HR: 1340h
AN: OS23D-1339    [Abstracts]
TI: Estimating Uncertainty and Frequency of High-Velocity Paleotsunami Inundation From Geologic Records in Back Barrier Settings, Test Locality Cannon Beach, Oregon, Central Cascadia Margin, USA
AU: * Peterson, C D
EM: petersonc@pdx.edu
AF: Geology Dept. Portland State University, Portland State University, Portland, OR 97207
AU: Jaffe, B
EM: bjaffe@usgs.gov
AF: United States Geological Survey Pacific Science Center, 400 Natural Bridges Drive, Santa Cruz, CA 95060
AU: Peters, R
EM: rpeters@usgs.gov
AF: United States Geological Survey Pacific Science Center, 400 Natural Bridges Drive, Santa Cruz, CA 95060
AB: Historic-tsunami sand deposits (1964 Alaskan far-field source) in Cascadia back barrier settings have been examined for distinctive criteria from Seaside, OR, Crescent City, CA, and Port Alberni, BC. These historic Pacific Northwest criteria have been combined with similar data from recent tsunami investigations in Papua New Guinea, Peru, and Japan to discriminate high-velocity paleotsunami inundation (at least 0.5 m/s) from non-tsunami deposition such as river flood, storm surge, extreme spring-tide, debris flow, coseismic-fluidization, and anthropogenic activities. The criteria are semi-quantitatively analyzed to evaluate the certainty of paleotsunami inundation records from geologic deposits. The criteria used for a representative test locality Cannon Beach, Oregon, include (1) event dating, (2) anomalous sand lithology, (3) debris cap, (4) distinct layer(s), (5), fining-upward grain-size, (6) laterally continuous deposition (10's of meters), (7) landward-thinning deposition (100's of meters), (8) landward-fining deposits (100's of meters), (9) beach sand mineralogy, and (10) marine diatoms. At least three nearest-neighbor' distal sites of target sand sheets were evaluated for the ten criteria, yielding a certainty index (CI). Four of the events (2-5) demonstrate high certainties (7-9) for tsunami origins. Event data as follows: Event (1/Far-field) Age (1964) CI (6) ID (50m) RI (260yr), Event (2/Cascadia) Age (1700AD) CI (8) ID (250m) RI (325yr), Event (3/Unknown) Age (0.8-0.9ka) CI (7) ID (400m) RI (434yr), Event (4/Cascadia) Age (1.1ka) CI (8) ID (500m) RI (651yr), Event (5/Cascadia) Age (1.3ka) CI (9) ID (900m) RI (1301yr). High-velocity inundation distances (ID) of 50-900m were measured by straight line from the crest of the beach barrier (6 m elevation contour). The tsunami recurrence interval (RI) where RI=(n+1)/m and n=record length (1300yr) is calculated for the 5 events. The event recurrence interval is then plotted against inundation distance (Y intercept=0) yielding a linear relation y=0.71x with Rsq2=0.9 for this small data set. Using the inundation versus recurrence relation the following estimates are found: 500yr recurrence=300-400m inundation, 1000yr recurrence=700-800m inundation. At this locality 3 out of 5 tsunami (2,4,5) are directly correlated to central Cascadia coseismic-subsidence events, so are classified as near-field sources. Only one certain far-field tsunami (Alaska 1964) is recorded at this locality for the 1300yr period. It has been 300 years since the last Cascadia-generated tsunami (1700AD) and the probability for an associated high-velocity tsunami inundation of greater than 250m at Cannon Beach, estimated from a mean recurrence interval of 325yr, is 50 percent in the next fifty years.
DE: 9355 Pacific Ocean
DE: 7221 Paleoseismology
DE: 4564 Tsunamis and storm surges
SC: Ocean Sciences [OS]
MN: 2004 AGU Fall Meeting