ButModerately sized, potentially damaging earthquakes could occur in this area at any time. Insurance industry estimates also include the Kern County Earthquake, M7. Cascadia Subduction Zone: Anytime. The geologic record shows that plus great earthquakes have occurred in the past 10, years.
About half of the past earthquakes have been very large estimated at a magnitude 8. The calculated odds that a Cascadia earthquake will occur in the next 50 years range from 7 percent to 15 percent for a great earthquake affecting the entire Pacific Northwest to about 37 percent for a very large earthquake affecting Southern Oregon and Northern California.
Facebook Twitter Email. Side-by-side comparison of dueling faults. 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 next earthquake arrived thirty-four years later at p. July 2, , 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.
Chimneys toppled, gas lines leaked, and power went out throughout much of Grays Harbor County. 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.
We were waiting for a news conference when it hit, an earthquake. 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. 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. Old 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 Compass Center, a facility for eighty homeless men in Pioneer Square, had to be abandoned. 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 , 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 Chunks of concrete fell sixty feet from the top of support pillars in the Garfield High School gym.
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. 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. 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; 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 liquefation. 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.
In short, this was not the Big One. What about the onshore Gorda Plate in northern California? An earthquake of M 6. After this earthquake, cracks in the ground appeared on the trail between Crescent City and Gasquet in the Smith River Valley, and all the chimneys were knocked down.
The highest intensity recorded was VIII, in a limited area in the northwestern corner of California, but intensities of V were felt over a broad area from Red Bluff in the south to McMinnville, Oregon, in the northern Willamette Valley.
Newspaper accounts did not report any aftershocks. Why should seismicity within the subducting oceanic plate be concentrated in the Puget Sound region? Oddly, this lower-plate seismicity does not extend very far south into Oregon Figure If subduction is taking place all along Cascadia, why should seismicity be concentrated only in Washington? To answer this question, we look at the contours of the subducting Juan de Fuca Plate, and we observe that the plate has an eastward-convex bend in Washington, curving from a north trend in Oregon to a northwest trend in southwest British Columbia Figure This bend is also reflected in the distribution of Cascade volcanoes Figure In northern California, Oregon, and southern Washington, these volcanoes line up north-south, parallel to the subduction-zone contours.
But in southwest British Columbia and northern Washington, including Mt. Baker and Glacier Peak, the volcanoes line up northwest-southeast, parallel to the subduction-zone contours.
This arch in the subduction zone may explain why the Olympic Mountains are so much higher than the Coast Range of Oregon or the hills of southwest Washington. The Olympic Mountains are arched up where the subduction zone bends the most, in map view.
To imagine the effect of this eastward-convex arch, consider a tablecloth hanging over the corner of a table. The tablecloth is straight along the sides of the table, but it makes a fold at the corner. Now suppose that, instead of a tablecloth, the table is covered by a sheet of hard plastic, the edges of which stick out over the side of the table. This is the same difficulty I have in gift-wrapping a present in a box. The wrapping folds neatly down the sides of the box, but in order to make the corners neat, I have to make a fold in the wrapping paper where it goes around the corner.
I do not excel at this, and so I generally have the present gift-wrapped at the store or by my wife. The plate can bend easily beneath Oregon or beneath southwest British Columbia, where the subduction zone is straight, but in trying to bend beneath the curved arch beneath Washington, internal stresses are built up that generate earthquakes.
Slab earthquakes occur there in two zones, even though the downgoing Juan de Fuca Plate there is relatively straight. One zone is a northward continuation of the Puget Sound deep zone, and it dies out near Vancouver Figure The other zone is beneath the west coast of Vancouver Island and it has lots of earthquakes Figure Leiph Preston and Ken Creager of the University of Washington have found earthquakes in this western zone as far south as southwestern Washington.
These earthquakes tend to occur in the oceanic mantle of the Juan de Fuca Plate whereas earthquakes of the eastern zone are more likely in the oceanic Juan de Fuca crust Figure Why should the slab have earthquakes beneath the straight subduction zone in British Columbia, but not the straight subduction zone in Oregon? The zone beneath the west coast of Vancouver Island may correspond to a shallower bend, but seismologists disagree on this point.
We have assumed that Oregon has a hazard from slab earthquakes, just as Washington does, even though it has not had any big slab earthquakes in historical time, with the possible exception of the earthquake near the California border, considered further below.
Perhaps the Puget Sound earthquakes are in a temporal cluster, an increase in slab earthquakes over nearly a century, and at some future time, Oregon might have a similar cluster. But not only does Oregon lack large slab earthquakes, it also has essentially no small ones either, whereas these are abundant in western Washington and in northern California Figure It is difficult to explain the lack of slab seismicity by saying that the slab is fully locked, because the earthquakes farther north are broadly distributed and are not localized on a few faults within the slab.
In addition, the crust of western Oregon is underlain by Siletzia basalt, which, as stated above, keeps the temperature of underlying Juan de Fuca Plate too hot to generate slab earthquakes. Wong also reanalyzed the Brookings earthquake, previously believed to have occurred in the Juan de Fuca Plate, and concluded that this earthquake was crustal, like similar earthquakes in northern California.
However, this idea is speculative, and not confirmed enough for Oregonians to relax and not worry about slab earthquakes like those in Washington. Another mystery is that wherever the deep slab is seismically active, the overlying continental crust is active, too.
The crustal seismicity is high beneath Puget Sound where the slab seismicity is high. In Northern California, both the Gorda Plate and the overlying and adjacent continental crust are characterized by frequent earthquakes. On Vancouver Island, the largest crustal earthquakes occurred on the onshore projection of the Nootka Transform Fault, and they were characterized by left-lateral strike-slip faulting, just as earthquakes on the Nootka Fault are.
If our speculations about a bending origin for the localization of seismicity are correct, there should be no relationship between earthquakes in the slab and earthquakes in the crust. Yet they appear to be somehow tied together, even though the seismicity zones in North American crust and in the Juan de Fuca Plate are generally separated by lower crust that is too hot and ductile to produce earthquakes Figure These questions, now being addressed by seismologists in Canada and the United States, are of practical importance because they bear on estimates of hazards in the Pacific Northwest and the Vancouver-Victoria region.
In summary, the three largest slab earthquakes in the Puget Sound region were characterized by very large areas of intensity VII, but only the earthquake had a very large area of intensity VIII. Earthquakes are studied using global seismometer networks that produce data about their locations, times, sizes and the nature of faulting.
Because earthquakes generally result from the motions of the plates, knowledge of the direction and amount of motion is important for understanding plate motions and the forces that cause them. Such studies are key to assessing the societal hazards posed by earthquakes. Welcome to the IRIS Image Gallery — a diverse collection of photographs and visuals that encompass the range and breadth of seismology and the seismological community.
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