Hypocenter and Magnitude Analysis of Aftershocks of the 2018 Lombok, Indonesia, Earthquakes Using Local Seismographic Networks

Abstract

The island of Lombok in Indonesia is located between the Indo-Australian and Eurasian subduction trenches and the Flores back-arc thrust, making it vulnerable to earthquakes. On 29 July 2018, a significant earthquake Mw 6.4 shook this region and was followed by series of major earthquakes (Mw>5.8) on 5, 9, and 19 August, which led to severe damage in the northern Lombok area. In this study, we attempt to reveal the possible cause of the sequences of the 2018 Lombok earthquakes based on aftershock monitoring data. Twenty stations were deployed to record earthquake waveform data from 4 August to 9 September 2018. In total, 3259 events were identified using 28,728 P- and 20,713 S-wave arrival times during the monitoring. The aftershock hypocenters were determined using a nonlinear approach and relocated using double-difference method. The moment magnitude (Mw) of each event was determined by fitting the displacement spectrum amplitude using a Brune-type model. The magnitudes of the aftershocks range from Mw 1.7 to 6.7. The seismicity pattern reveals three clusters located in the Flores oceanic crust, which fit well with the occurrences of the four events with Mw>6. We interpret these events as the main rupture area of the 2018 Lombok earthquake sequence. Furthermore, an aseismic zone in the vicinity of Rinjani extending toward the northwestern part of Lombok was observed. We propose that the crust in this area has elevated temperatures and is highly fractured thus inhibiting the generation of large earthquakes. The aseismic nature is therefore an artifact of the detection threshold of our network (Mw 4.6).

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

“…However, earthquakes in this tectonic setting are rare and generally not well monitored. In 2018, an unusual and destructive cascade of earthquakes occurred over a 3-week period at the island of Lombok, Indonesia (7,8). These earthquakes were produced by cascading rupture of the Flores Thrust, which extends beneath the active Sunda volcanic arc (9).…”

Delineation of Upper Crustal Structure Beneath the Island of Lombok, Indonesia, Using Ambient Seismic Noise Tomography

“…Figure 11 depicts vertical cross sections of our pseudo-3D Vs model for the upper crust of Lombok overlayed with the relocated hypocentres during the August -September 2018 period from Sasmi et al (2020). As indicated in cross-sections C-C' and B-B' , the low Vs associated with R3 and the northern part of R1 extend to greater than 4 km depth.…”

“…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). This tectonic activity results in a sequence of destructive earthquakes that occurred on July 29th to August 19th, 2018 (Afif et al, under review;Harsuko et al, 2020;Sasmi et al, 2020).…”

Investigation of Hilbert–Huang Transform and Fourier Transform for Horizontal-to-Vertical Spectral Ratio Analysis: Understanding the Shallow Structure in Mataram City, Lombok, Indonesia

“…Even though Mataram City is located in the western region of Lombok Island, the seismicity near this region was able to create severe destruction. Sasmi et al (2020) suggest that the Flores oceanic crust is divided into three segments, and the northwestern segment, which is relatively closer to Mataram City, has the largest magnitude of M 5.9 (August 9, 2018). This is relatively small compared to the M 7.0 earthquake (August 5, 2018) in the northern segment and M 6.9 earthquake (August 19, 2018) in the northeastern segment but does not rule out the possibility of creating an equally large, or even larger, magnitude earthquake in the future, considering the complex structure beneath the Lombok Island.…”

“…The interaction between the two creates an east-west trending fault beneath the northern part of Lombok Island with thrust fault mechanism, adjacent to the Flores back-arc thrust fault. The study by Sasmi et al (2020) also proposed that this structure of the Flores oceanic crust is segmented, and the failure of each successive segment triggers the next segment to fail, creating a large earthquake shortly afterward.…”