2 Apr 2009, 2:14pm
Cultural Landscapes Native Cultures
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Impacts Of Earthquake Tsunamis On Oregon Coastal Populations

Leland Gilsen. 2002. Impacts Of Earthquake Tsunamis On Oregon Coastal Populations. Association of Oregon Archaeologists Occasional Papers No. 7, 2002.

Leland Glisen was the State Archaeologist for Oregon from 1978 to 2002 within the State Historic Preservation Office of Oregon State Parks. He has a Ph.D. in Anthropology with a specialization in Archaeology from the University of Arizona. This paper was downloaded and reposted with permission from Dr. Glisen’s excellent website, Oregon-Archaeology [here].

Full text [here]

Selected excerpts:


What were the impacts to human populations from great subduction earthquakes and resulting tsunamis? This key question has not been addressed by archaeologists. What happens to population growth curves along the northern California, Oregon, and southern Washington coasts when, on an average of every 572 years, a great-subduction earthquake (between 8 and 9+ on the Richter scale with up to 10 hours of tsunami waves) hits prehistoric settlements?

First, there must have been significant loss of life among the prehistoric populations. Second plant and animal resources would have been disturbed or destroyed to some degree. Third, communication and travel would have been disrupted. Fourth, water transport (canoes) on estuaries and along the coast would have been lost or destroyed. Fifth, shelters (housing) would have been lost or damaged.

If in mid-winter and at night, like the quake of 1700, people would have been in coastal winter villages. They must have faced massive damage when weather conditions were poor. Stored foods may have been lost or damaged. Sites on cliff edges (including winter houses) may have crashed into the sea.

Physical Evidence

Adams (1992) summarized research into quakes in the Seattle area and found local tsunamis in the narrow channels, rock avalanches in the Olympic Mountains that dammed streams to produce lakes, and block landslides in Lake Washington. Atwater and Moore (1992) verified the local Puget Sound tsunamis north of Seattle. Karlin and Abella (1992) found steep basin landslides in Lake Washington in at least three locations that included large block slides that submerged forest habitats. Schuster et al. (1992) suggested that eleven rock avalanches in the southeastern Olympics were the result of quakes. “The rock avalanches that formed Jefferson, Lower Dry Bed, and Spider Lakes, and perhaps Lena Lake, provide evidence that strong shaking accompanied abrupt tectonic displacement in western Washington” (Schuster et al. 1992:1621). Logan and Walsh (1995) documented two drowned forests in lake Sammamish as evidence for one or two large block landslides into the lake as the result of a major earthquake.

So the physical damage from such events must have been massive. But the cultural and social damage must have been just as great. The loss of life in a small scale society would have a disproportionate effect on population growth curves, flattening them out, and modifying the region’s demography. This in turn, would have long term effects on human ecology or population adaptation. With the overall population being hit by periodic episodes of catastrophic death and destruction, there simply was a slower change in carrying capacity and a reduced need for changes in any of the key economic technological systems (production, distribution, consumption, or storage) or political systems (access to, and control over, important resources in the physical, biotic of cultural environmental context). …

Archaeological Evidence

Thomas Newman’s excavation at 35-TI-1 resulted in the following observation: …

… Drucker (1943) notes that several middens discovered during his survey of the northern Northwest Coast were being eroded by the ocean. The base of the middens were covered by high tides. This suggests that it may not be purely local subsidence of land which is responsible for inundation of sites, but the general phenomenon of eustatic adjustment as well (Newman 1959:35 – 36).

… [Emory Strong (1973)] notes that: 1) Shoto Village, which was dug by the Oregon Archaeological Society from 1964 – 1966, hit submerged midden; 2) a 1971 University of Washington excavation on Lake River ran into submerged deposits; 3) Drucker (1938:110) recorded submerged sites in a 1938 survey and thought “there might have been sudden submergence of the area; 4) Dougherty (WSU) in response to Strong’s inquiry, noted a number of recently submerged sites; 5) Gibbs published Indian myths about earthquakes; 6) Mead notes rock art below tide levels; 7) Newman’s excavations on the Tillamook Spit noting water problems (noted above).

Minor (et. al 1989) examined the relationship between the deposits and C14 dates for the North Yaquina Head site (35-LNC-50) (Figure 2). He found that his oldest date predated events, his single date for the non-shell occupation fell into the period between the 3rd and 2nd buried marsh, and his six dates for the shell midden deposits fell between the 2nd and 1st buried marshes. …

Woodward et al. (1990) report on excavation sat the Wilson River site (35-TI-2) and the Nehalem Bay site (35-TI-4). At the former site:

Yet, while there is no direct support for earthquake-generated catastrophic change at this locality, the abrupt disappearance in shellfish utilization can be interpreted as indicating that the rapid habitat change resulted from the periodic building and breaching of the sand spit protecting Tillamook Bay. That beaching and disappearing of the sand spit could have been caused by wave erosion from tsunamis and/or bay sieching (Woodward et al. 1990:61).

… Roberta Hall’s work at 35-CS-43 investigated tsunami events in core samples. In 1990, two large and very fragile unfired clay bowl-shaped objects were recovered in deposits that suggested they had been buried by a 30 cm thick sand layer that had preserved them. Hall believed the deposits were tsunami caused. …

Cole (et al. 1996) outlined evidence from two archaeological deposits found buried in peat and tsunami layers. The Copalis River Site (45-GH-104) was exposed by erosion of Cedar Creek in a buried subsidence peaty layer dated to the 1700 event. The Niawiakum site (45-PC-102) was in the uppermost of six buried peaty horizons exposed at extreme low tides. They concluded that the 1700 earthquake and tsunami event buried these two sites, and that more sites may be in buried contexts, thus creating a survey bias in the archaeological record.

Seismic activity about A.D. 1700 at the Cascadia subduction zone caused submergence, tsunami inundation, and consequential burial and tidal flooding of low-lying coastal sites that native peoples occupied during previous centuries. This may help to explain why relatively few prehistoric sites are known from a region whose rich microenvironments supported a large prehistoric population in early historic times. …

I suggest in this paper that the periodic earthquakes and tsunami events had a major impact on population growth in northern California, Oregon, and southern Oregon. As a result, there were simply fewer people here throughout the prehistory of the region. In turn, there are fewer sites and sites of less depth. In addition, since 50% of the currently recorded sites are below the tsunami runup zone, a significant percentage may have been destroyed or buried. This may be especially true because of their relatively small size and shallow depth. The impacts to cultural development must have been great, as a significant portion of the population is lost, and the landforms and resources changed in their relative location, predictability, and abundance. …

Coastal Populations

The groups along the Oregon coast at contact, from north to south, were the Chinook, Tillamook, Alsea, Siuslaw, Coos, and Athapaskan Tututni. Boyd, in the Handbook of North American Indians (Vol 7), notes the before 1774 there were about 200,000 people in the Nothwest culture area that includes coastal Oregon. His calculations for population size for Oregon groups are presented in Table 2.

Mooney’s total for Alsea and Siuslaw were combined. Seaburg and Miller (1990:561) list population estimates for the Tillamook: Lewis & Clark at 1,000 in 1805 – 1806; a Hudson’s Bay Company estimate of 1,500 in 1838, but that included some Alsea and Siuslaw. Zenk (1990a:570) indicated a population for the Alsea at 1,700 based on Lewis & Clark in 1806. Zenk (1990b:578) also gave an estimate for 1806 of 900 Siuslaws (proper), from 500 to 1,700 Lower Umpquas, and 1,500 Coosans.

Given Boyd’s total, a great subduction earthquake and tsunami death rate of 1% would be 175 people; 5% would be 875 people; 25% would be 4373; and 50% would be 8745 dead. Since 208 (50%) of the recorded 417 archaeological sites in Oregon are below the George Priest tsunami runup lines for such quakes, and 97 (23%) were too close to call, the loss of life among prehistoric populations from such events may have been very high. If the “too close to call” are added to the “below”, there are 305 (73%) sites at risk. …

Could this be used to explain the relatively low populations on the Northern California to southern Washington coast? I think it has some explanatory power, but cannot be tested with our current levels of data. I suspect that great subduction quake events with their tsunamis merit greater attention when trying to understand Oregon coastal prehistory and population demography. …

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