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.
The research does not allow scientists to conclude where the next big earthquake is most likely to strike. But it does add an important new piece to the seismic puzzle on the West Coast, heightening interest in the potential hazards on the Olympic Peninsula. And it points to new research that's needed.
"One of the things we need to understand better is the degree of variation in the subsurface [from] Vancouver Island to northern California … we also need to try and follow the structures that we've identified off shore, to the region where the fault approaches the surface," Prof. Calvert said.
The plates on the West Coast regularly become "stuck," but in parts of the fault there is regular slippage, every 14 months or so, which causes tremors.
Prof. Calvert said people don't feel those tremors because they are relatively small and are released over several weeks.
A megathrust event occurs when there is a sudden and rapid release of seismic energy over a huge area.
"In the 2004 Sumatra megathrust earthquake, the fault started to slip in the south and didn't stop until it was 1,200 kilometres north, three minutes later, which was why the earthquake was so large," he said.
Prof. Calvert said every time there is a small slip on the West Coast, "it is probably loading the shallow part of the fault and so that … eventually will lead to a very large earthquake, a magnitude 9 earthquake probably."
He said it is impossible to predict when a megathrust earthquake will occur, but it is only a matter of time before an event similar to the one in Sumatra strikes the West Coast.
"I'm not sure we are overdue, but it is certainly getting more likely," he said, noting there is a megathrust earthquake on the Pacific Coast every 500 to 600 years.
The last massive earthquake in the region was in 1700. Native legends both on the Olympic Peninsula and Vancouver Island recall that event, with stories of villages being swept away by giant waves and canoes ending up in treetops. And scientists have found ample geological evidence to support those stories, including ocean sand deposited far inland.
The co-authors of the study with Prof. Calvert are Leiph Preston, from the Department of Geophysics at the Sandia National Laboratories in New Mexico, and Amir Farahbod, at the Department of Earth Sciences, Simon Fraser University.
Editor's Note: The original newspaper version and an earlier online version of this article both had inaccurate headlines. This online version has been corrected.
Mark Hume