Adaptive Weighting for the WUS

For the June 1996 maps, we used the adaptive weighting scheme to include the background zones in the WUS without lowering the hazard values in the high seismicity areas. As with the CEUS, we checked the a-value for each source cell to see whether the rate from the historic seismicity exceeded that from the appropriate background zone. If it did, we just used the a-value from the historic seismicity. If the historic seismicity a-value was below the background value, then we used a rate derived from using 0.67 times the historic rate plus 0.33 times the background rate. This does not lower the a-value in any cell lower than the value from the historic seismicity. The total seismicity rate in this portion of the WUS in the new a-value grid is 16% above the historic rate (derived from M4 and greater events since 1963). Figure 23 is a map based on the adaptive weighting. Note that the areas of higher hazard in the Intermountain west have the same ground motions as the map with only historic seismicity and faults (compare Figures 23 and 19). Areas of low seismicity have the same probabilistic ground motions as the map made from models 1 and 2 (compare Figure 23 and 21).