Preprints
https://doi.org/10.5194/nhess-2021-343
https://doi.org/10.5194/nhess-2021-343

  20 Nov 2021

20 Nov 2021

Review status: this preprint is currently under review for the journal NHESS.

Tsunami hazard in Lombok & Bali, Indonesia, due to the Flores backarc thrust

Raquel Felix1, Judith Hubbard1,2, Kyle Bradley1,2, Karen Lythgoe2, Linlin Li3,4, and Adam Switzer1,2 Raquel Felix et al.
  • 1Asian School of the Environment, Nanyang Technological University, Singapore
  • 2Earth Observatory of Singapore, Nanyang Technological University, Singapore
  • 3School of Earth Sciences and Engineering, Sun Yat-sen University, Zhuhai, China
  • 4Southern Marine Science and Engineering Guangdong Laboratory (Zhuhai), Zhuhai, China

Abstract. The tsunami hazard posed by the Flores backarc 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 1–2.5 m-high local tsunamis along the northern coast of Lombok (Wibowo et al., 2021). 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+), 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 (Lythgoe et al., 2021), together with a high-resolution bathymetry dataset developed by combining direct measurements from 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 < 8 minutes and Denpasar in ~10–15 minutes, with multiple waves. For an earthquake with 3–5 m of coseismic slip, Mataram and Denpasar experience maximum wave heights of ~1.3–3.3 m and ~0.7 to 1.5 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 city 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.

Raquel Felix et al.

Status: open (until 03 Jan 2022)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse

Raquel Felix et al.

Raquel Felix et al.

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Short summary
The Flores thrust lies along the north coasts of Bali and Lombok. We model how an earthquake on this fault could trigger a tsunami that would impact the capital cities of Mataram and Denpasar. We show that for 3–5 m of slip on the fault (a Mw 7.7–7.9+ earthquake), the cities would experience a wave 1.3–3.3 and 0.7–1.5 m high, arriving in < 8 and ~10–15 mins, respectively. The earthquake would also cause subsidence of the cities by 20–40 cm, resulting in long-term exposure to coastal hazards.
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