A wide depth distribution of seismic tremors along the northern Cascadia margin

Tremors beneath Cascadia The Cascadia subduction zone, stretching from Vancouver Island in British Columbia to northern California, is thought likely to generate a ‘great earthquake’ every few hundred years. An intriguing phenomenon called episodic tremor and slip (ETS) has been observed in the region, and these events were interpreted as a sign of stress beneath the locked portion of the fault, where great earthquakes are thought to arise. A large subduction earthquake is more likely to happen during an ETS event, the thinking went. Anticipating an ETS event in early 2003, the Geological Survey of Canada made extra seismic recordings in the Vancouver Island area to determine the exact locations of ETS tremors. They discovered that the tremors span a wide depth range, from the upper crust in the overriding plate down to within the subducting oceanic crust, rather than simply following the plate interface. Certain characteristics of ETS are significantly different from those of local earthquakes, suggesting that the tremors are probably associated with a different seismogenic process.