Brief Comparison with Previous USGS Maps

One of the major outcomes of the new maps for the CEUS is that the ground motions are about a factor of 2-3 times lower, on average, than the PGA values in Algermissen et al. (1990) and the spectral values in Algermissen et al. (1991) and Leyendecker et al. (1995). We discussed this difference at the workshops. The primary cause of this difference is the magnitudes assigned to pre-instrumental earthquakes in the catalog. Magnitudes of historic events used by Algermissen et al were based on I$$_max (maximum observed intensity), using magnitude-I$$_max relations derived from WUS earthquakes. This overestimates the magnitudes of these events and, in turn, overestimates the rates of M4.9 and larger events. The magnitudes of historic events used in the new maps were primarily derived by Seeber and Armbruster (1991) from either felt area or I$$_max using relations derived from CEUS earthquakes (Sibol et al., 1987). Thus, rates of M4.9 and larger events are much lower in the new catalog, compared to those used for the previous USGS maps.