FRACTURE OR FAULT? USING 3D IMAGING TO DETERMINE THE DILATION VECTOR OF THE EARTHQUAKE FAULT NEAR MAMMOTH LAKES, CALIFORNIA
We constructed 3D models of the two segments using PhotoScan software and 1038 photos taken from within the fissure. To orient the models in space we surveyed 18 control points using a laser rangefinder and compass. We then located a number of piercing points (generally fracture intersections) and calculated the vectors connecting them. These vectors were then decomposed into dilational, dip-slip, and strike-slip components. The crack exhibited significant dilation at both sites. At the southern location these components were 1.34 m dilation, 0.12 m right-lateral strike-slip movement, and 0.17 m normal (down to the west) dip-slip movement. At the northern location these components were 2.11 m dilation, 0.90 m right-lateral strike-slip movement, and 1.27 m normal dip-slip movement. The dominance of dilation at both sites indicates that stretching above the rising Inyo dike tip could account for most of the observed deformation, but both sites exhibit measurable oblique, right-lateral, west-side-down movement.
Our findings support the hypothesis that magmatic activity, not tectonic activity, is the primary driver of opening at the EF. The EF projects into the western edge of the town of Mammoth Lakes and could be a site of future eruptive activity if the Inyo dike continues to ascend.