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 zone, record evidence of peats that were buried during coseismic subsidence and postseismic deposition of taunami sands and estuarine sediments as a result of great subduction zone earthquakes (magnitudes greater than 8). The identification of great earthquakes in these estuaries complements similar evidence in other estuaries in northern Oregon. The same number of burial events (6) are found within the top 3 m and within approximately the last 2,800 C-14 years in at least seven estuaries on the northern oregon coast. Unequivocal evidence for great earthquakes could not be provided for ad buried peats identified in the stratigraphic record, The stratigraphic records of buried pears vary because of differences in morphology, sediment supply, wetland surface elevations, and preservation potential within and among estuaries. However, consistent records of abrupt subsidence and corresponding tsunami deposition are observed in each of the estuaries, confirming the potential for great earthquakes along the northern Oregon coast Wetland sites that record the bear history of earthquakes are those with 1) elevations that are low enough to document rapid environmental changes that are easily recognized in the stratigraphic record, 2) an adequate sediment supply that allows wetlands to redevelop after coseismic subsidence, 3) locations that are in the path of sand-laden taunami surges, and 4) locations that pre away from migrating channels which erode the geologic record