° to the south at a depth of 20.1 ± 0.05 km, which indicates that the earthquakes in the sequence occurred within a rupture radius of the mainshock. Detailed seismic analysis of the sequence indicates that closely located subfaults or small weak zones were related to the mainshock. Key words: focal mechanism, Haenam earthquake sequence, hypocenter, stress drop Manuscript received June 7, 2020; Manuscript accepted August 24, 2020
1. INTRODUCTION The 2020 Haenam earthquake sequence occurred over a twoweek period, from 26 April to 8 May 2020, at the southwestern tip of the Korean Peninsula. The Korea Meteorological Administration (KMA), which is responsible for earthquake monitoring in South Korea, observed 74 earthquakes with local magnitudes (ML) from 0.9 to 3.1 in the epicentral region. The main shock occurred on 3 May. It was preceded by 38 foreshocks that occurred over a week and was followed for another week by 35 aftershocks (Fig. 1). After two weeks of a quiescence period, a ML 1.4 earthquake occurred on 23 May. KMA has cataloged earthquakes greater than or equal to ML 2.0 since 1978 and has documented microearthquakes smaller than ML 2.0 since 2009 (Fig. 2). Although the period of instrumental earthquake monitoring is short compared to the characteristic *Corresponding author: Dong-Hoon Sheen Department of Geological Environment, Faculty of Earth Systems and Environmental Sciences, Chonnam National University, Gwangju 61186, Republic of Korea Tel: +82-62-530-3454, Fax: +82-62-530-3459, E-mail: [email protected]
©The Association of Korean Geoscience Societies and Springer 2020
earthquake cycle, the earthquake catalog indicates that seismicity in the epicentral region is low, and not even a microearthquake had been observed for a decade within a 10-km radius from the epicenter. However, three earthquakes among the foreshocks larger than ML 2.0 – greater than the t
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