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Group 2 – FARM | Poster Abstracts
2-072
SEISMICITY IN A MODEL GOVERNED BY COMPETING FRICTIONAL
WEAKENING AND HEALING MECHANISMS Hillers G, Carlson JM, and Archuleta RJ
Observations spanning a wide range of space and time scales suggest a strain dependent
progressive evolution of material properties that control the stability of earthquake faults. The
associated weakening mechanisms are counterbalanced by a variety of restrengthening
mechanisms. The efficiency of the healing processes depends on local crustal properties such as
temperature and hydraulic conditions. We investigate the relative effects of these competing
nonlinear feedbacks on seismogenesis in the context of evolving frictional properties, using a
mechanical earthquake model that is governed by slip weakening friction. Weakening and
strengthening mechanisms are parameterized by the evolution of the frictional control variable--the
slip weakening rate R--using empirical relationships obtained from laboratory experiments.
Weakening depends on the slip of a model earthquake and tends to increase R, following the
behavior of real and simulated frictional interfaces. Healing causes R to decrease and depends on
the time passed since the last slip. Results from models with these competing feedbacks are
compared with simulations using non-evolving friction. Compared to fixed R conditions, evolving
properties result in a significantly increased variability in the system dynamics. We find that for a
given set of weakening parameters the resulting seismicity patterns are sensitive to details of the
restrengthening process, such as the a lower cutoff time, tc, up to which no significant change in
the friction parameter is observed. For relatively large and small cutoff times, the statistics are
typical of fixed large and small R values, respectively. However, a wide range of intermediate
values leads to significant fluctuations in the internal energy levels. The frequency-size statistics of
earthquake occurrence show corresponding nonstationary characteristics on times scales over
which negligible fluctuations are observed in the fixed-R case. The progressive evolution implies
that--except for extreme weakening and healing rates--faults and fault networks possibly are not
well characterized by steady states on typical catalog time scales, thus highlighting the essential
role of memory and history dependence in seismogenesis. The results suggest that an extrapolation
to future seismicity occurrence based on temporally limited data may be misleading due to
variability in seismicity patterns associated with competing mechanisms that affect fault stability
2-073
DILATANCY STABILIZATION VS THERMAL PRESSURIZATION AS A MECHANISM
FOR DETERMINING SLOW VS FAST SLIP Segall P, Rubin A, Rice JR, and Schmitt SV
We have previously discussed the possibility that rate-state friction nucleates slip under drained
conditions but that as slip accelerates dilatancy-induced pore-pressure reductions quench the
instability, resulting in slow slip. Accelerating slip also leads to shear heating and consequent
thermal pressurization of pore-fluids which destabilize slip, suggesting that competition between
thermal weakening and dilatant hardening may control whether slip is ultimately fast or slow.
We have studied isothermal friction-dilatancy interactions assuming 2D elasticity, rate-state
friction and a highly simplified dilatancy law. Pore-pressure fluctuations are governed either by
simplified, isothermal membrane diffusion, or by one-dimensional diffusion into the surrounding
medium computed by finite difference. For the membrane diffusion model, dimensional analysis
shows that dilatant strengthening scales with the dilatancy coefficient, and inversely with effective
stress. Linearized stability analysis suggests a boundary between slow and fast slip, which is
supported by numerical simulations. Theory predicts that stable slip is favored by low effective
stress, consistent with some seismic observations.
2008 SCEC Annual Meeting | 179