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
) was sheared under constant normal stresses ranging from
5 to 200 kPa and at rates ranging from 10-6
gouge exhibited rate-weakening behavior at rates lower than
m/s and rate-strengthening at rates above 5×10-4
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
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.