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The fault line where tectonic plates are colliding in the Pacific Northwest is much deeper than previously thought, which could mean the Olympic Peninsula will be hit hard when a megathrust earthquake next occurs on the West Coast, a new study suggests.
A team of Canadian and U.S. scientists, led by Andrew Calvert, a professor in Earth Sciences at British Columbia’s Simon Fraser University, made the discovery by pouring through data collected by a research project known as SHIPS, for Seismic Hazards Investigations in Puget Sound, which since 1998 has been examining earthquake dangers in the region.
“What we have put forward in our paper is that beneath the Olympic Peninsula, in northwest Washington State, there’s a large volume of sedimentary rock that extends actually from the Olympic Mountains at surface … down to a depth of almost 40 kilometres. That large volume of rock actually sits above the fault, or close to the fault, between the subducting Juan de Fuca plate and the overriding North American continent,” said Prof. Calvert.
He said the fault is about seven kilometres deeper than scientists had previously thought, but the implications of that aren’t fully understood.............
A new study just published in Nature Geoscience indicates the depth of the fault between the two tectonic plates forming the Earth’s surface in the Pacific Northwest is seven kilometres deeper than previously proposed.
Simon Fraser University earth scientist Andy Calvert, the study’s lead author, says he and his colleagues aren’t sure what to make of the discovery. But he speculates it may mean part of the fault’s locked zone — where a megathrust earthquake can occur — could be beneath the Olympic Peninsula.
Calvert’s team studied a 200-kilometre section of a fault formed by the subduction of the oceanic Juan de Fuca plate under the continental North America plate. In this region, the two plates are converging by four centimetres annually.
The section runs north-south from Victoria on Vancouver Island to southern Washington state. Scientists call it a transient slip zone because the fault between two plates slips gradually every 14 months or so. This gradual slip takes place over a two-week period and triggers tremors that are so small people don’t notice them.
In this study the slowly slipping section of the fault beneath Washington state was found to be 27 to 42 kilometres deep instead of 25 to 35.