LATE HOLOCENE TECTONIC DEFORMATION RECORDED ALONG THE STRAIT OF GEORGIA COAST IN NORTHERNMOST WASHINGTON

At least two earthquakes in the late Holocene have tectonically warped the coast of northernmost Washington east of the Strait of Georgia. Evidence for these earthquakes comes from three Whatcom County sites that record late Holocene relative sea level changes: Sandy Point, Terrell Creek and Birch Bay. At Sandy Point, two late Holocene uplifted beach berms testify to two instances of coseismic uplift. Soil pits excavated on the lower of the uplifted beaches indicate a tsunami probably invaded the beach immediately after uplift. At Terrell Creek marsh, which is on the coast 12 km north-northwest of the Sandy Point uplifted beaches, the coast has abruptly subsided at least once in the late Holocene, as indicated by cores that show a buried marsh soil overlain by tide flat deposits overlain by the modern marsh soil. Two km to the north at Birch Bay, the emergent Birch Bay lowland is underlain by tide flat deposits, indicating that the lowland was coseismically uplifted at least 1.5 m in the late Holocene. Four km further north, there is no indication that Drayton Harbor was affected by the uplift that occurred at Birch Bay. One reasonable interpretation of these preliminary observations is that an active late Holocene anticline, more westerly trending than northerly trending, is centered over Birch Bay; late Holocene tightening of the anticline occurred by slip on a underlying blind fault, uplifting the Birch Bay lowland and at the same time subsiding the Terrell Creek marsh. One of the two Sandy Point coseismic uplifts may have occurred during the same time, although it is unlikely that the same fault could have caused both the coseismic uplift at Birch Bay and the coseismic uplift at Sandy Point. Further support for at least one coseismic uplift at Sandy Point is evidence for an uplifted late Holocene beach in the Tennant Lake area, 10 km to the east-northeast of Sandy Point and 2 km southeast of Ferndale. Further work at all these sites will better define ages of earthquakes and better constrain amount of coseismic uplift and subsidence and the geometry of the buried fault or faults involved in the deformation.