HR: 1340h
AN: OS23D-1338    [Abstracts]
TI: Using Tsunami Deposits in a Probabilistic Inundation Study at Seaside, Oregon
AU: * Jaffe, B E
EM: bjaffe@usgs.gov
AF: United States Geological Survey Pacific Science, 400 Natural Bridges Drive, Santa Cruz, CA 95060
AU: Peterson, C D
EM: petersonc@pdx.edu
AF: Geology Dept. Portland State University, Portland State University, Portland, OR 97207
AU: Peters, R
EM: rpeters@usgs.gov
AF: United States Geological Survey Pacific Science, 400 Natural Bridges Drive, Santa Cruz, CA 95060
AB: Tsunami deposits in Seaside, Oregon, record information on inundation frequency and magnitude that is being integrated into a multidisciplinary probabilistic study of tsunami inundation. Deposits from 5 tsunamis in the past 2000 years have been found up to 2 km inland along a 5 km stretch of coast at 167 sites. Deposits are found primarily in marshes fringing the Necanicum River and Neawanna Creek, which flow parallel to the coast between 5-10 m high beach ridges. Deposits were formed by tsunamis generated by local Cascadia subduction zone earthquakes (e.g. 78 sites with deposits link to the 1700 AD Cascadia tsunami by depth below the surface and association with a subsided marsh) and by tsunamis generated by far-field earthquakes (e.g. 68 sites with deposits linked to the 1964 Alaska tsunami by depth below the surface). The most fundamental interpretation of the tsunami deposits is as a minimum area of inundation. Comparison of the 1964 deposit distribution and eyewitness accounts of inundation showed that the area of inundation was well documented by the deposits. The sole exception is along the marshes fringing the Necanicum River in the last 1 km of inundation where deposits were not formed. Interpretation of deposits from paleotsunamis impacting Seaside becomes more difficult because of inlet migration and the potential for changing topography and bathymetry. For example, the pattern and characteristics of deposits from the 1300 ybp tsunami suggests that it entered through an inlet approximately 4 km south of the present inlet. Without accounting for the change in inlet location, the extent of inundation would be misinterpreted. However, when accounting for paleogeography, tsunami deposits are an extremely useful tool to determine the magnitude and frequency of inundation in Seaside, Oregon, and as a component of a probabilistic study to aid in delineating future tsunami hazard areas.
DE: 9355 Pacific Ocean
DE: 7221 Paleoseismology
DE: 4564 Tsunamis and storm surges
SC: Ocean Sciences [OS]
MN: 2004 AGU Fall Meeting