This study
investigated the source process of the 2020 Bengkulu doublet earthquake (Mw 6.9
and Mw 7.2 by this study) using the high degree of freedom finite fault
inversion method recently developed by Shimizu et al. (2020). In the process,
we applied some unique treatments by considering an identical fault plane and
using the same data set for both earthquakes. Consequently, we have succeeded
in reconstructing a reliable interaction of the foreshock and mainshock source
processes by showing good waveform fitting between the synthetic waves and the
observed waves.
We found that the rupture in the
fault plane was divided into two distinct areas between foreshock and
mainshock. The foreshock earthquake broke the asperity located in the southeast
with a total maximum slip of 1.07 meters, while in the mainshock earthquake,
the asperity broke with a total maximum slip of 1.17 meters in the northwest
direction. Furthermore, the extension pattern of the 2020 Bengkulu doublet
earthquake rupture shows as if the mainshock earthquake continued the
unfinished foreshock earthquake rupture but with a more comprehensive area
coverage. It seems that the final stage of the foreshock rupture triggered the
mainshock hypocenter, which is closely located. Meanwhile, the suspending
pattern of the foreshock's and mainshock's rupture extension to the deeper part
appears to be related to the 2007 Bengkulu Great Earthquake (Mw 8.5). Moreover,
using the high degree of freedom finite fault inversion method, which allows us
to extract the fault plane geometry information, we discover that the dip angle
changes along the fault plane as the depth increases.
Keywords: doublet earthquake, source process, 2020 Bengkulu earthquake, rupture interaction, trigger