Remote monitoring of weak aftershock activity with waveform cross correlation: the case of the DPRK September 9, 2016 underground test

📝 Abstract

The method of waveform cross correlation (WCC) allows remote monitoring of weak seismic activity induced by underground tests. This type of monitoring is considered as a principal task of on-site inspection under the Comprehensive nuclear-test-ban treaty. On September 11, 2016, a seismic event with body wave magnitude 2.1 was found in automatic processing near the epicenter of the underground explosion conducted by the DPRK on September 9, 2016. This event occurred approximately two days after the test. Using the WCC method, two array stations of the International Monitoring System (IMS), USRK and KSRS, detected Pn-wave arrivals, which were associated with a unique event. Standard automatic processing at the International Data Centre (IDC) did not create an event hypothesis, but in the following interactive processing based on WCC detections, an IDC analyst was able to create a two-station event . Location and other characteristics of this small seismic source indicate that it is likely an aftershock of the preceding explosion. Building on the success of automatic detection and phase association, we carried out an extended analysis, which included later phases and closest non-IMS stations. The final cross correlation solution uses four stations, including MDJ (China) and SEHB (Republic of Korea), with the epicenter approximately 2 km to north-west from the epicenter of the Sept. 9 test. We also located the aftershock epicenter by standard IDC program LocSAT using the arrival times obtained by cross correlation. The distance between the DPRK and LocSAT aftershock epicenters is 25.5 km, i.e. by an order of magnitude larger than that obtained by the WCC relative location method.

💡 Analysis

The method of waveform cross correlation (WCC) allows remote monitoring of weak seismic activity induced by underground tests. This type of monitoring is considered as a principal task of on-site inspection under the Comprehensive nuclear-test-ban treaty. On September 11, 2016, a seismic event with body wave magnitude 2.1 was found in automatic processing near the epicenter of the underground explosion conducted by the DPRK on September 9, 2016. This event occurred approximately two days after the test. Using the WCC method, two array stations of the International Monitoring System (IMS), USRK and KSRS, detected Pn-wave arrivals, which were associated with a unique event. Standard automatic processing at the International Data Centre (IDC) did not create an event hypothesis, but in the following interactive processing based on WCC detections, an IDC analyst was able to create a two-station event . Location and other characteristics of this small seismic source indicate that it is likely an aftershock of the preceding explosion. Building on the success of automatic detection and phase association, we carried out an extended analysis, which included later phases and closest non-IMS stations. The final cross correlation solution uses four stations, including MDJ (China) and SEHB (Republic of Korea), with the epicenter approximately 2 km to north-west from the epicenter of the Sept. 9 test. We also located the aftershock epicenter by standard IDC program LocSAT using the arrival times obtained by cross correlation. The distance between the DPRK and LocSAT aftershock epicenters is 25.5 km, i.e. by an order of magnitude larger than that obtained by the WCC relative location method.

📄 Content

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REMOTE MONITORING OF WEAK AFTERSHOCK ACTIVITY WITH WAVEFORM CROSS CORRELATION: THE CASE OF THE DPRK SEPTEMBER 9, 2016 UNDERGROUND TEST

Bobrov D.I., I.O. Kitov, and M.V. Rozhkov

Abstract The method of waveform cross correlation (WCC) allows remote monitoring of weak seismic activity induced by underground tests. This type of monitoring is considered as a principal task of on-site inspection under the Comprehensive nuclear-test-ban treaty. On September 11, 2016, a seismic event with body wave magnitude 2.1 was found in automatic processing near the epicenter of the underground explosion conducted by the DPRK on September 9, 2016. This event occurred approximately two days after the test. Using the WCC method, two array stations of the International Monitoring System (IMS), USRK and KSRS, detected Pn-wave arrivals, which were associated with a unique event. Standard automatic processing at the International Data Centre (IDC) did not create an event hypothesis, but in the following interactive processing based on WCC detections, an IDC analyst was able to create a two-station event with local magnitude ML=1.8. Location and other characteristics of this small seismic source indicate that it is likely an aftershock of the preceding explosion. Building on the success of automatic detection and phase association, we carried out an extended analysis, which included later phases and closest non- IMS stations. The final cross correlation solution uses four stations, including MDJ (China) and SEHB (Republic of Korea), with the epicenter approximately 2 km to north-west from the epicenter of the Sept. 9 test. We also located the aftershock epicenter by standard IDC program LocSAT using the arrival times obtained by cross correlation. The distance between the DPRK and LocSAT aftershock epicenters is 25.5 km, i.e. by an order of magnitude larger than that obtained by the WCC relative location method.

Key words: waveform cross correlation, International monitoring system, aftershock, underground explosion, location, CTBT

Introduction Seismic network of the International monitoring system (IMS) is monitoring the compliance with the Comprehensive nuclear-test-ban-treaty (CTBT). The IMS uses modern methods of data recording, acquisition, and transfer. The data are collected by the International data centre where they are processed with standard techniques. New advanced methods of data analysis, such as the method of waveform cross correlation (WCC), allow significant enhancement of monitoring capabilities by reducing the magnitude threshold of nuclear test detection at regional and teleseismic distances. Moreover, remote detection and location of weak aftershock activity becomes feasible.

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In addition to significant reduction in detection threshold, the method of waveform cross correlation is characterized by dramatic improvement in the accuracy of relative location as well as magnitude estimation of small events compared to standard seismological methods [1]. In this paper, we present an example of superior WCC performance. It allowed to automatically detecting a low-magnitude seismic event that occurred on 11.09.2016 at 01:50:49.83 (UTC) near the epicenter (41.299°N, 124.049°E) of underground test conducted by North Korea on September 9, 2016 at 00:30:00.87, as estimated by the International Data Centre (IDC). At the same time, routine automatic processing did not create an event hypothesis, and thus, missed this event. Location and other characteristics of this small seismic source indicate that it is likely an aftershock of the preceding explosion. Therefore, the WCC provides unique and crucial information on the post-seismic processes induced by the announced underground test.
According to data from two seismic arrays of the International Monitoring System (IMS) of CTBT Organization (CTBTO), USRK (Russia) and KSRS (Republic of Korea), body-wave magnitude of the found event is 2.1. Location, seismic energy and the time elapsed since the moment of testing suggest the possibility of a causal link with the explosion. Therefore, it is possible to consider this event as an aftershock of the explosion. Magnitude 2.1 corresponds to an underground explosion of no more than ten to twenty tons of TNT, which determines the threshold for the biggest clandestine tests within the DPRK test site. Almost any event with magnitude of 2 or higher can be detected by the nearest IMS stations using cross correlation with signals from several previous explosions. Seismic waves from five announced DPRK underground tests were measured by IMS stations [1] and many other seismic networks - from global [2] to local [3]. According to the IDC, body-wave magnitude, mb(IDC), of the smallest of the five tests was 4.1, and the biggest test was conducted on September 9, 2016 and reached magnitude 5.1. Aftershocks associated with collapsing roof of the explosion cavity and ind

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