12/6/17

Discrimination of the DPRK underground explosions and their aftershocks using the P/S spectral amplitude ratio

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Abstract

We have estimated the performance of discrimination criterion based on the P/S spectral amplitude ratios obtained from six underground tests conducted by the DPRK since October 2006 and six aftershocks induced by the last two explosions. Two aftershocks were detected in routine processing at the IDC. Three aftershocks were detected by a prototype waveform cross correlation procedure with explosions as master events, and one aftershock was found with the aftershocks as master event. Two seismic arrays USRK and KSRS of the IMS and two non-IMS 3-C stations SEHB (South Korea) and MDJ (China) were used. With increasing frequency, all stations demonstrate approximately the same level of deviation between the Pg/Lg spectral amplitude ratios belonging to the DPRK explosions and their aftershocks. For a single station, simple statistical estimates show that the probability of any of six aftershocks not to be a sample from the explosion population is larger than 99.996% at the KSRS and even larger at USRK. The probability of any of the DPRK explosion to be a representative of the aftershock population is extremely small as defined by the distance of 20 and more standard deviations to the mean explosion Pg/Lg value. For network discrimination, we use the Mahalanobis distance combining the Pg/Lg estimates at three stations: USRK, KSRS and MDJ. At frequencies above 4 Hz, the (squared) Mahalanobis distance, D2, between the populations of explosions and aftershocks is larger than 100. In the frequency band between 6 and 12 Hz at USRK, the aftershocks distance from the average explosion D2>21,000. Statistically, the probability to confuse explosions and aftershocks is negligible. These discrimination results are related only to the aftershocks of the DPRK tests and cannot be directly extrapolated to the population of tectonic earthquakes in the same area

Probability of the North Korea nuclear testing mountain collapse is increasing

Seismic stations in South Korea, China and Russia detected increasing aftershock activity within the mountain where the biggest DPRK nuclear test was conducted on September 3, 2017. A few aftershock events occur three months after the test and about two months after the previous aftershock on October 12. This activity might be related to a complete collapse of the mountain or chimney collapse with opening of a direct access from the explosion cavity filled with radioactive debris to the atmosphere. This is the worst case scenario, but one cannot exclude this effect especially in view of high international tension in this region. China, Russia and South Korea are just in tens of kilometers from the test site, and Japan is not too far away. Seismologists continue detailed study without interruption and delay. 

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