Tremor and Slip

December 17, 2008

Anomalies at the plate boundary intrigue researcher Anne Trehu

Beneath the Pacific Northwest, the Juan de Fuca plate is slipping inexorably, millimeter by millimeter, under the North American plate. Over time, this motion generates the Cascade volcanoes, the Coast Range and an underwater chain of mountains along the continental margin. But the slippage is intermittent: The plates get stuck, straining the Earth's crust. When they snap - as happens about every 300 to 600 years - the Northwest experiences a large earthquake and possible tsunami. The most recent quake happened on January 26, 1700. In the time between great earthquakes, the plates appear to be locked together.

EarthScope scientists, including Anne Tréhu, are investigating the mechanisms that may trigger these events. Already, intriguing anomalies are turning up, episodes of "tremor and slip," in which GPS instruments record slippage accompanied by seismic tremor along the plate boundary inland from the "locked" zone. In central Oregon, the time between slip episodes appears to be infrequent and irregular. In contrast, EarthScope stations to the north (Puget Sound and Vancouver Island) record tremor and slip episodes at remarkably regular intervals of about 14 months, according to Tréhu. "It's pretty mysterious. We're not sure exactly what it means."

EarthScope, Tréhu says, will move scientists closer to answering two of the Big Questions facing geoscientists over the next decade: First, what drives the movement of plates and how do they get incorporated into the mantle? "This," says Tréhu, "is important for understanding the whole evolution of the continent." Second, what triggers an earthquake, how does it propagate, what controls its size, and what makes it stop? "This," she says, "could lead to strategies for early-warning systems."