Abstract Magnitude statistics (log N = a − bM) were measured for mine tremors and for the Parkfield earthquakes, listed by McEvilly, Bakun and Casaday (1967), in conditions of varying rates of aseismic ground deformation. Aseismic deformation rates were estimated using a tiltmeter in the ERPM gold mine and using results from small geodetic networks, reported by Smith and Wyss, for the region near Parkfield, California. In the magnitude-frequency relationship, b is independent of the rate of aseismic deformation, ε˙,anda=logε˙+Κ, where K is a constant. Thus, the level of seismicity appears to be determined by the rate of aseismic ground deformation. Changes in shear stress of the order of 100 bars do not have any measurable effect on the magnitude statistics.