The 1964 Great Alaska Earthquake & Tsunami

THE EARTHQUAKE

At 5:36 PM Alaska Standard Time on Good Friday March 27, 1964, rupture began near the north shore of Alaska’s Cook Inlet, 36 miles WSW of Valdez and 88 miles ESE of Anchorage.  It would take four minutes for the rupture to propagate, eventually rupturing a 500 mile by 125 mile fault area uplifting some areas by as much 30 feet and dropping down others by nearly 8 feet.

1964 Alaska earthquake areas of uplift and subsidence

 

This map shows the epicenter (red star) of the earthquake and the approximate area of fault rupture.  Uplifted areas are shown in red, and areas of subsidence shown in blue (Graphic provided by US Geological Survey.)

 

Ground Shaking

The earthquake was felt throughout most of mainland Alaska and from Ketchikan in the Southeast to the eastern Aleutian Islands, an area of over 800,000 square miles.  If centered in Humboldt County, It would have been felt from Los Angeles to Seattle and inland to Utah and Idaho.

 

Shake Map of 1964 Alaska earthquake

This maps shows the USGS felt pattern (isoseismal map) of the 1964 Alaska earthquake.  Most of Alaska’s population at the time were in the area of violent shaking.  The map in the lower right superimposes the 1964 earthquake shaking on California using the same scale and assuming the epicenter were in Humboldt Bay.  It would have been felt as far away as Los Angeles.

 

Experience what it was like in Anchorage during the earthquake: Bob Pate worked for radio station KHAR in Anchorage and recorded what happened in his home.

Listen here!

More than 100,000 people in the greater Anchorage and other southern Alaska towns and villages experienced violent ground shaking.  But only nine deaths were attributed to ground shaking effects.  Many homes, like those of Bob Pate’s, experienced little structural damage.  The low casualty rate was in part due to the time of day when most businesses and schools were closed and the wood frame construction typical of most structures in the area.

Liquefaction and Landslides

Much of the shaking damage was caused by landslides exacerbated by liquefaction.  The friction during shaking melted the frozen ground triggering numerous failures in the greater Anchorage area.  The most damaging was in Turnagain Heights where 130 acres of land slid as much as 2000 feet during the shaking, taking 75 homes and four lives with it.  Other failures in downtown anchorage claimed businesses and a school.

Landslide damage at Government Hill Elementary School

Government Hill Elementary School broke into several pieces; no one was in the school at the time.

Turnagain landslide damage

Turnagain Heights landslide, USGS photo.

Liquefaction damage in downtown Anchorage Alaska

Liquefaction slumping in downtown Anchorage, USGS photo.

Fault rupture land level changes

The earthquake ruptured the subduction zone interface (megathrust) between the Pacific and North American plates.  It also caused slip on a number of smaller upper plate faults related to the subduction zone.  Plate tectonic theory was in its infancy when the earthquake struck and subduction zones had yet to be recognized.  USGS geologist George Plafker carefully mapped surface faulting and how the land surface changed after the earthquake.  The only interpretation of his data that made physical sense was slip along a gently dipping thrust fault.

Uplift observed at Montague Island in Alaska showing areas now above sea level

Uplifted seafloor on Montague Island where the highest uplift (33 feet) was measured (USGS photo).

Earthquake impacts in Homer, AK resulting in subsidence and flooding

The town of Homer was in the area of subsidence.  This is the Homer spit where 5.6 feet of downdropping was measured (Pratt Museum photo)

 

For more scientific background on the earthquake, visit the link below.

The Good Friday Earthquake: 1964 Alaska

THE TSUNAMI WARNING SYSTEM IN 1964

The Honolulu Observatory was established as the U.S. Tsunami Warning Center in 1949, in response to the 1946 earthquake centered in Alaska’s Aleutian Islands that killed 159 people in Hawaii. The only real time instrumentation on site was a seismograph. The observatory was staffed during regular weekday work hours; in the evenings and weekends an alarm on the seismograph would alert duty personnel to return to the site.  If the seismic record suggested a significant Pacific basin earthquake, staff would teletype requests to other observatories throughout the Pacific to telephone or teletype their seismic data to Honolulu. After the data was received, an epicenter was determined and information sent to civil defense offices of states which might be affected.

Honolulu Observatory

Honolulu Observatory in 1965 (NOAA illustration)

In 1964, the Alaska earthquake knocked out all communications within Alaska and between Alaska and other states. The only warning Alaskans received of a potential tsunami was the long duration of shaking. The Alaska Observatory did send alerts to Hawaii and West Coast states. The table below shows the timeline of actions taken by the Observatory. It took one hour and 16 minutes to estimate that the earthquake was centered near Seward, Alaska.

Timeline of alert levels for the 1964 tsunami

The first alert bulletin was issued at 9:02 PM PST. The term “tsunami” was not in use at the time and surges following earthquakes were called tidal waves or seismic sea-waves.

Tsunami Bulletin for the 1964 Alaska tsunami

THE 1964 TSUNAMI

The earthquake was followed by tsunami surges that were recorded as far away as Antarctica and New Zealand. The tsunami caused more damage and casualties than the earthquake shaking.  106 people died in Alaska, 13 in California, and 5 in Oregon.

This numerical model shows propagation across the Pacific.

Tsunami Forecast Model Animation: Alaska 1964

The Tsunami in Alaska

The tsunami in Alaska was complex, caused by both the seafloor deformation associated with the M9.2 earthquake and secondary landslides.  In Valdez, the shaking triggered a large submarine landslide that arrived only minutes after the earthquake.  Outside of the Prince William Sound area, first surges arrived in 20 to 50 minutes.

Observed tsunami heights recorded in Alaska after the 1864 earthquake

Compilation of measured peak tsunami water heights in 1964.  Red circle is the epicenter, yellow bars are the relative water heights, numbers show select amplitudes in feet. Data from NOAA’s National Center for Environmental Information.

The Tsunami in California

In 1964, tsunami alerts messages were sent to State Civil Defense offices and not sent directly to counties.  California’s Civil Defense Office received information from the Honolulu Observatory at 9:30 PM PST on March 27, but did not issue alerts to counties until 11:08 PM. By that time, the estimated time of arrival of the tsunami in Crescent City was midnight and Del Norte Sheriff’s deputies had less than an hour to evacuate coastal areas.

Model of 1964 tsunami across Pacific Ocean and arrival in Crescent City

Numerical model of the tsunami about three hours after the earthquake, roughly the time Del Norte County officials were notified a tsunami could arrive around midnight.  Vertical scale greatly exaggerated.

What happened in Crescent City?

A tide gauge on the Lumber Dock at Crescent Harbor had been installed in 1933 and was operating on March 27.  Here are a series of time shots of the tide gauge record.

Crescent City tide gauge at time of bulletin

The tide gauge record at about the time of the Civil Defense notification.  It was a clear night, a full moon, and high tide.  The vertical axis is water height in meters relative to Mean Lower Low Water, the standard reference for tidal heights.

Crescent City tide gauge at time of tsunami arrival

Tide gauge recording just after midnight.  The first surge peaked at 11:50 PM on March 27th, with an amplitude about 6.4 feet above the tide, for a total of 14.5 feet.  It caused flooding in the dock area and up to Front Street.

Extent of inundation from the 1964 tsunami in Crescent City

Blue area is an estimate of the inundation caused by the first tsunami surge in 1964.  Eye witnesses described the flooding as very similar to what happened in May 1960 when a tsunami from the Chile magnitude 9.5 earthquake caused minor damage.  Photo on the right shows 1960 water levels in the harbor area.

Crescent City tide gauge at time of second wave arrival

A second surge arrives 30 minutes after the first.  It is smaller than the first but still causes some flooding in the waterfront area.

Crescent City tide gauge after the second surge

After the second surge, about 40 minutes elapses with no wave action.  This is the point when some survivors reported returning to the coastal area.  They assumed the tsunami was over, in part based on what they had experienced in the 1960 tsunami.

Crescent City tide gauge at time of third surge arrival

At a little before 1 AM, a third surge begins and the water quickly rises, surpassing the first surge and knocking the tide gauge housing over.  After this point, the tsunami behavior is uncertain (shown by dashed line).  Most survivors described four major surges and the fourth was the largest.  After the third surge, the harbor drained completely and people could see the seafloor in the moonlight.

Crescent City tide gauge at time of the fourth surge arrival

Most of the eyewitness accounts report the fourth surge was the highest.  It was just high enough to top the sea barrier on the west side of Crescent Harbor, an amplitude of 15.7 feet.  The tide was still relatively high and boosted the peak water elevation to nearly 22 feet.  Smaller surges likely continued in the harbor for days but never came close to the size of that fourth surge.

Peggy Coons and her husband were the lighthouse keepers of Battery Point and described the fourth surge,

Suddenly there it was, a mammoth wall of water barreling in toward us, a terrifying mass of destruction, stretching from the floor of the ocean upwards:  it looked much higher than the island, black in the moonlight. Roxey shouted, "Let’s head for the tower."  It was too late.  As we turned toward the tower, he yelled, "Look out!"  We both ducked.  It struck, split and swirled around both sides of the island with such speed we felt like we were sailing right along with it.  It took several minutes for us to realize the island hadn’t moved.  It crashed into the shore, picking up the driftwood logs and other debris lodged in our roadway and along the beachfront.  It looked as though it would push them on the pavement at the end of A street leading past the Seaside Hospital.”

Read Peggy Coons’ complete description of the 1964 tsunami here.

This CBS news report with Terry Drinkwater gives a feel of the impacts in Crescent City.

1964 tsunami at Crescent City

The Drinkwater story overstates the area affected and understates how high the water reached.  Orville Magoon of the US Army Corps of Engineers did a detailed study of the Crescent City area and mapped the extent of inundation and structural damage. 

That mapped area is presented in the image below, noted by the shaded region along the coastline. The flooding coverd 29 city bocks.

Flooding mapped in Crescent City by Magoon

 

Crescent City's Tsunami Walk, shares survivor accounts of what happened in 1964. To hear the accounts, take the Tsunami Walk below.

Take the Tsunami Walking Tour

Take the Tsunami Walk Tour to learn about the impacts of the tsunami in Crescent City

 Take the Tsunami Walking Tour