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them as well; activity on the entire region now ceases. If the fault network is studded with occasional asperity contacts in the fluidized state, the redistribution of stress may take on a filamentary geometry, as observed in experiments on granular assemblages, the grains having mainly free surfaces and only occasional bridging contacts (7476).

If, however, an asperity having a dimension of let us say a few tens of kilometers does not break completely through, then a high stress drop earthquake will occur; because of failure to connect to the fluidized regions, these events will not cause significant changes in activity at distances of the order of several hundred kilometers. The agency of extended regions of fluid penetration into fault networks may help to understand the observations of an increase in intermediate-magnitude activity on a time scale of several years before a strong earthquake and its subsequent, rapid extinction on a distance scale of many times the classical dimension of rupture of a large earthquake that has been recently noted as part of the phenomenology of strong earthquakes in California (77). Reactivation takes place on the tectonic time scale.

If the faults of the network are somewhat farther apart in comparison with the length of the zone of activation, faults at intermediate distance may become reactivated after an episode of quiescence by the stress corrosion mechanism (73); the stress-corrosion reactivation time scale is much shorter than the tectonic reactivation time scale on an individual fault zone. Reactivation on a time scale of decades is consistent with some patterns of seismicity in Japan and is consistent with a greater width of the fault network in Japan than in California.

Summary

Because of scale independence, homogeneous fault models develop power-law distributions of seismicity that reproduce well the G-R distribution for small events. Unfortunately, the seismicity is only a transient state; for any finite fault, sooner or later seismicity interacts with inhomogeneities at the boundaries that leave an imprint on future seismicity that is characteristic of the nature of the boundaries. We have therefore

FIG. 5. Distribution of seismicity for a part of the time sequence for two faults coupled as in Fig. 4. The distribution of fracture thresholds is shown on the right. Periodic end conditions on both faults. Seismicity is largely complementary, with quiescence on one fault matched by activity on the other at the same coordinate and time. Around time 1600, the fault with stronger barriers becomes quiet over almost its entire length, which is matched by activity over almost the entire length of the other. Around time 1920, the pattern is almost completely reversed. Noteworthy is the interval from times 1300 to 1800, when the barrier at coordinate 200 on the upper fault does not tear; after time 1800, it tears frequently.

FIG. 6. Cumulative distribution of interval times for fractures of the strong barrier at coordinate 200 on the upper fault of the model (M), compared with the distribution of interval times for great earthquakes at Pallett Creek (PC). The model curve is well fit by a truncated exponential distribution except for the instabilities at the lacunae, such as that between times 1300 and 1800 (see Fig. 5). Time scales of the two curves have been adjusted to have a common median.

considered inhomogeneous fault models, with geometrical fluctuations of fracture strengths, to guarantee that large events are localized and do not interact with boundaries and to guarantee that large earthquakes will not obey the power-law distribution. Inhomogeneous fault models favor the characteristic earthquake model for large events—namely, those that are limited by the strongest inhomogeneities—while the smaller earthquakes may continue to be described by power-law distributions. However, we find that the character of the seismicity is extremely sensitive to the geometrical distributions of inhomogeneities of fracture thresholds.

Most models of the seismicity of the largest events on single inhomogeneous faults show little dispersion of interval times. To generate significant dispersion of interval times, we have explored the nature of seismicity on a pair of parallel faults, whose stress redistribution patterns influence the occurrence of earthquakes on one another. Model simulations show that the instabilities of seismicity on a single fault have a different character in the coupled cases; seismicity now displays complementarity behavior as well as episodes of lacunarity. The distribution of interval times for the strongest earthquakes is a truncated exponential. We conclude that the popular assumption that the average rate of slip on individual faults is a constant is not likely to be valid.

Intermediate-term precursory activity in Southern California is simulated well by an asperity model that assumes that faults are fluidized and hence are weak over much of their length. Accelerated weakening of strength, coupled with the infusion of fluids from below the seismogenic zone, can account for local precursory quiescence for an increase of intermediate-magnitude activity at long range and the abrupt extinction of the latter by the occurrence of strong earthquakes with low average stress drop; the range of interaction can be many times the dimensions of the zone of brittle fracture in a large earthquake. The model of a fluidized fault network, sutured by occasional asperities, can explain the heat flow paradox, the orientation of the stress field near the SAF, and the low average stress drop in some strong earthquakes.

This research was supported by a grant from the Southern California Earthquake Center. This paper is publication no. 4622 of the Institute of Geophysics and Planetary Physics, University of California, Los Angeles, and is publication no. 323 of the Southern California Earthquake Center.

1. Sieh, K.E. (1978) J. Geophys. Res. 83, 3907–3939.

2. Sieh, K., Stuiver, M. & Brillinger, D. (1989) J. Geophys. Res. 94, 603–623.



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