HR: 11:50h
AN: S22A-07    [Abstracts]
TI: Investigation of Magmatic Processes in the Alaska Subduction Zone Using $b$-Value Imaging and Seismic Body Wave Tomography
AU: * van Stiphout, T
EM: thomas@vanstiphout.ch
AF: ETH Zuerich, Institute of Geophysics, HPP P3, Zuerich, ZH 8093 Switzerland
AU: Kissling, E
EM: kiss@tomo.ig.erdw.ethz.ch
AF: ETH Zuerich, Institute of Geophysics, HPP P13, Zuerich, ZH 8093 Switzerland
AU: Wiemer, S
EM: stefan.wiemer@sed.ethz.ch
AF: ETH Zuerich, Swiss Seismological Service, Institute of Geophysics, HPP P5, Zuerich, ZH 8093 Switzerland
AB: We investigate source region and ascend paths of magma in the Alaska subduction zone using two complementary techniques: 1) Imaging the earthquake size-distribution, or $b$-value, of earthquakes in the subducting slab; 2) imaging seismic body wave velocities using seismic tomography. The earthquake size distribution in the Cook inlet region (CIR) and interior Alaska (IAL) is analyzed using cross-sections, each about 250 km in length, 150 km in depth and 100 km in width, using data from the Alaska Earthquake Information Center (AEIC) for the period of 1989.2 to 2002.5. The seismicity above the magnitude of completeness ($M_c$ 2.1-2.4) is sampled in circles of 10 km radius and centered in a 1x1 km-grid. Different earthquake size distributions are found for IAL and CIR. The cross-sections beneath CIR reveals a high $b$-value anomaly ($b >$ 1.4) near the top of the slab in a depth of 80 km to 100 km, which we interpret to be related to the dehydration of the slab. The cross-section beneath IAL, were no volcanoes are present, on the other hand shows a high $b$-value along the entire upper edge of the slab. To test if the process of dehydration also alters seismic wave velocities, we invert a set of about 75000 P and about 25000S arrivals from about 5000 high quality earthquakes to obtain a tomographic image of the three-dimensional (3D) velocity structure for $V_p$ and $V_s$ down to a depth of about 140 km. The data and model setup allows to resolve structures of 20 km minimal length with certainty at depths of 100 km. The 3D seismic body wave velocity fields are also used to relocate the hypocenters and hence provide improved $b$-value imaging. Combined $b$-value and seismic tomography imaging is a promising new approach for improving our understanding of the processes that lead to subduction volcanism.
DE: 8180 Tomography
DE: 7230 Seismicity and seismotectonics
DE: 7280 Volcano seismology (8419)
DE: 8123 Dynamics, seismotectonics
DE: 7218 Lithosphere and upper mantle
SC: Seismology [S]
MN: 2004 AGU Fall Meeting