Measuring Deformation Band Orientations in the Etchegoin Sandstone: Implications for the Stress Field in Central California and the Accuracy of Emerging Orientation Measuring Technology

The nuances of stress and strain surrounding the creeping section of the San Andreas fault are poorly understood by structural geologists. Many have conducted studies near Parkfield, California because of its close proximity to the fault trace and unusual seismological history. This study contributes two data sets to the existing data surrounding Parkfield. First, I compare the orientations of planar deformation bands in a 53 m long transect as measured by three separate devices: a Brunton compass, the FieldMove Clino smartphone app, and extraction from a virtual model using CloudCompare’s Compass plugin Trace tool. I determine that smartphone measurements are moderately effective when compared against a Brunton, but that virtual extraction is largely unreliable. I then use the Brunton-derived orientations and observed slip amounts to characterize strain using directions of maximum and minimum elongation, from which I infer stress directions including maximum horizontal compression (SHmax). I find a value for SHmax of 010°, or 50° from the San Andreas fault strike, and a percent elongation between 2-6%. These findings corroborate previous studies in the same region and support a model of wrench transpression with a mechanically weak zone of distributed shear adjacent to the fault