An Analysis of Seismic Hazards in Indiana

Current USGS probabilistic seismic hazard estimates (Frankel et al., 2002) show a higher seismic hazard in southwestern Indiana than for the rest of the state. This is expected based on past work documenting large pre-historic events in the Wabash Valley, and also based on isoseismal levels of shaking from the New Madrid 1811-1812 earthquakes. When assessing the hazard on a county by county basis, the local soil conditions produce amplification of shaking in regions with soft or unconsolidated sediments relative to firm-rock sites. We have reproduced the probabilistic seismic hazard calculation following the USGS methodology for the state of Indiana and provided details for the shaking level on each county of the state We describe the datasets that we have collected and used to establish the input geology-based velocity model, including comparisons with independent datasets that allow us to estimate the true uncertainty of the measurements. Future versions of PSHA maps with site effects are planned that will use a database with higher resolution information on shear wave velocity structure. These future maps will provide information at the level necessary for planning and budgeting, though site specific studies will still be needed for engineering.