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

Dating the 1700 Cascadia Earthquake: Great Coastal Earthquakes in Native Stories Ruth S. Ludwin1 , Robert Dennis2 , Deborah Carver3 , Alan D. McMillan4 , Robert Losey5 , John Clague6 , Chris Jonientz-Trisler7 , Janine Bowechop8 , Jacilee Wray9 , and Karen James10 INTRODUCTION 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 treering studies (Yamaguchi et al., 1989) suggested that the most recent earthquake happened about 300 years ago. The exact date and approximate time of the most recent CSZ earthquake, 9 PM on 26 January 1700, were determined from Japanese historic records of a tsunami arriving with no reports of associated shaking (Satake et al., 1996). The year was con- firmed through close study of tree-ring patterns of ghost cedar roots (Yamaguchi et al., 1997). The magnitude estimate of 9.0 implies rupture along the entire length of the CSZ (Satake et al., 2003). Figure 1 shows the geographic extent of the likely rupture area. TRADITIONS FROM CASCADIA At the time of initial European contact, Cascadia native groups spoke more than a dozen distinct languages (Thompson and Kinkade, 1990) and lived in a complex social landscape with both similarities and differences between groups. Collection and recording of native stories began in the 1860’s, more than 350 years after the first European contacts in North America, almost 100 years after initial contact in Cascadia, and nearly 50 years after European settlement began. As a result, as much as 95% of native oral traditions may have been lost (Jacobs, 1962), and available recorded examples may not be a representative sampling of the original material. Storyteller, transcriber, and language and cultural issues all affect the story that ends up in print. Nonetheless, versions of oral traditions are preserved in hundreds of sources, and numerous stories describe shaking or marine flooding. Artifacts, dances, songs, ceremonies, and personal and place names supplement the range of information available for study. We are deeply indebted to the many informants who shared their stories and allowed them to be preserved in written form. Figure 1 shows source locations for 40 native stories from 32 independent sources. These stories represent less than a third of the known stories that refer to shaking or marine 1. Department of Earth and Space Sciences, University of Washington 2. Huu-ay-aht First Nation 3. Carver Geologic, Inc. 4. Department of Anthropology, Douglas College 5. Department of Anthropology, University of Alberta 6. Department of Earth Sciences, Simon Fraser University 7. FEMA Region X 8. Makah Museum and Cultural Center 9. Olympic National Park 10. Bainbridge Island, Washington Seismological Research Letters March/April 2005 Volume 76, Number 2 141 flooding and were selected on the basis of clarity, descriptions of phenomena notable in megathrust earthquakes, and geographic distribution. Some of these stories have been discussed in earlier studies (Heaton and Snavely, 1985; Clague, 1995; Carver and Carver, 1996; Minor and Grant, 1996; Hutchinson and McMillan, 1997; Losey, 2002; McMillan and Hutchinson, 2002). Figure 2 tabulates story elements and gives date estimates. Stories referenced in Figures 1 and 2A have been broadly grouped into three time categories: stories from which dates can be estimated, stories that are clearly historical but impossible to date, and apparently mythic stories without any clear timeframes. Historical stories cannot be distinguished from myth by style or content alone, however (story ref. 23, p. ix), and stories that appear to be mythological may be based on historical events. Stories designated as historical in the source texts are identified as historical in Figures 1 and 2A. Stories vary considerably in content and style along the Cascadia coast. At the southern end, many stories explicitly mention both earthquakes and tsunami. At the northern end, there are explicit earthquake stories and explicit flood stories, but only a few stories including both phenomena. In the middle portion of the CSZ, along the coast of Oregon and Washington, direct mention of earthquakes is rare and stories of marine floods are common. The differences likely result from differences in the collection and preservation of stories, and may also reflect differences in native cultures and lifestyles along the Cascadia coast or variations in earthquake effects. HISTORICAL TRADITIONS Nine