Frictional properties of the Mount St. Helens gouge

Frictional properties of gouge bounding the solid dacite plug that extruded at Mount St. Helens during 2004 and 2005 may have caused stick-slip upward motion of the plug and associated seismicity. Laboratory experiments were performed with a ring-shear device to test the dependence of the peak and steady-state frictional strength of the gouge on shearing rate and hold time. A remolded gouge specimen (~0.012 m³ ) was sheared under constant normal stresses ranging from 5 to 200 kPa and at rates ranging from 10^-6 to 10^-3 m/s. The gouge exhibited rate-weakening behavior at rates lower than 1×10^-4 m/s and rate-strengthening at rates above 5×10^-4 m/s. Peak strengths occurred during the onset of shearing, when displacements were generally less than 0.5 mm. In slide-holdslide tests, the peak strength of the gouge increased logarithmically as hold times increased from 3 s to almost 10⁵ s. Rate-weakening friction is a requirement for stick-slip behavior that is satisfied by the Mount St. Helens gouge. Indeed, regular stick-slip oscillations were observed in two experiments performed at the highest normal stress and lowest rates of shear. The conditions under which this stick-slip motion occurred indicate that the gouge also satisfies a second criterion for stick-slip behavior of materials exhibiting rateand-state dependent friction-gouge stiffness exceeds that of the ascending magma that drives upward motion of the plug. The presence of highly compliant magma as a driving element may be crucial for generating stick-slip instabilities at the shallow earthquake focal depths observed during the eruption.