| dc.description.abstract | Study on Seismic Activity at The Sumatra Subduction Zone: Large Earthquake Sequences, Seismic Hazard and Subduction Slab Properties
ABSTRACT
The Sumatra subduction zone exhibit as one of the most actives convergence plate boundary. The study on the seismic activity along this margin will give new perspectives on how the interaction between the earthquake occurrences on the governing active unidentified-identified faults, the hazard that might result by these faults activity and the properties of the subducting slab are considered important. The tectonic setting of the Sumatra, as well as the increasing amount of observational geophysical data, gives the opportunity to study these problems.
The oblique subduction of Australian oceanic plate beneath the continental Sunda plate has produced large – mega earthquake with most of them has induced significant lost not only at regional scale, but also worldwide. Three major faults system occupied the tectonic processes at present day Sumatra subduction zone, i.e. 1. Thrust fault at the subduction zone, 2. The Mentawai Fault (MF) (right-lateral strike-slip) and 3. The Sumatra Fault Zone (SFZ) (right-lateral strike-slip). Thus, having a broad scale problem along this subduction zone, the study brought into this dissertation is covering those major faults system though some limitations are unavoidable due to the availability of the geophysical dataset and or observation along this margin. Though, the data are somewhat limited, however, most of the dataset is obtained from local observation with limited access. Therefore, this dissertation is expected to provide new information and at a higher level of observational resolution. The detailed structure of the dissertation and the relationship of the studies within each chapter is discussed in Chapter 1.
An opportunity to analyze the earthquake event catalogs with their travel time provided by the Badan Meteorologi, Klimatologi dan Geofisika (BMKG) Indonesia is established in Chapter 2. This work showed that the data records by the local institution BMKG on a regional scale within two years (2009 to 2011) cover a higher resolution of datasets compare to the freely available dataset managed by the international institution, e.g. International Seismological Centre (ISC). Accordingly, by having a dense and high resolution dataset, we relocate the earthquake events based on hypocentre double difference techniques with a refined velocity model. The result shows quality improvement of the dataset with clear large earthquake sequences being observed, i.e. the Padang earthquake, the Banyak earthquake, and the Mentawai tsunami earthquake sequences. A southeast dipping of earthquake lineation was identified within the Banyak Islands earthquake sequence and considered as the activity of the upper splay fault. Limited earthquake activity was identified at the Mentawai gap region and considered as a locked asperity. Those faults could be as a major threat of a great source of tsunamigenic fault.
Although only limited time range of the earthquake event catalog used in Chapter 2, limited earthquakes along the SFZ were observed. The seismicity along SFZ has its magnitude between 6 and 7.7. On that account, the SFZ is capable to produce a large magnitude of earthquakes. We investigate the northern end of the SFZ and its hazard in Chapter 3. In this chapter, based on two reprocessed seismic reflection profiles, the image shows that the extended Sumatra Fault Zone strongly related to some chaotic facies indicating an evidence of large triggered submarine landslide ever occurred in this region. Tsunami scenarios were simulated with a combined source of fault activity and submarine landslide. In extreme case, by these combined mechanisms, if the earthquake as large as 7 Mw or larger and triggered a submarine landslide with a length of 600 meters and width of 300 meters, it could produce a tsunami as high as 4 meters along the coast. Hence, in this chapter it is strongly showed the potential tsunami hazard related to the submarine landslide triggering of a strike slip fault system, this mechanism may explain also some history tsunami in this area without a clear earthquake signature inland, and imply the necessity of landslide tsunami early warning system in this area.
The two chapters (Chapter 2 and 3) discussed in this dissertation are covered the major faults system along the Sumatra subduction zone. The other earthquake activities that are considered important but being paid less attention and or sometimes neglected are the intermediate depth earthquake which has a depth range greater and equal to 100 km depth. Acknowledged the advantages of the Sumatra tectonic setting, i.e. which has the forearc islands close to the trench and just 20-30 km above the subduction plate interface, it gives the opportunity to investigate the detailed properties of the subducting plate. In Chapter 4, by utilizing the observation of body wave dispersion at the forearc stations, we could estimate the dispersion curve that caused by a waveguide structure which acts as a low velocity layer (LVL) from the former oceanic crust existed at the intermediate depth (greater and equal to 100 km). A velocity reduction of 2 – 4 % resulted from this dispersion showed an agreement to the observation at other subduction zones with similar age of the subducting plate around the world (e.g. Alaska subduction zone). The significant delay for a certain frequency range that observed from the dispersion curve may indicate the thickness of the LVL. The finding of the LVL is considered important in understanding the occurrence of the intermediate – deep earthquake as well as the fate of the subducting plate that undergo the subduction processes. | en_US |