The 1965 Puget Sound earthquake occurred at 08:28 AM PDT (15:28 UTC) on April 29 within the

Puget Sound Puget Sound ( ) is a sound of the Pacific Northwest, an inlet of the Pacific Ocean, and part of the Salish Sea. It is located along the northwestern coast of the U.S. state of Washington. It is a complex estuarine system of interconnected m ...

region of

Washington state Washington (), officially the State of Washington, is a U.S. state, state in the Northwestern United States, Pacific Northwest region of the Western United States. Named for George Washington—the first President of the United States, U.S. p ...

. It had a magnitude of 6.7 on the

moment magnitude scale The moment magnitude scale (MMS; denoted explicitly with or Mw, and generally implied with use of a single M for magnitude) is a measure of an earthquake's magnitude ("size" or strength) based on its seismic moment. It was defined in a 1979 pa ...

and a maximum perceived intensity of VIII (''Severe'') on the

Mercalli intensity scale The Modified Mercalli intensity scale (MM, MMI, or MCS), developed from Giuseppe Mercalli's Mercalli intensity scale of 1902, is a seismic intensity scale used for measuring the intensity of shaking produced by an earthquake. It measures the ef ...

. It caused the deaths of seven people and about $12.5–28 million in damage. There were no recorded aftershocks.

Tectonic setting

The western part of Washington State lies above the

Cascadia subduction zone The Cascadia subduction zone is a convergent plate boundary that stretches from northern Vancouver Island in Canada to Northern California in the United States. It is a very long, sloping subduction zone where the Explorer, Juan de Fuc ...

, where the Juan de Fuca Plate is being subducted beneath the

North American Plate The North American Plate is a tectonic plate covering most of North America, Cuba, the Bahamas, extreme northeastern Asia, and parts of Iceland and the Azores. With an area of , it is the Earth's second largest tectonic plate, behind the Paci ...

. The seismicity of this region consists of rare great megathrust earthquakes, like the

1700 Cascadia earthquake The 1700 Cascadia earthquake occurred along the Cascadia subduction zone on January 26, 1700, with an estimated moment magnitude of 8.7–9.2. The megathrust earthquake involved the Juan de Fuca Plate from mid- Vancouver Island, south along th ...

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 Basalt (; ) is an aphanitic (fine-grained) extrusive igneous rock formed from the rapid cooling of low-viscosity lava rich in magnesium and iron (mafic lava) exposed at or very near the surface of a rocky planet or moon. More than 90 ...

gabbro Gabbro () is a phaneritic (coarse-grained), mafic intrusive igneous rock formed from the slow cooling of magnesium-rich and iron-rich magma into a holocrystalline mass deep beneath the Earth's surface. Slow-cooling, coarse-grained gabbro is ...

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 The focal mechanism of an earthquake describes the deformation in the source region that generates the seismic waves. In the case of a fault-related event it refers to the orientation of the fault plane that slipped and the slip vector and ...

indicates that it resulted from normal faulting within the Juan de Fuca slab. There were no recorded

aftershock In seismology, an aftershock is a smaller earthquake that follows a larger earthquake, in the same area of the main shock, caused as the displaced crust adjusts to the effects of the main shock. Large earthquakes can have hundreds to thousa ...

s, similar to observations from the 1949 Olympia and

2001 Nisqually earthquake The 2001 Nisqually earthquake occurred at on February 28, 2001 and lasted nearly a minute. The intraslab earthquake had a moment magnitude of 6.8 and a maximum Mercalli intensity of VIII (''Severe''). The epicenter was in the southern Puge ...

s 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 Alluvium (from Latin ''alluvius'', from ''alluere'' 'to wash against') is loose clay, silt, sand, or gravel that has been deposited by running water in a stream bed, on a floodplain, in an alluvial fan or beach, or in similar settings. ...

, although there were some exceptions. The observed pattern of intensities was very similar to those from the 1949 Olympia and

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 The Boeing Company () is an American multinational corporation that designs, manufactures, and sells airplanes, rotorcraft, rockets, satellites, telecommunications equipment, and missiles worldwide. The company also provides leasing and ...

plants at Renton and

Seattle Seattle ( ) is a seaport city on the West Coast of the United States. It is the seat of King County, Washington. With a 2020 population of 737,015, it is the largest city in both the state of Washington and the Pacific Northwest region o ...

, both built on artificial fill and

mudflat Mudflats or mud flats, also known as tidal flats or, in Ireland, slob or slobs, are coastal wetlands that form in intertidal areas where sediments have been deposited by tides or rivers. A global analysis published in 2019 suggested that tidal f ...

s, suffered major damage. The

State Capitol building This is a list of state and territorial capitols in the United States, the building or complex of buildings from which the government of each U.S. state, the District of Columbia and the organized territories of the United States, exercise its ...

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 U.S. Route 101, or U.S. Highway 101 (US 101), is a north–south United States Numbered Highway that runs through the states of California, Oregon, and Washington, on the West Coast of the United States. It is also known as (The Royal Roa ...

sinking . 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

peak ground acceleration Peak ground acceleration (PGA) is equal to the maximum ground acceleration that occurred during earthquake shaking at a location. PGA is equal to the amplitude of the largest absolute acceleration recorded on an accelerogram at a site during a par ...

of 0.204''g'' was measured at Olympia.

Aftermath

The damage and deaths in the 1965 earthquake helped bring about the installation of the

Pacific Northwest Seismic Network The Pacific Northwest Seismic Network, or PNSN, collects and studies ground motions from about 400 seismometers in the U.S. states of Oregon and Washington. PNSN monitors volcanic and tectonic activity, gives advice and information to the publi ...

in 1969.

References

External links

* {{DEFAULTSORT:1965 Olympia Earthquake Earthquakes in Washington (state) 1965 earthquakes 1965 in Washington (state) 1965 natural disasters in the United States Puget Sound region