Read Me

Western U.S.

The files in this zipfile display different SA ratio maps, or PGA ratio maps. The file name tries to convey the region. The comparisons are of the current PSHA model, that of April 2007, with the last USGS published model, often called the 2002 model, which was released in October 2003.

Most of the ratio maps compare the complete model. By complete, I mean that all of the seismic sources are included. Usually, if the ratio is substantially different locally, it is because a fault's geologic hazard model has changed, or the subduction model has changed. There are several regional changes that are mostly due to attenuation model changes (lower 1hz SA for example in much of southwest US), and some to a combination of source and attenuation model (Cascadia source model plus Gregor attenuation model).

Some ratio maps included here use data confined to a small subset of the sources. The two examples as this is written are 1hzSAratio.EXTmap.jpg and 1hzSAratio.WUSmapC.jpg. The EXTmap figure compares the hazard when just Extensional-zone gridded seismicity is compared between the two models, that of 2007 with that of 2002. Extensional-zone region is mostly Basin and Range, but extends southwest into northwestern Mexico. The WUSmapC map compares Washington-Oregon (transpressional region) background seismicity hazard between 2007 and 2002.

These ratio maps show that the background or gridded seismicity model is sometimes driving the change in the collective-hazard ratio maps. Places where gridded seismicty changes cause a substantial increase (percentage increase that is ) are eastern Nevada and north-central Arizona. There are also places where reduction in rate estimates for background seismicity significantly lower the overall hazard. Many of the fault-specific changes are discussed in the text being distributed with these maps.

Steve Harmsen, USGS, harmsen@usgs.gov
April 4, 2007

Central U.S.

The files in this zip file contain PGA, SA, and SA-ratio maps for the Central and Eastern U.S. The ratio is of the April, 2007 PSHA model over the 2002 model, released in Oct 2003.

In general, the 2007 model is the one described in the text. This model gives 1/2 weight to each of two New Madrid Seismic Zone hazard models. One plot, ceus.2sc.jpg, gives full weight to the no-clustering model, for comparison with the one with 50% weight to the clustering model. The alternate period maps (.1, .3, .5, and 2.0-s SA) are available at this time for CEUS but not for WUS.

Steve Harmsen
USGS, harmsen@usgs.gov