We construct an earthquake instability model to estimate precursory fa
ulting and ground deformation before the next moderate (M 5.5-6) earth
quake on the San Andreas fault near Parkfield, California. The quasi-s
tatic model simulates fault slip, fault shear stress, and ground defor
mation for all stages of repeated earthquake cycles. Unstable slip, wh
ich is the analog of a moderate Parkfield mainshock, is caused by over
all failure of a postulated strong area or patch of the fault zone, Th
e brittle patch and surrounding weaker viscous fault are defined by th
e sign of a spatially varying coefficient of a slip rate dependent fau
lt law. Stable failure of the patch leading up to mainshock failure ca
uses slip rate to increase at depth. Amplitudes and timescales of the
resulting preseismic deformation anomalies are found for the current e
arthquake cycle at Parkfield, starting just after the 1966 mainshock,
in two ways. In the first way we vary parameter values in a sequence o
f simulations so as to find the simulation giving best agreement with
locations, moments, and recurrence times of past moderate earthquakes
and also with fault creep and trilateration line length changes since
the 1966 mainshock. In the second way we assign values of most paramet
ers from laboratory friction experiments and a temperature-depth profi
le inferred from heat flow data. Overall agreement between model resul
ts and observations is comparable for the two methods. In all simulati
ons, preseismic deformation anomalies arise from slip rate increase ne
ar the model earthquake focus and start several years or less before i
nstability. Preseismic anomalies in surface fault creep and line lengt
h are always too small to detect in current measurements. However, for
some simulations constrained mainly by field data the predicted anoma
lies in borehole dilatation are large enough to detect.