Local crustal extension at Mount St. Helens, Washington

Seismicity and the orientation of fault planes from focal mechanisms indicate that Mount St. Helens is located at a dextral offset along the St. Helens seismic zone (SHZ): earthquake swarms occurring in this offset are related to volcanic eruptions. Because motion on the SHZ is in a right‐lateral strike‐slip sense, this dextral offset creates extension within a volume of the crust between the offset fault segments. This offset geometry is similar to that of geothermal areas along the San Andreas fault system. We apply a model derived from these geothermal areas to Mount St. Helens and find that the major differences between Mount St. Helens and the geothermal areas can be related to the ratio of the width of the offset between fault segments (l), to the seismogenic depth (h). At Mount St. Helens this ratio is < 1, whereas in the geothermal areas the ratio is ≈ 1. We propose that when l/h < 1 as at Mount St. Helens, the regional minimum principal stress does not completely dominate the small volume under extension, and as a consequence, the opening geometry is poory established compared to the oblique crustal spreading that characterizes the geothermal areas where l/h ≈ 1. Late Quaternary volcanic vents near Mount St. Helens strike northeast, similar to the strike of a set of pre‐Quaternary faults and intrusive rocks that are mapped north of the volcano; in addition, the deepest earthquakes occurring within the extensional volume are aligned along a northeast striking fault. Since these northeast striking features are aligned approximately prependicular to the regional minimum principal stress, we infer that the spatial position of Mount St. Helens is controlled by the junction of the right‐stepping offset of the SHZ with the older set of fractures and that these fractures are favorably aligned with respect to the contemporary regional tectonic stress directions for the transport of magma through the brittle crust. The sense of fault motions predicted by our model for local crustal extension is consistent with an apparent component of right‐lateral shear measured from geodetic lines around Mount St. Helens during June and July 1980.