Ancient Quakes

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Puget Sound residents live in the Cascadia subduction zone, home of a massive fault plane that extends from Vancouver Island all the way down to northern California. The Cascadia zone is the region where the Juan de Fuca plate is sliding underneath North America in a great tectonic collision. Such subduction zones are notorious for causing powerful earthquakes, including the Chile quake of 1960 (magnitude 9.5) and the Anchorage quake of 1964 (magnitude 9.2). Earthquakes occur when the plates lock and then release abruptly, rather than sliding smoothly and gradually past each other. Geologists are still debating the potential size and frequency of subduction earthquakes in our region, but evidence is strong that giant earthquakes have occurred here in the past.footnote 1comma footnote 2

Geologist Brian F. Atwater, USGS scientist and UW affiliate faculty member, started searching in 1986 for evidence of ancient quakes in the Pacific Northwest. Atwater looked along the Washington coast at creek sediments, with the rationale that he might find the tell-tale signs of land rising or dropping, signs that have been found in other subduction zones where massive quakes have occurred.

In 1987, Atwater reported that Washington's Pacific coast appeared to have subsided repeatedly in the past several thousand years. Along with UW geology professor Joanne Bourgeois, he also found that some of the subsidence events were accompanied by tsunamis that deposited sand on coastal land. The most recent such event happened about 300 years ago. UW's radiocarbon lab, directed by geology professor Minze Stuiver, has provided the most precise age measurements obtained for this event. Stuiver's lab dated coastal trees that were killed when the land suddenly dropped and plunged their roots into salt water. The average interval is about 500 years but individual intervals have been as large as 1,000 years and as short as a couple of centuries.

In 1992, researchers reported evidence that something very unusual happened in the Seattle area about 1,000 years ago. Something catastrophic.

Atwater and colleagues found a sheet of sand in Cultus Bay on southern Whidbey Island that came from a surge of water in Puget Sound some 1,000 to 1,100 years ago. The surge was, most probably, a tsunami, or tidal wave, from a massive quake along the Seattle fault which runs underneath the city.footnote 3

The corroborating evidence is impressive. Twenty feet of abrupt uplift south of the Seattle fault, sometime between 500 and 1,700 years ago; a landslide at Lake Washington between 1,000 and 1,100 years ago; rock avalanches in the Olympic Mountains between 1,000 and 1,300 years ago; a ground-water eruption along the Washington coast between 900 and 1,300 years ago; and abrupt subsidence of land at West Point in Seattle between 1,000 and 1,100 years ago.footnote 4 Atwater suspects a large earthquake on the Seattle fault probably generated the tsunami by causing an abrupt lifting of land south of the fault, which runs underneath the Kingdome, and a complementary lowering of land to the north. Such a movement, he believes, would have caused water in Puget Sound to surge northward across the fault and deposit the sand sheet in Cultus Bay some 18 miles to the north.

The realization that great earthquakes of magnitude 8 to 9 have repeatedly struck the Pacific Northwest, most recently about 300 years ago, has caused engineers to rethink building codes. The Uniform Building Code contains standards for designing structures in relation to six levels of earthquake-shaking hazard. Prior to 1994, the code had given the Seattle area the second highest hazard rating and placed the rest of Oregon and Washington in the third. But in view of the new evidence, as of 1994, all of Oregon and Washington have been placed in the second highest hazard category.

Discoveries about the ancient earthquakes also have helped bring about reinforcement of existing structures. More than $130 million was invested in seismic upgrades to structures between 1988 and 1995 by public and private organizations in the Pacific Northwest.footnote 5

  1. "Rattling the Northwest: Signs of prehistoric superquakes may portend a future shock," Science News, 137, 105 (1990).
  2. "A Fault Runs Through It," Columns, p.25, March 1993.
  3. "A Tsunami About 1000 Years Ago in Puget Sound, Washington," Brian F. Atwater and Andrew L. Moore, Science, 258, 1614 (1992).
  4. See references in (3.).
  5. "Averting Surprises in the Pacific Northwest," Brian F. Atwater, Thomas S. Yelin, Craig S. Weaver, and James W. Hendley II, U.S. Geological Survey Fact Sheet 111-95, March 1995, GPO 1995-684-845.

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