Source Investigation of the 2018 Lombok (Indonesia) Earthquake Sequences

The western extension and deformation mechanism of Flores back-arc thrust in eastern Sunda-Banda Arc (Indonesia) are poorly investigated, and, thus, poorly constrained. From late July to August 2018, a sequence of large earthquakes (M6.4+) took place in the north of Lombok Island that marked the previously westernmost termination of the continuous zone of the back-arc thrusting. The 2018 Lombok earthquake sequences that began with Mw 6.4 (28 July 2018), and followed by Mw 6.9 (5 August 2018), and Mw 6.9 (19 August 2019) with massive subsequent aftershocks claimed on more than 500 casualties, nearly 500,000 people displaced and serious damages on Lombok Island. Here we relocate the aftershocks and perform the finite fault inversions of M6.4+ earthquake sequences constrained with teleseismic body and surface waves. Both refined hypocenters of aftershocks and rupture processes of large earthquakes provide detail kinematic descriptions of the source mechanisms of the sequences. The aftershocks distributions and slip model suggest that the earthquakes occurred on south-dipping low angle thrust faulting that striking to the east while it also activated aftershocks on surrounding complex shallow faulting with distinguishing distributions. The source inversions of large earthquakes over the entire of the western part of Flores back-arc thrust resulted as simple circular rupture propagations initiated from ~15 km depth for all events except the westernmost events (Mw 6.9 on 5 August 2019) that had a more complex rupture and initiated from shallower depth, and with slip distributed cross over the former identified westernmost termination of the Flores back-arc thrust. Our study suggests the further extension of back-arc thrusting and the possible structures revealed from the subsequence aftershocks. The source characterizations revealed in this study would be important for further seismic hazard analysis in this region.