Abstract
We have successfully conducted the first ambient noise tomography on the island of Lombok, Indonesia using local waveform data observed at 20 temporary stations. Ambient noise tomography was used to delineate the seismic velocity structure in the upper crust. The waveform data were recorded from August 3rd to September 9th, 2018, using short-period and broadband sensors. There are 185 Rayleigh waves retrieved from cross-correlating the vertical components of the seismograms. We used frequency-time analysis (FTAN) to acquire the interstation group velocity from the dispersion curves. Group velocity was obtained for the period range of 1 s to 6 s. The group velocity maps were generated using the subspace inversion method and Fast Marching Method (FMM) to trace ray-paths of the surface waves through a heterogeneous medium. To extract the shear wave velocity (Vs) from the Rayleigh wave group velocity maps, we utilize the Neighborhood Algorithm (NA) method. The 2-D tomographic maps provide good resolution in the center and eastern parts of Lombok. The tomograms show prominent features with a low shear velocity that appears up to 4 km depth beneath Rinjani Volcano, Northern Lombok, and Eastern Lombok. We suggest these low velocity anomalies are associated with Quaternary volcanic products, including the Holocene pyroclastic deposits of Samalas Volcano (the ancient Rinjani Volcano) which erupted in 1257. The northeast of Rinjani Volcano is characterized by higher Vs, and we suggest this may be due to the presence of igneous intrusive rock at depth.
Highlights
Lombok is part of the eastern Sunda Arc, a region of complex and active tectonics, including the Flores Back Arc Thrust which extends east-west from north of Flores to north of Bali Island, transform faults to the east and west of Lombok, and the margin of the Australian Plate to the south of Lombok (Figure 1)
The study described here was undertaken in order to constrain shallow crustal structure that may influence the propagation of seismic waves, so that better forecasts can be made of ground motion in future earthquakes
The body wave tomogram (Afif et al, under review) shown in Figure 2 at depth 0 km has roughly similar velocity pattern as in Figure 10 at depth 0.5, 1, and 2 km with a low velocity anomaly extending from north to east Lombok, whereas on the North-East side of Lombok there is a high velocity anomaly
Summary
Lombok is part of the eastern Sunda Arc, a region of complex and active tectonics, including the Flores Back Arc Thrust which extends east-west from north of Flores to north of Bali Island, transform faults to the east and west of Lombok, and the margin of the Australian Plate to the south of Lombok (Figure 1). From the tomographic group velocity maps, there are two prominent features shown in the west and east of northern Lombok that are relatively fast, whereas there is a low velocity anomaly that appears below Rinjani Volcano which continues northward and persists until a period of 4 s.
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