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Pacific Northwest Geologic Mapping and Urban Hazards

Geophysical Maps

Variations in density and magnetic content of rock units in the subsurface cause measurable changes in the earth's gravity and magnetic fields. Maps of the field variations are used to determine crustal structure at depth and to prospect for fault zones, basins, and deposits of economic significance.

Rocks have widely varying physical properties. Basalt, for example, is typically denser and more magnetic than a sedimentary rock of equal volume. Geophysicists capitalize on these characteristics by making measurements of gravity and magnetic fields on or just above the earth’s surface. By analyzing subtle variations in gravity and magnetic fields, they can infer the density and magnetization and, thus, the types of rocks beneath the surface, even though the rocks might be concealed completely beneath water, vegetation, or other rocks. The spatial association of concealed rocks, thus determined, provides a way to map faults and other tectonic or hydrologic structures.

Seattle Aeromagnetic map
Aeromagnetic anomalies, Seattle Quadrangle. Included in Puget Sound aeromagnetic maps and data, U.S. Geological Survey Open-File Report 99-514.

The Pacific Northwest is an ideal place for these kinds of geophysical studies. For example, volcanic rocks of diverse origins underlie much of western Oregon and Washington. Deep sediment-filled basins in this volcanic terrain produce distinctive gravity "anomalies", regions where the gravitational attraction of the earth is slightly less than normal. These gravity lows appear as circular blue regions on gravity maps, most notably the Seattle basin, which lies beneath the City of Seattle. From the shape of this gravity anomaly, we can infer the three-dimensional shape of the Seattle basin, an important part of understanding the seismic hazard of the Seattle region.

Geophysical anomalies also provide information about concealed faults. The Seattle fault zone, for example, lies beneath Seattle, Bremerton, and Bellevue and is known to have generated a M 7 earthquake 1100 years ago. Much of the bedrock geology in and around the fault zone is concealed beneath water and young glacial deposits. Because some of the bedrock is slightly magnetic, magnetic anomalies serve as a substitute for bedrock geology. In particular, magnetic anomalies can be used to map individual strands of the fault zone with high resolution.

Similarly, the Portland area is underlain in many places by rocks of the Columbia River Basalt Group. These volcanic rocks cooled as horizontal layers. Subsequent deformation has offset or folded the layers, and these features produce magnetic anomalies. For example, the Mount Angel fault, which produced a M 5.6 earthquake in 1993, is almost entirely concealed beneath young flood deposits. But the Mount Angel fault also offsets underlying Columbia River basalt, and the resulting aeromagnetic anomalies permit mapping of the fault over long distances.

Aeromagnetic

Blakely, R.J., Wells, R.E., and Weaver, C.S., 1999, Puget Sound aeromagnetic maps and data, U.S. Geological Survey Open-File Report 99-514.

Gravity

Robert L. Morin, Karen L. Wheeler, Darcy K. McPhee, Philip A. Dinterman, and Janet T. Watt, 2007, Principal Facts of Gravity data in the Northern Willamette Valley and Vicinity, Northwestern Oregon and Southwestern Washington, U.S. Geological Survey Open-File Report 2007-1058

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