Local earthquake tomography of the source region of the 2018 Lombok earthquake sequence, Indonesia

Abstract

Summary We develop and present a three-dimensional (3D) seismic velocity model of the source region of the 2018 Lombok, Indonesia earthquakes by employing local earthquake tomography. The data consist of 28,728 P- and 20,713 S-wave arrival times from 3,259 events which were recorded by 20 local seismic stations. The results show that most of the significant earthquakes occur to the edge of high-velocity regions. We interpret these to represent coherent blocks of the Flores Oceanic Crust underthrusting Lombok. At depths shallower than the nucleation area of the largest earthquake, many triggered aftershocks are located within a low-velocity, high-Vp/Vs region which is probably a highly fractured fault zone with a large amount of fluid. This fault zone is parallel to the dip of the Flores Back Arc Thrust and probably ruptured during this earthquake sequence. A prominent low-velocity, high-Vp/Vs region is co-located with the northwest and southern flank of the Rinjani volcanic complex. This large aseismic region is probably related to a wide area of the crust containing fluids due to ongoing magma intrusion beneath the volcano. To the east of Rinjani Volcano a cooled intrusive complex was imaged. It is characterized by high-velocity and low-Vp/Vs, supported by the presence of a high Bouguer anomaly. We confirm the existence of the Sumbawa Strait Strike-Slip Fault and find it is characterized by an elongated low-velocity, high-Vp/Vs zone.

Shear Wave Splitting of the 2018 Lombok Earthquake Aftershock Area, Indonesia

“…Sasmi et al (2020) explained that the 2018 Lombok earthquake aftershocks were mostly distributed to the depth of < 20 km with an average hypocenter slope of ~30 o and thought to be related with the rupture zone of 2018 Lombok earthquake. These significant events of earthquake were suspected as a result of the oceanic crust slabbed to the north of Lombok Islands and caused fault segmentation (Sasmi et al, 2020;Afif et al, 2021;Priyono et al, 2021;Wang et al, 2020;Salman et al, 2020;Yang et al, 2020;Lythgoe et al, 2021). The results from these studies confirm that the lack of correlation between event depths and δt suggests the shallow anisotropic fabrics may control the main possible sources of anisotropy in this area.…”

“…This may be related to the influence of the crack zone formed during the 2018 Lombok significant earthquake (Sasmi et al, 2020;Afif et al, 2021;Priyono et al, 2021;Wang et al, 2020;Salman et al, 2020). Sasmi et al (2020) explained that the 2018 Lombok earthquake aftershocks were mostly distributed to the depth of < 20 km with an average hypocenter slope of ~30 o and thought to be related with the rupture zone of 2018 Lombok earthquake.…”

“…Rinjani volcanic system (Afif et al, 2021;Sundhoro et al, 2000), or fluid related to the subduction system of oceanic crust in the north of Lombok (Afif et al, 2021;Priyono et al, 2021;Yang et al, 2020) contributes to the crustal anisotropy beneath the Lombok Island. The presence of fluid in the microcrack systems can preclude the microcracks to close producing higher degree of anisotropy.…”

Existence of Faults that Cause Earthquakes on Lombok Island: A Critical Literature Review

“…However, the difference is in the structure's location, which is in Lombok's northern region (Widyarta et al, 2020). Another tomographic study by Afif et al (2021) and Priyono et al (2021) found that aftershocks formed an east-west cluster due to the low seismic velocities of the local medium. High temperature, fluid, and numerous fractures are indicated by this.…”

Hypocenter Determination using a Non-Linear Method in Bali, Lombok, and Nusa Tenggara Regions: Preliminary Result

“…One method to obtain information on subsurface structures uses seismic tomography, which requires precise earthquake hypocenters. Previous studies were related to evaluating the seismicity, structure, and subduction dynamics beneath the study area, such as local tomography inversion [3,5], Double-difference relocation method [3], and GPS observation [6]. This study used the non-linear method from Lomax to determine hypocenters around Bali and Nusa Tenggara.…”