J. Korean Earth Sci. Soc., v. 39, no. 4, p. 342−348, August 2018 ISSN 1225-6692 (printed edition) https://doi.org/10.5467/JKESS.2018.39.4.342 ISSN 2287-4518 (electronic edition) Temporal and Spatial Variations of the ML 5.8 Gyeongju Earthquake on September 12, 2016 1 1, 2 Gyeong Su Lee , Jai Bok Kyung *, and Sang Jun Lee 1 Department of Earth Science Education, Korea National University of Education, Chungbuk 28173, Korea 2 School of Earth and Environmental Sciences, Seoul National University, Seoul 08826, Korea Abstract: An earthquake of ML 5.8 hit the Gyeongju area on September 12, 2016. A sequence of foreshock-mainshock- aftershock of 588 events with equal to or greater than magnitude 1.5 occurred for six months in this area. Around ninety- nine percentage (98.8%) of the total energy was released intensively within a day, and about 80% of the total events took place within a month after the Gyeongju earthquake. The epicentral distribution of aftershocks of major events (ML 5.1, o 5.8, 4.5, and 3.5) were elongated in the direction of N30 E. They correlate well with the focal mechanism solution. These facts support the inference that the Gyeongju earthquakes occurred on a sub-parallel subsidiary fault of the Yangsan fault zone or on the linking damage zones between Deokcheon and Yangsan fault. During the last six years before the Gyeongju earthquake, there were few events within 10-km radius from the epicenter. This seismic gap area was filled with a sequence of the Gyeongju earthquakes. The b value for aftershock of the Gyeongju earthquakes is 1.09. Keywords: Gyeongju earthquake, Yangsan fault, aftershocks, seismic gap, b value Introduction maximum MM intensity in the epicentral area was reported as VIII (Park et al., 2016) and instrumental A sequence moderate-sized earthquake of ML 5.8 intensity was also VIII (Hong et al., 2017). Most of occurred about 10 km south of Gyeongju city on the focal depths range from 10 to 16 km (Kim et al., September 12, 2016. It is the largest earthquake since 2016). the seismic network operated in South Korea in 1905. In the Gyeongju area, about ten destructive The earthquake was followed by numerous aftershocks earthquakes generated in historical time (Lee and of about 550 events from September 12, 2016 to Yang, 2006). Recently, there was an earthquake of ML March 11, 2017 around the epicentral area. 4.2 on June 26, 1997 (Kyung and Lee, 1998). The Recently the characteristics of Gyeongju earthquake prominent geological features in the epicentral area is was studied by several authors (eg., Kim et al., 2016; the NNE trending Yangsan fault system, a dominantly Kim et al., 2016; Hong et al., 2017; Chung and Iqbal, right-lateral strike-slip fault, which has been actived 2017). The main ML 5.8 event ruptured the fault plane during the late Quaternary (Kyung and Lee, 2006). o o o o with a strike of 115 (or 24 ), a dip of 86 (or 78 ), In this study we analyzed the spatial and temporal o o and a rake of 12 (or 176 ) (Kim et al., 2016). It occurrence of the sequence of Gyeongju earthquakes. produced strong ground shaking and large felt area covering most of the southern peninsula. The A Sequence of Gyeongju Earthquakes *Corresponding author: [email protected] *Tel: +82-43-230-3742 About 47 minutes before the Gyeongju earthquake *Fax: +82-43-232-7176 (ML 5.8) at 20:32:54 (KST), a medium-sized foreshock This is an Open-Access article distributed under the terms of the (ML 5.1) occurred followed by twenty-nine aftershocks. Creative Commons Attribution Non-Commercial License (http:// After the mainshock of ML 5.8, 557 aftershocks with creativecommons.org/licenses/by-nc/3.0) which permits unrestricted non-commercial use, distribution, and reproduction in any medium, local magnitude equal to or larger than 1.5 occurred provided the original work is properly cited. for six months. Figure 1 shows the epicentral Temporal and Spatial Variations of the ML 5.8 Gyeongju Earthquake on September 12, 2016 343 zone or on the linking damage zones between Deokcheon fault and Yangsan fault (Kim et al., 2017). Figure 2 shows the distribution of occurrence time for six months. The largest aftershock (ML 4.5) occurred one week after the mainshock on September 19. Temporal Distribution Figure 3 shows the daily frequency and cummulative frequency of Gyeongju earthquakes, respectively. Almost 46 and 59% of the total 588 events occurred Fig. 1. Epicentral distribution of 588 events observed by KMA network for six months (September 12, 2016~March within 2 days and 1 week after the start of Gyeongju 11, 2017). USF: Ulsan fault, YSF: Yangsan fault, MoRF: events sequence, respectively. About 80% of the total Moryang fault, MiRF: Miryang fault, and JIF: Jain fault. events occurred within a month. Although the occurrence decrease gradually after the mainshock, the number increase again after the largest distribution of the total events, provided by National aftershocks (ML 4.5) occur, and the frequency Earthquake Comprehensive Information System (NECIS), decreases steadly as time elapses. for six months from September 12. Figure 4 shows the daily and cummulative energy Most of the events occurred condensely within 5- release of these events, respecctively. The amount of km radius from the epicenter of mainshock. They released energy was calculated from Gutenberg and mainly occurred in between the Yangsan fault and Richter relation (1956) as Deokcheon fault, which extends parallel on the log E=11.8+1.3 M western side. The Gyeongju earthquakes was inferred to occur on a subsidiary fault of the Yangsan fault where E is energy and M is magnitude of earthquake. Fig. 2. Relation between magnitude and occurrence time of foreshocks-mainshock-aftershocks around Gyeongju for six months. 344 Gyeong Su Lee, Jai Bok Kyung, and Sang Jun Lee Fig. 3. Daily (left) and cumulative (right) frequency of earthquake occurrences. Fig. 4. Daily (left) and cumulative (right) energy release of earthquakes. Almost 98.8% of total energy was released within a The comparison between the latitudinal and day just after the start of Gyeongju events series. longitudinal distributions of aftershock events for 6 months (Figs. 7 and 8) show narrower distribution for Space-Time Distribution logitude than that for latitude after a period of earthquake occurrence. This indicates that the epicenters The spatial distributions of epicenters are shown are closely related to the latitudinal geological every one month interval in Fig. 5. Whereas the structures, such as sub-parallel subsidiary fault of the events occurred intensively and widely in the direction Yangsan fault zone. of NNE during the first month, the occurrence number During the last six years before the Gyeongju abruptly decrease with the passage of time. earthquakes, there were very few events within 10-km The spatial distributions are compared for events radius from the epicenter, which were mainly between foreshock and mainshock (Fig. 6a), one-day occupied by a sequence of Gyeonju earthquakes (Figs. events just after the mainshock (Fig. 6b), those after 9 and 10). This can be interpreted that the epicentral ML 4.5 (Fig. 6c) and those after ML 3.5 (Fig. 6d). region of Gyeongju earthquakes has been remained as Foreshock events and the one-day events just after a seismically gap area for more than 6 years. mainshock display elliptical shape in the direction of NNE. Temporal and Spatial Variations of the ML 5.8 Gyeongju Earthquake on September 12, 2016 345 Fig. 5. The aftershock activity every one month interval. The major and minor radius of ellipse are 5.7 and 4.9 km, respectively. b value The b value for aftershock events is 1.09. Finally, the aftershock of the Gyeongju earthquake Conclusion was analyzed by the Gutenberg-Richter's magnitude- frequency relation (1942) as The temporal and spatial variations of the Gyeongju earthquakes can be summarized as follows: Log N=a−b×M. 1. A sequence of foreshock-mainshock-aftershock of where N is the number of earthquakes greater than or 588 events occurred with equal to or greater than equal to a magnitude M, and the constants a and b magnitude 1.5 for six months in Gyeongju area. For value can be obtained by the maximum-likelihood the Gyeongju earthquakes series, there was about 80% method (Utsu, 1966). As shown in Fig. 11, the of the total events within a month and was 98.8% of relation for the aftershocks for six months yield the total energy release within a day. equation 2. The epicentral distribution of aftershocks of major events (ML 5.1, 5.8, 4.5, and 3.5) are elongated Log N=4.36−1.09×M o in the direction of N30 E. They correlate well with 346 Gyeong Su Lee, Jai Bok Kyung, and Sang Jun Lee Fig. 6. Epicentral distribution of earthquakes. (a) is events between foreshock and mainshock. (b), (c), and (d) are one-day events just after ML 5.8, ML 4.5, and ML 3.5, respectively. Fig. 7. Latitudinal distribution of epicenters for six months. Temporal and Spatial Variations of the ML 5.8 Gyeongju Earthquake on September 12, 2016 347 Fig. 8. Longitudinal distribution of epicenters for six months. Fig. 10. Relation between distance of events from epicen- ter of the Gyeongju earthquake and their occurrence time from January 1, 2011 to December 31, 2016. the focal mechanism solution studied by others. These facts support the inference that the Gyeongju earthquakes occurred on a sub-parallel subsidiary fault of the Yangsan fault zone or on the linking damage zones between Deokcheon fault and Yangsan fault.