Seismic hazards in the central United States are typically based on occurrences of earthquakes in the New Madrid Seismic Zone. However, paleoliquefaction evidence shows that large prehistoric earthquakes also occurred in the Wabash Valley region of Indiana and Illinois. A geotechnical study of the soil conditions at paleoliquefaction sites there is used to estimate both the magnitudes and accelerations of the prehistoric earthquakes.

This study covers an area of the Wabash River drainage approximately 250 km north to south and 180 km east to west, in southern Indiana and Illinois. In situ soil strength parameters were measured at 22 sites. The measured strength parameters are used in conjunction with liquefaction susceptibility analyses to estimate moment magnitude (M) and peak surface accelerations of four separate paleoearthquakes. In addition, site response studies based on the Atkinson and Boore (1995) model of bedrock motions for eastern North American earthquakes are used to develop attenuation relations. This allows a comparison of the results of the geotechnical study with a seismological prediction of peak surface accelerations due to the paleoearthquakes.

Using a regionally appropriate relationship between magnitude and maximum distance to liquefaction effects (Obermeier et al., 1993) leads to preliminary magnitude estimates of M6.8, M6.9, M7.2, and M7.8 for the four paleoearthquakes. Also, a method we developed, an energy-stress approach, is used to estimate the peak surface accelerations at these magnitudes that would be required to induce the observed liquefaction effects. Next, the acceleration estimates are compared with peak surface motions predicted by a seismological attenuation model for various magnitudes in order to determine the best-fitting magnitude. For the four paleoearthquakes studied, the geotechnical and seismological estimates of surface accelerations most closely agree for M6.9, M7.1, M7.3, and M7.8. Thus, two different methods yield basically the same paleomagnitudes, and therefore provide our best estimates.

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