STRESS, SLIP, AND EARTHQUAKES IN MODELS OF COMPLEX SINGLE-FAULT SYSTEMS INCORPORATING BRITTLE AND CREEP DEFORMATIONS

Numerical simulations of slip evolution along a cellular vertical stri ke-slip fault in an elastic half-space are performed for several model s representing discrete fault systems embedded in three-dimensional el astic continua. The geometry and imposed boundary conditions correspon d approximately to the central San Andreas fault. The simulations inco rporate brittle and creep deformations in series; the net fault zone d eformation rate is the sum of creep rate and frictional slip rate. Bri ttle fault properties are given by various distributions of earthquake stress drops on failing segments (numerical cells). The assumed distr ibutions represent two idealized situations corresponding to different extreme states along an evolutionary path of a fault: (1) a strongly disordered state characterized by a wide range of size scales, represe nting immature fault zones and extended spatial domains, and (2) a rel atively regular state having a narrow range of size scales, representi ng mature highly-slipped faults. The assumed creep properties ale iden tical in all cases. These are prescribed in terms of coefficients char acterizing a power law dependency of creep-slip rate on stress. The co mbined brittle-creep process and employed parameters lead to an overal l ''pine-tree'' stress-depth profile with a ''brittle-ductile'' transi tion depth of about 12.5 km, and variable stress-along-strike profiles with ''brittle-creep'' transition around 65 km NW of the 1857 rupture . The spatial patterns of simulated hypocenters are statistically simi lar to observed data. The results indicate that the range of size scal es characterizing strong fault zone heterogeneities has important mani festations on the seismic response of a fault system. A narrow range o f size scales leads to frequency-size statistics of earthquakes resemb ling the characteristic earthquake distribution, and quasi-periodic te mporal distribution of large events as in the seismic gap hypothesis. On the other hand, a wide range of size scales leads to Gutenberg-Rich ter power law frequency-size statistics, and random or clustered tempo ral distribution of large events. The simulations demonstrate that tre atment of the various observed forms of frequency-size and temporal st atistics of earthquakes can be unified through the concept of range of size scales characterizing fault zone heterogeneities. This has a cle ar physical interpretation in terms of structural properties of a give n fault zone or broad lithospheric domain, and is supported by observe d earthquake and fault data. In some simulated cases the frequency-siz e statistics of small earthquakes fall sharply below the self-similar Gutenberg-Richter line. The results indicate that small earthquakes pr epare the fault for the occurrence of a large event by smoothing, duri ng gradual tectonic loading, the long-wavelength components of stress on the fault. This is done through short-wavelength stress roughening associated with the numerous ruptures of the small events. The above p attern of smoothing/roughening of long/short wavelengths of stress on the fault is reversed during large-scale ruptures of the big events.