What fraction of interseismic uplift at subduction zones becomes permanent? Observations from southwestern Washington

Juliet Crider, University of Washington
Kelsay Stanton, University of Washington (now at Washington Dept. Natural Resources)

Abstract

The spectacular Late Holocene estuarine deposits in southwestern Washington State show an unambiguous record of repeated co-seismic subsidence from subduction zone megathrust earthquakes; yet the same coastline hosts a flight of Pleistocene terraces recording long-term permanent rock uplift. A simple conceptual model of subduction zone earthquakes posits that co-seismic subsidence results from release of accumulated interseismic strain. We propose that a fraction of the interseismic strain must become permanent, resulting in coastal uplift accumulating over many seismic cycles. Stanton et al. (2023) mapped and dated the terraces, yielding an average uplift rate of 0.4 ± 0.1 mm/yr over the last 100 ky., the first rates from southwestern Washington. We use this rate a simple vertical deformation budget for Holocene events to argue that about one-tenth of the interseismic uplift becomes permanent. Furthermore, in the southwestern Washington, this uplift is at least partially accommodated by oblique slip on shallow crustal faults.

Session

Neotectonics
earthquake geology