Large 19th century earthquakes in eastern/central North America: A comparative analysis

Abstract

For the understanding of seismogenesis as well as seismic hazard assess- ment in the North American mid-continent, two historical events are of paramount importance: the 1811-1812 New Madrid, central U.S,. sequence and the 1886 Charleston, South Carolina earthquake. Published estimates of magnitudes of the four principal New Madrid earthquakes have ranged from M∼7-8.75. In contrast, published estimates of the magnitude of the Charleston earthquake have almost all been within a range of Mw6.8-7.3. Upon cursory inspection, the macroseismic ef- fects of the New Madrid mainshocks appear to be more severe at regional distances than those of the Charleston mainshock. I compare the intensity distributions more carefully, focusing on key indicators rather than the poorly constrained overall dis- tribution of intensities. I conclude that the primary difference between the intensity distributions of the Charleston and New Madrid earthquakes is that the former has much better sampling, in particular of the low intensity field. These results sug- gest that the largest New Madrid mainshocks were not substantially larger than the Charleston earthquake.