Observations of non-volcanic tremor during the northern Cascadia slow-slip event in February 2002

Abstract

We locate in the Cascadia subduction zone nonvolcanic tremors from an episodic tremor and slip event in February 2002. The tremors occurred during two 10-day periods separated by a short lull. Tremors that occurred during the first period are broadly distributed between 10 km and 40 km depth at the southern end of Vancouver Island, and over a >50 km wide region measured normal to the margin. During the second period of tremor activity, most tremors were concentrated in a narrower zone, and many tremors occurred near the subduction megathrust at approximately 35 km depth. This relatively large number of tremors near the subduction megathrust is in contrast to the broad depth distribution of the subsequent Match 2003 tremor sequence. The change in the pattern of tremor activity observed during the February 2002 sequence may indicate a change in the slow slip process, or in its migration along the margin. Copyright 2006 by the American Geophysical Union.