Ground Motion and Liquefaction Simulation of the 1886 Charleston, South Carolina, Earthquake

Abstract

As part of a comprehensive earthquake loss and vulnerability evaluation of the state of South Carolina, ground motions were simulated for a moment magnitude ( M ) 7.3 “1886 Charleston-like” earthquake using finite-fault and point-source stochastic numerical modeling. The probability for liquefaction was also predicted based on factors of safety computed from average cyclic stress and shear-wave-velocity-based cyclic resistance ratios, clay content, and saturation. Because there is considerable uncertainty regarding the 1886 source, the rupture plane of the 1886 event was modeled as both a 100-km-long, 20-km-wide fault (static stress drop of 27 bars) and a 50-km-long, 16-km-wide fault (107-bar stress drop). The source was assumed to be a north-northeast-striking, strike-slip fault coincident with the Woodstock fault. Based on comparing the computed and observed 1886 liquefaction areas and ground motions for both the low and high stress drop events, the two cases were weighted 0.8 and 0.2, respectively. To accommodate epistemic uncertainty in eastern U.S. earthquake source processes, three region-specific point-source attenuation models were also developed and used: a single-corner frequency model with both a constant stress drop and a magnitude-dependent stress drop, and the double-corner frequency model. The finite-fault and point-source models were weighted 0.8 and 0.2, respectively. To incorporate site effects into the ground-motion estimates, an extensive effort was made to characterize the thicknesses, shear-wave velocities ( V S), and dynamic material properties of unconsolidated sediments. Characteristic V S profiles were developed using the available subsurface information, which incorporated a wide range of soil and rock conditions. Amplification factors were computed for four site response categories, each of which were a function of soil thickness, input hard-rock motion, and spectral frequency. From the five weighted stochastic ground-motion models (two finite fault and three point source) and amplification factors, rock and soil ground motions were computed to produce statewide ground-motion maps for the M 7.3 scenario event. Weighting of the Charleston source and ground-motion models was implemented so that the resulting liquefaction areas matched the 1886 areas of liquefaction. Peak horizontal ground acceleration (PGA) values as high as 0.6 g -0.7 g were estimated in the vicinity of the modeled rupture. PGAs in the range of 0.3 g -0.4 g were estimated for Charleston consistent with the observed building damage and liquefaction. Significant ground shaking (PGA > 0.2 g ) extends out to distances of 50-60 km. Strong long-period (≥1.0 sec) ground motions are predicted throughout the state. The probabilities for liquefaction were highest in the epicentral region (>50%), consistent with the observed occurrences of liquefaction in 1886. Manuscript received 5 February 2003.