Tectonic Geodesy: An Analysis of the Crustal Deformation of the Western Sundaland Plate from Nearly Two Decades of continuous GPS Measurements
Yong, Chien Zheng
The Sundaland plate (hereinafter, Sunda plate) is located in a tectonically active region where the Eurasia, India-Australia, Yangtze, Burma, Molucca Sea, Banda Sea, and Timor plates converge and share common boundaries. It is known that great earthquakes typically rupture along subduction plate boundaries, and this is the case for the Sunda megathrust. On 26th December 2004, a thrust earthquake of Mw 9.0 that ruptured a ~1,300 km long segment, initiated from off the coast of northern Sumatra (Aceh) to the south of Andaman Island, and marked the beginning of an active seismological period. Three months later, another thrust earthquake of Mw 8.6 ruptured to the south of the 2004 event. Further south, the 2007 Mw 8.5 Bengkulu thrust earthquake ruptured off the shore of Bengkulu and had two massive aftershocks of Mw 7.9 and Mw 7.0 in less than 24 hours’ following the event. Most recently, the 2012 Mw 8.6 and Mw 8.2 Wharton Basin (WB) strike-slip earthquakes ruptured a diffuse boundary of the Indian and Australian plate that accommodates the present-day plate motion of the Sunda plate. This dissertation presents the geodetic analysis of 143 continuous GPS (cGPS) measurements that span a large region of the Sunda plate, including Indonesia (Sumatra), Malaysia (Peninsular Malaysia, Sabah and Sarawak), India (Nicobar–Andaman Islands), Thailand, the Philippines and Singapore, between 1999.0 and 2015.9. During the 2004 and 2005 earthquakes, the cGPS measurements recorded significant coseismic displacements and postseismic deformation in the western margin of the Sunda plate. Subsequently, the regional velocity field for distances of up to 1,400 km from the epicentre shows a clear deviation from the course of motion of prior to the 2004 and 2005 earthquake events, implying that a significant postseismic relaxation process is undergoing in the elastic crust and the underlying mantle. The velocity field deviation has decreased significantly following the 2012 Wharton Basin strike-slip earthquakes, which postseismic of 2012 earthquake is reaching closer to the interseismic state. The geodetic-based strain rate field indicates a high shear strain rate following the 2004 and 2005 megathrust earthquakes, mainly concentrated on the Sunda forearc at the segment north of the equator. It is likely to be caused by postseismic relaxation from both ruptures, which produces a complex deformation pattern on the overriding plate surface. The dilatation strain rate analysis reveals localised subsidence in northern Kelantan, a northeast coast state in the Peninsular Malaysia, which is likely induced by groundwater extraction. The region shows higher ground deformation rates (0.22 ppm/yr) than the other parts of Peninsular Malaysia. The observed vertical measurements indicated a maximum subsidence rate of 4.22±0.17 mm/yr (1𝜎 confidence level), as well as a corresponding horizontal deformation signal that manifests as high shear strain rate. Dilatation strain rate analysis shows a contraction pattern along the SW-NE trend of the Kelantan River, associated with the extensional pattern as moving further away from the river. A study of the interseismic plate locking coefficient also reveals that the subduction interface for this segment, which was fully locked before the 2004 and 2005 earthquakes is freely slipping after the ruptures. The present-day (2007–2016) interseismic velocity fields have shown that this segment of the plate is now regaining locking with >0.5 coupling coefficient. While to the south, the plate interface along the Siberut segment was locked before the 2007 Bengkulu earthquake and it remains partially locked after the rupture. This finding is also consistent with published results, that suggest that the 2007 ruptures only slipped a part of the rupture length of the 1797 and 1833 earthquakes. This study reveals the temporal variation of the coupling coefficient along the Sunda subduction interface following the series of great earthquakes. The coseismic displacements are inverted to study two key ruptures, the 2004 Aceh earthquake and 2012 Wharton Basin earthquakes since they have significant effect on the present-day geodynamics of the Sunda plate. The preferred model in this study agrees with the published models that the 2004 Aceh earthquake ruptured an at least 1,300 km long segment along the Sunda subduction trench, and it resulted in a moment of 4.3×1022 Nm and the moment magnitude of Mw 9.0. The recent 2012 strike-slip earthquake shows a preference fault rupture of NNE trending. The rupture may have triggered some motion on the Sunda subduction interface, that is suggested to be one of the contributing sources for the small-scale tsunami that was recorded at the regional tide gauges.
Advisor: Denys, Paul H.; Pearson, Christopher F.
Degree Name: Doctor of Philosophy
Degree Discipline: School of Surveying
Publisher: University of Otago
Keywords: Sunda; GPS; Deformation; Tectonic; subduction; Malaysia; Geodetic; Earthquake; Strain; Slip_Partitioning; Plate_Boundary; Sumatra; Groundwater_Extraction; land_subsidence
Research Type: Thesis