Seismic parameters for the central United States based on paleoliquefaction evidence in the Wabash Valley

Abstract

Seismicity in the central United States has typically been defined by the New Madrid Seismic Zone. Recent discoveries of paleoliquefaction evidence outside that Zone suggest prehistoric earthquake magnitudes in the Wabash Valley far exceed historical events. This geotechnical engineering study of paleoliquefaction sites in southern Indiana and along the Wabash River in Illinois has provided an opportunity to estimate magnitudes and attenuation characteristics associated with strong prehistoric Wabash Valley earthquakes. In-situ soil strength parameters have been measured at 22 sites over an area in the Wabash River drainage approximately 250 km N-S, and 180 km E-W. These parameters have been used in liquefaction susceptibility analyses to estimate moment magnitude (M) and surficial ground motion amplitudes associated with each of four separate paleo-earthquakes. In addition, site response studies based on a semi-theoretical model of eastern North America bedrock motions have been used to compare results of the engineering study with seismological predictions of ground motions associated with large earthquakes in the region. The site response study has been used to develop an attenuation relationship for eastern North America soil sites that mirrors attenuation relationships for the western United States. Minimum magnitude estimates are based on both distribution of paleoliquefaction evidence and on geotechnical estimates of stresses required to induce the evidence observed. A seismic energy intensity approach is then used to help improve the magnitude and surface acceleration estimates. The approach allows revised magnitude estimates by comparing peak accelerations required to induce the liquefaction effects observed to the peak surface motions predicted by the seismological model. The results of this study suggest the Wabash Valley paleoseismic events had magnitudes of M6.9, M7.1, M7.3 and M7.8. Uncertainty exists in the analysis, but these estimates are considered reliable to within 0.25 to 0.5 magnitude unit. The geotechnical estimates of peak meizoseismal accelerations and regional attenuation characteristics were also found to be consistent with seismological predictions. The estimates of seismic parameters based on this study greatly exceed those of all historical events, suggesting the Wabash Valley seismic hazard may be underestimated by local building codes, and that seismic design requirements should be reassessed.