Abstract

We use 28 calibration events (3.7 ≤ M ≤ 7.3) from Texas to the Grand Banks, Newfoundland, to develop a Modified Mercalli intensity (MMI) model and associated site corrections for estimating source parameters of historical earthquakes in eastern North America. The model,  
\[\mathrm{MMI}=1.41+1.68{\times}\mathbf{M}=0.00345{\times}{\Delta}-2.08\mathrm{log}{\ }({\Delta}),\]

where Δ is the distance in kilometers from the epicenter and M is moment magnitude, provides unbiased estimates of M and its uncertainty, and, if site corrections are used, of source location. The model can be used for the analysis of historical earthquakes with only a few MMI assignments. We use this model, MMI site corrections, and Bakun and Wentworth's (1997) technique to estimate M and the epicenter for three important historical earthquakes. The intensity magnitude MI is 6.1 for the 18 November 1755 earthquake near Cape Ann, Massachusetts; 6.0 for the 5 January 1843 earthquake near Marked Tree, Arkansas; and 6.0 for the 31 October 1895 earthquake. The 1895 event probably occurred in southern Illinois, about 100 km north of the site of significant ground failure effects near Charleston, Missouri.

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