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.