Energy Budget Analysis of Slow-Slip Tremor Events Along the Cascadia Subduction Zone Using Continuous GPS Array Data

James Chapman
September 2008

Abstract

Seismic hazards poised to cities by subduction zones are strongly controlled by fault slip along the deeper extent of the two plate interaction closest to population densities. In Cascadia, where Mw=9 size events are known to occur from a variety of sources, modeling of leveling data has suggested that the region of maximum slip lies well offshore and diminishes rapidly inland. However, over two dozen slow slip distributions have been imaged using Global Positioning System (GPS) along the lower reaches of the northern Cascadia locked zone between 30 and 40 km in depth. Averaged over many episodic tremor and slip events, the upper limit of transient slip in the vicinity of Seattle, Washington and Vancouver, British Columbia comes close to the heavily urbanized regions. Moreover, these events appear to dissipate approximately half of the total tectonic convergence energy in the region, implying that approximately half of the energy will be available in the next megathrust earthquake. This inference is supported by agreement with observed interseismic deformation patterns, which is consistent with significant plate coupling extending closer to urbanized areas than has been previously thought. The hazard potential incurred by this scenario necessitates a sober mitigation readjustment given that the stress is likely accumulating much closer to the population centers of the Pacific Northwest than previously supposed.

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