02739nas a2200241   4500008004100000050003800041245014700079210006900226260000900295300001400304490000700318520197900325653002002304653001202324653001302336100001502349700001202364700002002376700001502396700001502411700001602426856005502442       1998                        eng d  aOSU Libraries: Valley QE696 .Q35500aTidal marsh stratigraphy, sea-level change and large earthquakes-II: submergence events during the last 3500 years at Netarts Bay, Oregon, USA0 aTidal marsh stratigraphy sealevel change and large earthquakesII  c1998  ap.365-3930 v173 aThis is the second investigation where we apply the methods and scientific framework common to sea-level investigations in northwest Europe to test a model of coseismic land subsidence followed by land uplift during interseismic strain accumulation, used to explain alternating peat-mud couplets of late Holocene age found in many of the estuaries of the Pacific Northwest of the USA and Canada. Recent studies in this region interpret such couplets as the product of repeated large (magnitude 8 or 9) earthquakes on the Cascadia subduction zone. Stratigraphic investigations of the intertidal sediments at Netarts Bay, Oregon, using lithological, pollen and diatom analyses reveal different types of submergence associated with peat-mud couplets during the last 3500yr. Three peat-mud couplets record gradual sedimentation changes within an infilling body of water. Four other couplets record rapid submergence, burial of tidal marshes and their replacement by low marsh environments. In three of the events rapid submergence is small, 0-0.5m. The most recent marsh submergence event, around AD 1700, was 0.4+/-0.3m. Most of the buried peats contain microfossil evidence for a gradual increase in marine influence before the rapid change from organic to minerogenic sedimentation. Similar gradual changes are common in comparable pale environments on non-seismic coasts and are strong evidence against a simple interseismic strain accumulation-coseismic subsidence model. One hypothesis is that the gradual increase in marine influence before each submergence represents pre-seismic relative sea-level rise caused by a reduction in the rate of land uplift. The microfossil analyses provide evidence of three stages within an earthquake deformation cycle: coseismic subsidence; interseismic land uplift; and pre-seismic reduction in the rate of land uplift. At present the microfossil data do not differentiate between rapid post-seismic land uplift and rapid sedimentation.10acoastal hazards10ageology10atsunamis1 aShennan, I1 aLong, A1 aRutherford, M M1 aInnes, J B1 aGreen, F M1 aWalker, K J  uhttps://netarts.library.oregonstate.edu/node/13901