Dating the 1700 Cascadia Earthquake: Great Coastal Earthquakes in Native Stories

Although scientific recognition of the earthquake hazard presented by the Cascadia subduction zone (CSZ) is relatively recent, native peoples have lived on the Cascadia coast for thousands of years, transferring knowledge from generation to generation through storytelling. This paper considers the ways in which information on coastal earthquakes is presented in native traditions and estimates the date of the most recent event from them. The primary plate-boundary fault of the CSZ separates the oceanic Juan de Fuca Plate from the continental North America Plate (Figure 1). It lies about 80 km offshore and extends roughly parallel to the coast from the middle of Vancouver Island to northern California. Although recognized as early as the mid-1960's, the CSZ was initially assumed to be incapable of producing great megathrust earthquakes. It is seismically quiet, and no sizable earthquake has occurred on it since European settlement began. As the theory of plate tectonics matured, studies of subduction zones worldwide identified characteristics associated with megathrust earthquakes. These earthquakes are most common in areas where hot, young, buoyant crust is rapidly subducted (Heaton and Kanamori, 1984). Although the rate of subduction in Cascadia is relatively slow, the subducted crust is among the youngest and hottest anywhere. Field investigations soon located geological evidence of abrupt land-level changes characteristic of megathrust earthquakes in “ghost forests” of dead cedar trees in coastal estuaries in Washington and Oregon (Nelson et al. , 1995). The cedars, originally above the limit of the tides, were killed when their roots were suddenly plunged into salt water. Beneath the surface of these same estuaries, soil cores revealed layered deposits showing a repeated cycle of slow uplift and rapid submergence. Preliminary age estimates based on radiocarbon dating (Nelson et al. , 1995) and tree-ring studies (Yamaguchi et al. , 1989) suggested that the …

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