Significant tsunami hazards in Indonesia from landslide and volcanic sources

Tsunamis from landslide and volcanic sources have been responsible for significant destruction and fatalities worldwide as evidenced most recently during the January 2022 Tonga volcanic tsunami (Heidarzadeh et al., 2022: https://doi.org/10.1016/j.oceaneng.2022.112165). Indonesia is a hot spot for such tsunamis from landslide and volcanic sources as the region suffered from destructive events in the past, such as the 1883 tsunami following the Krakatau eruption which costed at least 36,000 lives. More recently the region was struck by the 2018 Anak Krakatau volcanic tsunami with approximately 450 deaths, and the 2018 Palu (Sulawesi) tsunami with more than 4,000 casualties. Therefore, it is vital to further study the generation potential and mechanisms of such tsunamis and to improve hazard knowledge base.

Here, we study three recent tsunamis in Indonesia, two of which occurred following an earthquake while the other one occurred following a volcanic eruption. All three have a landslide component in their sources: the June 2021 Seram Island tsunami (earthquake), the December 2018 Palu tsunami (earthquake), and the December 2018 Anak Krakatau tsunami (volcanic eruption).

A tsunami was observed on 16^th June 2021 in Seram Island following an M_w 5.9 earthquake. The tsunami amplitude was approximately 50 cm at Tehoru tide gauge whereas two other stations showed amplitudes of less than 4 cm. Such a relatively large tsunami (50 cm) is unexpected from a normal-faulting M_w 5.9 earthquake. We hypothesize that that a secondary source (i.e., a landslide) was involved. We applied tsunami modelling and source analysis to examine this hypothesis. Tsunami simulations confirmed that that the earthquake could only have contributed to a few centimeters of the tsunami and thus cannot reproduce the 50 cm waves. However, we could reproduce the tsunami observations using a landslide source. For more information see here: https://doi.org/10.1785/0120210274.   

Regarding the September 2018 Palu tsunami, it is now commonly accepted that a submarine landslide should have most likely contributed to the tsunami generation in addition to the earthquake. However, the nature of the landslide whether submarine or subaerial, and the contribution of the two sources are not clear. We propose a novel dual landslide-earthquake source that explains most of the observation of the 2018 Palu event. Our dual model comprises the USGS earthquake model (length = 264 km, width = 37 km, slip = 0 – 8.5 m) combined with a submarine landslide with a length of 1.0 km, a width of 2.0 km, and a thickness of 80.0 m. For more information see here: https://doi.org/10.1080/21664250.2022.2122293.         

For the December 2018 Anak Krakatau tsunami, we present the results of our field surveys. We surveyed 29 locations and measured tsunami runups from 0.9 m to 5.2 m, tsunami heights from 1.4 to 6.3 m, and inundation distances from 18 to 212 m. For more information, see here: https://doi.org/10.1007/s00024-020-02587-w.

We also discuss future directions towards expanding our limited understanding of tsunamis from landslide and volcanic sources in Indonesia which are often unpredictable and deadly. This research is funded by The Royal Society (UK), grant number CHL/R1/180173.