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. The Cascadia subduction zone is thought to be capable of generating major earthquakes with moment magnitude as large as M w = 9 at an interval of several hundred years 1 , 2 , 3 . The seismogenic portion of the plate interface is mostly offshore and is currently locked, as inferred from geodetic data 4 , 5 , 6 . However, episodic surface displacements—in the direction opposite to the long-term deformation motions caused by relative plate convergence across a locked interface—are observed about every 14 months with an unusual tremor-like seismic signature 7 , 8 , 9 . Here we show that these tremors are distributed over a depth range exceeding 40 km within a limited horizontal band. Many occurred within or close to the strong seismic reflectors above the plate interface where local earthquakes are absent, suggesting that the seismogenic process for tremors is fluid-related. The observed depth range implies that tremors could be associated with the variation of stress field induced by a transient slip along the deeper portion of the Cascadia interface or, alternatively, that episodic slip is more diffuse than originally suggested.