Imaging Earthquake Scarps and Tsunami Deposits in the Pacific Northwest, USA

Abstract

A considerable amount of interdisciplinary research has been conducted during the past several decades to establish evidence for prehistoric earthquakes along the Cascadia margin of the United States and Canada. Several methods have been proposed to aid in this process including the mapping of earthquake induced effects (e.g. scarps) along the coastline and the mapping of paleotsunami deposits within low‐lying coastal wetlands of the Pacific Northwest. As these subsurface features depend on local geomorphology, there is a need to accurately identify and characterize them along the coastal plain. The major objectives of our experiments were to test if ground penetrating radar (GPR) would be able to detect these features in the subsurface, earthquake induced scarps and tsunami deposits, and provide images so that their thickness and internal stratigraphy could be determined. GPR transects were collected with both a pulseEKKO 100 and 1000 GPR systems using various antennae frequencies ranging from 50 to 450 MHz. Our test localities included Long Beach, Washington and Seaside, Oregon. Near Long Beach, a series of 8 coseismic subsidence induced scarps have been previously mapped. With this project we were able to collect a higher resolution grid (225 MHz) over a scarp and visualize the associated geomorphic features in three dimensions. At Seaside, the 1700 AD tsunami inundated the low‐lying area via channelized flow and pour‐overs which left several deposits including a 10–30 cm thick sand sheet. This fan has been mapped using GPR and in association with the sediment cores the tsunami sand sheet was identified. From data collected with higher frequency antennae (450 MHz), the internal layers of the sand sheet including cross beds can be resolved. Based on the interpretation of the GPR data one can hypothesize that when mega earthquakes occur along the Cascadia margin the associated coastal depositional and/or erosional processes can be both imaged and inferred.