CORVALLIS, Ore. – A pair of modest earthquake clusters that occurred off the Oregon coast in 2004 may be more significant than scientists initially realized after detailed analysis of the main shocks by researchers at Oregon State University.
The newly published study found that the quakes probably occurred on the fault that forms the boundary between the North American and Juan de Fuca plates at a depth where the fault is thought to be “locked.” While earthquakes occur frequently in the upper and lower plates in the Cascadia subduction zone, there haven’t been any instrumentally recorded quakes on this boundary, according to Anne Trehu, a professor in the College of Oceanic and Atmospheric Sciences at Oregon State University and lead author.
Results of the study were announced this week in the journal Geology, published by the Geological Society of America.
“We don’t really know what they mean,” Trehu said of the clusters. “But this is an area with a great deal of paleoseismic evidence for very large earthquakes, so documenting such activity is significant. As we learn more about seismic behavior and associate that with the structure of the plates and faults, we will be in a better position to develop models for what might happen and when.”
Research by one of Trehu’s colleagues, OSU’s Chris Goldfinger, suggests that there may have been as many as 23 major earthquakes in the Cascadia subduction zone over the past 10,000 years. The last known major plate boundary earthquake took place on Jan. 26 in the year 1700, rupturing the entire subduction zone. Scientists know a fair amount about that earthquake, estimated at magnitude 9.0, because of written records of a corresponding tsunami in Japan, as well as plentiful geologic evidence.
The moderate 2004 earthquakes off the central Oregon coast were felt as far east as the Willamette Valley, but did little if any damage to coastal communities. The first, of magnitude 4.9, occurred on July 12 just off Newport. It was followed on Aug. 19 by a magnitude 4.8 earthquake just to the south, near Waldport. Smaller earthquakes occurred near the same spots in August of 2007.
The research by Trehu and colleagues Jochen Braunmiller and John Nabelek found that they occurred about 10-15 kilometers below the surface, and just 10-15 kilometers off the coast.
Most of the earthquakes in the Pacific Northwest have occurred in the upper plate and some have occurred in the lower plate, Trehu said. But portions of the boundary region – where the Juan de Fuca plate is being subducted beneath the North American plate – have been without recent seismic activity and are thought to be locked.
“These earthquakes are telling us something about the nature of the plate boundary,” Trehu said. “It could be that they result from sudden slips on strong patches of the plate boundary, whereas most of the plate boundary slips without generating earthquakes large enough to be recorded on land. Or they could reflect weak patches on an otherwise locked fault.”
Trehu recently installed a temporary onshore/offshore seismic array to try to determine which of those possibilities has the most merit. Her project was funded by the Marine Geophysics and EarthScope programs of the National Science Foundation.
The subduction of the Juan de Fuca plate beneath the North American plate is a slow and intermittent process. Averaged over geologic time – in this case, many millions of years – the plates move past each other at a rate of about 4.5 centimeters a year, Trehu said.
Beneath the continental margin, where the plate boundary is relatively cold, this motion takes place during infrequent and irregular intervals – as do the large earthquakes that may take place. Where the plate boundary is deeper, the plate motion occurs in slow events that take place at regular intervals in a phenomenon known as episodic tremor and slips, or ETS.
Another objective of the NSF-funded seismic array is to test whether ETS can trigger slips on the shallow part of the subduction fault.
“Evidence in the paleoseismic record shows that these plates tend to get stuck for a while, and then unstuck, resulting in major earthquakes,” Trehu noted. “Just what it is that triggers major quakes, though, is the million dollar question. For seismic researchers, it is the Holy Grail.”