No 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.

Temperature plays a critical role in defining the seismogenic zone, the area of the crust where earthquakes most commonly occur; however, thermal controls on fault ruptures are rarely observed directly. We used a rapidly deployed seismic array to monitor an unusual earthquake cascade in 2018 at Lombok, Indonesia, during which two magnitude 6.9 earthquakes with surprisingly different rupture characteristics nucleated beneath an active arc volcano. The thermal imprint of the volcano on the fault elevated the base of the seismogenic zone beneath the volcanic edifice by 8 km, while also reducing its width. This thermal “squeezing” directly controlled the location, directivity, dynamics, and magnitude of the earthquake cascade. Earthquake segmentation due to thermal structure can occur where strong temperature gradients exist on a fault.

As we strive to understand the most remote region of our planet, one critical area of investigation is the uppermost inner core since its structure is related to solidification of outer core material at the inner core boundary (ICB). Previous seismic studies have used body waves to show that the top ∼100 km of the inner core is isotropic. However, radial anisotropy cannot be uniquely determined by body wave observations. Alternatively, normal mode center frequencies are sensitive to spherically symmetric Earth structure, therefore may provide a constraint on the existence of radial anisotropy in the inner core. Here we show that normal mode center frequency measurements are compatible with 2–5% radial anisotropy in the top ∼100 km of the inner core with a fast direction radially outward and a slow direction along the ICB. Given the uncertainties in the mineral physics and processes that produce anisotropy, the observed radial anisotropy may be reconciled with predictions based on either solidification processes or from texturing due to anisotropic growth.