Analysis of the April 10, 2021 (Mw 6.1) destructive intra-slab earthquake, East Java, Indonesia

A destructive earthquake (Mw 6.1) struck East Java, Indonesia, on 10 April 2021, resulting in at least 10 deaths and damage to over 3000 buildings. Preliminary hypocenter determinations indicate that it was located ~20 km south of Java, at a depth of ~60 km, suggesting that it likely occurred within the subducting oceanic slab beneath the Sunda Arc. In order to better understand the origins of this important event, we have undertaken non-linear hypocenter determination of the mainshock and its aftershocks through picking of P- and S-wave arrival times recorded by nearby BMKG seismic stations, followed by precise relative relocation using the double-difference method. We have also conducted source mechanism analysis to determine the type of fault slip, and obtained a rupture model of the mainshock by performing a finite fault inversion. Our results show that the mainshock was intra-slab, with an estimated depth of 58 km and was followed by, on average, deeper aftershocks that ranged in depth from 57 to 69 km. The source mechanism solution of selected aftershocks reveal that they rupture along a thrust fault oriented almost parallel to the trench, which is consistent with the mainshock source solution. The rupture modelling results indicate that the earthquake is characterised by a high stress-drop and compact rupture area, which together with its depth of nearly 60 km, may explain the relatively small number of aftershocks that were recorded.