Properties of three seismic events in September 2017 in the northern Korean Peninsula from moment tensor inversion

📝 Abstract

Moment tensor inversion is conducted to characterize the source properties of the September 3, M6.3, the September 3, M4.6, and the September 23, M3.4 seismic events occurred in 2017 in the nuclear test site of DPRK. To overcome the difficulties in the comparison, the inversion uses the same stations, the same structural model, the same algorithm, and nearly the same filters in the processing of waveforms. It is shown that the M6.3 event is with predominant explosion component, the M4.6 event is with predominant implosion component, while the M3.4 event is with a predominant double couple component (~74%) and a secondary explosion component (~25%). The three seismic events are with a similar centroid depth. The double couple component of the M3.4 event shows a normal fault striking northeastward.

💡 Analysis

Moment tensor inversion is conducted to characterize the source properties of the September 3, M6.3, the September 3, M4.6, and the September 23, M3.4 seismic events occurred in 2017 in the nuclear test site of DPRK. To overcome the difficulties in the comparison, the inversion uses the same stations, the same structural model, the same algorithm, and nearly the same filters in the processing of waveforms. It is shown that the M6.3 event is with predominant explosion component, the M4.6 event is with predominant implosion component, while the M3.4 event is with a predominant double couple component (~74%) and a secondary explosion component (~25%). The three seismic events are with a similar centroid depth. The double couple component of the M3.4 event shows a normal fault striking northeastward.

📄 Content

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Properties of three seismic events in September 2017 in the northern Korean Peninsula from moment tensor inversion

Libo Han, Zhongliang Wu, Changsheng Jiang*, Jie Liu Institute of Geophysics, China Earthquake Administration, 100081 Beijing, China China Earthquake Networks Center, 100036 Beijing, China

• Corresponding author, e-mail: Jiangcs@cea-igp.ac.cn

ABSTRACT Moment tensor inversion is conducted to characterize the source properties of the September 3, M6.3, the September 3, M4.6, and the September 23, M3.4 seismic events occurred in 2017 in the nuclear test site of DPRK. To overcome the difficulties in the comparison, the inversion uses the same stations, the same structural model, the same algorithm, and nearly the same filters in the processing of waveforms. It is shown that the M6.3 event is with predominant explosion component, the M4.6 event is with predominant implosion component, while the M3.4 event is with a predominant double couple component (~74%) and a secondary explosion component (~25%). The three seismic events are with a similar centroid depth. The double couple component of the M3.4 event shows a normal fault striking northeastward. Key words: Full moment tensor; gCAP inversion; DPRK nuclear test site INTRODUCTION In September 2017 the northern Korean Peninsula dramatically caused the attention of the world by a series of seismic events of M6.3 and M4.6 on September 3, and M3.4 2

on September 23, respectively. Table 1 lists the parameters of such seismic events as reported by China Earthquake Networks Center (CENC). DPRK authorities announced that the M6.3 event was a “successfully conducted hydrogen-bomb test”. The M4.6 event, some 8 minutes followed, was regarded as a collapse. Different from previous similar seismic events which have been studied in various aspects, for example the yield and the accurate/precise location (e.g. Kim and Richards, 2007; Kvaerna et al., 2007; Bonner et al., 2008; Koper et al., 2008; Kim et al., 2009; Schlittenhardt et al., 2010; Shin et al., 2010; Wen and Long, 2010; Chun et al., 2011; Rougier et al., 2011; Pasyanos et al., 2012; Zhao et al., 2012; Zhang and Wen, 2013; Zhao et al., 2014), properties of, and relation between, these three seismic events, especially the source property of the last small event, raises scientific problems to be discussed by seismological data.
Shortly after the occurrence of the M3.4 event on September 23, some results were reported by the Weibo/WeChat (Chinese version of Twitter/Facebook). The result from the P/S spectrum ratio indicated that this event is not within the cluster of the UNEs (http://www.igg.cas.cn/xwzx/kyjz/201709/t20170923_4863959.html) , but for the source property of this event there were two different opinions, namely natural earthquake, or collapse (http://wemedia.ifeng.com/30848756/wemedia.shtml; Chen Huizhong, 2017, WeChat communication, 2017/09/23; Liu Jie, 2017, ibid; Su Jinrong, 2017, ibid; Peng Zhigang, 2017, ibid; Zhao Lianfeng, 2017, ibid. Reports from different experts varied with time, yet from the related media coverage, “traces” of such discrepancy can be seen clearly). Analysts of China Earthquake Networks Center 3

(CENC) insisted that the event is associated with an explosion (http://www.cenc.ac.cn/cenc/dzxx/336221/index.html) . Due to the concern of the security situation of the Korean Peninsula as well as the potential environmental effects associated with the UNE detonation (Console and Nikolaev, 1995), the properties of this seismic event, as well as the discrepancies between different agencies, caused the attention, and even critical comments, in the public.
The spirit of forensic seismology (Bowers and Selby, 2009), like the works associated with jurisdiction, is to exploit the data available, even if with limited coverage and quality, to get some evidences of, at least clues to, the reliable and/or persuasive conclusions. Characterization of the three seismic events has obvious limit from the data available due to poor geographic coverage of seismic stations, distance from the recording stations to the epicenter, the small magnitude of the last seismic event, and the mixing of the seismic waveform of the second event with that of the first one. However, by some semi-quantitative, e.g. comparative, analysis, yet some qualitative conclusions could be obtained, which may help in the judgment of the properties of these seismic events. In this research letter we try to discuss this question in the perspective of seismic moment tensor, which seems not mentioned in the discussion among different agencies. FULL MOMENT TENSOR INVERSION Characterization of seismic events (including underground nuclear tests) in the perspective of seismic moment tensor, as well as its theoretical discussion, has a long history (e.g. Wu and Chen, 1996; Dufumier and Rivera, 1997; Dreger and Woods, 4

2002; Ford et al., 2009a,

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