News flash: In the summer of 1999, the residents of Washington State, British Columbia, and the surrounding area didn’t experience a magnitude 6.7 earthquake.
That’s right, they didn’t feel a thing, although lower portions of the Earth’s crust moved 20 millimeters. In a sudden surge, such a shift would have spawned an impressive quake, but the movement happened over a period of 6 to 15 days. Even seismometers in the area missed the motion because there wasn’t any shaking going on. Only a Global Positioning System (GPS) network that continuously monitors surface motion in the region picked up the abnormal movement, says Herb Dragert of the Geological Survey of Canada in Sidney, British Columbia.
Off the coast of the Pacific Northwest, a small chunk of ocean floor called the Juan de Fuca plate moves to the northeast at the blazing speed of about 37 mm per year. At the so-called Cascadia subduction zone, the plate dives beneath the North American plate, which in that area also moves to the northeast but at a more stately 8 mm/year. Sudden release of stress along the boundary between the two plates can generate earthquakes up to magnitude 8, says Dragert.
In August and September 1999, seven GPS sites in southwestern British Columbia and northwestern Washington showed an unusual reversal of direction. The surface stations moved to the southwest between 2 and 4 mm over a period of 6 to 15 days before they resumed their normal journey toward the northeast, says Dragert. His team reports its findings in the April 19 Science Express, an online version of Science.
This backtracking is best explained by a slow release of stress deep within the subduction zone, about 30 to 40 kilometers below Earth’s surface, the researchers say. There, where temperatures are about 500C, partially melted rocks lubricate the fault, Dragert notes.