New data on the stick-slip mechanics of seismogenic faults from rotary
shear experiments
Abstract
Rotary shear (RS) experiments have been used to characterize the
deformational behavior of materials and attempt to understand
earthquakes. Typical RS experiments test materials under a prescribed
slipping velocity and normal load. Yet, in natural earthquakes, fault
nucleation, growth, termination, and slipping velocity are not
predefined, but a result of the stored and released energy around the
seismic fault. Here we present new measurements performed with a RS
apparatus designed to be more representative of a natural system. The
device uses a clock spring that when loaded by a motor imposes a
linearly increasing torque to the sample. Thus, events occur
spontaneously when the shear stress exceeds the static shear stress
acting on the surfaces in contact. We report the results of experiments
using solid poly(methyl methacrylate) (PMMA) and granular samples of
polyvinyl chloride powder (figure), glass microbeads, silica powder, and
crushed quartz. PMMA experiments were started at a spring loading rate
(SLR) of ~2.5 RPM, where we observed low amplitude
stick-slip events occurring at regular recurrence intervals. The SLR was
then increased to ~12.5 RPM where after a transition
period, temperatures during slip events exceeded the melting point of
PMMA (~160ºC). This formed a melt layer that cooled and
bonded the slipping surfaces. The friction coefficient just before
rupture and the amount of weakening increased as a function of the
amount of melt produced. Granular experiments were conducted at a SLR of
~2.5 RPM and variable normal stresses (0.1-0.5 MPa). The
granular samples show strain hardening just before rupture, followed by
strain softening and marked changes in behavior with varying water
content. Since the behavior of PMMA is comparable to that of rocks at
depth (McLaskey and Glaser, 2011), results of PMMA tests yield insight
into precursory and coseismic events, fault strengthening/weakening
mechanisms, and perhaps, the formation of pseudotachylite glass.
Experiments with granular samples allow us to characterize each
material’s behavior in response to variable water content, SLR, and
normal stress. We conclude that analog materials are valuable to
simulate the behavior of the seismogenic brittle lithosphere. From such
experiments, we can gain insight into stick-slip mechanisms relevant to
earthquakes.