REGIONAL STRONG GROUND MOTION DURING THE 1811-12 NEW MADRID EARTHQUAKES INFERRED FROM PALEOLIQUEFACTION

Peak ground accelerations (PGA) caused by the 1811-1812 New Madrid earthquakes were inferred from surficial paleoliquefaction features in Quaternary sediments in the Mississippi embayment, Arkansas and Missouri. Both the regional pattern and magnitudes of PGA were inferred within the 11,000 km² land area in the embayment that is partially covered by surface manifestations of liquefaction. Surface manifestations consist of sand blows and sand-filled lateral spreading and ground cracks. PGA estimates were derived from liquefaction probability curves, which relate the percentage of land affected by liquefaction to a magnitude-scaled PGA (Holzer, 2008, ASCE Geotechnical Special Publication 181). Probabilities were based on complementary cumulative frequency distributions of the liquefaction potential index (LPI), which were computed with 258 cone penetration test soundings. The soundings were conducted in the surficial geologic units that exhibit paleoliquefaction features. We relied on geologic mapping by others. The magnitude-scaled PGA was computed by dividing PGA by the liquefaction magnitude scaling factor (MSF). Both a moment magnitude and water table depth are required for the analysis. We considered both uncertainty in MSF and the potential impact of sand aging because the glacial outwash or valley train deposits that liquefied in 1811-1812 are of Late Pleistocene age. The inferred maximum PGA ranged from 0.20 to 0.46 g, depending on the assumed magnitude, MSF, water table depth, and effect of sand aging.

Proposed moment magnitudes of the 1811-1812 earthquakes were also evaluated by considering the intensity of the paleoliquefaction features as a function of distance from the seismic source. Our evaluation suggests that the magnitudes were approximately middle sevens (~7.5). The paleoliquefaction features are inconsistent with smaller magnitudes (<7) that have been assigned by some investigators to the earthquakes.