Characteristics and Processes of Degradation on Normal Fault Scarps in Basalt, Central Oregon and Northern California

Normal faults that break the surface create scarps. Scarps in alluvium degrade predictably so that time since formation can be inferred from scarp profile, however, scarps in jointed bedrock, such as basalt, do not degrade according to previous models. Understanding the processes involved in degradation of scarps in jointed basalt may lead to the formation of a degradation model. I use survey data and statistical analyses from 36 scarps in central Oregon and northern California, to determine the characteristics that play a dominant role in scarp degradation in jointed basalt. These data indicate that scarp facing direction, column height, and joint spacing influence degradation while vesicularity, scarp height, and scarp location play less integral roles. These data also indicate that basalt columns larger than 0.45 m diameter do not easily topple and likely need regional ground motion to induce toppling, thereby aiding scarp degradation. I use measurements from toppled columns to calculate the quasi-static peak acceleration necessary for topple. I use the acceleration to determine the Modified Mercalli intensity, which I use as a proxy for earthquake magnitude. I compare the results to published acceleration probability maps as well as to a compilation of earthquake magnitudes since 1970 in central Oregon and northern California. I also estimate the probable maximum earthquake magnitude typical faults in the region could produce based on fault segment length. The curve infers the frequency of local earthquakes with magnitudes sufficient to topple, which I use to infer a degradation rate assuming ground motion is a primary geomorphic agent. I create a geometric simulation of scarp degradation in jointed bedrock, with varying retreat increments to represent jointing, as well as variable initial scarp angles, and talus repose angles. A comprehensive census of characteristics at more scarps would provide a database to improve understanding of the primary variables involved in degradation, to validate the methodology presented using toppled columns as strong motion sensors, and further refine the geometric simulation.