Tsz Man Fisher (Lau) and Dr. Ronald Harris, Department of Geological Science

The original project was titled, “Journey to the Center of the Earth: Investigating Subduction Channel Processes in Southern Tibet.” Complications came up when we began to work on this project last summer. With the permission of the ORCA committee, we changed the project into tsunami modeling instead of investigating subduction channel in Tibet.

The purpose of this project is to re-analyze the earthquake and tsunami hazards in Eastern Indonesia through historical records. We are able to model two historical events so far. And computer modeling of these two historical tsunamis has already reviled a high potential for large mega-thrust earthquakes and tsunamis in the Eastern Indonesia region.

Pre-instrumental tectonic events are documented through the effort of Arthur Wichmann (Wichmann 1918, 1922). This record extends up to 400 years of extreme geophysical events that have inflicted heavy damage at Dutch out-posts throughout the Indonesian region. This record has been recently translated into English and organized by students in the Department of Geological Science. The result of their investigations of historical earthquake and tsunami sites was presented in a previous American Geological Union meeting(Major, 2009).

Hoping to further utilize the historical records, we modeled some of these events to locate possible earthquake epicenters. Ancient earthquake are hints to hidden present seismic threads.

Based on the described tsunami heights during the events, the Flores Thrust Earthquake in 1820 and Banda Arc Earthquake in 1852 are recreated through a seimic deformation model (Okada, 1985) and a numerical tsunami model (Satake, 2002). By inputting the parameters and magnitudes of each earthquake, we get the tsunami height at designated tidal gauges from modeling. These results are then compared to the records to verify. We were able to locate the earthquakes and their parameters without much surprise. Our results show that these historical earthquakes initiated along a developing backarc thrust system in the Flores Sea and along the thrust front of the Banda arc-continent collision zone respectively. However, when we tried to determine the magnitudes of these earthquakes, we have some new discovery.

At least a Mw 7.8 earthquake was responsible for the 18-24 m tsunami in 1820 along the south coast of Makassar. Shaking was felt up to 850 km across the Flores Sea. The most likely source for this event is the Flores Thrust, which also produced a much smaller tsunami from a Mw 7.8 earthquake in 1992. To account for much larger wave heights observed locally (18-24 m) we infer there was either a nearby submarine
landslide or another earthquake took place at the south coast of Makassar. We further suggested that the latter took place because of the violent ground shaking that was observed by the victims. This finding implies reactivation of a buried ancient fault.

Another >Mw 8.5 earthquake took place along the Banda Arc in 1852. This earthquake was felt as far away as Java in Western Indonesia, 2000 km away. We initially identified the source as the southern part of the Banda Arc. However, the tsunami wave heights from our modeling did not match the historical record. More modeling needs to be done to investigate the origin of this event. Nevertheless, some of our models has suggested that the source of this earthquake as the northern part of the Banda Arc.

Some of the most notable events during the 19th century in the Sumatran region have already reoccurred and claimed over 300,000 lives. Many of the events have yet to reoccur. Based on the seismic history of the Banda Arc region and the large amount of strain that has accumulated during the past 160 years of relative seismic quiescence, it is highly likely that more mega-thrust earthquakes and tsunami are imminent. These events will affect 10 times more people and much larger cities than those of the past.

We presented our research in the American Geological Union meeting this past December. Many modeling expertise gave us some positive feedback and suggestion to improve our studies. It was a very valuable experience. We realized that our modeling gave us a general idea and confines of the two earthquake events mentioned. However, there are a lot of improvements can be made in the continuation of this research.

Reference

• Okada, Y. (1985). Surface deformation due to shear and tensile faults in a half-space. Bulletin of the Seismological Society of America, 75(4), 1135-1154. Retrieved from http://search.proquest.com/docview/51292948?accountid=4488
• Satake, K., Smith, J. R., & Shinozaki, K. (2002). Three-dimensional reconstruction and tsunami model of the nuuanu and wailau giant landslides, hawaii. Geophysical Monograph, 128, 333-346. Retrieved from http://search.proquest.com/docview/51932701?accountid=4488
• Wichmann, A., Die erdbeben des Indischen Archipels bis zum Jahre 1857 (Earthquakes of the Indian Archipelago Until the Year 1857), in Afdeling Natuurkunde Nederlandse Verhandelingen, 20 (1918): 193p.
• Wichmann, A., Die erdbeben des Indischen Archipels von 1858 bis 1877 (Earthquakes of the Indian Archipelago From 1858 to 1877), in Afdeling Natuurkunde Nederlandse Verhandelingen, 22 (1922): 209 p.