Seismo Lab Brown Bag Seminar
Title: The Enduring Enigma of the 1886 Charleston, South Carolina Earthquake: Are Railroads the Rosetta Stone?
Abstract: The 1886 Charleston, South Carolina, earthquake was felt over much of the eastern United States, with moment magnitude, Mw 6.8-7.3, previously estimated from felt reports. In spite of extensive contemporaneous documentation, the earthquake and Charleston Seismic Zone have remained enigmatic. Bilham and Hough (TSR, 2023) recently developed an elastic deformation model for the 1886 earthquake using three identified geodetic constraints, including a ~4.5-m dextral offset in a railroad line southeast of Summerville where the track crosses the previously mapped Summerville fault. The offset was described in contemporary accounts but never considered by subsequent studies. Unlike other railroad disturbances in the epicentral region it cannot be explained by site response. The preferred deformation model yields moment magnitude 7.3, but in contrast to conclusions of previous studies, we found no support for significant slip south of the Ashley River. Apart from the one lateral offset, other documented disturbances along three railroad lines are most readily explained by dynamic strain effects on saturated Atlantic Coastal Plain (ACP) sediments. I will also discuss the near- and far-field intensity distribution using extensive archival accounts. I compare the intensity distribution, constrained at nearly 1300 locations, with predictions from modern ground motion models (GMMs) and ground motion-intensity conversion equations. Although GMMs are characterized by significant epistemic uncertainty for large earthquakes, we show that the intensity distribution is consistent with average model predictions, assuming Mw 7.3. Given the expected influence of site response on ACP sediments throughout the epicentral region, the fit to near-field intensities is insensitive to the rupture length or other detailed rupture properties. Macroseismic intensities can be used together with model predictions to improve a ShakeMap for the earthquake but cannot improve the independently determined rupture model or Mw. An apparent concentration of high intensities near Rantowles, including a famous photograph of a buckled railroad track, was defined by a multiplicity of environment effects that are readily explained by dynamic stress effects and site response in the low-lying marshy regions surrounding Charleston.