The 1965 Puget Sound earthquake occurred at 08:28 AM PDT (15:28

UTC Coordinated Universal Time (UTC) is the primary time standard globally used to regulate clocks and time. It establishes a reference for the current time, forming the basis for civil time and time zones. UTC facilitates international communica ...

) on April 29 within the

Puget Sound Puget Sound ( ; ) is a complex estuary, estuarine system of interconnected Marine habitat, marine waterways and basins located on the northwest coast of the U.S. state of Washington (state), Washington. As a part of the Salish Sea, the sound ...

region of

Washington state Washington, officially the State of Washington, is a state in the Pacific Northwest region of the United States. It is often referred to as Washington State to distinguish it from the national capital, both named after George Washington ...

. It had a magnitude of 6.7 on the

moment magnitude scale The moment magnitude scale (MMS; denoted explicitly with or Mwg, 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. was defined in a 1979 paper ...

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

Mercalli intensity scale The Modified Mercalli intensity scale (MM, MMI, or MCS) measures the effects of an earthquake at a given location. This is in contrast with the seismic magnitude usually reported for an earthquake. Magnitude scales measure the inherent force or ...

. 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, about off the Pacific coast of North America, that stretches from northern Vancouver Island in Canada to Northern California in the United States. It is capable of producing 9.0+ m ...

, where the Juan de Fuca plate is being subducted beneath the North American plate. The seismicity of this region consists of rare great

megathrust earthquake Megathrust earthquakes occur at convergent plate boundaries, where one tectonic plate is forced underneath another. The earthquakes are caused by slip along the thrust fault that forms the contact between the two plates. These interplate earthq ...

s, 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 the P ...

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 aphanite, 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 planetary surface, surface of a terrestrial ...

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

eclogite Eclogite () is a metamorphic rock containing garnet ( almandine- pyrope) hosted in a matrix of sodium-rich pyroxene ( omphacite). Accessory minerals include kyanite, rutile, quartz, lawsonite, coesite, amphibole, phengite, paragonite, zoisit ...

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 Fault (geology)#Slip.2C heave.2C throw, deformation in the Hypocenter, source region that generates the seismic waves. In the case of a Fault (geology), fault-related event, it refers to the ori ...

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 Epicenter, the same area of the Mainshock, main shock, caused as the displaced Crust (geology), crust adjusts to the effects of the main shock. Large earthq ...

s, 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 Alluvium (, ) 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. Alluvium is also sometimes called alluvial deposit. Alluvium is ...

, 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 The Boeing Company, or simply Boeing (), is an American multinational corporation that designs, manufactures, and sells airplanes, rotorcraft, rockets, satellites, and missiles worldwide. The company also provides leasing and product support s ...

plants at Renton and

Seattle Seattle ( ) is the most populous city in the U.S. state of Washington and in the Pacific Northwest region of North America. With a population of 780,995 in 2024, it is the 18th-most populous city in the United States. The city is the cou ...

, 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 ...

s, 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 U.S. Route 101, or U.S. Highway 101 (US 101), is a major north–south highway that traverses the states of California, Oregon, and Washington on the West Coast of the United States. It is part of the United States Numbered Highway Syst ...

sinking . The

Tacoma Narrows Bridge The Tacoma Narrows Bridge is a pair of twin bridges, twin suspension bridges that span the Tacoma Narrows strait of Puget Sound in Pierce County, Washington, Pierce County, Washington (state), Washington. The bridges connect the city of Tacom ...

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 wikt:accelerogram, accelerogram at a ...

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 public ...

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 Puget Sound earthquake