The neighboring cities of Portland, Oregon, and Vancouver, Washington, are situated in a very interesting area, geologically speaking. About 220 km to the west is a major fault zone, where the huge Pacific tectonic plate is sliding eastward beneath the North American plate. The sinking plate is thought to lie about 50 kilometers below Portland-Vancouver. To the east about 80 kilometers is the Cascade Range, a chain of active volcanoes that includes Mount Hood and Mount St. Helens. It is not surprising, therefore, that this metropolitan area of roughly 1.5 million people is susceptible to earthquake, volcanic, and landslide hazards. Moderate-sized earthquakes occur in the Portland-Vancouver area with magnitudes up to about 5. Indeed, on March 25, 1993, an earthquake of magnitude 5.6 struck an area about 50 km south of downtown Portland and caused about $30 million in damages.
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(Click on image for a full-size version, 65 Kbytes.) The cities of Portland and Vancouver are located between a major thrust fault to the west (shown by the sawtoothed black line) and the Cascade volcanic chain to the east (shown by the pink and red colors). |
The earthquakes beneath Portland-Vancouver occur along faults in the earth's crust. Although the earthquakes originate at depths of 10 to 20 km, the faults usually extend upward to near the earth's surface. Even so, the faults are difficult to find and hard to study because they are usually concealed beneath vegetation, sediments, water, and urban development.
In 1992, the U.S. Geological Survey conducted an airborne survey of the Portland-Vancouver area to help locate and understand some of these concealed hazards. Using a specially designed airplane and instrumentation, scientists measured the magnetic field of the earth at an altitude of 250 meters (about 800 feet) above the ground throughout the metropolitan area.
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(Click on image for a full-size version, 60 Kbytes.) This specially designed aircraft was used to conduct the airborne magnetic survey of the Portland-Vancouver area. The white boom at the rear of the aircraft is the magnetic sensor. The aircraft and instrumentation is owned and operated by the USGS and stationed in Denver, Colorado. |
Faults in the earth's crust often produce slight variations in the magnetic field just above the earth. Maps showing the strength of the magnetic field may include distinctive patterns (called "anomalies" by geophysicists) that indicate the presence of faults. Careful analysis of magnetic maps, therefore, can help to locate and describe unknown or poorly understood faults. The aircraft and instrumentation are owned by the U.S. Geological Survey and are used for a variety of studies related to geologic hazards, resource exploration, and environmental problems throughout the United States.
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(Click on image for a full-size version, 84 Kbytes.) The rainbow colors on this map represent "anomalies" in the strength of the magnetic field of the earth over the Portland-Vancouver area. Reddish colors indicate anomalously strong magnetic intensities, bluish colors relatively weaker intensities. The arrows highlight the Portland Hills fault zone. The gray area in the inset map shows the most populated areas of Portland and Vancouver. |
The Portland-Vancouver survey was planned and the data were interpreted in cooperation with scientists from the Oregon Department of Geology and Mineral Industries and from Portland State University. These data are now being used by city, county, and state planners to assess the seismic hazard potential of the area.
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(Click on image for a full-size version, 146 Kbytes.) This map shows the magnetic field over downtown Portland. Rainbow colors represent the relative intensity of the magnetic field. Dashed lines are faults known from geologic mapping. Note the strong magnetic lineation parallel to both the Portland Hills fault and the East Bank fault. |
The magnetic patterns confirm the existence of the East Bank fault and suggests that it may be more dangerous than previously suspected. In particular, the magnetic pattern associated with this fault extends at least 50 km to the southeast (as indicated by the arrows on the large magnetic map), to near the town of Estacada, Oregon, and considerably beyond the previously suspected extent of the fault. Scientists believe that the East Bank fault, the Portland Hills fault, and other northwest-southeast trending faults in the Portland metropolitan area are part of a broad zone of faulting, called the Portland Hills fault zone. If seismically active along its entire length, the Portland Hills fault zone poses a more significant seismic hazard to the Portland-Vancouver community than previously suspected.
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(Click on image for a full-size version, 86 Kbytes.) An "aerial view" of the Portland Hills fault zone. Scene is viewed from the southeast, with topography vertically exaggerated. The red dashed line is the location of the Portland Hills fault zone, as determined from geologic mapping and the airborne magnetic survey |
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