Down-depth Seismicity Gaps and the Shape of the Seismic Zone along the entire Indonesian arc from Relocated Hypocenters

HR: 16:30h
AN: U44A-05 [Abstracts]
TI: Down-depth Seismicity Gaps and the Shape of the Seismic Zone along the entire Indonesian arc from Relocated Hypocenters
AU: * Das, S
EM: das@earth.ox.ac.uk
AF: Department of Earth Sciences, University of Oxford, Parks Road, Oxford, OX1 3PR United Kingdom
AB: Using thousands of handpicked P, S, pP, sP, PcP, and ScP phases from digitally recorded seismograms, together with International Seismological Centre reported phases, we obtain improved hypocentral locations for ~2600 earthquakes deeper than 50 km with m_b > ~5.0 earthquakes, for the period 1962 to September 1996, along the Indonesian subduction zone. The seismicity distribution is found to be very non-uniform both along the arc and in depth. Gaps in the relocated hypocenters exist along depth in most places of the arc, with its upper edge varying from 100-450 km depth and its lower edge from 350-670 km in different portions of the arc. The relocated hypocenters show that (1) a portion of the Indonesian arc between ~110°E and 123°E longitude, and deeper than ~500 km, is dipping southward at an angle of ~75°, that is, in a direction opposite to the upper part of the north dipping slab, suggesting southward lateral flow in the mantle, relative to the plate motion vector here. (2) East of about 108°E, the seismic zone is wider near 670 km than near the 500 km depth. (3) The seismic zone between 129--131°E in the 100--200 km depth range is the widest along the arc both in strike and downdip. This region, near the highest arc curvature, has the highest seismic activity, and is the only part of the arc with earthquakes continuously occurring from the surface down to below 600 km. (4) The very deep earthquakes under Sulawesi are shown to be part of the west-southwest dipping Seram slab. (5) In the westernmost part of the Banda arc, the slab is under downdip tension in the 50-250 km depth range, while the deepest portion of the slab in this region is under compression. From 128-131°E the slab between 100--200 km depth is under mainly horizontal compression. Our study supports the "two-slab" model for the Banda arc. 3-D computer animations of the subduction zone will be presented. Das, S. (2004) Seismicity Gaps and the Shape of the Seismic Zone in the Banda Sea Region from Relocated Hypocenters,J. Geophys. Res., 109, doi:10.1029/2004JB003192. H.-J. Schöffel and S. Das (1999). Fine details of the Wadati-Benioff zone under Indonesia and its geodynamic implications, J. Geophys. Res., 104, 13101-13114.
DE: 8120 Dynamics of lithosphere and mantle--general
DE: 8166 Stresses--deep-seated
SC: Union [U]
MN: 2005 Joint Assembly