HR: 1345h
AN: T22C-0931
TI: Structure of the Subduction Plate Boundary Offshore Nicaragua
AU: * McIntosh, K D
EM: kirk@ig.utexas.edu
AF: Univ of Texas Inst for Geophysics, 4412 Spicewood Sprgs Rd, Bldg 600, Austin, TX 78759-8500 United States
AU: Silver, E A
EM: esilver@es.ucsc.edu
AF: Earth Sciences Department, Univ of California, Santa Cruz, A112 EMS Bldg, Santa Cruz, CA 95064 United States
AU: Ranero, C R
EM: cranero@geomar.de
AF: GEOMAR, Wischhofstrasse 1-3, Kiel, 24148 Germany
AU: Ahmed, I
EM: ahmed@ig.utexas.edu
AF: Univ of Texas Inst for Geophysics, 4412 Spicewood Sprgs Rd, Bldg 600, Austin, TX 78759-8500 United States
AB: Data from a deep penetration seismic reflection survey off the Pacific coast of Nicaragua mark the Cocos-Caribbean plate boundary from the trench to depths $>$ 30 km. Coverage of the plate boundary includes most, if not all, of the interplate seismogenic zone and also includes 21 lines crossing the updip portion that ruptured in the 1992 tsunami earthquake. Studies of this earthquake and tsunami suggest that the downdip limit of interplate seismogenic rupture is relatively shallow off Nicaragua, 20-30 km, compared to about 40 km beneath the Nicoya Peninsula, Costa Rica and that rupture extended to very shallow depths near the trench. Our seismic data delineate the plate boundary through the seismogenic zone and in some cases a shallower reflection intersects this boundary at depths of 20-30 km. We suspect that this reflection may originate from the base of the upper plate crust. The apparent correspondence of these intersecting reflections with the downdip limit of the seismogenic zone suggests that upper plate mantle in contact with the subducting Cocos plate is aseismic, similar to interpretations at other subduction zones. This conceptual crustal model is consistent with rapidly, seaward-thinning upper plate crust interpreted from a GEOMAR wide-angle seismic transect in this area and with the depth distribution of upper plate seismicity, which is largely confined to the crust. Our future work will be to refine our images of the plate boundary and other key reflectors and use available wide-angle seismic data to independently identify the upper plate crustal structure.
DE: 1030 Geochemical cycles (0330)
DE: 3640 Igneous petrology
DE: 8120 Dynamics of lithosphere and mantle--general
DE: 8123 Dynamics, seismotectonics
DE: 8180 Tomography
SC: T
MN: 2001 AGU Fall Meeting