NEW AEROMAGNETIC DATA FROM THE PACIFIC NORTHWEST ILLUMINATE FOREARC GEOLOGY AND EARTHQUAKE HAZARDS

Over the past decade the U.S. Geologic Survey has systematically acquired 11 high-resolution aeromagnetic surveys over critical parts of the Cascadia forearc. The surveys cover a total area in excess of 80,000 km² and include the major cities of Everett, Bremerton, Seattle, Tacoma, and Olympia, Washington, and Portland and Eugene, Oregon. Nearly the entire surveyed region is underlain by Tertiary and younger volcanic rocks of diverse origin, ideal for aeromagnetic studies of tectonic structures that post-date the volcanic units.

The focus of these studies has been the Puget Lowland-Willamette Valley urban corridor of Oregon and Washington, a region largely covered by surficial deposits, dense vegetation, and urban development but also underlain by numerous active faults. The Seattle fault zone, for example, lies beneath the cities of Seattle, Bremerton, and Bellevue and is known to have generated a M 7 earthquake 1100 years ago. Aeromagnetic anomalies serve as a substitute for the hanging-wall geology of the Seattle fault zone, and stratigraphic relations thus determined allow mapping of individual strands of the fault zone with high resolution. In the northern Willamette Valley and Portland basin, the underlying Columbia River Basalt Group (CRB) serves as a strain marker for post-Miocene deformation that is reflected in aeromagnetic anomalies. The Mount Angel fault, which produced a M 5.6 earthquake in 1993, is almost entirely concealed beneath Pleistocene Missoula flood deposits. But the Mount Angel fault also offsets underlying CRB, and the resulting aeromagnetic anomalies permit mapping of the fault over long distances.

Compilations of these new data with surrounding aeromagnetic surveys provide insights to the regional extent and internal structure of the Cascadia forearc. Joint analysis with regional gravity data reveals discrete forearc blocks separated by northwest-striking structural zones. The forearc in Oregon behaves as a continuous block but is segmented into eight to ten blocks beneath Washington and Vancouver Island. Historic earthquakes tend to fall along block boundaries, but the association remains problematic in some areas, reflecting vastly different time spans of the historic record of earthquakes and the geologic record represented by potential-field anomalies.