GEOPHYSICAL MAPPING OF SUBSURFACE STRUCTURES IN THE UPPER COOK INLET BASIN, ALASKA

The upper Cook Inlet is the infrastructure nexus and logistical hub of Alaska. In addition to the road, rail, and utility grid for the Anchorage urban area, the region hosts offshore/onshore oil and gas platform and pipeline facilities; these are all vulnerable to seismic disruption. To quantify seismic risk for the region, it is important to (1) map shallow structures such as faults with known Holocene fault scarps and folds with suspected Quaternary activity and to (2) understand the deeper basin structure and geometry as it relates to focusing of seismic energy. As part of the Anchorage Urban Region Aeromagnetic (AURA) project of the USGS, we are applying a number of potential field modeling and interpretation techniques to low-level, high resolution, aeromagnetic data. At the shortest wavelengths, the magnetic anomalies reflect truncation edges and dip attitudes of relatively magnetic intervals within the youngest (Pliocene and Quaternary) units of the Cook Inlet basin stratigraphy. The patterns of these anomalies indicate greater lateral continuity of fold anticlines than indicated on published maps. If the faults coring these fold structures are active, this greater continuity indicates an increased potential for larger magnitude seismic events than previously recognized. Aeromagnetic expression of magnetic sediments in the anticline north of the historically active Castle Mountain Fault shows the extent of this shallow fault-related folding. Gravity and aeromagnetic data in the Anchorage urban region allow creation of a 3D model showing the shallow basin structure and the underlying lateral discontinuity along the inactive Border Ranges fault. Modeling the 3D geophysical properties in this region can contribute to better predictive maps of seismic shaking.