"Geologic hazards in the Portland-Vancouver area include faults entirely hidden by river sediments, vegetation, and urban development. A recent aerial geophysical survey revealed patterns in the Earth's magnetic field that confirm the existence of a previously suspected fault running through Portland. It also indicated that this fault may pose a significant seismic threat. This discovery has enabled the residents of the populous area to better prepare for future earthquakes."
U.S. Geological Survey: http://www.usgs.gov/