Geologic evidence for earthquakes during the past 2000 years along the Copalis River, southern coastal Washington

Abstract

Evidence for several prehistoric earthquakes is present in deposits less than 2000 years old that crop out at the Copalis River estuary (47°07.2′N, 124°09.7′W). The deposits show that land subsided decimeters into the intertidal zone on as many as three occasions and that sand vented while the land underwent little or no subsidence on another occasion. The evidence for earthquake‐induced subsidence consists of buried marsh and forest soils that evince sudden estuarine submergence attended on at least one occasion by a landward directed surge of sandy water. This combination of submergence and surge implies subsidence and tsunami. Exposed evidence for subsidence and tsunami along the Copalis River is strong for a time within a few decades of 300 years ago, moderate for a time 1400–1900 years ago, and weak (subsidence) or lacking (tsunami) for an intermediate time also about 1400–1900 years ago. The history of sudden submergence and tsunami in the past 2000 years at the Copalis River estuary resembles that shown by buried soils farther south in coastal Washington at Grays Harbor and Willapa Bay. The history can be explained most simply by great (Mw of 8 or 9) thrust earthquakes at the boundary between the Juan de Fuca and North America plates, with one or two earthquakes occurring 1400–1900 years ago and another close to 300 years ago. The evidence for earthquake‐induced venting along the Copalis River consists of sand intrusions and vented‐sand volcanoes. The sand rose through muddy estuarine deposits at least 3 m thick, entraining these deposits in fragments as much as 0.3 m long. Intrusion occurred at one or more times in the past 2000 years, and at least some of the venting occurred 900–1300 years ago. The sand bodies do not imply artesian flow of flood‐pressured groundwater because the sand vented forcefully and infrequently through a lowland that probably lacked natural levees. The earthquake implied by the venting of sand 900–1300 years ago could have come from a source below, above, or at the boundary between the Juan de Fuca and North America plates. If from the subducted Juan de Fuca plate the implied earthquake was farther west or larger than the largest earthquakes within that plate during the past 100 years. Evidence permitting a source within the North America plate includes a fault scarp, an uplifted tideflat, and landslides near Puget Sound dating from the interval 700–1700 years ago. A plate boundary source would explain buried soils whose estuarine submergence and burial imply coseismic subsidence for parts of coastal Washington in the interval 600–1300 years ago. However, stratigraphy shows that little or no coseismic subsidence occurred along the Copalis River during the venting 900–1300 years ago. If a plate boundary earthquake struck coastal Washington 900–1300 years ago, the earthquake produced a different distribution of land level change than did the plate boundary earthquakes inferred for 1400–1900 and 300 years ago.