  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 ended with the chimney
reaching the free surface.
