Cascading Partial Rupture of the Flores Thrust during the 2018 Lombok Earthquake Sequence, Indonesia

Abstract

A series of four Mw>6 earthquakes struck the northern region of Lombok, eastern Indonesia, in a span of three weeks from late July to mid-August 2018. The series was thought to be associated with the Flores thrust, but the exact mechanism causing the unusual earthquake series has remained elusive. Our Interferometric Synthetic Aperture Radar analysis, combined with insights from seismology, indicates that the events originated at different hypocenter depths with differing fault geometries, which may explain the cascading behavior of the events, and indicates that better imaging of active fault geometry might provide some insight into future rupture behavior on other similar thrust systems. Our static stress change calculations suggest that the earlier events in the sequence played a role in promoting the later events. In addition, the second event brought the most significant impact on a nearby volcano, by causing volumetric expansion at its shallow magma plumbing system and unclamping its magma ascent zone, which may potentially have an impact on its future eruptive activity. However, no volcanic activity has so far occurred after the earthquakes. Finally, our damage proxy maps suggest that the second event caused the greatest damage to buildings.

Thermal squeezing of the seismogenic zone controlled rupture of the volcano-rooted Flores Thrust

“…We did not use all the possible InSAR images [such as in Salman et al, (7)], but select one ascending (ALOS-2) and one descending (Sentinel-1) InSAR image for each earthquake, and because these satellites have similar-looking angles to the ground, ascending/descending images from ALOS-2 is similar to that from Sentinel-1 [see more details in Salman et al, (7)]. For the slip model inversion, derived displacement measurements were downsampled using the quadtree structures (37).…”

Tsunami hazard in Lombok & Bali, Indonesia, due to the Flores backarc thrust

“…A relationship between faulting and volcanic activity was observed for the 2018 Lombok earthquake sequence, which generated four >Mw 6 events between 28 th July to 19 th August. These earthquakes did not occur offshore on the northern frontal thrust of the Flores Thrust, but instead involved slip along the deeper part of the fault and associated imbricate thrusts beneath Lombok, to the north of the active Rinjani volcano (Salman et al, 2020;Yang et al, 2020;Lythgoe et al, 2021). While these earthquakes were not 105 directly caused by volcanic activity, the presence of the volcano constrained the earthquake distribution by elevating the downdip limit of the seismogenic zone in the crust (Lythgoe et al, 2021).…”

Shear Wave Splitting of the 2018 Lombok Earthquake Aftershock Area, Indonesia

“…The significant earthquakes that occurred several times were indicated as the result of segmentation ruptures formed before the occurrence of a strong earthquake (Wang et al, 2020;Sasmi et al, 2020). Salman et al (2020) using InSAR data also explained that the significant earthquake sequences came from different hypocenters, and formed a rupture of a thrust fault that tilted to the south. This earthquake sequence might trigger secondary disasters such as landslides (Ferrario, 2019).…”

“…This earthquake sequence might trigger secondary disasters such as landslides (Ferrario, 2019). In addition, it is suggested that the 2018 Lombok earthquake might cause a volumetric expansion in the shallow magma plumbing system at Rinjani volcano, which might affect the volcanic activity in the future (Salman et al, 2020).…”

“…On the other hand, the 2018 Lombok earthquake caused fault segmentation with a different strike orientation. The focal mechanism analysis of the regional seismic network (Salman et al, 2020) indicates an east-west relative strike orientation between 85.4880° -91.5725° of the Lombok earthquake fault system. For this reason, we implemented a crustal anisotropy study to prove whether crustal seismic anisotropy in the Lombok region is more influenced by the regional strain rate or the local fault system associated with the 2018 Lombok earthquake rupture zone.…”