Using Ground Penetration Radar for Imaging and Mapping of Thin, Shallow Tsunami Deposits in Washington, Pacific Northwest United States

Abstract:

A tidal marsh at Discovery Bay, on the Strait of Juan de Fuca, has the longest record of tsunami deposition in Washington, with nine described tsunami deposits. One of the youngest continuous deposits Bed 1is likely from the 1700 A.D. M9+ Cascadia Earthquake, based on its stratigraphic position and radiocarbon age. Bed 1 is typically found at a depth of ~ 0.45 m, has a maximum thickness of 8 cm, and is composed grains of silt to fine sand. Ground Penetration Radar (GPR) is useful to study such tsunami deposits, because it can “see” characteristics of the deposits that could be missed in cores or outcrops. Tsunami deposits typically extend over wide areas. GPR imaging can trace a layer over a wide area in the subsurface of a tidal marsh. Correlation of layers between coring or outcrops is often difficult across distances in a marsh. GPR technology allows in situ correlation of potential tsunami deposits in the subsurface. We used GPR to map subsurface images of previously described tsunami deposits in the top 2 m at Discovery Bay. We used MALA 450 MHz antenna and recording unit, and ran the survey during the low tide time range (3-4 hours). After adjusting the soil velocity (dielectric constants) and scan parameters we ran various transects correlated the shallow soil cores in 0-1.5m of the soil column. Tsunami sand layer is relatively distinct among other layers on radargrams. Maximum penetration depth reached was about 2 meters and saltwater effect is dominant at 2 meters and greater depths. In addition to this success, there is also the potential to use GPR to“see” characteristic tsunami deposit features such as draping and infilling of low spots. This imaging could help guide locations to sample with strategic cores or pits.

We think that our preliminary results are promising,and plan to use the GPR technology to investigate potential tsunami deposits inPuget Sound and other coastal areas of Washington.