Non linear dynamical triggering of slow slip,

Abstract:

Among the most fascinating, recent discoveries in seismology have been
the phenomena of triggered slip, including triggered earthquakes and
triggered-tremor, as well as triggered slow, silent-slip during which
no seismic energy is radiated. Because fault nucleation depths cannot
be probed directly, the physical regimes in which these phenomena
occur are poorly understood. Thus determining physical properties that
control diverse types of triggered fault sliding and what frictional
constitutive laws govern triggered faulting variability is
challenging.

We are characterizing the physical controls of triggered faulting with
the goal of developing constitutive relations by conducting laboratory
and numerical modeling experiments in sheared granular media at
varying load conditions. In order to simulate granular fault zone
gouge in the laboratory, glass beads are sheared in a double-direct
configuration under constant normal stress, while subject to transient
perturbation by acoustic waves. We find that triggered, slow,
silent-slip occurs at very small confining loads (~1-3 MPa) that are
smaller than those where dynamic earthquake triggering takes place
(4-7 MPa), and that triggered slow-slip is associated with bursts of
LFE-like acoustic emission. Experimental evidence suggests that the
nonlinear dynamical response of the gouge material induced by dynamic
waves may be responsible for the triggered slip behavior: the
slip-duration, stress-drop and along-strike slip displacement are
proportional to the triggering wave amplitude. Further, we observe a
shear-modulus decrease corresponding to dynamic-wave triggering
relative to the shear modulus of stick-slips. Modulus decrease in
response to dynamical wave amplitudes of roughly a microstrain and
above is a hallmark of elastic nonlinear behavior. We believe that
the dynamical waves increase the material non-affine elastic
deformation during shearing, simultaneously leading to instability and
slow-slip. The inferred triggered slow-slip on the San Andreas Fault
at Parkfield, CA., due to December, 2003 Mw6.5 San Simeon Earthquake
(Breguier et al., Science 321, p.1478, 2008) shows very similar
characteristics to what we observe in the laboratory, suggesting an
extremely low in situ effective stress or a weak fault and a
nonlinear-dynamical triggering mechanism.


http://www.lanl.gov/orgs/ees/ees11/geophysics/nonlinear/nonlinear.shtml