Local earthquake tomography of the source region of the 2018 Lombok earthquake sequence, Indonesia

We develop and present a 3-D seismic velocity model of the source region of the 2018 Lombok, Indonesia earthquakes by using local earthquake tomography. The data consist of 28 728 P- and 20 713 S-wave arrival times from 3259 events which were recorded by 20 local seismic stations. The results show that most of the significant earthquakes occur to the edge of high-velocity regions. We interpret these to represent coherent blocks of the Flores Oceanic Crust underthrusting Lombok. At depths shallower than the nucleation area of the largest earthquake, many triggered aftershocks are located within a low-velocity, high-Vp/Vs region which is probably a highly fractured fault zone with a large amount of fluid. This fault zone is parallel to the dip of the Flores Back Arc Thrust and probably ruptured during this earthquake sequence. A prominent low-velocity, high-Vp/Vs region is colocated with the northwest and southern flank of the Rinjani volcanic complex. This large aseismic region is probably related to a wide area of the crust containing fluids due to ongoing magma intrusion beneath the volcano. To the east of Rinjani Volcano a cooled intrusive complex was imaged. It is characterized by high-velocity and low-Vp/Vs, supported by the presence of a high Bouguer anomaly. We confirm the existence of the Sumbawa Strait Strike-Slip Fault and find it is characterized by an elongated low-velocity, high-Vp/Vs zone.