Potential Risk of a Tsunami in Southern Java Originating from the Sunda Megathrust Earthquake

We have always been taught that Indonesia is a country rich in resources, gemah ripah loh jinawi. Fertile land, rich and prosperous nature and peaceful conditions. However, behind the wealth and tranquility of the land, lies the risk of real danger. Sitting on the Pacific Ring of Fire, the territory of Indonesia lies at the confluence of 3 giant plates. Indonesia actually inhabits an area that is very tectonic and volcanically active. The consequence of this confirms the harsh reality that geological disasters and their derivatives will become a scourge that is familiar to people's lives.

With 150 million people, Java Island is the island with the most population, not only in Indonesia, but throughout the world. With unique geographical and geological conditions, from the western tip to the eastern tip of Java Island, there are many mountains, mountains, and volcanoes. An island, Java Island is surrounded by the sea in all directions. To the north is the Java Sea, to the west is the Sunda Strait, to the east is the Bali Strait, and to the south is the Indian Ocean. This means that the risk of a tsunami on the coast of Java Island exists and will continue to exist.

The threat of this danger is very serious, considering that in the south of the island of Java there is a giant plate fault which is a meeting between 2 plates, namely the Indo-Australian Plate and the Sunda Plate. The area where the two plates collide is called the subduction zone. Specifically, the confluence of the Indo-Australian Plate and the Sunda Plate is called the Sunda Megathrust Subduction Zone. This fault stretches for 5,500 km from Bangladesh, curving to the west and south of Sumatra Island, Java Island, Bali Island and eastern Indonesia to end in northwest Australia (Kerry S., 2007).

Sunda Megathrust

Before discussing Sunda Megathrust, the author will try to explain what a "Megathrust" is. The contact surface between the underthrusting and overriding plates in the subduction zone is called a megathrust fault (Bilek & Lay, 2018). Meanwhile, a subduction zone is a meeting zone between two colliding plates. The mechanism of earthquakes that occur in the Megathrust zone is usually a thrust-fault or rising fault.

Megathrusts generally extend from troughs on the deep ocean floor and beneath the fringes of continental rocks. The fact that most large megathrust earthquakes are underwater poses other dangers besides the vibrations caused by the earthquake itself. If the megathrust suddenly ruptures, the result is that the energy from the vibration can push a large volume of seawater onto it and cause a tsunami. Tsunamis are large waves that radiate out from the epicenter of vibration and can quickly cross the open ocean and even oceans and can land tens to thousands of kilometers away as a series of waves that can reach tens of meters high.

The Sunda megathrust stretches south from Bangladesh, curving around the western and southern sides of Sumatra, Java, Bali and eastern Indonesia to northwestern Australia for about 5500 km. (Kerry, S., 2007). In general, the Sunda Megathrust is segmented into several parts. These include the Aceh-Andaman segment, the Mentawai segment, the Nias-Simeulue segment, the southern Java segment, and the northern Australian segment. Each segment has a different earthquake risk.

Earthquake and Tsunami Risk from the Sunda Megathrust

The southern segment of Java from the Sunda Megathrust has a long history of recorded seismic activity dating back hundreds of years. Located off the southern coast of Java, it is estimated that the seabed setting is dated to the Jurassic era, accompanied by thick layers of sediment and subducting plates beneath the Sundaland margin in the Java Trench (Harris, R., & Major, J., 2017).

The distribution of the earthquake epicenter in South Java with a magnitude > 4.0 (Widiyantoro, et al., 2020)

This southern segment of Java poses a major threat to the population on the South Coast of Java Island. The recent absence of major earthquakes can be an indication that a tsunamigenic (tsunami-generating) earthquake in the South Coast of Java is a potential threat. Therefore, the initiation of an effective early warning system needs to be a top priority. This is because many people who live in coastal areas at high risk of a tsunami have little time to evacuate (Widiyantoro et al., 2020).

The results of the tsunami modeling that Widyantoro, et al., (2020) carried out showed that in the worst case scenario, in which the Sunda Megathrust of the Southern Java Segment broke simultaneously, then this event could cause a tsunami off the southern coast of Java. This worst-case scenario earthquake is modeled to generate tsunami heights of up to 20.2 m and 11.7 m in West Java and East Java, respectively.

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Modeling of tsunami sources off the South Coast of Java (Widiyantoro, et al., 2020)

The maximum height of the tsunami in the entire model area for the duration of the simulation (Widiyantoro, et al., 2020)

The maximum height of the tsunami along the southern coast of Java based on the location of east-west longitude (Widiyantoro, et al., 2020)

The simulation of the Megathrust earthquake scenario for the Southern Java segment modeled by Widiyantoro, et al (2020) is the worst scenario in which 2 sub-segments of this segment break at once and cause a Megaquake or a large earthquake with a magnitude of 9.1 (Mw 9.1). The simulation shows that the average tsunami height will be around 4.5 meters across the South Coast of Java Island. However, the worst-case scenario simulation here is still a conservative model that does not take into account the potential for underwater landslides that can be a factor that strengthens a tsunami as happened in the 2018 Palu Earthquake and Tsunami Disaster.

REFERENCE

Bilek, S. L., & Lay, T. (2018). Subduction zone megathrust earthquakes. Geosphere, 14(4), 1468-1500.

1. Harris, R., & Major, J. (2017). Waves of destruction in the East Indies: the Wichmann catalogue of earthquakes and tsunami in the Indonesian region from 1538 to 1877. Geological Society, London, Special Publications, 441(1), 9-46.
2. Philibosian, B., Sieh, K., Avouac, J. P., Natawidjaja, D. H., Chiang, H. W., Wu, C. C., ... & Wang, X. (2017). Earthquake supercycles on the Mentawai segment of the Sunda megathrust in the seventeenth century and earlier. Journal of Geophysical Research: Solid Earth, 122(1), 642-676.
3. Sieh, K. (2007). The Sunda megathrust---past, present and future. Journal of Earthquake and Tsunami, 1(01), 1-19.
4. Widiyantoro, S., Gunawan, E., Muhari, A., Rawlinson, N., Mori, J., Hanifa, N. R., ... & Putra, H. E. (2020). Implications for megathrust earthquakes and tsunamis from seismic gaps south of Java Indonesia. Scientific reports, 10(1), 1-11.