Megathrust earthquakes, with similarities to the December 2004 earthquake in Sumatra, are thought to occur along the Cascadia subduction zone with average repeat intervals of roughly 500 years. These earthquakes accommodate the relative motion between the Juan de Fuca and North American plates along a fault that is mostly off shore, but may extend onshore in places such as the Olympic Peninsula. It has been recently discovered that much, if not all, of the slip along the portion of the plate boundary just down dip (east) of the megathrust earthquake zone is accommodated by episodic tremor and slip (ETS) events that occur every 14 months in the regions from the Olympic Peninsula north into Vancouver Island. We propose to deploy arrays of seismometers and Global Positioning System instrumentation in order to record and better characterize the next ETS event which is expected to occur sometime in August, September or October, 2005.
