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5.5.3: Slab Earthquakes in the Juan de Fuca Plate Beneath the Continent in the Puget Sound Region

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    clipboard_e4a2436f67a5999fdef77b6ee3dccf5c9.pngThe greatest amount of seismicity generated by the Juan de Fuca Plate itself (not including the Explorer and the Gorda plates) is beneath western Washington, where it is being subducted beneath North America (Figures 3-21, 5-1). These are called slab earthquakes or Benioff zone earthquakes. Most of the damage and loss of life in the Pacific Northwest has been as a result of these earthquakes, including the largest known historical shocks to strike either Washington or Oregon

    The first of these, on April 13, 1949, really should have been no great surprise. The southwesternmost Puget Sound region had been struck by earthquakes on November 13, 1939 (M 5.5-5.9), and on February 15, 1946 (M 6.3). Both were slab earthquakes, and both had produced intensities as high as VII, which meant minor damage and collapse of chimneys. The 1949 earthquake of M 7.1 struck the southern Puget Sound region just before noon on April 13. Strong shaking lasted about thirty seconds. Most people were at work, getting ready to go to lunch. Most schools were on vacation, which turned out to be a blessing because of the collapse of many unreinforced brick school buildings. The epicenter was between Olympia and Fort Lewis, and the high-intensity damage zone extended from Rainier, Oregon, on the Columbia River, north to Seattle (Figures 5-3 to 5-5). The earthquake was felt from Vancouver, B.C., to Klamath Falls and Roseburg, Oregon. A sidewalk clock outside a jewelry store at 1323 Third Avenue in Seattle stopped at the moment of the earthquake: 11:56.

    Eleven-year-old Marvin Klegman was killed, and two other children were injured by falling bricks as they played outside the Lowell School in Tacoma. Jack Roller was killed when part of the Castle Rock School building collapsed on him. Five students and two teachers were injured at Adna School 10 miles west of Centralia. One little girl was critically injured as she left her second-grade classroom. Tons of bricks fell from the Lafayette School building in Seattle, but school was not in session, and children were playing in the schoolyard far from the building. The Lafayette School was one of ten Washington schools condemned after the earthquake. The auditorium collapsed at Puyallup High School (Figure 5-3), but no one was in it at the time. Part of the Boys Training School at Chehalis crumpled and fell, injuring two boys.

    There were many narrow escapes. Freda Leaf, seventy-one, jumped into the Duwamish River but was rescued by a neighbor, D. V. Heacock. Part of the roof of the Busy Bee Restaurant on Second Avenue in Seattle fell in, and the patrons headed for the exit. The proprietor, George Pappas, immediately saw the danger and ordered the bartender, a big man named Bill Given, to block the exit. Moments later, tons of bricks cascaded onto the sidewalk in front of the restaurant. Water spilled out of an old water tower at the reservoir at Roosevelt Way and East 86th Street; a few minutes before, painters working at the tower had knocked off for lunch. At the Tacoma Narrows Bridge, under repair at the time, a twenty-three-ton steel saddle mounted to hold up a suspension cable dislodged and plunged off the bridge and through a scow on the water below, injuring two people. In Olympia, the Old State Building (Figure 5-4) and the State Insurance Building were the worst hit. Governor Arthur Langlie and his assistant, Dick Everest, were in their offices in Olympia and were showered with falling plaster.



    At the Blue Mouse Theater in Tacoma, people were watching the earthquake scene from The Last Days of Pompeii as the earthquake struck. In a bizarre coincidence, a crucifixion scene with accompanying earthquakes was being shown at the time of the earthquake at the nearby Roxy Theater. At Second and Occidental in Seattle, a man was seen walking rapidly down the street after the earthquake clad only in underwear, sportscoat, and shoes.

    In Oregon, broken water pipes flooded the basements of two stores in Astoria, plaster cracked in Florence, and dishes crashed from their shelves in Newport. Chimneys crashed at Reed College in Portland, and office workers on the twelfth floor of the new Equitable Building were knocked to the floor.

    Fortunately, perhaps amazingly, only seven lives were lost and the damage was only $15 million, even though the magnitude was 7.1. In today’s dollars, the losses would be perhaps twenty times that; the losses to Washington schools alone would have been $60 million in 1998 dollars. But losses were still remarkably low. Probably the main reason, aside from school being out of session, was that the focal depth of the earthquake was about thirty-five miles below the surface, meaning that the shock waves had thirty-five miles to weaken in amplitude before reaching the surface. Because it was such a deep earthquake, the Intensity VIII zone was very large, but there were no areas of Intensity IX or X, as there would have been with a shallower crustal earthquake of the same magnitude.

    On April 29, 1965, at 8:29 in the morning, a second large slab earthquake with magnitude 6.5 struck between Kent and Des Moines, south of Sea-Tac Airport between Seattle and Tacoma. Like the 1949 earthquake, its focus was more than thirty miles beneath the surface.

    Adolphus Lewis, seventy-five, a retired laborer, was on his way from his hotel room to have breakfast when he was killed by falling debris (Figure 5-6). Raymond Haughton, fifty-two, was killed, and Eugene Gould, fifty, critically injured when a fifty-thousand-gallon wooden water tank on a two-hundred-foot tower collapsed at the Fisher Flouring Mills. In total, six people were killed, including those suffering heart attacks, and property damage was estimated at $12,500,000, $60 million in 1998 dollars.


    As in 1949, there was considerable damage to school buildings. In Seattle, parts of Broadview Elementary School collapsed, and there was damage to the Ballard High School auditorium. The greatest damage was to West Alki Elementary School, where a chimney sixty feet high fell into the boiler room, narrowly missing the custodian. Unlike in 1949, no pupils were injured.

    The 8:15 mass at St. James Cathedral was interrupted when low-hanging chandeliers began to swing violently. Two hundred parishioners fled the cathedral but returned for the remainder of the service when the tremors subsided. At the Rainier Brewing Company, two thousand-barrel aging tanks were knocked off their platforms. One split open, spilling enough beer for fifteen thousand cases. Engineer John Strey found himself wading hip-deep through the foamy beer. The restaurant at the top of the Space Needle was full of customers when it began to sway, “like riding the top of a flagpole.” No one ran for the elevators, and all finished breakfast after the violent shaking had ceased.

    The next earthquake arrived thirty-four years later at 6:44 p.m. July 2, 1999, at Satsop, Washington, ironically the site of a nuclear power plant proposed by the Washington Public Power Supply System that, fortunately, never got built. The earthquake had a moment magnitude of 5.8 and was twenty-five miles deep. The lovely old Grays Harbor County Courthouse in Montesano, built in 1910, was severely damaged. The ceiling and an exterior wall of Moore’s Furniture Store in Aberdeen collapsed, causing extensive havoc inside. Chimneys toppled, gas lines leaked, and the power went out throughout much of Grays Harbor County. John Hughes of The Daily World in Aberdeen reported from the parking lot on State Street that “(s)treetlight poles shook, my Volkswagen Beetle did the Macarena while Dee Anne Shaw’s Chrysler coupe was undulating.”


    Then came 11:54 a.m. on Ash Wednesday, February 28, 2001.

    I was having a late-morning cup of coffee in Corvallis when I began to feel dizzy. The two people across the table from me continued to talk and obviously felt nothing, so I thought I was ill. Then I saw the swaying of a lamp and realized that I was feeling the long-period waves from a distant earthquake.

    Brian Wood of KIRO-TV was setting up for a press conference by Seattle Mayor Paul Schell, who was about to explain the city’s response to the Mardi Gras riots the previous night in which one person had been killed. Before the mayor arrived, the room began to shake, and Wood immediately began to broadcast: “This is Brian Wood, live in downtown Seattle, live on the twelfth floor of the mayor’s conference room. We were waiting for a news conference when it hit, an earthquake.” This made KIRO first with the story, which was broadcast nationwide. Later, ABC in New York would ask sheepishly if it could carry the story from KIRO, a CBS affiliate, because its ABC affiliate, KOMO-TV, had taken too much time getting organized.

    Curtis Johnny and his girlfriend, Darlene Saxby, headed for the exit of their South Park apartment as soon as they felt the earthquake. Suddenly, a chimney crashed through the ceiling, covering Johnny with bricks. “I was pretty hysterical,” Darlene said. “I was just throwing bricks off of him and screaming at the same time.” Neighbors had to break in the door to the apartment to get them out. Hin Pang and his wife Sim Pang were visiting friends at a Chinatown club when the earthquake hit. As they ran from the building, they were struck by a shower of bricks from a ledge three stories above them. Sim Pang suffered head, chest, and arm injuries but was released from Harborview Medical Center later in the day. She had been buried by the bricks, and she suffered chest injuries and a crushed pelvis; he remained in the hospital for a longer time but survived.

    clipboard_e8f1eb980263036916ce23c5061e4382e.pngOld buildings fared the worst. Tops of brick buildings crashed to the street along Alaskan Way Viaduct and along Second Avenue, crushing cars. A huge piece of the Fenix Undergound, a night club on Second Avenue South, fell on two parked cars; the inside wall collapsed, trapping club owner Mike Lagervall and his secretary inside. The roof of the Washington Federal Savings building partially fell in, and one of its façades covered a ninety-foot stretch of the sidewalk (Figure 5-7). The Compass Center, a facility for eighty homeless men in Pioneer Square, had to be abandoned. The Alaskan Way Viaduct itself, built in 1953 for $8 million, suffered damage but did not collapse; replacing it would cost $400 million. (A few years later the Alaskan Way viaduct would, in fact, be replaced.) The great stone columns of the Capitol Dome in Olympia, built in 1928, were knocked out of line. State employees were allowed to return at the end of April, but tours of the Capitol were not scheduled to resume until the end of 2004. Chunks of concrete fell sixty feet from the top of support pillars in the Garfield High School gym. In Centralia, the rooftop brick façade of Coast to Coast Hardware collapsed and punched holes in a lower roof of the rear addition.

    In the Grand Ballroom of the Westin Hotel in downtown Seattle, Bill Gates was onstage about to demonstrate Microsoft’s forthcoming Windows XP operating system when the shaking began. The talking stopped, and Gates looked around as ceiling tiles began to fall. Giant chandeliers swayed, and the audience started screaming and heading for the exits or crawling under chairs. Gates calmly walked offstage, perturbed at being interrupted, even as a piece of light fixture the size of a cereal box fell next to him. Asked later if he had been frightened, Gates said, “No, I was worried about what was going on, was there a bomb, or what was going on.”

    There were light moments. Joanne Smith, a third-grade teacher at St. Matthew Parish School in Hillsboro, Oregon, led her children out onto the damp playground where they watched dozens of earthworms come out of the ground, disturbed by the surface waves of the earthquake. In Seattle, Skyler Dufour, nine, collected rubble to be offered on eBay with bids opening at seven dollars. At De Laurenti’s Specialty Foods in the Pike Place Market, two hundred bottles of wine fell to the floor, with the fifty-five-dollar bottles on the top shelf falling the farthest. Steve Springston, a wine buyer, observed that “it was a very complex aroma.” Christopher Carnrick was participating in a videoconference when the room started to shake. He jumped on the table, took a surfing stance, and shouted, “I am RIDING this BABEE out,” not realizing that his surfing adventure was being viewed by astonished participants in San Francisco and Montana.

    Governor Gary Locke estimated the damage to be as great as two billion dollars. But on the other hand, only one person died, a Burien woman who had a heart attack during the earthquake; 396 people were injured. But on reflection, it became obvious that the damage could have been much worse. First, it was a deep earthquake, so that seismic waves had a longer distance between the hypocenter and the surface for waves to diminish, or attenuate. A subduction-zone earthquake would have had strong shaking over a much longer time, and a crustal earthquake of the same magnitude would have had much more powerful seismic waves and greater intensities. Second, the Puget Sound region was in its second straight dry winter, and water tables were the lowest in thirty years, reducing the potential for liquefaction. Finally, Seattle had just completed a Project Impact preparedness exercise; many structures had been retrofit, and people were much better informed than they had been. (Paula Seward, vice president of Northwest sales at Quakeproof, was in the middle of a presentation about earthquake preparedness to a group on the third floor of a downtown Seattle hotel when the quake struck. A participant asked her, “Is this part of your sales presentation?”)

    In short, this was not the Big One. As Bill Steele of the Pacific Northwest Seismograph Network put it, “If you’re going to have a magnitude 7 in the Puget Sound area, let it be a deep one.”

    This page titled 5.5.3: Slab Earthquakes in the Juan de Fuca Plate Beneath the Continent in the Puget Sound Region is shared under a CC BY-NC-SA 4.0 license and was authored, remixed, and/or curated by Robert S. Yeats (Open Oregon State) via source content that was edited to the style and standards of the LibreTexts platform; a detailed edit history is available upon request.