NERC Urgency Sulawesi tsunami 2018

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At 6pm local time on the September 28th 2018, a strike-slip earthquake, Mw 7.5, struck the west of Sulawesi Island in Indonesia. Earthquake shaking immediately destroyed large areas of Palu City, the nearest large town of 135,000 people. Further damage was caused by liquefaction of underlying, fine-grained fluvial sediments, which mobilised mudslides that swept through the city. Soon after the earthquake, tsunamis, with elevations of up to 11 m inundated local coastlines causing further destruction, especially to Palu City, and Donggal farther north. At present, 2,100 people are known to have died in the event with over 700 missing. A significant number of these died in the tsunamis.Recent tsunamis provide critical information on mechanisms and impacts which provide essential data on informing on hazard and risk. The mechanism of the Sulawesi earthquake is significant because it is different to recent devastating events, such as Papua New Guinea (1998), Indian Ocean (2004) and Japan (2011), which were triggered by thrust faulting. Sulawesi is a strike-slip rupture with dominantly horizontal movement. Present understanding of earthquake tsunami generation suggests that, because of the absence of significant vertical seabed elevation, it should not have generated the recorded (up to 11m) tsunamis. Preliminary numerical tsunami modelling confirms this understanding. An additional tsunami mechanism is required, and submarine landslides are most likely. Numerical modelling of tsunamis require validation from field observations to confirm their accuracy. These observations have to be made as soon as possible after impact because much of the evidence on the tsunami and its' scale is extremely fragile and short-lived. For example, the directions of flow may be identified by flattened grass; tsunami flow depth and elevation may be identified from water marks on buildings or building damage, vegetation stripping and debris caught in trees or bushes. Sediment deposited, especially in monsoonal conditions, is rapidly eroded or removed. It is vital therefore that after a major tsunami, field observations are made and the research carried out as soon as practicable. Here, for Sulawesi, we propose an urgent response field survey, based on pre-survey interpretations of before and after high-resolution satellite imagery. The results will identify in detail geographical variations on tsunami inundation which will inform on the impact, potential tsunami mechanisms and offer validation for numerical tsunami models. This methodology (desk study followed by field work) was successfully applied to the Japan 2011 tsunami, and is a first critical step towards developing an integrated system for interoperable digital field data collection. From the present field surveys there is information over much, but not all, of the area impacted by the tsunami (see 2). There is a close correlation between the offshore strike-slip rupture (mapped from remote - inSAR data) and field evidence for a tsunami from inundation measurements. There is also correlation between the offshore rupture and coastal subsidence, which supports coastal/submarine landslides as the tsunami mechanism. For the proposed survey, PI Tappin leads an international and multidisciplinary initiative (UK, Indonesia, US and Poland) to study the Sulawesi tsunami. This approach is most likely to fully understand the event, and is required because, at present tsunami warning in Indonesia is based on far-field earthquakes. Locally triggered events, such as Sulawesi, where there was minimal warning, are not at present addressed. More broadly, the project will contribute to an improved understanding of locally, strike-slip triggered, submarine landslide tsunamis and their hazard. The project will inform on improved mitigation strategies in this context. The research will form the basis for future, more in depth research.