Distinguishing the aftershock sequences of the DPRK5 and DPRK6 underground nuclear tests

Link to full paper in arxiv.org

Abstract

The rate of aftershocks in the sequence initiated by the DPRK underground tests has been increasing since January 2021. In total, 22 reliable aftershocks were detected between January 13 and October 1, 2021. Their characteristics are similar to the aftershocks in one of two clusters: 1) the fifth DPRK (DPRK5) test (mb(IDC)=5.09) conducted on September 9, 2016, which induced the first DPRK aftershock in the sequence detected at 1:50:48 UTC on September 11, 2016; 2) the sixth DPRK (DPRK6) explosion (mb(IDC)=6.07), which generates its aftershock sequence with characteristics significantly different from the aftershocks in the DPRK5 sequence. The length, intensity, and alternating character of these sequences suggest specific mechanisms of energy release likely associated with the interaction of the damaged zones of the DPRK5 and DPRK6 and the collapse of their cavities with progressive propagation of the collapsing chimneys to the free surface. According to the depth estimates based on the moment tensor modelling, the DPRK5 and DPRK6 were conducted at practically the same depths. The difference in magnitudes suggests that their damaged zones differ by a factor of 2 or more. The first aftershock of the DPRK6 (mb(IDC=4.12) 8.5 minutes after the test is evidence of the cavity collapse and creation of a chimney, which did not reach the surface. The activity in 2021 indicates that the chimney collapse is not finished yet. One can expect more aftershocks in the near future, likely ending with the chimney reaching the free surface.

Seismic events found by waveform cross correlation after the announced underground nuclear tests conducted by the DPRK on 12.02.2013 and 06.01.2016. Aftershocks or hidden nuclear tests?

Link to full paper in arxiv.org

Abstract

The multi-master method based on waveform cross-correlation has found several aftershocks of the third (DPRK3) and the fourth (DPRK4) announced underground nuclear tests conducted by the DPRK on 12.02.2013, and 06.01.2016, respectively. The second DPRK test had no reliable aftershock hypotheses at the level of the method sensitivity and resolution. The closest to the test site array station USRK was not operational before 20.12.2008 and the first DPRK test cannot be analyzed by the multi-master method based on waveform templates from two IMS stations - USRK and KSRS. The largest aftershocks of the DPRK3 and DPRK4 are close in magnitude to the first aftershock of the DPRK5 on 11.09.2016, and many aftershocks of the DPRK6. Three big aftershocks of the DPRK3 occurred on 25.05.2014, i.e. 467 days after the mainshock. The DPRK3 aftershock found at 6:43:03 UTC had a relative magnitude of 2.91. There were found two aftershocks of the DPRK4: on 19.02.2016 (0:28:07 UTC) and on 02.07.2016 (19:52:27 UTC). The largest aftershocks of the DPRK3 and DPRK4 could be interpreted as related to the cavity collapse process likely followed by a chimney collapse, not reaching the free surface, however, as no noble gases and/or radionuclide particles were detected. Seismic signals from all events found before the DPRK5 (09.09.2016), had a high level of cross-correlation with the waveform templates of the DRPK5 and DPRK6. These signals prove the shape similarity, and thus, the close spatial proximity of the seismic events after DPRK3/DPRK4 to the DPRK5/DPRK6 aftershocks. There were no other events found by the multi-master method since 01.01.2009, even when the highest resolution regime was used. The resolution and sensitivity of the multi-master method were tuned and tested using the data after 09.09.2016.