Earthquake ground-motion prediction equations for Eastern North America

In our recent ground-motion article (Atkinson and Boore, 2006), equation (6) describes an adjustment factor that can be applied to our ground-motion prediction equa-tions to accommodate a factor of 2 difference in stress drop from our preferred value of 140 bars. The text following this equation describes how the factor may be scaled to accom-modate other stress parameter values. This allows the user to adjust the equations for any arbitrary stress parameter of their choosing (within the tested range from 35 to 560 bars). The descriptive text of how the scaling should work is in error. The scale factor that multiplies log SF2 should be as follows: Scale factor log(stress/140) / log (2). Thus the scale factor by which we multiply log SF2 (where log SF2 is as given in equation 6) has a value of 0 for stress 140, a value of 1 for stress 280, and a value of1 for stress 70. For the example given in the text of a desired stress of 210 bars, the factor is log(210/140)/log(2) 0.58. In this case, we would add 0.58 log SF2 to the predicted log PSA values