Effect of Soil Aging on Assessing Magnitudes and Accelerations of Prehistoric Earthquakes Evangelia Leon, a… Sarah L. Gassman, b… and Pradeep Talwani c… Increase in strength due to aging of sands is reflected in higher blow counts and tip resistance values in penetration resistance measurements. This affects the magnitudes and peak ground acceleration estimates of prehistoric earthquakes obtained from an analysis of geotechnical observations at paleoliquefaction sites in the South Carolina Coastal Plain. In this study, corrections were made to account for the effects of soil aging, which were neglected in earlier estimates. The results show that when the effects of aging of soils on their geotechnical properties are incorporated, the resulting back- calculations reduced earlier magnitude estimates of prehistoric earthquakes by about 0.9 units. The peak ground acceleration estimates were reduced by about 15% for those earthquakes originally estimated at approximately 0.15 g. For those earthquakes whose original estimates were greater than 0.2 g, there was no noticeable change when a correction was made for the aging of soils. DOI: 10.1193/1.1949223 INTRODUCTION Paleoseismicity, the history of prehistoric earthquakes, is being used increasingly in seismic hazard analyses SHA. In the western United States, where faults are exposed, the effects of prehistoric earthquakes can be directly studied in suitably placed trenches. In the eastern United States, large earthquakes are less frequent and in the absence of surficial evidence, the indirect effects of prehistoric earthquakes, such as sand blows em- bedded in soft sediments, are studied. By dating trapped organic material and, in some cases, associated archeological artifacts in and around sand blows, it has now become possible to reconstruct the chronology of past earthquakes associated with liquefaction e.g., Talwani and Schaeffer 2001, Tuttle et al. 2002. This chronology of past earth- quakes is used to infer the recurrence rate of large earthquakes, one of the parameters needed in SHA. Until recently, another parameter needed in SHA, the magnitudes of prehistoric earthquakes, was estimated indirectly. The magnitude of the prehistoric or paleoearthquake was based on the distribution of associated sand blows. In the Charleston, South Carolina, region, the results of extensive paleoseismological investigations have revealed evidence of seven prehistoric earthquakes in the past 6,000 years Table 1. Based on the more recent events, these data suggested that, on average, a Florence & Hutcheson, Inc., 2700 Middleburg Drive, Suite 150, Columbia, SC 29204; lleon@flohut.com b University of South Carolina, Department of Civil & Environmental Engineering, Columbia, SC 29208; gassman@engr.sc.edu c University of South Carolina, Department of Geological Sciences, Columbia, SC 29208; talwani@geol.sc.edu 737 Earthquake Spectra, Volume 21, No. 3, pages 737–759, August 2005; © 2005, Earthquake Engineering Research Institute