Elliptical borehole enlargements or "breakouts" caused by systematic spalling of a borehole wall were identified in 18 wells drilled in the Coast Range and Willamette Valley of western Oregon. The breakouts generally indicate a NNW to NNE orientation of maximum horizontal compression (Oilmax) that agrees with the predominant direction of Oilmax determined from earthquake focal mechanisms, from post-middle Miocene structural features, and from alignmeres of Holocene volcanic centers in the Pacific Northwest.

This paper is one of a series dealing with earthquake hazards of the Pacific Northwest, primarily in western Oregon and western Washington. This research represents the efforts of U.S. Geological Survey, university, and industry scientists in response to the Survey initiatives under the National Earthquake Hazards Reduction Program. Subject to Director's approval, these papers will appear collectively as U.S. Geological Survey Professional Paper 1560, tentatively titled "Assessing Earthquake Hazards and Reducing Risk in the Pacific Northwest." The U.S. Geological Survey Open-File series will serve as a preprint for the Professional Paper chapters that the editors and authors believe require early release. A single Open-File will also be published that includes only the abstracts of those papers not included in the pre-release. The papers to be included in the Professional Paper are: Implementation Kockelman, W. J., "Techniques for reducing earthquake hazards--An introduction" Booth, D.B., and Bethel, J.P., "Approaches for seismic hazard mitigation by local governments-An example from King County, Washington" May, P.J., "Earthquake risk reduction prospects for the Puget Sound and Portland Areas" Perkins, J.B., and Moy, K.K., "Liability for earthquake hazards or losses and its impacts on Washington's cities and counties" Preuss, Jane,