NEW EVIDENCE OF PALEOTSUNAMI RUNUP IN THE CENTRAL CASCADIA MARGIN

The Cascadia subduction zone is capable of generating earthquakes that can produce catastrophic tsunamis. Computer simulations of possible, tsunami excitation scenarios yield runup heights ranging from 5 to 30 m in height. Other approaches are needed to estimate likely tsunami runup in the Cascadia margin for purposes of evacuation planning. Our study focuses on finding evidence for paleotsunami runup elevations in the Central Cascadia Margin, specifically at upland sites 5 to 13 m elevation MSL, located south of the city of Newport, Oregon. Core samples were collected in reducing marsh type environments from creeks and Pleistocene dune deflation areas where Holocene silt and mud deposits have accumulated. Samples taken at 20 to 100 cm subsurface depth show several sand deposits (0.5 to 5 cm in thickness), with sharp bottom contacts that grade upward to silt and mud layers. The core site elevations (NAD 83) were surveyed by EDM total station (+/- 1 cm) and tied into either published benchmarks or GPS stations. It is possible that these anomalous sand layers have been deposited by tsunamis that reached the measured elevation of the investigation sites. Further investigation of the deposits and others in the Central Cascadia Margin, including radiocarbon age dating and marine diatom testing are required to confirm that the anomalous sand layers were deposited from nearfield tsunami runup.