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Crustal Deformations, Earthquake and Tsunami Hazards of the Sumatran Plate Margin : Presentasi RUTI IV



Crustal Deformations, Earthquake and Tsunami Hazards
of the Sumatran Plate Margin

Danny Hilman Natawidjaja
RC Geotechnology, Indonesian Institute of Sciences (LIPI), danny@geotek.lipi.go.id
Kerry Sieh
Tectonic Obsevatory, California Institute of Technology (Caltech), sieh@gps.caltech.edu

Abstract

The Sumatran plate boundary is one of the highest seismicity in the world. In the past dozens of large earthquakes (M≥7) have occurred along the Sumatran fault on land, and several giant earthquakes (M≥8) on the shallow subduction zone. However, these have been largely underestimated until the 2004 Aceh-Andaman megathrust-tsunami event occurred. Now, many experts from multi-disciplines have been focusing their studies on Sumatran earthquake and tsunamis. This RUTI project aims to understand better the short-term and long-term behaviors of the Sumatran earthquakes, especially the megathrust. We use multi-disciplinary approach to investigate how the tectonic-strain slowly accumulating for a very long time and then suddenly releasing the strain on a very large earthquake event. First, before this project started, we have been using coral microatolls that abundantly fringe the islands west of Sumatra to study the past tens to hundreds of years of crustal-deformation processes related to very large earthquake cycles. The growths of microatolls are very sensitive to sea-level fluctuations or uplift-subsidence, so they serve as natural geodetic recorders. We have been able to discover the old giant tsunami earthquakes that occurred in 1797 and 1833 and the tectonic processes before and after the events. In 2005, after the giant Aceh-Andaman 2004 and Nias-Simelue 2005 earthquakes we have thoroughly documented the tectonic uplift and subsidence using the microatolls that were raised or subsided tens of centimeters to meters using a similar principal. The results of this study enable us to tightly constraint the sources of these two events very accurately. Continuing this work, in 2006 and 2007 (ongoing) we use microatolls to investigate the predecessor, old giant earthquakes of the 2004 and 2005-like events, including the 1861 megathrust event in Nias-Simelue that produce large tsunami. Second, we use the continuous Global Positioning System (cGPS ) technology to study recent crustal movements related to subduction earthquake processes. In 2002, we had started to deploy 6 cGPS stations thoughout Mentawai and Batu Islands. We called our new GPS network SuGAr (Sumatran GPS Array). By the end of 2004, we have installed 12 cGPS that covered Mentawai and Batu islands. During the RUTI period, in 2005 and 2006 years, we deployed 15 more cGPS, so now we have 27 cGPS stations that cover Aceh, Simelue, Banyak, Nias, Batu, Mentawai, West Sumatra, and Bengkulu areas. In 2007 (ongoing) we plan to install 6 more cGPS stations in the region so then we will have more accurate observations of earthquake-crustal movements after the recent megathrusts and before the next one. Our GPS data have been able to determine the locked subduction interface around the Menawai region, which is a locus of the future giat earthquake in this region. In 2005 and 2006, we had successfully applied satellite-phone system and developed related hardwares and softwares to automatically transfer the cGPS data in daily basis. In 2007, to improve the capability of the system, we will be experimenting with high-rate cGPS technology and real-time GPS processing, so then they will be ready if we want to use SuGAr for GPS-based Tsunami Early Warning System (TEWS). Third, since 2004, we have equipped SuGAr with six short-period digital seismometers and 4 strong-motion seismometers. In 2005-2006, we have analyzed data of the 6 SP seismometers and to locate aftershocks of the 2004 and 2005 giant earthquakes and hence to determine the geometry of the subduction interface more accurately. In 2007 (ongoing) we will deploy 6 broadband seismometers in collaboration with BMG. Three of them will be operating in real-time mode for the national TEWS. We have used geophysical and seismological modeling to characterize the Sumatran-Plate margin’s earthquakes. Based on seismicity and crustal models, we have been able to constraint where the high-potential areas for the next very large earthquakes in the region. Fourth, using the earthquake sources constrained by our coral, GPS, and seismic data, we have been developing plausible tsunami models for Sumatra. These include constructing tsunami models for the 2004 and 2005 earthquakes as well as for an expected future very-large tsunami in West Sumatra and Bengkulu if the megathrust under Mentawai ruptures. In 2006, we had updated our Sumatran tsunami models and developed tsunami inundation models for Aceh, Nias, west Sumatra and Bengkulu coasts. Beside, these technical and scientific aspects, we have also created several public posters and brochures based on our discoveries to educate and raise awareness of people living these vulnerable areas.



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