HR: 14:55h
AN: T43C-06    [Abstracts]
TI: Improving Seismic Constraints on Subduction Zone Geometry
AU: * Syracuse, E M
EM: syracuse@bu.edu
AF: Boston University, Department of Earth Sciences, 675 Commonealth Avenue, Boston, MA 02215, United States
AU: Abers, G A
EM: abers@bu.edu
AF: Boston University, Department of Earth Sciences, 675 Commonealth Avenue, Boston, MA 02215, United States
AU: Fischer, K M
EM: karen_fischer@brown.edu
AF: Brown University, Department of Geology, Box 1846, 324 Brook Street, Providence, RI 02912, United States
AU: van Keken, P E
EM: keken@umich.edu
AF: University of Michigan, Department of Geological Sciences, 2534 CC Little Building, 1100 North University Avenue, Ann Arbor, MI 48109, United States
AU: Kneller, E A
EM: ekneller@umich.edu
AF: University of Michigan, Department of Geological Sciences, 2534 CC Little Building, 1100 North University Avenue, Ann Arbor, MI 48109, United States
AU: Rychert, C A
EM: crychert@ucsd.edu
AF: University of California San Diego, Institute of Geophysics and Planetary Physics, Scripps Institution of Oceanography, 9500 Gilman Drive, La Jolla, CA 92093, United States
AB: Accurate slab geometries are necessary for 3D flow modeling, and for understanding the variations in temperature and melting geometry between different subduction zones. Recent studies have shown that the depth to slab beneath arc volcanoes varies by as much as a factor of two between subduction zones, but these results are based on teleseismic earthquake catalogs with potentially large errors. When available, local seismic arrays provide better constraints. The TUCAN array (Tomography Under Costa Rica and Nicaragua) deployed 48 three component broadband PASSCAL instruments for 18 months with station spacing of 10-50 km across the Central America arc. This dataset provides some of the best control anywhere for ground-truth comparison of teleseismic catalogs in steeply dipping subduction zones. Joint inversion of TUCAN arrival times for velocity and hypocenters illuminate a 10-15 km thick Wadati-Benioff zone (WBZ), with absolute hypocenter uncertainties of 1-5 km. Besides providing accurate hypocenters, the tomographic images provide independent constraints on melting and temperature, through the imaging of low Vp (7.5-7.8 km/s) and highly attenuating (40<Qs<100) mantle wedge, and the imaging of a high Vp/Vs (≥ 1.78) column inferred to be melt. International Seismic Centre (ISC) and EHB (Engdahl et al., 1998) hypocenters show a teleseismic Wadati- Benioff zone (TWBZ) that lies 15 km below that of the TUCAN catalog on average at 80-200 km depth, with similar results for local catalogs based on a 1D velocity model. However, the width of the TWBZ is 30-80 km or 3-5 times that indicated by TUCAN hypocenters; this additional width suggests errors of +/- 10-33 km. Commonly, the top of the subducting slab is assumed to lie at the top of the WBZ seismicity, for example if double seismic zones are expected. Because of the large scatter, the TWBZ is biased too shallow compared to the TUCAN data, vertically by as much as 50 km for the steeply-dipping Nicaragua slab. Relative relocations of hypocenters from ISC arrival times reduce this scatter to be more consistent with TUCAN hypocenters, with a center of seismicity less than 5 km deeper and a WBZ thickness 2-3 times greater. Thus, in regions with no local array, relative relocations of teleseismic catalogs can provide slab geometries consistent with regional earthquakes. We extend the slab inferred from TUCAN using relative relocations, to generate an accurate 3D description of the slab suitable for high-resolution geodynamic modeling.
DE: 7203 Body waves
DE: 7230 Seismicity and tectonics (1207, 1217, 1240, 1242)
DE: 7240 Subduction zones (1207, 1219, 1240)
DE: 8180 Tomography (6982, 7270)
DE: 8185 Volcanic arcs
SC: Tectonophysics [T]
MN: 2007 Fall Meeting