The 2018 Lombok earthquake cascade was controlled by thermal squeezing of the seismogenic zone from an active arc volcano

Abstract:

A sequence of unusual earthquakes occurred at the volcanic island of Lombok, Indonesia over a three week period in 2018. The earthquakes were produced by rupture of the Flores Thrust, which dips beneath the coast of Lombok and intersects the root zone of the isolated and active arc volcano Gunung Rinjani. The earthquakes are puzzling in a number of ways: two magnitude 6.9 earthquakes occurred within two weeks of each other on the same fault, nucleating near the same point but rupturing in opposite directions. Despite their proximal locations, our finite fault inversions show that the rupture characteristics of the two earthquakes were very different. The first event ruptured several small asperities with a rougher moment rate function and a longer duration compared to the second event, which has a broader distribution of slip from a single major asperity.

We analyse seismic and geodetic data, including from a local seismic array deployed just prior to the mainshocks. Relocated seismicity and slip models show that the earthquakes occurred in a continuous narrow seismogenic zone (SZ) which is elevated near to the volcanic centre. The mainshocks nucleated at the elevated part of the SZ and propagated away from it. Slip of the first Mw 6.9 event was contained within a shallow SZ, while the second event ruptured a deeper and wider SZ. We test whether fault geometry is controlling the extent of rupture by building a 3D geometric model of the fault by combining relocated seismicity, moment tensors and seismic reflection data. The data are well fit by a semi-planar structure, indicating that structural variations are not the origin of the spatially restricted rupture. Instead we propose that the thermal structure of the crust restricts the SZ, with the thermal imprint of the volcano raising the SZ. We construct a simple thermal model by assuming that the base of 95% of seismicity (D95) corresponds to a brittle ductile boundary of 450°C, consistent with a feldspar composition. Our model shows that the shallow limit of the SZ is closely approximated by an isotherm, corresponding to ~250°C.

Additionally, we find that the first Mw 6.9 event is characterized by many repeating aftershocks occurring between asperities, in contrast with very few repeating aftershocks for the single-asperity second event. This suggests the occurrence of stronger afterslip between the asperities of the first event. The well-resolved broad spectrum of the slip behavior in an area with a strong thermal gradient calls for customized dynamic simulations to further reveal the fundamental mechanism of earthquakes.