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
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.
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.
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.
