| Aerial photo interpretations and geologic field investigations in southern and central Oregon, combined with previous fault and seismicity studies nearby, indicate that late Pleistocene and Holocene fault activity is concentrated along four zones stretching northward into the Cascade volcanic arc and across the northwestern Basin and Range Province. Placed in a regional context, these active fault zones serve to separate Western Oregon from "stable" North America. Regional geodetic measurements, historic earthquake moment tensors, and the orientations and slip rates of faults that cross Oregon, are the basis for constructing a kinematic model which reveals that the active faults accommodate overall motion in a direction ;The kinematic model of faulting, together with data describing the locations, lengths, and slip rates of late Quaternary and Holocene faults, are the bases for constructing predictive maps of earthquake ground motions, that will help to understand better the seismic hazards in Oregon. Potential earthquakes are hypothesized to occur along eleven major source zones which include the Cascadia subduction zone, the subducting Juan de Fuca plate, and nine zones of onshore and offshore crustal faults. Empirical relationships and the active fault data are used to estimate the magnitudes and recurrence intervals of hypothetical earthquakes. This information is encoded as map elements in a Geographical Information System (GIS) which is used to apply earthquake attenuation relations and to establish contours of peak horizontal ground acceleration and velocity across Oregon. A regional map of surficial geology is used to improve predictions of site response by accounting for potential seismic amplification at sites underlain by soft young sediments. All data are stored as GIS database layers which are combined with population density and various algorithms to produce maps of seismic ground motion, duration of shaking, probability of exceeding a damaging level of shaking, and seismic risk. Results indicate that, by area, as much as one-half of Oregon can expect peak ground accelerations to exceed 0.2 g at the 5% probability level in a 100-yr interval. As much as 80% of the Oregon population resides in areas that can experience strong ground motions (... |
