Seismicity in a model governed by competing frictional weakening and healing mechanisms

Gregor Hillers, Jean M. Carlson, & Ralph J. Archuleta

Published 2009, SCEC Contribution #1190

Observations from laboratory, field and numerical work 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 healing and restrengthening mechanisms. The efficiency of these processes depends on local crustal properties such as temperature and hydraulic conditions, which are usually hard to constrain.
We investigate the relative effects of these nonlinear feedback and memory processes on seismogenesis in the context of evolving frictional properties, using a mechanical earthquake model that is governed by slip weakening friction.
Competing weakening and strengthening mechanisms are parameterized by the evolution of the frictional control variable---the slip weakening rate $---using empirical relationships obtained from laboratory experiments.
Weakening mechanisms depend on the slip of a model earthquake and tend to increase $, following the behavior of real and simulated frictional surfaces.
Healing mechanisms cause $ to decrease and depend on the time passed since the last slip.
Results from models with competing feedbacks are compared with simulations using non-evolving properties.
In contrast to the fixed attractor emerging from homogeneous, slip and time independent $ conditions, evolving properties result in a significantly increased number of accessible system states.
We find that for a given set of weakening parameters the resulting seismicity pattern are sensitive to details of the restrengthening process, such as the healing rate $ and a lower cutoff time $, which control the degree of memory in a system.
For relatively large and small cutoff times, the system develops typical response statistics governed by large and small $ values, respectively, similar to the results using constant, homogeneous conditions.
A wide range of intermediate values, however, leads to significant fluctuations in measurements of the time dependent internal energy or stress levels. Consequently, the frequency-size statistics of earthquake occurrence show corresponding nonstationary characteristics.
The progressive evolution implies that---except for extreme amplitudes of weakening and healing processes---faults and fault networks possibly do not reach a (statistically) steady state, thus highlighting the essential role of memory and history dependence in seismogenesis.
The results suggest that an extrapolation to future seismicity occurrence on the basis of available, temporally limited seismicity data may be misleading considering likely fluctuations due to competing mechanisms that affect fault stability.

Hillers, G., Carlson, J. M., & Archuleta, R. J. (2009). Seismicity in a model governed by competing frictional weakening and healing mechanisms. Geophysical Journal International, 1363. doi: 10.1111/j.1365-246X.2009.04217.x.