Consequently, the area is susceptible to earthquakes on the numerous faults caused by the stress. These faults, however, are difficult to find and study because they are often concealed beneath sediments deposited by the Willamette and Columbia Rivers or hidden by vegetation and urban development. The USGS uses such aircraft to study geologic hazards, mineral resources, and environmental problems throughout the United States. To help locate and understand these concealed hazards, the USGS in 1992 conducted the aerial geophysical survey of the Portland Vancouver area that led to the March 1993 press conference. Using a specially designed airplane and instruments, scientists measured the magnetic field of the Earth at an altitude of about 800 feet above the ground.
Faults often produce distinctive patterns in the Earth's magnetic field.
The USGS uses aircraft with magnetometers to study geologic hazards, mineral resources, and environmental problems throughout the United States. The Portland Vancouver magnetic survey was planned in cooperation with scientists from the Oregon Department of Geology and Mineral Industries and from Portland State University. The data from the survey were interpreted by cooperating scientists and the USGS and are now being used by state and local planners to assess the seismic-hazard potential in the area. The best known fault in the Portland Vancouver area is the Portland Hills Fault, which runs northwest-southeast through the heart of downtown Portland. The aerial survey found two parallel and distinct magnetic patterns with the same northwest-southeast trend.