Tsunami hazard in Lombok and Bali, Indonesia, due to the Flores back-arc thrust

Abstract

Abstract. The tsunami hazard posed by the Flores back-arc thrust, which runs along the northern coast of the islands of Bali and Lombok, Indonesia, is poorly studied compared to the Sunda Megathrust, situated ∼250 km to the south of the islands. However, the 2018 Lombok earthquake sequence demonstrated the seismic potential of the western Flores Thrust when a fault ramp beneath the island of Lombok ruptured in two Mw 6.9 earthquakes. Although the uplift in these events mostly occurred below land, the sequence still generated local tsunamis along the northern coast of Lombok. Historical records show that the Flores fault system in the Lombok and Bali region has generated at least six ≥Ms 6.5 tsunamigenic earthquakes since 1800 CE. Hence, it is important to assess the possible tsunami hazard represented by this fault system. Here, we focus on the submarine fault segment located between the islands of Lombok and Bali (below the Lombok Strait). We assess modeled tsunami patterns generated by fault slip in six earthquake scenarios (slip of 1–5 m, representing Mw 7.2–7.9+) using deterministic modeling, 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, together with a high-resolution bathymetry dataset developed by combining direct measurements from the General Bathymetric Chart of the Oceans (GEBCO) with sounding measurements from the official nautical charts for Indonesia. Our results show that fault rupture in this region could trigger a tsunami reaching Mataram in <9 min and Denpasar in ∼ 23–27 min, with multiple waves. For an earthquake with 3–5 m of coseismic slip, Mataram and Denpasar experience maximum wave heights of ∼ 1.6–2.7 and ∼ 0.6–1.4 m, respectively. Furthermore, our earthquake models indicate that both cities would experience coseismic subsidence of 20–40 cm, exacerbating their exposure to both the tsunami and other coastal hazards. Overall, Mataram is more exposed than Denpasar to high tsunami waves arriving quickly from the fault source. To understand how a tsunami would affect Mataram, we model the associated inundation using the 5 m slip model and show that Mataram is inundated ∼ 55–140 m inland along the northern coast and ∼230 m along the southern coast, with maximum flow depths of ∼ 2–3 m. Our study highlights that the early tsunami arrival in Mataram, Lombok, gives little time for residents to evacuate. Raising their awareness about the potential for locally generated tsunamis and the need for evacuation plans is important to help them respond immediately after experiencing strong ground shaking.

A preliminary report on seismicity declustering methods and completeness magnitude in eastern Sunda Arc

“…This is possible due to the presence of the subduction megathrust zone close to the Sunda Trench with high seismotectonic activities [17,25,27]. However, large events with 𝑀 w ≥ 6.5 are more recorded in the northern than in the southern, indicating the potential threats of the Flores Back-arc for generating tsunamigenic events, consistent with previous studies concerning with tsunamis generated by back-arc thrust faults [16,24]. Hence, the results obtained from this study could partially contribute to improve seismic hazard analysis and risk assessment in NTB and NTT, the eastern provinces in Indonesia prone to seismic threats.…”

“…Using these zones, we are able to capture seismicity rate and vulnerability in each province and in the northern side or the southern side of the provinces. This has considered two possible sources of past and future events, the one associated with the Flores Back-arc Thrusting Fault to the north and the other, the subduction zone to the south of the provinces [16,17,18,24,25].…”

“…Regarding the 1992 severe Flores tsunami following a strong ground motion of 𝑀 w 7.8 epicentred to the north of the island [24,26] and the same possible future event in the regions of interest, we need to explore the data in Table 2 by changing them into qualitative statement based on the local vulnerability for each region of interest. Hence, we provide in Table 3 the final results of all the calculations in terms of qualitative information, showing whether the choices of algorithm and technique of 𝑀 c determination are self-consistent.…”

Sensitivity Analysis of Tsunami Heights to Shallow Bathymetric Resolution

“…All the points are then interpolated using the 'Topo to Raster' tool in ArcGIS which is based on the ANUDEM model 24 designed to generate elevation models with hydrologically correct morphology. Although this method was developed for subaerial water flow, it has been effectively used in recent tsunami numerical modeling studies [25][26][27] . The geoprocessing workflow of the whole interpolation process, from the random point extraction to the bathymetry interpolation, is semi-automated using the visual programming language ModelBuilder in ArcGIS.…”

SEATANI: hazards from seamounts in SouthEast Asia, Taiwan, and Andaman and Nicobar Islands (eastern India)

“…First of all, we would need to improve our collaborative effort with private and government agencies, which may have seismic and bathymetry data that may improve our understanding of volcanic hazards from submarine volcanoes in the region. Second, we can partially improve the existing bathymetry datasets, by combining direct bathymetric information from Gebco and from local nautical charts (Felix et al, 2022), this will help with a better regional seamount characterisation, hazard assessment, and eventually hazard modelling and impact analysis at key locations. Third, we will use new satellite altimetry data of the sea surface, which will be made available from NASA in 2023 through the SWOT (Surface Water and Ocean Topography) mission, which was launched in December 2022.…”