Imaging the magma system beneath Toba caldera, North Sumatra and aftershock study of the 1996 Biak earthquake, Irian Jaya, Indonesia.

摘要

This thesis comprises seismic studies using arrival time data recorded by two temporary seismic networks deployed in Indonesia. Local earthquakes recorded by a seismic network deployed for about four months around Toba caldera, North Sumatra, are used to study the distribution of the magma beneath the caldera and its seismicity. Aftershocks of the 1996, Biak earthquake, Irian Jaya, recorded by a three week temporary seismic network are used to study the rupture zone of the earthquake.;Three-dimensional P wave velocities beneath Toba Caldera are estimated using inversion of arrival time data of local earthquakes recorded by a 40-station temporary seismic network. Inversions reveal the presence of low velocities within the crust and upper-most mantle beneath Toba caldera. Assuming the low P velocity structures are associated with high temperatures, the 3D velocities suggest that the magma system of Toba caldera is distributed in two separate zones. Beneath the northern end of Toba caldera, magma is likely distributed in the upper crust, associated with Haranggaol caldera. This magma system is separated from a larger magma system which is likely distributed within the upper and middle crust beneath the middle to southern Toba caldera.;Shallow earthquakes around Toba caldera reveal three active faults. The first two active faults are the Renum and Toru segments, west and south of Toba caldera respectively. The length of the active fault is about 40 km and 65 km for Renum and Toru respectively, and the depth of seismogenic zone is about 15--20 km. The third lineation of earthquakes is associated with a previously unknown fault beneath Toba caldera. The earthquakes in this fault are about 20 km to 40 km deep beneath Pusubukit volcano. The occurrence of these earthquakes suggests that magma was moving from the upper mantle to the deep crust.;Location of aftershocks of the 1996 Biak earthquake are determined using a two-dimensional velocity model, estimated using inversion of aftershock arrival times. The aftershocks cover an area about 300 x 80 km2 and reveal a gentle south dipping rupture zone. Estimation of stress drop, about 1.16 MPa, suggests that Biak earthquake is a common interplate event in subduction zone environment. The distribution of aftershocks also shows vertical rupture zones south of the main shock which is probably responsible for the generation of tsunami and north of Biak island which is consistent with three extensional mechanisms of larger aftershocks in the north of Biak island. (Abstract shortened by UMI.)