T161 Characterizing Cascadia’s Earthquakes—Reexamining Open Questions about Cascadia Seismic and Tsunami Hazards (Posters)

Monday, 23 October 2017
9:00 AM - 6:30 PM
Halls 4EF

IDENTIFYING THE SOURCE OF ANOMALOUS SAND DEPOSITS IN JOSEPH WHIDBEY STATE PARK, WHIDBEY ISLAND, WA

Paper number: T161-6
Program: Technical Sessions
Time: 9:00 AM – 6:30 PM

Authors

Abstract

Six studies conducted in Puget Sound and Strait of Juan de Fuca found evidence of tsunamis in the form of sand deposits in marsh areas. Three such deposits are attributed to tsunamis from the Cascadia subduction zone, while the other deposits are from other tectonic sources. Because the body of research on this topic is small, we conducted a study in Joseph Whidbey State Park coring for potential tsunami deposits along five transects. Within the peat and mud of the marsh, we found two distinct sand layers. Samples of these sand layers were analyzed using diatom, radiocarbon, and grain size analyses. Diatom analysis within sand and peat layers distinguishes freshwater deposits from saltwater deposits to confirm that the sand deposits have an oceanic source. Samples of the sand layers consistently had a higher percentage of marine diatoms than peat samples. Radiocarbon dates show that the upper, less extensive deposit dates to an event between ~1218-1390 AD and the lower deposit to ~995-1170 AD. These dates possibly correlate to a local source tsunami found also in Discovery Bay and the Cascadia W tsunami, respectively. However, the Cascadia W tsunami is not found elsewhere in Puget Sound. Analysis of grain size shows that the sediment source location is most likely the top berm. Using grain size and assuming a tsunami origin of the deposits, the advection equation indicates the velocity and water depth of the flow were ~10 mph and ~2 m deep for the upper deposit and ~10 mph and ~5 m deep for the lower. The deposits thin rapidly which is more characteristic of storm overwash deposits. Modelling with GeoClaw-STRICHE shows that deposition of wedge-shaped deposits by a tsunami is possible in this high-friction environment, but storm deposition cannot yet be ruled out as a source for the deposits.

Geological Society of America Abstracts with Programs. Vol. 49, No. 6

doi: 10.1130/abs/2017AM-304065

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