xi, 114 p. : ill. A print copy of this thesis is available through the UO Libraries. Search the library catalog for the location and call number. / Different models have been proposed for the "drumbeat" earthquakes that accompanied recent eruptive behavior at Mount St. Helens. Debate continues as to whether seismicity is related to brittle failure during the extrusion of solid dacite spines or is the result of hydrothermal fluids interacting with a crack buried in the volcanic edifice. My model predictions of steady-state conduit flow confirm the strong control that degassing exerts on eruptive behavior. I discuss the necessary role of degassing for extruded material to attain the high density (low vesicularity) of the observed spine material and discuss the implications for generating seismicity. A brittle-failure source of seismicity requires that the gouge elastic properties accommodate some strain, since the magma compressibility in the upper conduit is too low to do so on its own. I also report on a novel method for generating high-resolution digital elevation models of fault surface textures. / Committee in Charge: Dr. Alan Rempel, Chair; Dr. Katharine Cashman; Dr. David Schmidt