A HEALING-RELOADING FEEDBACK-CONTROL ON THE GROWTH-RATE OF SEISMOGENIC FAULTS

Spatial and temporal variations in the growth rates of faults are expl ained in terms of a stress feedback mechanism operating in the seismog enic upper crust. It is based on the idea that seismic rupture of a fa ult perturbs the surrounding stress field, advancing the occurrence of future earthquakes on some faults that are optimally oriented while r elaxing stress levels on others. If post-slip healing is geologically rapid, then the earthquakes that are thus induced will contribute to r eloading along the earlier rupture zone because of the symmetry of the optimal geometry. A positive feedback is set up so that, even in area s that are undergoing uniform tectonic straining, some faults develop higher displacement rates and grow more rapidly while others experienc e reduced rates or become inactive. Using a thin plate elastic model f or lithospheric-scale faulting, it is shown that this heating-reloadin g feedback mechanism drives rapid localisation and the formation of ma jor through-going faults moving at plate boundary velocities. Enhanced displacement rates (compared to an isolated fault) develop shortly af ter the onset of deformation along those faults which are optimally po sitioned in the overall fault population. Thus the formation of a new plate boundary fault zone is predetermined and is a consequence of, ra ther than the precursor of, preferentially high displacement rates. Al so, fault segments located at points of rupture symmetry, e.g. the cen tral portion of a fault zone, are reloaded more frequently and develop higher displacement rates and consequently have longer segment length s and/or larger displacement to length ratios. Episodic fault movement through time is a general feature of the model. These predictions are consistent with available field observations over a wide range of sca les. Thus, elastic-brittle failure and healing appear to be important rheological components of the lithosphere on long time scales (10(4)-1 0(6) y), as well as on the time scale of earthquake recurrence. (C) 19 98 Elsevier Science Ltd. All rights reserved.