Introduction

As oceanic plates subduct down into the mantle, friction on the interface with the overriding plate causes stick-slip behavior in the megathrust zone, pulling the upper plate down until it pops back up during a potentially devastating earthquake. Recent observations have also revealed slow slip episodes (SSE) that occur regularly on parts of the deeper plate interface with motion indicating release of accumulated strain (Dragert et al., 2001; Lowry et al., 2001). Their frequency and amount of slip ($M_{w}$ $\sim$6.5-7.5) (Melbourne et al., 2005) imply they are a substantial portion of the interplate deformation budget. The duration of these episodes is much greater than earthquakes, yet they are accompanied by weeks of non-volcanic tremor (NVT) (Obara, 2002; Rogers and Dragert, 2003; Szeliga et al., 2004). As such, they represent another section of the strain rate continuum between earthquake and geologic time scales. Processes that govern ETS or potential relationships to major earthquakes and local geology remain unknown, although ETS has been proposed to impact the likelihood of megathrust earthquakes (Mazzotti and Adams, 2004).

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