Probing changes in frictional state due to normal stress perturbations
using controlled-source ultrasonics
Abstract
We perform a suite of laboratory friction experiments on saw-cut
Westerly Granite surfaces and probe frictional state evolution in
response to step changes in normal stress. The experiments are conducted
with the objective of illuminating the origin of friction memory effects
and the fundamental processes that yield friction rate and state
dependence. In contrast to previous works, we measure directly the fault
slip rate and account for changes in slip rate caused by normal stress
perturbations. Further, we complement mechanical data acquisition by
continuously probing the faults with ultrasonic pulses. We conduct the
experiments at room temperature and humidity conditions in a servo
controlled biaxial testing apparatus in the double direct shear
configuration. The normal stress perturbations are carried out during
steady shearing over a range of shear velocities, from 0.02 - 100 μm/s.
We report observations of a transient shear stress and friction
evolution with step increases and decreases in normal stress.
Specifically, we show that shear stress evolves in a two-stage fashion
– first linear-elastically, then inelastically in response to the
normal stress step. We find that the excursions in slip rate resulting
from the changes in normal stress must be accounted for in order to
accurately predict fault strength evolution. The effects of induced
changes in fault slip rate are also apparent in elastic wave properties.
Ultrasonic wave amplitudes increase instantly in response to normal
stress steps and then gradually decrease to a new steady state value, in
part due to changes in fault slip rate. This decrease is strongly
related to accelerated creep at the fault interface. We also demonstrate
that steady state amplitudes are a reliable proxy for real contact area
(RCA) at the fault interface. Previous descriptions of frictional state
evolution during normal stress perturbations have not adequately
accounted for large slip velocity excursions. Here, we do so by using
the measured ultrasonic amplitudes as a proxy for frictional state
during transient shear stress evolution. Our work improves understanding
of induced seismicity and triggered earthquakes with particular focus on
simulating static triggering and stress transfer phenomena using
rate-and-state frictional formulations in earthquake rupture models.