The introduction of wide dynamic-range digital seismographs and accelerometers has made possible the direct calculation of ML from the amplitudes of seismic waves. We have computed the ML of 76 earthquakes that occurred from 2000-2003 with 2.5 On average, the magnitude corrections are correlated with, and can be predicted from, the NEHRP soil classification assigned to each site. Computed magnitudes tend to be higher at sites where the estimated surface shear wave velocity is low. The variations in computed magnitude do not seem to depend on the presence of deep-basin structures in the subsurface. On the other hand, such structures have been shown to affect the amplitudes of low-frequency seismic waves. This apparent discrepancy is probably due to the fact that Richter magnitudes are intrinsically sensitive to higher frequency ground motions. We conclude that variations in Richter magnitude can be used to detect shallow subsurface site conditions likely to affect earthquake shaking.