P-wave Attenuation Structure around the Source Region of 2018 Lombok, Indonesia, Earthquakes

Abstract:

On July-August 2018, a series of significant earthquakes (Mw 5.9 – Mw 6.9) struck Lombok Island, Indonesia. Taking advantage of the newly deployed 20 temporary stations of the Lombok Island starting from August 4, 2018, we aim to study the three-dimensional P-wave attenuation ( ) structures around the source area of the 2018 Lombok earthquake, which has never been done before. The attenuation structure can be estimated based on the amplitude decay of seismic waves that can be quantified through the parameter as a function of the travel time and dimensionless quality factor . We applied Simul2000 code to estimate the structure using 6,244 P-wave data from 914 events. Our result shows that the major earthquakes (≥ Mw 5.9) in a high- and low to moderate-velocity zone indicate that the mainshock took place in a brittle seismogenic layer. In the north-west part of Lombok Island, the aftershocks clustered in a low- with a depth <15 km while in the north-east part, aftershocks clustered in a high- with a depth of >15 km. These results suggest that the Flores Back-arc Thrust (FBT) is segmented, and only the fault system in the north-west part is affected by the presence of fluid, which causes high attenuation in this area. In addition, we also observe an apparent low- anomaly in the northern part of Rinjani Volcano. We proposed that this anomaly is associated with high-temperature areas that contribute to terminate the rupture, thus during a series of earthquakes, the activity of Rinjani Volcano did not increase significantly. The newly established attenuation structure in Lombok Island provides an essential complement to the previous velocity structure in understanding more details about the 2018 Lombok earthquake mechanism and the geometry of the Flores Back-arc Thrust.