With four years of data from 268 seismometers on the ocean floor and several hundred on land, researchers have found anomalies in the upper mantle below both ends of the Cascadia Subduction Zone. They may influence the location, frequency and strength of earthquake events along the U.S. Pacific Northwest.
The anomalies, which reflect regions with lower seismic wave velocities than elsewhere beneath the fault line, point to pieces of the Earth’s upper mantle that are rising and buoyant because of melting rock and possibly elevated temperatures, said Miles Bodmer, a University of Oregon doctoral student who led a study now online as an accepted paper by the journal Geophysical Research Letters.
The 620-mile subduction zone, which hasn’t experienced a massive lengthwise earthquake since 1700, is where the Juan de Fuca ocean plate dips under the North American continental plate. The fault zone stretches just offshore from northern Vancouver Island to Cape Mendocino in northern California.
The mantle is rising under the southern Gorda deformation zone at the north edge of the San Andreas Fault and under the Olympic Peninsula and southern Vancouver Island.
“What we see are these two anomalies that are beneath the subducting slab in the northern and southern parts of the subduction zone,” Bodmer said. “These regions don’t have the same behavior as the entire fault. There are three segments that have their own distinct geological characteristics. The north and south segments have increased locking and increased tremor densities.”
Locking refers to how strongly two plates stick. “If they are stuck together tightly, as is the case here, they are building up stress, and you have the potential for the release of that stress, or energy, in large earthquake events,” Bodmer said.
Such quakes, while strong, are below that of the 9-plus magnitude event projected should all of Cascadia rupture at once, he said. Locking is much weaker in Cascadia’s central section, which includes most of Oregon, where infrequent, smaller quakes tend to occur from creeping along the plates.