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

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 International Data Centre of the Comprehensive Nuclear-Test-Ban Treaty Organization. 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 International Monitoring System (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.

Introduction The main difference between seismic sources of underground explosions and earthquakes, which is useful for discrimination, is related to the efficiency of P- and S-wave generation. Explosions demonstrate more efficient generation of compressional (P) waves, while earthquakes generate shear waves (S) of relatively higher amplitude. As a result, the difference between body (P) wave, mb, and surface (related to S) wave, Ms, magnitudes measured at teleseismic distances successfully serves for discrimination of earthquakes and underground explosions.
At regional distances, seismic wave-field is characterized by a much higher fluctuation in amplitudes of P and S-waves than that at teleseismic distances because of inhomogeneous velocity and attenuation structures of the crust and the uppermost mantle. No reliable magnitude scale can be derived for discrimination purposes. Therefore, finer characteristics are needed to distinguish between earthquakes and explosions. It was found that the spectrum of P-wave falls faster for earthquakes than that of explosion sources. Based on this observation, a spectral discriminant was introduced as a frequency dependent ratio of P- and S-wave spectral amplitudes. In the past, we conducted a thorough study of spectral discriminants and other discrimination methods, such as statistical identification, artificial neural networks classification and moment tensor estimates, using data from the Soviet nuclear tests, regional chemical explosions and earthquakes [1-7]. It was tested in a number of geological/seismological regions (chiefly around test sites and PNEs) and demonstrated good performance at the level of (teleseismic) Ms-mb criterion, but for much smaller events, which are not measured at teleseismic stations.

Data and method We test the P/S spectral amplitude ratio criterion as a possible tool to discriminate six underground tests conducted by the DPRK since October 2006 (see Table 1) and six smaller events found near these explosions just after the DPRK5 and DPRK6. These six smaller events were detected by waveform cross correlation at regional stations of the International monitoring system (IMS) of the Comprehensive Nuclear-Test-Ban Treaty Organization (CTBTO) and non- IMS stations (see Table 2). Since their waveforms demonstrate high correlation coefficients with those from the DPRK explosions and the arrival times found by cross correlation do not depend on the length of correlation window one may consider these low-magnitude events as located very close or within the DPRK tests site. Taking into account that the smaller events occurred just after the two biggest tests, their low magnitudes (2 and more units of magnitude lower than their corresponding main shocks), and the spatial closeness

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