Geosciences Journal pISSN 1226-4806 eISSN 1598-7477 Vol. 20, No. 6, p. 753  757, December 2016 DOI 10.1007/s12303-016-0034-9 ⓒ The Association of Korean Geoscience Societies and Springer 2016 LETTER The 12 September 2016 Gyeongju earthquakes: 2. Temporary seismic network for monitoring aftershocks ABSTRACT: The M L 5.8 earthquake in Gyeongju, southeastern Korea, on September 12, 2016 11:32:54 (UTC) was the largest earthquake on the Korean Peninsula since instrumental monitor- ing began in 1978. It was preceded by an M L 5.1 foreshock and is being followed by numerous aftershocks. Within an hour of the mainshock, the first temporary seismic station to monitor after- shocks was installed at about 1.5 km east of the announced epi- center. The current temporary seismic network consists of 27 stations equipped with broadband sensors covering an area of ~38  32 km in the mainshock region. This is the first high-density aftershock monitoring array in the Korean Peninsula. Initial results, using data from both the regional seismic networks and the after- shock monitoring array, indicate that earthquakes during the first 10 days following the mainshock are related to the Yangsan Fault System. Establishment of an official rapid-response team to mon- itor aftershocks of major earthquakes is advised. Key words: Gyeongju earthquake, aftershock monitoring network, Yangsan Fault System, rapid earthquake response 1. INTRODUCTION On September 12, 2016, at 11:32:54 UTC (20:32:54 Korea Standard Time; GMT + 9 hours), a moderate earthquake (M L 5.8) occurred in Gyeongju, southeastern Korea. The earthquake was preceded by an M L 5.1 foreshock at 10:44:32 UTC and followed by numerous aftershocks, including 17 events with local magnitude larger than 3.0 during the rest of the month. The largest aftershock, M L 4.5, occurred on 19 September 2016, at 11:33:58 UTC. In a companion paper to this study (Kim et al., 2016), the source parameters of the main events are discussed in detail. Immediately after the M L 5.1 foreshock, teams for aftershock monitoring were dispatched from Busan to Gyeongju. Approximately one hour after the main earthquake, the first temporary seismic station to record aftershocks in the epicentral area began recording continuous seismic data at a sampling frequency of 200 Hz. Three additional seismographs were installed in the epicen- tral area by midnight on the day of the main earthquake. There has been a long academic dispute concerning the seismic activity of the Yangsan Fault System, including the potential for reactivation (e.g., Kyung et al., 1999, 2010). Although seismic activity is documented in historic records, events during the instrumental recording period have been ignored due to the very low seismicity rate (Chiu and Kim, 2004; Lee and Yang, 2006; Kyung et al., 2010). The M L 5.8 Gyeo- ngju earthquake and its aftershocks have settled the debate. Kwang-Hee Kim Tae-Seob Kang* Junkee Rhie YoungHee Kim Yongcheol Park Su Young Kang Minhui Han Jeongmu Kim Jechan Park Minook Kim ChangHwan Kong Dabeen Heo Heekyoung Lee Euna Park Hyejin Park Sang-Jun Lee Sungwon Cho Jeong-Ung Woo Sang-Hyun Lee Juhwan Kim } } } } Department of Geological Science, Pusan National University, Busan 46241, Republic of Korea Division of Earth Environmental System Science, Pukyong National University, Busan 48513, Republic of Korea School of Earth and Environmental Sciences, Seoul National University, Seoul 08826, Republic of Korea Division of Polar Earth-System Sciences, Korea Polar Research Institute, Incheon 21990, Republic of Korea Department of Geological Science, Pusan National University, Busan 46241, Republic of Korea Division of Earth Environmental System Science, Pukyong National University, Busan 48513, Republic of Korea Earthquake and Volcano Bureau, Korea Meteorological Administration, Seoul 07062, Republic of Korea Division of Earth Environmental System Science, Pukyong National University, Busan 48513, Republic of Korea School of Earth and Environmental Sciences, Seoul National University, Seoul 08826, Republic of Korea *Corresponding author: tskang@pknu.ac.kr