Earthquake rate, slip rate, and the effective seismic thickness for oceanic transform faults of the Juan de Fuca plate system

Abstract

The earthquake rate, average fault slip rate, and the effective seismic thickness have been examined for the Revere-Dellwood-Wilson, Sovanco, Nootka, Blanco and Mendocino transform faults, bordering the Juan de Fuca plate system. Seismicity statistics are related to the rate of slip along a given fault due to earthquakes, using the concept of seismic moment. There are significant sources of uncertainty, including: the incompleteness and limited history of the earthquake catalogue, 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 and 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. An agreement between the deformation rate predicted by seismicity statistics for the fault zones and observed deformation from GPS and other geophysical data is used to show the soundness of the method and parameters used. 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. The selection of a thin effective seismic layer of about 3 km 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 similarity of the deformation estimates based on seismicity and those from plate models shows a remarkable consistency in these rates over a significant temporal range from tens to millions of years. © 2005 RAS.