ABSTRACT

Eleven small seismic events that occurred after the North Korean underground nuclear test (UNT) on 3 September 2017 have been reported. We detected three additional events not found using traditional methods by using a waveform‐matching technique. The magnitude of these fourteen shocks ranges from Mw 2.1 to 4.0. The first and largest of them, occurring about 8½ minutes after the nuclear explosion itself, has been identified by several authors as a cavity collapse. We analyzed the remaining 13 events, first by using regional P‐ and S‐wave spectral amplitude ratios observed at station MDJ (Mudanjiang, China), applying Mahalanobis methods to identify them. Applying a previously established linear discriminant function for the region to observed P/S spectral ratios, 12 of the events are classified as earthquakes, and 1 other, the first of a doublet pair on 9 December 2017, appeared to be an explosion. However, waveform similarity of this event with its doublet pair, and application of the MDJ discriminant function to data from two other regional stations suggests that it, too, is likely to be an earthquake. It appears that damage caused by strong ground motion from the large UNT of 3 September 2017 has led to slow adjustments in the surrounding region entailing stress relief via aftershocks. Additional small seismic events (earthquakes) may be expected in the region.

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