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We have estimated the performance of discrimination criterion based on theP/S spectral amplitude ratios obtained from six underground tests conducted bythe DPRK since October 2006 and six aftershocks induced by the last twoexplosions. Two aftershocks were detected in routine processing at the IDC.Three aftershocks were detected by a prototype waveform cross correlationprocedure with explosions as master events, and one aftershock was found withthe aftershocks as master event. Two seismic arrays USRK and KSRS of the IMSand two non-IMS 3-C stations SEHB (South Korea) and MDJ (China) were used. Withincreasing frequency, all stations demonstrate approximately the same level ofdeviation between the Pg/Lg spectral amplitude ratios belonging to the DPRKexplosions and their aftershocks. For a single station, simple statisticalestimates show that the probability of any of six aftershocks not to be asample from the explosion population is larger than 99.996% at the KSRS andeven larger at USRK. The probability of any of the DPRK explosion to be arepresentative of the aftershock population is extremely small as defined bythe distance of 20 and more standard deviations to the mean explosion Pg/Lgvalue. For network discrimination, we use the Mahalanobis distance combiningthe Pg/Lg estimates at three stations: USRK, KSRS and MDJ. At frequencies above4 Hz, the (squared) Mahalanobis distance, D2, between the populations ofexplosions and aftershocks is larger than 100. In the frequency band between 6and 12 Hz at USRK, the aftershocks distance from the average explosionD2>21,000. Statistically, the probability to confuse explosions and aftershocksis negligible. These discrimination results are related only to the aftershocksof the DPRK tests and cannot be directly extrapolated to the population oftectonic earthquakes in the same area.

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