A remaining question is whether upper plate structure controls plate interface behavior or vice versa. Both models have been proposed for fore-arc basins, with either basins developing in response to locking on the subduction interface (Song and Simons, 2003; Wells et al., 2003) or thickness of the upper plate critical wedge controlling the frictional behavior on the plate interface (Fuller et al., 2006). For ETS recurrence, the accreted terranes comprising the upper plate above ETS generate inherently sizable along-strike variations in structure, composition and age that are presumably more significant than long-term effects of ETS on upper plate structure. This supports an interpretation where variations in the Wrangellia, Siletzia, and Klamath blocks control behavior of the ETS source zone. A clue to how continental blocks could be responsible for differences in ETS recurrence is geochemical evidence that the different terranes have different fluid content (Schmidt and Grunder, 2006), which could trigger ETS via high pore fluid pressures (Kodaira et al., 2004; Obara, 2002). An intriguing hypothesis is that different terrane composition affects rheology of the upper plate and hence the plate interface. For example, the Siletzia terrane would represent denser, stronger, more oceanic-like crust, while the Klamath terrane represents lighter, weaker, more continental-like crust. Such a scenario would suggest that the low-lying Siletzia region has a longer recurrence interval because the upper plate has the strength to accumulate strain for longer periods between SSE.

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