Large megathrust earthquakes occur on average every 550 years on the northern Cascadia subduction zone, where the Juan de Fuca tectonic plate subducts underneath the North American plate off the coast of Washington State. Scientists had thought that major earthquakes were likely to occur only offshore, but a new analysis by Chapman and Melbourne shows that a giant earthquake could extend deeper along the subduction zone, and thus farther inland.
The authors analyzed 15 small, nonearthquake periodic slip events known as episodic tremor and slip (ETS) detected between 1997 and 2008 to develop a model of seismogenic coupling between the two tectonic plates.
Their model accurately predicts the deformation of the overlying North American plate as measured by Global Positioning System stations and shows that seismic stress accumulation reaches to about 25 kilometers (15.5 miles) depth, or about 60 km (37.3 mi) inland, rather than stopping offshore at about 15 km (9.3 mi) depth, as had been thought. This indicates that the Seattle metropolitan areas could be subject to a magnitude 8.9 earthquake (from just the Washington State segment of Cascadia).
The authors conclude that results point to a need to reassess the Cascadia megathrust seismic hazard.
The research is published in Geophysical Research Letters. Authors include James S. Chapman and Timothy I. Melbourne: Department of Geological Sciences, Central Washington University, Ellensburg, Washington, USA
- Chapman et al. Future Cascadia megathrust rupture delineated by episodic tremor and slip. Geophysical Research Letters, 2009; 36 (22): L22301 DOI: 10.1029/2009GL040465
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