Potential role of volcanic glass-smectite mixtures in slow earthquakes
in shallow subduction zones: Insights from low- to high-velocity
friction experiments
Abstract
Volcanic glass and its mixture with smectite are commonly observed in
shallow parts of subduction zones. As volcanic glass layers often act as
a glide plane to induce mass transportation such as submarine
landslides, and because its alteration product, smectite, is one of the
frictionally weakest geological materials, the frictional
characteristics of volcanic glass-smectite mixtures are important for
fault slip behavior in shallow parts of subduction zones. We performed a
series of friction experiments on volcanic glass-smectite mixtures with
different smectite contents at various velocity conditions from 10 μm/s
to 1 m/s under an effective normal stress of 5 MPa and pore pressure of
10 MPa. In general, friction coefficients negatively depend on the
smectite content at any velocity tested. We found that samples with
smectite contents of 15-30 % showed a drastic slip-weakening behavior
at intermediate velocities of 1-3 mm/s with a characteristic slip
displacement of ~0.1 m. Finite element method modeling
shows that thermal pressurization does not contribute to the observed
weakening behavior. We propose that gouge fluidization or
compaction-induced pore pressure increase may be the cause of the
weakening. The slip-weakening behavior at intermediate velocities
enlarges a critical nucleation length for frictional instability to 1-30
km, or prevent acceleration to seismic slip velocities. Therefore,
gouges with minor amount of clay, such as subducting volcanic ash
layers, may contribute to the occurrence of the at shallow depths in
subduction zones.