Tsunami Hazard in Eastern Indonesia: Source Identification and Reconstruction for Historical Case Studies
Creators: name: Pranantyo, Ignatius
Publisher: name: The Australian National University
Year: 2020

description: The archipelagic country of Indonesia is vulnerable to tsunami hazard due to its tectonic setting. An updated tsunami catalogue numbers at least 133 tsunamis documented from 1608 to 2018. Approximately 80% of tsunamis in Indonesia were generated by earthquakes. Eastern Indonesia experienced almost double the number of tsunamis than western of Indonesia, as separated by the Wallace Line. It is almost certain that the Sunda subduction zone and Krakatau (including Anak Krakatau) generated all tsunamis in the western part of Indonesia. However, it is more difficult to determine the primary source of tsunamis in the eastern region. Observations of these tsunamis are documented in several tsunami catalogues. Most of the events begin with a description of ground motion felt by local people at various locations, which as then followed by a tsunami. For several major events, there was detailed information on the physical tsunami behaviour observed at several places. For events in eastern Indonesia, there is no detailed information on the primary source of the ground motion and the tsunami. The aims of this study are 1) to develop techniques to optimise information from sparse and incomplete historical accounts using three case studies from eastern Indonesia: a) the Ambon Island 1674, b) the Banda Sea 1852, and c) the Flores Island 1992 tsunamis, and 2) to identify and reconstruct the primary source of the ground motion and tsunami for each event. The Ambon Island 1674 earthquake and tsunami has the oldest detailed historical account in Indonesia. It was also the largest tsunami run-up height ever documented in Indonesia, reaching about 100 m only on the northern shore of Ambon, whereas minor tsunamis were observed at other locations. The accounts gave detailed information on the earthquake intensities and tsunami observations from Ambon and its surrounding islands. Through a process of eliminating the well-known faults around the island and tsunami modelling, the most credible source to explain the tsunami observation was determined to be a landslide from the northern shore of Ambon. The earthquake source is still unclear. However, the ground motions were caused by a local and shallow depth earthquake. This study found that the Banda Sea 1852 earthquake and tsunami was the first event known in which a major tsunami was generated by a very low-angle normal fault, in this case known as the Banda Detachment. This conclusion is reached by combining a tsunami inverse travel time simulation, an earthquake intensity inversion, and tsunami modelling. An earthquake from the Banda Detachment can generate high intensity ground motion on the Banda Islands that gradually decreases towards Ternate in the north. Moreover, a landslide triggered by the Banda Detachment explains why people at Banda Neira and Ambon observed a tsunami that arrived with a positive phase polarity, unlike previous studies hypothesizing a source on the Tanimbar Trough. The source of the Flores Island 1992 earthquake and tsunami is constrained using a finite-fault source inversion technique. In this study, multiple data types are utilised together to provide an alternative solution to the rupture area, which has never been done in previous studies of this event. Through this technique and careful analysis of the fault plane model, the strike of the earthquake is confirmed to be 70deg. This fault geometry raises new questions about segmentation on the Flores back-arc thrust. Lastly, this study recommends a major modification for tsunami and earthquake hazard in eastern Indonesia. Firstly, all of events studied potentially involved landslides, so that landslides have to be considered in any tsunami hazard assessment. Secondly, the Banda Detachment is a major tsunami and earthquake source in the Banda Sea region. Lastly, the Flores back-arc thrust is a segmented zone. These factors will dramatically change the potential seismic and tsunami hazard distribution in this region.