Geologic Evidence of Historic and Prehistoric Tsunami Inundation at Seaside, Oregon

Abstract

Over the past decade, research conducted along the Cascadia subduction zone coast established evidence for coseismic subsidence, liquefaction, and nearfield tsunami deposition. Seaside is a low lying northern Oregon coastal city potentially at risk for nearfield tsunami inundation from a Cascadia earthquake. The 1964 Alaskan farfield tsunami impacted Seaside, and deposits from that event serve as a model for interpreting prehistoric tsunami deposits in the Seaside area. A reconnaissance subsurface study of potential tsunami inundation sites was performed by trenching and gouge coring in the coastal wetlands along the Necanicum River, Neacoxie Creek, drainage to the east of Neacoxie Creek, Stanley Lake, and Neawanna Creek. A total of 278 core sites were logged for shallow lithologic stratigraphy and contact relations. To establish tsunami depositional trends from the 1964 farfield event, 71 observation sites, 62 core logs, two grids, and eight trenches were evaluated. Wave amplification occurred in the Necanicum River/estuary mouth and north of 12th Avenue, south of the G Street bridge crossing Neacoxie Creek, and south of the HWY 101 bridge crossing Neawanna Creek. These areas contain anomalously thick sand deposits compared to the deposits along the Necanicum River and Neawanna Creek where the wave attenuated, depositing a sand layer thinning up stream. Neawanna Creek wetlands contain most of the preserved 1700 AD earthquake subsidence horizons and sand layers. Within the Seaside wetlands, 90 core sites contain the 1700 AD subsidence horizon. No subsided peaty horizons were observed west of Neawanna Creek. At the Mill Creek/Stanley Lake area, the 1700 AD tsunami deposition is minimal to non-existent. In the southernmost Neawanna wetlands, 1700 AD tsunami deposits are restricted to a narrow zone southeast of the Mill Ponds and north of S Avenue bridge. Overland inundation of the 1700 AD tsunami, interpreted from core records, did n ot reach the central Neawanna wetlands (1. 5 km east of the present coastline) and crossed a narrow cobble ridge entering the Neawanna wetlands from a southern Necanicum channel.