T11H-08
Megathrust Slip Varied During Past Cascadia Subduction Zone Earthquakes at Siletz Bay, Central Oregon
Monday, 14 December 2015: 09:45
304 (Moscone South)
Robert Carleton Witter, USGS Alaska Science Center, Anchorage, AK, United States, Simon E Engelhart, University of Rhode Island, Kingston, RI, United States, Alan R Nelson, US Geological Survey, Denver, CO, United States, Yvonne Milker, University of Leipzig, Institute of Geophysics and Geology, Leipzig, Germany, Andrea D Hawkes, UNC Wilmington, Wilminton, NC, United States, Dawei Gao, University of Victoria, Victoria, BC, Canada, Ben Horton, Rutgers University, Institute of Marine and Coastal Sciences, New Brunswick, NJ, United States and Kelin Wang, Geological Survey of Canada Sidney, Sidney, BC, Canada; University of Victoria, School of Earth and Ocean Sciences, Victoria, BC, Canada
Abstract:
Microfossil-based estimates of subsidence during the AD 1700 Cascadia earthquake suggest that slip on the megathrust varied along strike. Here, we use microfossil foraminiferal assemblages to reconstruct coastal subsidence during predecessors of the AD 1700 earthquake to test if slip varied in successive earthquakes. Subsidence estimates come from two salt marshes that show stratigraphic evidence of sudden submergence 6 times in the past ~3 ka at Siletz Bay, Oregon (44.9°N, -124.0°W). Evidence for subsidence during earthquakes includes: sharp contacts of salt marsh peat buried by overlying intertidal mud that extend >2 km across the estuary; shifts in fossil foram assemblages across the contacts that indicate rapid submergence; and the thickness of overlying mud that suggests lasting submergence. On 4 of 6 contacts, sand—probably spread by tsunamis—sharply overlies peat along the bay margin of Salishan Spit. Median calibrated 14C ages on fossil plants from above and below 6 contacts give the times of submergence: 140, 780, 1225, 1550, 2510, and 3040 cal yr BP. The times correlate well with regional earthquake chronologies; the youngest contact probably marks subsidence in AD 1700. To reconstruct subsidence using fossil foram assemblages, we apply a foraminiferal transfer function developed from measured elevations of modern assemblages. Changes in fossil forams across the AD 1700 contact at Salishan Spit indicate 0.69±0.28 m of subsidence; subsidence during prior earthquakes varied from as little as 0.18±0.28 m at 780 cal yr BP to as much as 0.83±0.28 m at 1225 cal yr BP and 1.01±0.28 m at 1550 cal yr BP. Fault dislocation models show that variations in subsidence at Siletz Bay from one earthquake to the next can be explained by nearby variations in megathrust slip. Seismic hazard assessments should use a suite of rupture models that vary slip along strike and down dip to account for the variable subsidence over time that we find in central Oregon.
