HR: 16:30h
AN: T44A-03    [Abstracts]
TI: Seismic Slip Rate and Effective Seismic Thickness for Oceanic Transform Faults Bounding the Juan de Fuca
AU: * Willoughby, E C
EM: ele.willoughby@nrcan.gc.ca
AF: Geological Survey of Canada, Pacific Geoscience Centre, 9860 West Saanich Rd., P.O. Box 6000, Sidney, BC V8L 4B2 Canada
AU: Hyndman, R D
EM: rhyndman@nrcan.gc.ca
AF: Geological Survey of Canada, Pacific Geoscience Centre, 9860 West Saanich Rd., P.O. Box 6000, Sidney, BC V8L 4B2 Canada
AU: Hyndman, R D
EM: rhyndman@nrcan.gc.ca
AF: School of Earth and Ocean Sciences, University of Victoria , P.O. Box 3055 STN CSC, Victoria, BC V8W 3P6 Canada
AU: Mazzotti, S
AF: Geological Survey of Canada, Pacific Geoscience Centre, 9860 West Saanich Rd., P.O. Box 6000, Sidney, BC V8L 4B2 Canada
AU: Mazzotti, S
AF: School of Earth and Ocean Sciences, University of Victoria , P.O. Box 3055 STN CSC, Victoria, BC V8W 3P6 Canada
AB: The oceanic transform faults which bound the Juan de Fuca plate system: the Revere-Dellwood-Wilson, Sovanco, Nootka, Blanco and Mendocino faults, have deformation rates as predicted by plate models and observed, indirectly using GPS. We examine the degree to which this slip is accommodated seismically. The earthquake rate, derived average slip rate, and the effective vertical seismic thickness have been examined for each fault. The moment release rates are estimated by integrating the moment contribution rate over the magnitude versus frequency of occurrence relation up to a maximum magnitude. Seismicity statistics are related to the rate of slip along a given fault from earthquakes, using the concept of seismic moment. There are significant sources of uncertainty, including: the incompleteness and limited history of the earthquake catalog, the variety of magnitude definitions which can only be related empirically, empirical moment-magnitude relations (and the effect of their stochasticity), uncertainty in fault lengths and the effective seismic thickness, the recurrence relation, the determination of maximum magnitude and how the recurrence relation is truncated at maximum magnitude. Nonetheless, this method has been used successfully to provide estimates of deformation in good agreement with those from plate models, assuming that only the crust is seismic. The similarity of the deformation estimates based on seismicity and on plate models, shows a remarkable consistency in these rates over a significant temporal range- from tens to millions of years. The least constrained parameter is the effective seismic thickness, thus the effect of a 2, 3, 6.5 and 10 km thick zone is investigated for each fault. Many believe that oceanic transform faults are fundamentally different from continental transform faults since it appears that slip cannot be accommodated by observed seismicity alone and hence seismic efficiency is deemed to be low with considerable aseismic slip. However, this finding is tied to the selection of an effective seismic thickness of the order of 10 km, based largely on poorly constrained earthquake depth solutions. The selection of a thin effective seismic layer of about 3 km involving only the lower oceanic crust can consistently explain most of the deformation in the region as being seismically accommodated. The upper mantle is inferred to be aseismic, which is consistent with evidence of its serpentinization beneath these faults. The upper crust has very high porosity and may be sufficiently fractured such that it cannot support significant earthquakes. These geological considerations lend support to the possibility that there is in fact a thin effective seismic layer and that deformation within this layer on these oceanic transform faults can be fully seismically accommodated.
DE: 7220 Oceanic crust
DE: 7230 Seismicity and seismotectonics
DE: 8123 Dynamics, seismotectonics
DE: 3040 Plate tectonics (8150, 8155, 8157, 8158)
DE: 1242 Seismic deformations (7205)
SC: Tectonophysics [T]
MN: 2004 AGU Fall Meeting