HR: 16:45h
AN: T44A-04    [Abstracts]
TI: An Overview of Results from the 2003-2004 Mariana Subduction Factory Passive Imaging Experiment
AU: * Wiens, D A
EM: doug@wustl.edu
AF: Dept. Earth and Planetary Sci., Washington University, 1 Brookings Dr., St. Louis, MO 63130 United States
AU: Shiobara, H
EM: shio@eri.u-tokyo.ac.jp
AF: Earthquake Research Institute, University of Tokyo, 1-1-1 Yayoi, Bunkyo-ku, Tokyo, 113-0032 Japan
AU: Pozgay, S
EM:
AF: Dept. Earth and Planetary Sci., Washington University, 1 Brookings Dr., St. Louis, MO 63130 United States
AU: Barklage, M
EM:
AF: Dept. Earth and Planetary Sci., Washington University, 1 Brookings Dr., St. Louis, MO 63130 United States
AU: Emry, E L
EM:
AF: Dept. Earth and Planetary Sci., Washington University, 1 Brookings Dr., St. Louis, MO 63130 United States
AU: Heeszel, D S
EM:
AF: Dept. Earth and Planetary Sci., Washington University, 1 Brookings Dr., St. Louis, MO 63130 United States
AU: Pyle, M
EM:
AF: Dept. Earth and Planetary Sci., Washington University, 1 Brookings Dr., St. Louis, MO 63130 United States
AU: Conder, J
EM:
AF: Dept. Earth and Planetary Sci., Washington University, 1 Brookings Dr., St. Louis, MO 63130 United States
AU: Sugioka, H
EM:
AF: IFREE, JAMSTEC, 2-15 Natsushima-Cho, Yokosuka, 237-0061 Japan
AB: We investigate the seismological structure and seismicity of the Mariana subduction zone and backarc basin using data from the Mariana Subduction Factory Imaging Experiment. The passive component of this experiment consisted of 20 broadband seismographs deployed on the island chain and 58 ocean-bottom seismographs (OBS) across the arc and in the backarc. This dense seismic network recorded from May-June, 2003 until April-May, 2004 and 68 instruments returned useful data. We obtained the 3D P-wave velocity structure of the Mariana mantle wedge from a tomographic inversion of P-wave arrivals from local earthquakes, arrival times from large teleseismic earthquakes determined by multi-channel cross correlation, and arrivals from Mariana earthquakes to stations around the world reported by the PDE bulletin. These results clearly show a high velocity slab extending to depths of ~600 km. A slow velocity anomaly of ~8% is present within the upper 150 km beneath the Mariana back-arc spreading center. Slow velocity anomalies are also found beneath the active volcanic arc and in the uppermost forearc mantle. We also investigate the anisotropy of the Mariana region using shear wave splitting from S waves propagating from intermediate and deep earthquakes in the Mariana slab to the OBS and land seismic stations. The northern part of the arc system shows approximately arc-parallel fast directions for intermediate depth events. Fast directions surrounding the spreading center also show N-S orientation, whereas raypaths sampling west of the spreading center show NW-SE and E-W fast directions, or approximately parallel to the the absolute plate motion of the subducting Pacific Plate. This overall pattern could be due to either the predominance of along-strike flow near the slab and slab induced counterflow in the far backarc, or to a different relationship between LPO and mantle flow near the water rich slab. Slab seismicity is characterized by a double seismic zone in the best-imaged part of the arc near 18N, with the lower zone initiating in the forearc at a depth of 60 km and extending to depths of about 200 km. Planes of the double seismic zone are initially separated by about 40 km, but the separation decreases with increasing depth. The forearc seismicity is dominated by a highly seismic region of the shallow thrust zone at depths of 30-40 km, containing several extremely active patches with diameters of about 20 km. The outer forearc, in the region of the serpentinite seamounts, is mostly aseismic, particularly immediately beneath the seamounts. We suggest that the lack of large (Mw > 7.0) thrust earthquakes in Mariana results from serpentinization of the outermost mantle forearc and from the the extreme heterogeneity of the shallow thrust interface, as revealed by the patches of high seismicity.
DE: 3001 Back-arc basin processes
DE: 7208 Mantle (1212, 1213, 8124)
DE: 7240 Subduction zones (1207, 1219, 1240)
DE: 8180 Tomography (6982, 7270)
SC: Tectonophysics [T]
MN: Fall Meeting 2005