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

Abstract

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.

Seismic Attenuation Tomography From 2018 Lombok Earthquakes, Indonesia

“…The aftershock pattern of the 2018 Lombok earthquake could be divided into two clusters: the northwest cluster shallower than 20 km; the events of July 29, August 5, and August 9, 2018, took place in this area; and the northeast cluster which has a depth greater than 20 km and is the area where the August 19, 2018 events took place, as shown in Figure 1. The aftershocks are first concentrated in the northwest cluster (starting from August 4) before moving to the northeast cluster (starting from August 19), as also shown by previous study of Lythgoe et al (2021).…”

“…The presence of fluid can cause pressure to move first in the northwest area, before triggering earthquakes in the northeast due to the differences in pore pressure. Alternatively, the differences in attenuation characteristics and aftershocks distribution in R1 and R2 are probably due to the thermal gradient associated with the presence of the Rinjani volcano as stated in Lythgoe et al (2021). The presence of a squeezing zone generated by a thermal push from the volcanic heat in the R1 might raise the seismogenic zone to the shallower crust so that the aftershocks in R1 (northwest area) are shallower than the aftershocks in R2 (northeast area).…”

“…The second factor that we suggest could play an essential role in this earthquake is the influence of thermal gradient. The recent study of Lythgoe et al (2021) stated that there is an effect of thermal gradient from the deep magmatic system underlying Rinjani volcano and it controlled the rupture in the seismogenic zone.…”

“…A bodywave travel time tomography study using a local network has also been done, which provide information about the characteristics of the mainshock and aftershock source regions (Afif et al, 2020). The recent study by Lythgoe et al (2021) shows the influence of thermal squeezing in controlling the rupture in the Lombok seismogenic zone.…”

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

“…We 20 assess modeled tsunami patterns generated by fault slip in six earthquake scenarios (slip of 1-5 m, representing Mw 7.2-7.9+), with a focus on impacts on the capital cities of Mataram, Lombok and Denpasar, Bali, which lie on the coasts facing the strait. We use a geologically constrained earthquake model informed by the Lombok earthquake sequence (Lythgoe et al, 2021), together with a high-resolution bathymetry dataset developed by combining direct measurements from GEBCO with sounding measurements from the official nautical charts for 25 Indonesia. Our results show that fault rupture in this region could trigger a tsunami reaching Mataram in <8 minutes and Denpasar in ~10-15 minutes, with multiple waves.…”