The Cascadia subduction zone contributes significantly to the hazard along the coastal Pacific Northwest of the United States. We have constructed a logic tree for assessing the uncertainty in the ground motions that includes variability in the location of the bottom of the rupture, the magnitude, the recurrence rate, and the ground motion attenuation relations. Results of this uncertainty analysis indicate an overall coefficient of variation for the Cascadia hazard model with 2% probability of exceedance in 50 years between 0.3 and 0.5. Variability in each of the individual parameters results in coefficient of variations less than 0.3. The logic tree for the Cascadia subduction zone and synthetic seismograms for hazard involve extensive computer calculations that may be facilitated by a supercomputer.