1965 Puget Sound earthquake

Tectonic setting

The western part of Washington State lies above the Cascadia subduction zone, where the Juan de Fuca plate is being subducted beneath the North American plate. The seismicity of this region consists of rare great megathrust earthquakes, like the 1700 Cascadia earthquake, and more common earthquakes originating from within the subducting slab. These events relate to normal faulting, associated with the bending of the slab, possibly related to a phase change below about 40 km from basalt/gabbro to eclogite.

Earthquake

The earthquake had an estimated magnitude of 6.6 , 6.5 and 6.7 . At 10–20 seconds the duration of strong ground motion was relatively short. The earthquake's focal mechanism indicates that it resulted from normal faulting within the Juan de Fuca slab. There were no recorded aftershocks, similar to observations from the 1949 Olympia and 2001 Nisqually earthquakes and a characteristic of such intraslab events.

Intensity

The pattern of shaking intensity was somewhat variable, with a large region with an intensity of VII (Very strong) containing localized areas of intensity VIII (Severe). These variations generally relate closely to the underlying geology, with higher intensities recorded where there was either artificial fill or alluvium, although there were some exceptions. The observed pattern of intensities was very similar to those from the 1949 Olympia and 2001 Nisqually earthquakes.

Damage

Three people were killed by falling debris in the Duwamish valley floor area of Seattle, and four others died from heart attacks. There was minor damage recorded over a large area, including fallen chimneys and cracked mortar. The two Boeing plants at Renton and Seattle, both built on artificial fill and mudflats, suffered major damage. The State Capitol building in Olympia suffered cracking to the dome and supporting buttresses, leaving it in a condition where a major aftershock could have caused complete collapse. Single-story unreinforced brick buildings performed the worst in the earthquake with wood-framed structures generally performing very well. Major highways had relatively little damage, with some sections of U.S. Route 101 sinking 1 to 2 ft. The Tacoma Narrows Bridge was closed for 30 minutes as a precaution but reopened with only damage to light fixtures and weather seals on cables.

Ground acceleration

A peak ground acceleration of 0.204g was measured at Olympia.

Aftermath

The damage and deaths in the 1965 earthquake helped bring about the installation of the Pacific Northwest Seismic Network in 1969.

References

References

1. National Geophysical Data Center / World Data Service (NGDC/WDS): NCEI/WDS Global Significant Earthquake Database. NOAA National Centers for Environmental Information.. (1972). "Significant Earthquake Information". NOAA National Centers for Environmental Information.
2. (October 21, 2009). "Puget Sound area, Washington 1965 04 29 15:28:43.7 UTC Magnitude 6.5 Intensity VIII". United States Geological Survey.
3. Oakeshott, G.B.. (July 1965). "The Seattle Earthquake". Mineral Information Service.
4. Wong, I.G.. (2005). "Low Potential for Large Intraslab Earthquakes in the Central Cascadia Subduction Zone". Bulletin of the Seismological Society of America.
5. (1967). "Geological Survey Research 1967". [[United States Government Printing Office]].
6. Cascadia Region Earthquake Workshop. (2008). "Cascadia Deep Earthquakes". Washington Division of Geology and Earth Resources.
7. David Postman. (March 4, 2001). "Is Capitol dome at risk? Huge stone columns knocked out of line". The Seattle Times.
8. (May 1965). "All The Fear A Word Can Make—Earthquake!". [[Washington State Department of Highways]].
9. (July 27, 2011). "ANSS – Advanced National Seismic System Pacific Northwest Region". United States Geological Survey.
10. (2002). "Intensity Distribution and Isoseismal Maps for the Nisqually, Washington, Earthquake of 28 February 2001". United States Geological Survey.