Tsunami Hazard in Eastern Indonesia: Source Identification and Reconstruction for Historical Case Studies

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

The 1674 Ambon Tsunami: Extreme Run-Up Caused by an Earthquake-Triggered Landslide

We present an analysis of the oldest detailed account of tsunami run-up in Indonesia, that of the 1674 Ambon tsunami (Rumphius in Waerachtigh Verhael van de Schuckelijcke Aerdbebinge, BATAVIA, Dutch East Indies, 1675). At 100 m this is the largest run-up height ever documented in Indonesia, and with over 2300 fatalities even in 1674, it ranks as one of Indonesia's most deadly tsunami disasters. We consider the plausible sources of earthquakes near Ambon that could generate a large, destructive tsunami, including the Seram Megathrust, the South Seram Thrust, and faults local to Ambon. We conclude that the only explanation for the extreme run-up observed on the north coast of Amon is a tsunami generated by an earthquake-triggered coastal landslide. We use a two-layer tsunami model to show that a submarine landslide, with an approximate volume of 1 km3, offshore the area on Ambon's northern coast, between Seith and Hila, where dramatic changes in coastal landscape were observed can explain the observed tsunami run-up along the coast. Thus, the 1674 Ambon tsunami adds weight to the evidence from recent tsunamis, including the 1992 Flores, 2018 Palu and Sunda Strait tsunamis, that landslides are an important source of tsunami hazard in Indonesia

Modelling of historical tsunami in eastern Indonesia: 1674 Ambon and 1992 Flores case studies

In order to reliably assess tsunami hazard in eastern Indonesia, we need to understand how historical events were generated. Here we consider two such events: the 1674 Ambon and the 1992 Flores tsunamis. Firstly, Ambon Island suffered a devastating earthquake that generated a tsunami with 100 m run-up height on the north coast of the island in 1674. However, there is no known active fault around the island capable of generating such a gigantic wave. Rumphius' report describes that the initial wave was coming from three villages that collapsed immediately after the earthquake with width as far as a musket shot. Moreover, a very high tsunami was only observed locally. We suspect that a submarine landslide was the main cause of the gigantic tsunami on the north side of Ambon Island. Unfortunately, there is no data available to confirm if landslide have occurred in this region. Secondly, several tsunami source models for the 1992 Flores event have been suggested. However, the fault strike is quite different compare to the existing Flores back-arc thrust and has not been well validated against a tide gauge waveform at Palopo, Sulawesi. We considered a tsunami model based on Griffin, et al., 2015, extended with high resolution bathymetry laround Palopo, in order to validate the latest tsunami source model available. In general, the model produces a good agreement with tsunami waveforms, but arrives 10 minutes late compared to observed data. In addition, the source overestimates the tsunami inundation west of Maumere, and does not account for the presumed landslide tsunami on the east side of Flores Island.This research was partially funded by the Australian Department of Foreign Affairs and Trade (DFAT) through the Australian Awards Scholarship and Research School of Earth Sciences, the Australian National University’s student grant

Tsunami risk communication and management: Contemporary gaps and challenges

Very large tsunamis are associated with low probabilities of occurrence. In many parts of the world, these events have usually occurred in a distant time in the past. As a result, there is low risk perception and a lack of collective memories, making tsunami risk communication both challenging and complex. Furthermore, immense challenges lie ahead as population and risk exposure continue to increase in coastal areas. Through the last decades, tsunamis have caught coastal populations off-guard, providing evidence of lack of preparedness. Recent tsunamis, such as the Indian Ocean Tsunami in 2004, 2011 Tohoku and 2018 Palu, have shaped the way tsunami risk is perceived and acted upon. Based on lessons learned from a selection of past tsunami events, this paper aims to review the existing body of knowledge and the current challenges in tsunami risk communication, and to identify the gaps in the tsunami risk management methodologies. The important lessons provided by the past events call for strengthening community resilience and improvement in risk-informed actions and policy measures. This paper shows that research efforts related to tsunami risk communication remain fragmented. The analysis of tsunami risk together with a thorough understanding of risk communication gaps and challenges is indispensable towards developing and deploying comprehensive disaster risk reduction measures. Moving from a broad and interdisciplinary perspective, the paper suggests that probabilistic hazard and risk assessments could potentially contribute towards better science communication and improved planning and implementation of risk mitigation measures

Multi-Data-Type Source Estimation for the 1992 Flores Earthquake and Tsunami

We revisit the source of the 1992 Flores earthquake and tsunami using finite-fault inversion. We simultaneously invert teleseismic body and surface waves together with coseismic uplift/subsidence datasets. We then verify the inverted source against tsunami run-up heights along the northern coast of Flores Island and the only tide gauge recording of the tsunami. Our preferred source model provides a good fit to all the datasets, whereas previous models only explained a subset of the available data. We show that the fault geometry implies segmentation of the back-arc thrust system in the eastern Sunda Arc.This work was partly supported by the Government of Australia’s Australian Awards Scholarship program

Earthquakes and tsunamis caused by low-angle normal faulting in the Banda Sea, Indonesia

As the world’s largest archipelagic country in Earth’s most active tectonic region, Indonesia faces a substantial earthquake and tsunami threat. Understanding this threat is a challenge because of the complex tectonic environment, the paucity of observed data and the limited historical record. Here we combine information from recent studies of the geology of Indonesia’s Banda Sea with Global Positioning System observations of crustal motion and an analysis of historical large earthquakes and tsunamis there. We show that past destructive earthquakes were not caused by the supposed megathrust of the Banda outer arc as previously thought but are due to a vast submarine normal fault system recently discovered along the Banda inner arc. Instead of being generated by coseismic seafloor displacement, we find the tsunamis were more likely caused by earthquake-triggered submarine slumping along the fault’s massive scarp, the Weber Deep. This would make the Banda detachment representative not only as a modern analogue for terranes hyper-extended by slab rollback but also for the generation of earthquakes and tsunamis by a submarine extensional fault system. Our findings suggest that low-angle normal faults in the Banda Sea generate large earthquakes, which in turn can generate tsunamis due to earthquake-triggered slumping.d I.R.P. by an Australian Awards scholarship and partially by a Japan Society for the Promotion of Science Bridge Fellowship awarded to P.R.C. We also thank TGS and GeoData Ventures and R. Hall for providing the multibeam data used to derive the bathymetry image in Fig. 1. P.R.C. and J.D.G. publish with the permission of the CEO, Geoscience Australi

The 2010 Mw 7.8 Mentawai earthquake : very shallow source of a rare tsunami earthquake determined from tsunami field survey and near-field GPS data

The Mw 7.8 October 2010 Mentawai, Indonesia, earthquake was a “tsunami earthquake,” a rare type of earthquake that generates a tsunami much larger than expected based on the seismicmagnitude. It produced a locally devastating tsunami, with runup commonly in excess of 6 m. We examine this event using a combination of high-rate GPS data, from instruments located on the nearby islands, and a tsunami field survey. The GPS displacement time series are deficient in high-frequency energy, and show small coseismic displacements (<22 cm horizontal and <4 cm subsidence). The field survey shows that maximum tsunami runup was >16 m. Our modeling results show that the combination of the small GPS displacements and large tsunami can only be explained by high fault slip at very shallow depths, far from the islands and close to the oceanic trench. Inelastic uplift of trench sediments likely contributed to the size of the tsunami. Recent results for the 2011 Mw 9.0 Tohoko-Oki earthquake have also shown shallow fault slip, but the results from our study, which involves a smaller earthquake, provide much stronger constraints on how shallow the rupture can be, with the majority of slip for the Mentawai earthquake occurring at depths of <6 km. This result challenges the conventional wisdom that the shallow tips of subduction megathrusts are aseismic, and therefore raises important questions both about the mechanical properties of the shallow fault zone and the potential seismic and tsunami hazard of this shallow region.Published versio

Tsunami risk communication and management: Contemporary gaps and challenges

Very large tsunamis are associated with low probabilities of occurrence. In many parts of the world, these events have usually occurred in a distant time in the past. As a result, there is low risk perception and a lack of collective memories, making tsunami risk communication both challenging and complex. Furthermore, immense challenges lie ahead as population and risk exposure continue to increase in coastal areas. Through the last decades, tsunamis have caught coastal populations off-guard, providing evidence of lack of preparedness. Recent tsunamis, such as the Indian Ocean Tsunami in 2004, 2011 Tohoku and 2018 Palu, have shaped the way tsunami risk is perceived and acted upon. Based on lessons learned from a selection of past tsunami events, this paper aims to review the existing body of knowledge and the current challenges in tsunami risk communication, and to identify the gaps in the tsunami risk management methodologies. The important lessons provided by the past events call for strengthening community resilience and improvement in risk-informed actions and policy measures. This paper shows that research efforts related to tsunami risk communication remain fragmented. The analysis of tsunami risk together with a thorough understanding of risk communication gaps and challenges is indispensable towards developing and deploying comprehensive disaster risk reduction measures. Moving from a broad and interdisciplinary perspective, the paper suggests that probabilistic hazard and risk assessments could potentially contribute towards better science communication and improved planning and implementation of risk mitigation measures

Tsunami risk communication and management: Contemporary gaps and challenges

Very large tsunamis are associated with low probabilities of occurrence. In many parts of the world, these events have usually occurred in a distant time in the past. As a result, there is low risk perception and a lack of collective memories, making tsunami risk communication both challenging and complex. Furthermore, immense challenges lie ahead as population and risk exposure continue to increase in coastal areas. Through the last decades, tsunamis have caught coastal populations off-guard, providing evidence of lack of preparedness. Recent tsunamis, such as the Indian Ocean Tsunami in 2004, 2011 Tohoku and 2018 Palu, have shaped the way tsunami risk is perceived and acted upon. Based on lessons learned from a selection of past tsunami events, this paper aims to review the existing body of knowledge and the current challenges in tsunami risk communication, and to identify the gaps in the tsunami risk management methodologies. The important lessons provided by the past events call for strengthening community resilience and improvement in risk-informed actions and policy measures. This paper shows that research efforts related to tsunami risk communication remain fragmented. The analysis of tsunami risk together with a thorough understanding of risk communication gaps and challenges is indispensable towards developing and deploying comprehensive disaster risk reduction measures. Moving from a broad and interdisciplinary perspective, the paper suggests that probabilistic hazard and risk assessments could potentially contribute towards better science communication and improved planning and implementation of risk mitigation measures