Migration characteristics of seismic tremors in the northern Cascadia margin

Seismic waveforms from a dense regional array were analyzed to study episodic tremor-and-slip (ETS) events in northern Cascadia. The April 2005 episode occurred mostly in northern Vancouver Island (VI) where the Explorer plate subducts beneath the North America plate, while the May 2005 episode was mainly in middle VI. All tremor occurrences are positively correlated with the presence of subducted slabs. Besides the steady along-strike migration and halting behavior found in previous ETS events, a new pattern (jumping) is found this time. Visual inspection of previous seismic records suggests that the jumping behavior is probably common to ETS tremors in all regions of VI. The majority of tremors occurred between 25 and 45 km depth in places where local seismicity is sparse or absent. Our observations are compatible with the interpretation that fluids released from dehydration of subducted materials are involved in tremor generation.