Shallow SH-wave seismic investigation of the Mt. Angel Fault, Northwest Oregon, USA Zhenming Wang a, * , Ian P. Madin a , Edward W. Woolery b a Oregon Department of Geology and Mineral Industries, 800 NE Oregon Street #28, Suite 965, Portland, OR 97232, USA b Department of Geological Sciences/Kentucky Geological Survey, University of Kentucky, Lexington, KY 40506, USA Received 17 May 2002; accepted 10 March 2003 Abstract The Mt. Angel Fault is likely one of the most active faults near the Portland metropolitan area, and was probably associated with the 1993 Scotts Mills earthquake. SH-wave seismic techniques used to image the Mt. Angel Fault suggest that the fault offsets late Pleistocene gravel (f 22 to 34 ka) at several locations. Within the study area, displacement of the late Pleistocene gravel along the strike of the Mt. Angel Fault increases from no obvious displacement on the northwest to approximately 18 m on the southeast. This trend of increasing offset along the strike of the fault is paralleled by topographic and geomorphic trends. A reconnaissance geologic investigation at an anomalous bend in the Pudding River near the projected trace of the Mt. Angel Fault revealed potential tectonic deformation in sediments younger than the late Pleistocene gravel imaged by SH-wave data. The results of this study have contributed to the paleoseismic record of the Mt. Angel Fault, laid the groundwork for future geologic investigations along the Pudding River, and determined potential sites for future paleoseismic trenching investigations. D 2003 Elsevier Science B.V. All rights reserved. Keywords: Seismic investigation; SH-wave reflection; SH-wave refraction; Mt. Angel Fault 1. Introduction The scientific and public understanding of earth- quake hazards in Oregon has increased dramatically in the last decade. This is in large part because of the growing body of evidence documenting the occur- rence of past great subduction earthquakes (Atwater, 1987; Yamaguchi et al., 1997) and two widely felt damaging crustal earthquakes in 1993 (Madin et al., 1993; Wiley et al., 1993). For much of the heavily populated northern Willamette Valley and Portland Basin, the greatest threat may come from local crustal earthquakes. For example, Wong et al. (2000) fore- casted peak ground acceleration (PGA) in excess of 0.7 g for much of the Portland area in the event of a rupture of the Portland Hills Fault. Several faults in the northern Willamette Valley and Portland Basin have been included in recent scenario and probabilistic hazard assessments as dis- crete sources (Geomatrix Consultants, 1995; Frankel et al., 1996; Wong et al., 2000). Currently, 1,715,000 Oregon residents (52% of Oregon’s population) live within 30 km of these faults, yet there are no reliable data on the exact location, dip, sense of motion, slip rate, or age of the most recent event for any of the 0040-1951/03/$ - see front matter D 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0040-1951(03)00153-7 * Corresponding author. Present address: Kentucky Geological Survey, University of Kentucky, Lexington, KY 40506, USA. Tel.: +1-859-257-5500; fax: +1-859-257-1147. E-mail address: zwang@kgs.mm.uky.edu (Z. Wang). www.elsevier.com/locate/tecto Tectonophysics 368 (2003) 105– 117