Melding CEUS and WUS Maps

One of the major tasks was combining the CEUS and WUS hazard calculations into one set of national maps. We did separate calculations for the two regions whose delineation was based on the inferred boundary of ground-motion attenuation. The plausible location of this transition between CEUS and WUS attenuation was discussed at the Intermountain West workshop. In that workshop we decided to use a boundary located along the eastern edge of the Basin and Range province (Figure 1). Both the Rocky Mountain region and the Colorado Plateau were assigned to the CEUS attenuation. The area of New Mexico and westernmost Texas was assigned to the WUS attenuation zone. The previous USGS maps (e.g., Algermissen et al., 1990) used an attenuation boundary further to the east along the Rocky Mountain front.

Two sets of hazard calculations were done using different attenuation relations. First we produced two catalogs of earthquakes separated by the attenuation boundary. For earthquakes west of the boundary we used the WUS attenuation relations and for earthquakes east of the boundary we used CEUS attenuation relations. The hazard calculations were done with a sizeable overlap between runs. The CEUS run was extended westward to 115 degrees W. The WUS run was extended eastward to 100 degrees W. For each site we then added the rates of exceedance for the two sets of runs so that the hazard contribution for earthquakes on the other side of the attenuation boundary was included. We should note that we used WUS attenuation relations for WUS earthquakes, even for sites located east of the attenuation boundary. It would have been computationally difficult to consider how much of the path was contained in the CEUS attenuation province. Also, the attenuation relation is dependent on the stress drop, so basing the relation that we used on the location of the earthquake rather than the receiver is reasonable.