Regularity of seismic slip along a 9 km segment of the Calaveras fault zone is believed to result from steady-state loading of a creeping fault to generate stresses on an isolated stuck patch which moves in a stick-slip event in the magnitude range 3 to 4 whenever a critical threshold is reached. The patch behavior can be described by a simple model similar to the spring-driven frictional models used in laboratory simulations of stick-slip. The (M?3) recurrence time for this model is directly proportional to the seismic slip (computed from magnitudes) since the last time the threshold was reached. If the model is correct, an (3?M?4) earthquake should occur at 37¿K 17'¿2' N, 121¿ 39'¿2' W within 48 days of January 1, 1977.