IMAGING OF 3-D SEISMIC VELOCITY STRUCTURE OF SOUTHERN SUMATRA REGION USING DOUBLEDIFFERENCE TOMOGRAPHIC METHOD

• Main Author: LESTARI (NIM : 22412304); Pembimbing : Dr. Andri Dian Nugraha, S.Si, M.Si , TITIK
• Format: Theses
• Language: Indonesia
• Online Access: https://digilib.itb.ac.id/gdl/view/20581
• Institution: Institut Teknologi Bandung

Southern Sumatra region has a high level of seismicity due to the influence of subduction system, Sumatran fault, Mentawai fault and stretching zone activities. The seismic activities of Southern Sumatra region are recorded by Meteorological Climatological and Geophysical Agency (MCGA’s) Seismograph network. In this study, we used earthquake data catalog compiled by MCGA for 3209 earthquake events from 50 seismic stations around Southern Sumatra region for time periods of April 2009 – April 2014 in order to invert for the 3-D seismic velocities structure. We applied double-difference seismic tomography method (tomoDD) to determine Vp, Vs and Vp/Vs ratio with hypocenter adjustment, so it is expected to provide a better understanding of tectonic and geodynamic settings for southern Sumatra region. For the inversion procedure, we started from the initial 1-D seismic velocity model of AK135 and constant Vp/Vs of 1.73. The synthetic travel time from source to receiver was calculated using pseudo-bending technique while the tomographic inversion was performed using a damped least square method. The resolution model was evaluated using checkerboard test and Derivative Weigh Sum (DWS). <br /> <br /> <br /> <br /> <br /> Tomographic inversion results tomogram of velocity structure and hypocenter relocation. Although the distribution of earthquake events and seismic stations are not favorable for high quality tomographic imaging, some important and robust anomalies were obtained. Low velocity anomaly along Bukit Barisan may be associated with weak zone of Sumatra fault and volcanic activity. High velocity anomaly at fore arc region should be interpreted as bedrock of continental crust at uppermost crust (0 – 30 km) and subducting slab of Indian oceanic plate at intermediate depth (> 50 km). The presence of low velocity anomaly beneath fore arc region may indicate fluids circulating which is released by dehydration slab processes in subducting oceanic crust and sediments. Fluids were expelled upward through cracks in the top of the oceanic plate and may lead to partial melting as a source of volcanism at a depth of about 100 km. The partial melted material moving up to the earth surface and is recorded as low velocity zone under volcanic arc line. The existence of volcanoes adjacent to Sumatran fault which is the main source of earthquakes in Sumatra mainland. Hypocenter relocation results travel time residuals are close to zero and the reduction of fixed depth, especially 10 km depth. Earthquake locations generally shifted northeast - southwest ward with average horizontal and vertical shifting are 5.5 km and 6.74 km, respectively. Eventhough the interpretation result is not absolute, this study provides valuable information to improve the understanding of tectonic and geodynamic setting in southern Sumatra region.