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