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Lifelines and earthquake hazards in the greater Seattle areaIntroduction
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Older residents can remember that in 1949 and 1965, earthquakes of magnitude 7.1 and 6.5, respectively, hit the Puget Sound region. Eight people were killed in each earthquake. Damage from smaller recent earthquakes, such as the 1996 Duvall earthquake that disrupted a Mariners' baseball game, has been very slight.
Despite the lack of recent, large, damaging earthquakes, earth scientists and engineers now understand that earthquake hazards in the Seattle area are greater than previously known. In the early 1990s scientists and engineers accepted geologic evidence that great subduction zone earthquakes, of magnitude 8 to 9, repeatedly strike along the Washington coast. In 1992, geologists recognized that raised and lowered beaches on Bainbridge Island recorded a large earthquake on the Seattle fault about 1100 years ago. Since then, geologic and geophysical field investigations at an accelerating pace have sought to understand the potential for large-magnitude shallow earthquakes in the Seattle area.
The results have been sobering. Changes in the elevation of beaches, particularly in southern Puget Sound, suggest one or more large events in addition to the Seattle fault event about 1100 years ago. Geologic study of trenches across the Seattle fault on Bainbridge Island has found evidence of as much as a meter of slip during an earthquake within the last 4500 years. Preliminary geophysical measurements show crustal contraction across the Seattle fault, clear evidence that strain is slowly building toward the next earthquake.
Western Washington lies in the contact between two of the Earth's large crustal plates. The Juan de Fuca plate, which forms the floor of the northeastern Pacific Ocean, moves northeastward with respect to the North American plate at an average rate of about 4 centimeters (1.5 inches) per year, as indicated by the arrow in Figure 1. As it collides with North America, the Juan de Fuca plate slides (or subducts) beneath the continent and sinks slowly into the earth's mantle (the 3,000-kilometer-thick rocky shell between the outermost crust and the molten outer core of the earth). The shallow, east-dipping zone of contact between the plates is the Cascadia fault. The collision of these plates produces the Cascade volcanoes and earthquakes in three source zones.
The area just offshore, where the Cascadia fault is near the surface of the Earth, is called the Cascadia subduction zone. Now, as at most times, there is little slip on the Cascadia fault in the subduction zone. Eastward motion of the Juan de Fuca plate is absorbed by compression of the North American plate. Records provided by buried soil layers, dead trees, and deep-sea deposits indicate to geologists that the Cascadia fault ruptures and releases this compression in large-magnitude 8 to 9-earthquakes about every 500-600 years. It is the upper portion of the shallowly dipping Cascadia fault that ruptures during these events; most of the rupture area is offshore. The last such earthquake occurred on January 26, 1700.
When the Cascadia fault ruptures, it will likely cause: 1) Severe ground motions along the coast, with shaking in excess of 1 g in many locations (1 g is equal to the acceleration of gravity, 0.5 g is half the acceleration of gravity). The greater Seattle area will see 0.2 to 0.3 g accelerations from a subduction-zone earthquake. 2) Because of the very large fault area involved, slip will produce strong motions that may last for two to four minutes as the earthquake propagates along the fault, and include seismic waves of very long period (20 seconds or more). These long-period waves may particularly effect very tall structures, and long structures such as bridges. 3) Tsunamis generated by sudden uplift of the sea floor above the fault. Effects of past tsunamis are among the evidence observed by geologists to infer the history of earthquakes in the subduction zone. 4) Effects in all of Cascadia's major population centers, from Vancouver, B.C., to Portland, putting strong stresses on the regional infrastructure.
Benioff (Deep) Zone
As the Juan de Fuca plate subducts beneath North America, it becomes denser than the surrounding mantle rocks and breaks apart under its own weight, causing Benioff zone earthquakes. Beneath Puget Sound the Juan de Fuca plate reaches a depth of 40-60 km and begins to bend even more steeply downward, forming a "knee" (Figure 1). It is at this knee where the largest Benioff zone earthquakes occur: both the 1949 event near Olympia (southwest of Tacoma) and the 1965 event near the Seattle-Tacoma International Airport occurred at the knee. But we expect that Benioff zone earthquakes as large as magnitude 7.5 are expected everywhere west of the eastern shores of Puget Sound.
Like subduction earthquakes, Benioff zone earthquakes have several distinctive characteristics. First, because they occur at depths of 40 kilometers or more, high frequency energy has been attenuated. On hard rock, peak ground accelerations are no more than about 0.2 to 0.3 g. Second, they tend to be felt over much broader areas than a shallow earthquakes of comparable magnitude. And third, significant aftershocks aren't expected, an important point for post-earthquake response.
The third source zone is the crust of the North American plate. Of the three source zones, this is the least understood. A variety of lines of evidence lead to the conclusion that the Puget Lowland area is currently shortening north-south at a rate of about 1/2 cm (one-fifth of an inch) per year. Where, and how, this shortening is occurring is not well understood, but at least some of it is occurring on the Seattle fault.
Other active faults may be present in the greater Seattle area (Figure 2), but geologists have only documented young (in the last 14,000 years) motion on the Seattle fault. How many other crustal faults pose significant earthquake hazards to the Puget Sound region is not yet known, but geologists and geophysicists are studying the South Whidbey Island fault, the Olympia fault (southwest of the map), and the Devils Mountain fault (north of the map) for evidence of young earthquakes.
As noted above, the seismic potential of the Seattle fault has been only recently appreciated. Because crustal earthquakes are shallow—at depths of 5 to 20 km—and may occur directly beneath urban areas, they have the potential to do great damage. The 1995 Kobe (Japan) and 1994 Northridge (California) earthquakes, with ground motions of 0.5 to 1.0 g, may be good analogs for a crustal earthquake in the greater Seattle area.
Probabilistic Ground Motion Map
The probabilistic hazard map (Figure 3 below) shows the expected peak horizontal ground motions on a hard rock site with a 2% probability of exceedance in 50 years, aggregated from all three sources. Note that along the coast the contour lines strike more or less south-north-in this region the hazard is dominated by the subduction zone source, and farther east shaking is likely to be less. Moving into Puget Sound, contours swing east-west-the effect of the Benioff zone, which appears to produce more earthquakes beneath Puget Sound than farther south or north. The bullseye over central Puget Sound reflects our new understanding of the Seattle fault. If geologic studies now in progress sustain our current view of Seattle fault zone hazards, it is likely that the greater Seattle area will see upward revisions in building codes.
The type and depth of near-surface deposits (down to about 30 meters) can greatly affect the intensity of earthquake shaking at a given site. Poorly consolidated, water-saturated soils (seismologists and engineers call the entire top 30 meters of such deposits soils) usually amplify incoming seismic motions, sometimes by as much as a factor of two. Typically, such poorly-consolidated soils are found in river and stream valleys. Areas of artificial fill are also often poorly consolidated.
Earthquake shaking may be prolonged above the Seattle basin. Modeling studies suggest that large buried lenses of sedimentary rocks with low seismic-wave velocities can act as reverberation chambers, trapping seismic waves and producing echoes.
When an earthquake occurs, there are a number of possible effects. Here we limit our discussion to some of the most important with respect to the lifelines shown on the map: surface rupture, ground shaking, liquefaction, and earthquake-induced landslides.
Surface rupture is a new concern in the Puget Sound region. Until the discovery of the changes in elevations of beaches on Bainbridge Island, it was unclear whether recent surface faulting had occurred in the area. With the documentation of surface rupture on Bainbridge Island in the last 14,000 years and detailed geophysical surveys along the Seattle fault, it now seems likely that there is a broad zone (striped pattern on the map) where surface rupture may occur. Although for any given earthquake the changes of surface rupture may be small, it may be desirable for lifeline system engineers to at least consider such a possibility. Several lifeline systems cut north-south through this zone of possible surface faulting.
Ground shaking occurs in a wide area following an earthquake. Because of the complexity of three source zones, it is useful to use the probabilistic hazard map (Figure 3) as an initial guide to areas of strong shaking. However, engineers know from experience that unconsolidated young deposits often amplify ground motion, sometimes by a factor of two or more. Areas of unconsolidated deposits shown on the map should be viewed with caution—ground motions in these areas will likely be more intense than predicted for hard rock sites. Significant sections of major sewer and petroleum pipelines south of Seattle cross such areas. In addition to unconsolidated-soil amplification, areas over the Seattle basin may experience prolonged ground shaking from the deep sedimentary basin effect. Earth scientists are actively working to quantify these effects in the greater Seattle area.
Liquefaction is another problem in areas of unconsolidated young deposits. Strong shaking may cause water stored within the soil to be suddenly released. When this happens, the soil loses its shear strength and its ability to support large loads. Structures may fail. If adjacent to a riverbank, or on a slope, ground can move laterally (lateral spread), carrying with it buried pipelines and foundations. Generally, many of the same areas subject to amplified ground shaking are also susceptible to liquefaction; thus the shaded areas on our map carry double hazard significance. Liquefaction susceptibility depends on the details of soil composition and structure. It can be (and in some cases has been) mapped in more detail than is shown here.
Finally, steep slopes may produce landslides during earthquakes. Some landslides do not occur in the first few minutes following an earthquake, but days later. There were numerous landslides during and after the 1949 and 1965 earthquakes; some closed roads and swept sections of railroad track into Puget Sound. Steep slopes throughout the greater Seattle area are candidates for earthquake-induced failure, though we have not attempted to delineate these areas.
1 U.S. Geological Survey, University of Washington, Box 351310, Seattle, WA 98195
This site is maintained by the Pacific Northwest Urban Corridor Geologic Mapping Project, part of the Geology, Minerals, Energy and Geophysics Science Center