The tectonic setting of the Puget lowland was simpler during late Tertiary time (106–107 yr) when the transfer of stress from subduction was not modulated by periodic glacial loading. Each advance of the Cordilleran Ice Sheet during the Quaternary Period created an additional 10–13 MPa of vertical load that may have stabilized the thrust sheet beneath the Puget lowland, causing background tectonic stresses to accumulate beyond the strength of active faults in their unglaciated condition. Conversely, deglaciation may have been accompanied by a brief episode of intense seismicity caused by decreasing overburden stress, elevated pore pressures, and elastic flexure in the shallow crust, and by viscous drag at the base of the crust during relaxation of isostatic anomaly. If accumulated tectonic stress was effectively “erased” during deglaciation, then some, or nearly all, of the present interglacial may have been seismically quiet relative to the modern regime. Such a scenario would complicate the use of paleoseismic approaches to establish the recurrence frequency of damaging earthquakes in this heavily populated region.

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