**FRICTIONAL WEAKENING AND SLIP COMPLEXITY IN EARTHQUAKE
FAULTS**
**SHAW BE**
**JOURNAL OF GEOPHYSICAL RESEARCH-SOLID EARTH**

100: (B9) 18239-18251 SEP 10 1995

**Abstract:**

Previous work has shown that velocity-weakening friction
produces slip complexity in simple dynamical models of earthquake faults
( Carlson and Langer, 1989). Here I show that a different type of dynamical
instability, caused by slip-weakening friction, also produces slip complexity.
The deterministically chaotic slip complexity produced by slip-weakening
friction in a simple one dimensional model is studied and the scaling of
the distribution of sizes of events with the parameters in the model examined.
In addition, a possible physical origin of frictional weakening is examined,
through a very simplified mathematical representation of a physical process
proposed by Sibson (1973), whereby frictional heating causes an increase
in pore fluid temperature and pressure, thereby reducing the effective
normal stress and friction. The two different types of frictional weakening
are derived from two opposing limits, with slip weakening occurring when
the dissipation of heating is slow compared to the rupture timescale, as
Lachenbruch (1980) has shown, while velocity weakening is shown to occur
when the dissipation is fast compared to the rupture timescale. Since both
end-member cases of frictional weakening are seen to produce slip complexity,
slip complexity is argued to be a generic feature of frictional weakening
and elastodynamics on a fault.