The characterisation of announced nuclear tests and seismic events in the vicinity of the DPRK test site using P/Lg ratios at regional seismic stations

Throughout the recent period of announced nuclear testing by the Democratic People's Republic of Korea there has been a series of small magnitude seismic events detected in the vicinity of the test site. These events, reported by the International Data Centre of the Comprehensive Nuclear Test-Ban-Treaty Organisation, are only detected at regional seismic stations. It is of interest to the global CTBTO community if these events and the announced nuclear tests can be characterised using regional observations. We investigate the ratio of P- to Lg-wave amplitudes to discriminate between announced nuclear tests and presumed earthquakes in the vicinity of the test site. Investigating amplitude ratios independently at seismic stations overcomes path effects assuming that the events of interest are all located near to each other. There is a clear separation between the P/Lg ratios of announced nuclear tests and presumed earthquakes on the 3-component sensors at USRK and MDJ for events within 50km of the test site. Interestingly, the signals in the vicinity of the test site have low coherence across the USRK seismic array, likely due to effects from local geology at the array site. Effective discrimination comes from averaging the root-mean-square amplitudes of all three-components of the seismometer, and not on the beam made using vertical array elements. The Pg/Lg amplitude ratios are more consistent over the range of passbands investigated (1-18Hz) compared to Pn/Lg ratios. The P/Lg ratios of assumed mining events (the majority generating infrasound detections) in the vicinity of the test site are higher than earthquakes, however being more than 100km from the test site could generate different path effects to USRK. Discrimination between earthquakes and explosions in the vicinity of the DPRK test site using regional signals supports the technical verification of the CTBT.

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