FRACTURE OR FAULT? USING 3D IMAGING TO DETERMINE THE DILATION VECTOR OF THE EARTHQUAKE FAULT NEAR MAMMOTH LAKES, CALIFORNIA

The origin of the Earthquake Fault (EF), a crevice up to 20 m deep and up to three meters in width in rhyodacite lava 3 km northwest of Mammoth Lakes, California, is not well understood. Faulting is one possibility, but the fracture could be volcanic in origin. The EF is situated along the eastern margin of the Sierra Nevada, where there are active strike-slip and normal faults related to northwest translation of the Sierran microplate. Thus, the EF could be a fault with a large dilational component. However, the EF is also part of an extensive system of faults and fissures that parallel the active Mono-Inyo volcanic system, and lies on the southern extension of a line of 600-year-old volcanoes in the Inyo chain. We constructed a 3D model of two segments of the EF using photogrammetry in order to determine the dilation vector. Purely dilational displacement would be consistent with stretching above a rising dike, whereas significant strike-slip or dip-slip displacement would indicate a tectonic origin.

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.