STRATIGRAPHIC EVIDENCE FOR GREAT SUBDUCTION-ZONE EARTHQUAKES AT 4 ESTUARIES IN NORTHERN OREGON, USA

Pour estuaries (Necanicum, Nestucca, Siletz, and Yaquina), located on the northern oregon coast and within the central Cascadia subduction z one, record evidence of peats that were buried during coseismic subsid ence and postseismic deposition of taunami sands and estuarine sedimen ts as a result of great subduction zone earthquakes (magnitudes greate r than 8). The identification of great earthquakes in these estuaries complements similar evidence in other estuaries in northern Oregon. Th e same number of burial events (6) are found within the top 3 m and wi thin approximately the last 2,800 C-14 years in at least seven estuari es on the northern oregon coast. Unequivocal evidence for great earthq uakes could not be provided for ad buried peats identified in the stra tigraphic record, The stratigraphic records of buried pears vary becau se of differences in morphology, sediment supply, wetland surface elev ations, and preservation potential within and among estuaries. However , consistent records of abrupt subsidence and corresponding tsunami de position are observed in each of the estuaries, confirming the potenti al for great earthquakes along the northern Oregon coast Wetland sites that record the bear history of earthquakes are those with 1) elevati ons that are low enough to document rapid environmental changes that a re easily recognized in the stratigraphic record, 2) an adequate sedim ent supply that allows wetlands to redevelop after coseismic subsidenc e, 3) locations that are in the path of sand-laden taunami surges, and 4) locations that pre away from migrating channels which erode the ge ologic record