EARTHQUAKE AND LANDSLIDE HAZARDS AT THE FAR ENDS OF THE PORTLAND HILLS FAULT ZONE

Recent geologic mapping of the Oregon City and Dixie Mtn Quadrangles at the SE and NW ends respectively of the Portland Hills Fault Zone have provided new constraints on Portland area faults, and have greatly improved the understanding of local landslide hazards. The new maps are supported by analysis of hundreds of water well logs, dozens of geochemical analyses and geomorphic analysis of high resolution LIDAR DEM's.

In the Oregon City quadrangle, the NW-trending Portland Hills Fault (PHF) and Bolton Faults offset basalt flows of the Boring volcanic field. The PHF is inferred to offset a flow dated at 1.2 Ma by at least 90 m vertically, yielding a slip rate of approximately 0.075 mm/yr. The Bolton Fault offsets a Boring flow dated at 2.4 Ma by 28 m, yielding a slip rate of 0.011 mm/yr. This rate is similar to the rate derived from offset of the 15.4 Ma Columbia River Basalt Group (CRBG) flows in the quadrangle (0.015 mm/yr). An apparent lack of surface traces in the LIDAR DEM where these faults cross 12-15 ka Missoula Flood deposits is consistent with the low slip rates.

At the NW end of the Zone, the PHF changes from a straight structure bounding the steep front of the Portland Hills, and extends N across the Dixie Mtn Quadrangle as a sinuous fault juxtaposing Miocene Scappoose Formation against the overlying the CRBG. The estimated offset is 150-200 m down to the east. Most of the trace of the PHF across the quadrangle is buried by Missoula Flood deposits and massive landslide complexes.

The combination of geologic field mapping and geomorphic analysis of LIDAR DEM's has allowed for accurate mapping of hundreds of landslides in both quadrangles, ranging from debris flow fans covering a few tens of square meters to huge bedrock slide complexes covering several square kilometers. In both areas, it was possible to map numerous small fans at the mouth of intermittent streams and minor gullies, which are interpreted to be the deposits of debris flows. This high-quality map of existing deposits is critical to making a susceptibility map for debris flow hazards. In the Dixie Mtn. area, large bedrock slides occur on sediment layers between CRBG flows, and two very large complexes involve the CRBG and underlying Scappoose Formation. Near Oregon City, large bedrock slides occur around the rims of nearly all canyons where basalt flows overlie weak mudstone.