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2018 Sulawesi earthquake and tsunami in Indonesia, reconstruction of the earthquake that caused 4300 victims

The September 28, 2018 at 5.02pm local time a violent earthquake occurred magnitude 7.5 it fell upon Sulawesione of the large Sunda islands, in the western sector of the Malay archipelago, in Indonesiacausing violent tsunamis and a disaster of exceptional proportions. 4340 people died, mainly due to the collapses of buildings and infrastructures triggered by the earthquake, as well as mud avalanches and phenomena of liquefaction of the soil. The epicenter was located in Donggala Regency and the hypocenter at a depth of 20 km. Among the victims, just over 1200 lost life due to the tsunamis that hit the island in the half hour following the earthquake, with waves that reached maximum heights of 10 meters. According to official data, the earthquake caused 10,679 injuries, 667 missing people and over 200,000 displaced people.

The Sulawesi earthquake and tsunami: the cause

The main shock of magnitude 7.5 hit the island of Sulawesi at 5.02pm (local time). The hypocenter of the earthquake was located at one depth of 20 kmWhile the epicenter was located at 78 km north of the city of Paluthe capital and largest city of Sulawesi province in Indonesia. However, numerous tremors preceded the main earthquake in the previous three hours, in some cases reaching a magnitude of six degrees. Over 14 tremors with a magnitude close to 5 followed the main earthquake, for a total of approximately 140 secondary or aftershocks recorded in the following days.

Geographical propagation of the Sulawesi earthquake. Source: Wikimedia Commons
Geographical propagation of the Sulawesi earthquake. Source: Wikimedia Commons

The island of Sulawesi is located in the so-called Belt of Fireor “Ring of fire” a strip of territory approximately long 40,000 km that surrounds thePacific Oceanor and is characterized by intense volcanism And extremely high and violent seismicity. The Belt of Fire is indeed the most tectonically active region in the worldlocated along continental plate margins and subduction zones. Several studies following the earthquake attributed its origin to the movement of the strike-slip fault known as Palu-Korowhich extends for a length of approximately 264 km and a width of 36.75 km within the Sunda tectonic plate. The earthquake appears to have been triggered by its rupture which caused blocks of rock to move about 5 metres.

Victims and damage caused by the earthquake and tsunami

As mentioned previously, the epicenter of the earthquake was located approximately 80 km north of the capital Palu, which then had approximately 350,000 inhabitants. However, its effects spread over a radius greater than 120kmhitting the city itself hard Palu and three other major cities south of the epicenter: Donggala, Sig And Paris Moutong.

The death count stands at 4,340to which they are added 667 missing, beyond 10,600 injured and almost 224,000 displaced in 122 areas. The earthquake was classified as catastrophicwith a magnitude equal to 12 on the Mercalli scale (MCS). The damage to infrastructure was considerable, with almost 70,000 buildings destroyed or damaged by the earthquake and the coseismic phenomena generated by it. According to what is reported on the website of the National Center for Environmental Information (NCEI), the economic impact of the disaster is around 13 trillion of American dollars. The estimated economic loss exceeds 1.5 billion of American dollars.

As is often the case, most of the casualties and structural damage were not caused by the earthquake itself, but by coseismic phenomenai.e. by the permanent deformations of the soil and surface morphologies induced by the propagation of seismic waves. In the case of the Sulawesi earthquake, recorded coseismic effects include mud avalanches and phenomena of soil liquefactionwhich significantly contributed to the number of collapses. To these are added tsunami waves with a maximum recorded height of 10 meterswhich affected coastal cities, especially the regions east of Palu Bay. Only tsunamis are estimated to have caused the deaths of over 1,200 people.

Damage caused by tsunamis (A, C and D) and soil liquefaction (B) observed in the city of Donggala. Credit: Goda et al. (2019) / Frontiers in Earth Science.
Damage caused by tsunamis (A, C and D) and soil liquefaction (B) observed in the city of Donggala. Credit: Goda et al. (2019) / Frontiers in Earth Science.

Sources

NOAA Goda, K., Mori, N., Yasuda, T., Prasetyo, A., Muhammad, A. and Tsujio, D., 2019. Cascading geological hazards and risks of the 2018 Sulawesi Indonesia earthquake and sensitivity analysis of tsunami inundation simulations. Frontiers in Earth Science, 7, p.261. Naik, S.P., Mohanty, A., Sotiris, V., Mittal, H., Porfido, S., Michetti, A.M., Gwon, O., Park, K., Jaya, A., Paulik, R. and Li, C., 2023. 28th September 2018 Mw 7.5 Sulawesi Supershear Earthquake, Indonesia: Ground effects and macroseismic intensity estimation using ESI-2007 scale. Engineering Geology, 317, p.107054. Reliefweb NOAA – Sulawesi, Indonesia, Earthquake and Tsunami, September 28, 2018 Cilia, M.G., Mooney, W.D. and Nugroho, C., 2021. Field insights and analysis of the 2018 Mw 7.5 Palu, Indonesia earthquake, tsunami and landslides. Pure and Applied Geophysics, 178(12), pp.4891-4920. USGS NASA