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ISSN : 1225-6692(Print)
ISSN : 2287-4518(Online)
Journal of the Korean earth science society Vol.39 No.4 pp.342-348
DOI : https://doi.org/10.5467/JKESS.2018.39.4.342

Temporal and Spatial Variations of the ML 5.8 Gyeongju Earthquake on September 12, 2016

Gyeong Su Lee1, Jai Bok Kyung1*, Sang Jun Lee2
1Department of Earth Science Education, Korea National University of Education, Chungbuk 28173, Korea
2School of Earth and Environmental Sciences, Seoul National University, Seoul 08826, Korea
Corresponding author: jbkyung@knue.ac.kr Tel: +82-43-230-3742 Fax: +82-43-232-7176
August 1, 2018 August 21, 2018 August 24, 2018

Abstract


An earthquake of ML 5.8 hit the Gyeongju area on September 12, 2016. A sequence of foreshock-mainshockaftershock of 588 events with equal to or greater than magnitude 1.5 occurred for six months in this area. Around ninetynine 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, 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.



초록


    Introduction

    A sequence moderate-sized earthquake of ML 5.8 occurred about 10 km south of Gyeongju city on September 12, 2016. It is the largest earthquake since the seismic network operated in South Korea in 1905. The earthquake was followed by numerous aftershocks of about 550 events from September 12, 2016 to March 11, 2017 around the epicentral area.

    Recently the characteristics of Gyeongju earthquake was studied by several authors (eg., Kim et al., 2016; Kim et al., 2016; Hong et al., 2017; Chung and Iqbal, 2017). The main ML 5.8 event ruptured the fault plane with a strike of 115° (or 24°), a dip of 86° (or 78°), and a rake of 12 ° (or 176 ° ) (Kim et al., 2016). It produced strong ground shaking and large felt area covering most of the southern peninsula. The maximum MM intensity in the epicentral area was reported as VIII (Park et al., 2016) and instrumental intensity was also VIII (Hong et al., 2017). Most of the focal depths range from 10 to 16 km (Kim et al., 2016).

    In the Gyeongju area, about ten destructive earthquakes generated in historical time (Lee and Yang, 2006). Recently, there was an earthquake of ML 4.2 on June 26, 1997 (Kyung and Lee, 1998). The prominent geological features in the epicentral area is the NNE trending Yangsan fault system, a dominantly right-lateral strike-slip fault, which has been actived during the late Quaternary (Kyung and Lee, 2006).

    In this study we analyzed the spatial and temporal occurrence of the sequence of Gyeongju earthquakes.

    A Sequence of Gyeongju Earthquakes

    About 47 minutes before the Gyeongju earthquake (ML 5.8) at 20:32:54 (KST), a medium-sized foreshock (ML 5.1) occurred followed by twenty-nine aftershocks. After the mainshock of ML 5.8, 557 aftershocks with local magnitude equal to or larger than 1.5 occurred for six months. Figure 1 shows the epicentral distribution of the total events, provided by National Earthquake Comprehensive Information System (NECIS), for six months from September 12.

    Most of the events occurred condensely within 5- km radius from the epicenter of mainshock. They mainly occurred in between the Yangsan fault and Deokcheon fault, which extends parallel on the western side. The Gyeongju earthquakes was inferred to occur on a subsidiary fault of the Yangsan fault 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 within 2 days and 1 week after the start of Gyeongju events sequence, respectively. About 80% of the total events occurred within a month.

    Although the occurrence decrease gradually after the mainshock, the number increase again after the largest aftershocks (ML 4.5) occur, and the frequency decreases steadly as time elapses.

    Figure 4 shows the daily and cummulative energy release of these events, respecctively. The amount of released energy was calculated from Gutenberg and Richter relation (1956) as

       log E=11.8+1.3 M

    where E is energy and M is magnitude of earthquake.

    Almost 98.8% of total energy was released within a day just after the start of Gyeongju events series.

    Space-Time Distribution

    The spatial distributions of epicenters are shown every one month interval in Fig. 5. Whereas the events occurred intensively and widely in the direction of NNE during the first month, the occurrence number abruptly decrease with the passage of time.

    The spatial distributions are compared for events between foreshock and mainshock (Fig. 6a), one-day events just after the mainshock (Fig. 6b), those after ML 4.5 (Fig. 6c) and those after ML 3.5 (Fig. 6d). Foreshock events and the one-day events just after mainshock display elliptical shape in the direction of NNE.

    The comparison between the latitudinal and longitudinal distributions of aftershock events for 6 months (Figs. 7 and 8) show narrower distribution for logitude than that for latitude after a period of earthquake occurrence. This indicates that the epicenters are closely related to the latitudinal geological structures, such as sub-parallel subsidiary fault of the Yangsan fault zone.

    During the last six years before the Gyeongju earthquakes, there were very few events within 10-km radius from the epicenter, which were mainly occupied by a sequence of Gyeonju earthquakes (Figs. 9 and 10). This can be interpreted that the epicentral region of Gyeongju earthquakes has been remained as a seismically gap area for more than 6 years.

    b value

    Finally, the aftershock of the Gyeongju earthquake was analyzed by the Gutenberg-Richter's magnitudefrequency relation (1942) as

       Log N=ab×M.

    where N is the number of earthquakes greater than or equal to a magnitude M, and the constants a and b value can be obtained by the maximum-likelihood method (Utsu, 1966). As shown in Fig. 11, the relation for the aftershocks for six months yield equation

       Log N=4.36−1.09×M.

    The b value for aftershock events is 1.09.

    Conclusion

    The temporal and spatial variations of the Gyeongju earthquakes can be summarized as follows:

    1. A sequence of foreshock-mainshock-aftershock of 588 events occurred with equal to or greater than magnitude 1.5 for six months in Gyeongju area. For the Gyeongju earthquakes series, there was about 80% of the total events within a month and was 98.8% of the total energy release within a day.

    2. The epicentral distribution of aftershocks of major events (ML 5.1, 5.8, 4.5, and 3.5) are elongated in the direction of N30oE. They correlate well with 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.

    3. During the last six years before the Gyeongju earthquakes, there were few events within 10-km radius from the epicenter. In this seismic gap area, a sequence of the Gyeongju earthquakes densely occurred.

    4. In the Gutenberg-Richter relation, the b value for aftershock of the Gyeongju earthquakes is 1.09.

    Figure

    JKESS-39-342_F1.gif

    Epicentral distribution of 588 events observed by KMA network for six months (September 12, 2016~March 11, 2017). USF: Ulsan fault, YSF: Yangsan fault, MoRF: Moryang fault, MiRF: Miryang fault, and JIF: Jain fault.

    JKESS-39-342_F2.gif

    Relation between magnitude and occurrence time of foreshocks-mainshock-aftershocks around Gyeongju for six months.

    JKESS-39-342_F3.gif

    Daily (left) and cumulative (right) frequency of earthquake occurrences.

    JKESS-39-342_F4.gif

    Daily (left) and cumulative (right) energy release of earthquakes.

    JKESS-39-342_F5.gif

    The aftershock activity every one month interval. The major and minor radius of ellipse are 5.7 and 4.9 km, respectively.

    JKESS-39-342_F6.gif

    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.

    JKESS-39-342_F7.gif

    Latitudinal distribution of epicenters for six months.

    JKESS-39-342_F8.gif

    Longitudinal distribution of epicenters for six months.

    JKESS-39-342_F9.gif

    Epicentral distribution of events occurred during the last six years before Gyeongju earthquakes (upper) and those for Gyeongju earthquakes (lower).

    JKESS-39-342_F10.gif

    Relation between distance of events from epicenter of the Gyeongju earthquake and their occurrence time from January 1, 2011 to December 31, 2016.

    JKESS-39-342_F11.gif

    Magnitude versus logN (N: cumulative number) for the aftershocks of ML 5.8 in the sequence of the Gyeongju earthquake.

    Table

    Reference

    1. Chung, T.W. , and Iqbal, M.Z. , 2017, Hypocentral depth determination of Gyeongju earthquake aftershock sequence , Geophysics and Geophysical Exploration, 20,49-55.
    2. Gutenberg, B. , Richter, C.F. , 1942, Earthquake magnitude intensity, energy, and acceleration . Bulletin of the Seismological Society of America, 32, 163-191.
    3. Gutenberg, B. , and Richter, C.F. , 1956, Earthquake magnitude, intensity, energy, and acceleration (second paper) , Bulletin of the seismological society of America, 46(2), 105-145.
    4. Hong, T.K. , Lee, J.H. , Kim, W.H. , Hahm, I.K. , Woo, N.C. , and Park, S.J. , 2017, The 12 September 2016 ML 5.8 midcrustal earthquake in the Korean Peninsula and its seismic implications , Geophysical Research Letters, 44(7), 3131-3138.
    5. Kim, Y.H. , Rhie, J.K. , Kang, T.S. , Kim, K.H. , Kim, M.O. , and Lee, S.J. , 2016, The 12 September 2016 Gyeongju earthquake: 1. Observation and remaining questions , Geosciences Journal, 20(6), 747-752.
    6. Kim, Y.-S. , Kim, T. , Kyung, J.B. , Cho, C.S. , Choi, T.-H. , and Choi, C.U. , 2017, Preliminary study on rupture mechanism of the 9.12 Gyeongju earthquake, Journal ofthe Geological Society of Korea, 53, 407-422.
    7. Kyung, J.B. and Lee, H.U. , 1998, Intensity analysis of the 26 June 1997 Kyongju earthquake and its geological significance , The Journal of Engineering Geology, 8,13-24.
    8. Kyung, J.B. and Lee, K. , 2006, Active fault study of the Yangsan fault system and Ulsan fault system, southeastern part of the Korean Peninsula , Journal of the Korean Geophysical Society, 9, 219-230.
    9. Lee, K. , and Yang, W.S. , 2006, Historical Seismicity of Korea , Bulletin of the Seismological Society of America, 3, 846-855.
    10. Park, S. , Min, K. , Lee, L. , Lee, B.S. , Choi, M. , Jeong, S. , Han, S. M. , Park, E.H. , Park, J. , and Lee, J.H. , 2017, Intensity distribution of the September 12, 2016 earthquake, Proceedings of EESK Conference. Utsu, T., 1966, A statistical significance test of the difference in b-value between two earthquake groups , Journal of Physics of the Earth, 14, 37-40.