IMPACT OF IMPROVED GULF COAST SEISMIC ATTENUATION BOUNDARY AND STRESS PARAMETER ESTIMATES ON OUR UNDERSTANDING OF SEISMIC HAZARD IN THE SOUTHEASTERN U.S

Recently published studies of the seismic attenuation (Q) boundary and stress parameter estimates for the Gulf Coast and southeastern U.S. (SEUS) improve our understanding of seismic hazard for the SEUS. First, the improved Gulf Coast Q boundary location indicates a more restricted region of high attenuation in the SEUS than previous studies that were based on limited Q observations, seismotectonic regionalization, and crustal structure regionalization. Second, for earthquakes outside the Gulf Coast region and at long periods, the high attenuation region appears to be more restricted to close to the Gulf Coast. Third, most of Florida has less attenuation than the Gulf Coast region and is more like the central and eastern U.S. (CEUS) mid-continental attenuation, in keeping with the differing geological history of the Florida Peninsula from the Gulf Coast. Fourth, stress parameter, which is related to the high frequency energy and ground motion expected for CEUS earthquakes, appears to be lower in the south central U.S. than the east coast, northeastern U.S., and eastern Canada, but still significantly higher than for the western U.S. And fifth, induced earthquakes in the CEUS appear to have similar stress parameter levels as shallow natural events. Thus seismic hazard from induced earthquakes is the same as for shallow natural earthquakes in a given region. In general seismic ground motions and hence hazard from large CEUS earthquakes is not reduced in Alabama, Georgia, and Florida due to Gulf Coast Q. At long periods for large earthquakes outside the Gulf Coast Q region, lower hazard from Gulf Coast Q is restricted to near the Gulf Coast in Texas, Louisiana, and Mississippi. This pattern of seismic hazard is reflected in the intensity observations from the 1886 Charleston, South Carolina, M7.0 earthquake.