MAGMATIC SYSTEM GEOMETRY AT MOUNT ST HELENS MODELED FROM THE STRESS-FIELD ASSOCIATED WITH POSTERUPTIVE EARTHQUAKES

Earthquakes following the May 18, 1980, and June 12, 1980, explosive eruptions of Mount St. Helens were concentrated at depths below 6 km. The hypocenters of these posteruption earthquakes define two seismic lobes separated by an aseismic volume of 2 km width that is located beneath the position of the crater. The timing and location of the posteruption seismicity suggests that the earthquakes occurred in the country rock surrounding a magma body located within the aseismic volume. The withdrawal of magma caused a pressure decrease within the reservoir, and the earthquakes occurred as a brittle response to the resulting stress change. Focal mechanism solutions for posteruption earthquakes distributed within the seismic lobes are calculated using polarity and amplitude data. Results show that a perturbation from the regional stress field occurs between depths of 7 and 11 km. This anomalous stress field is successfully modeled by a decrease in pressure within a cylindrical magma body located between the seismic lobes and subject to conditions representing the regional shear stress regime. In the best fitting model the magma body is centered at 46.204-degrees-N, 122.187-degrees-W, 600 m to the north of the present dome, and is under a regional shear stress of 20 bars. The radius of the chamber is calculated to be between 0.65 and 0.75 km when using pressure drops within the cylinder of 220 and 150 bars respectively. The magma body extends from 7 to 11 km and has a volume of between 5 and 7 km3. Posteruption earthquakes located below 11 km fall on a northeast striking fault that is preferentially aligned with the regional tectonic stress regime. This fault may be a conduit to transport magma to the shallow reservoir from greater depths within the crust.