Modified Mercalli Intensity (MMI) data are the only available ground-motion data for most of the large historical earthquakes that have occurred in eastern North America (ENA). Are these historical observations consistent with expected intensity levels based on regional ground-motion relations? It is important to answer this question, as it bears on the reliability of the results of seismic hazard analyses.

An empirical study of California strong-motion records and associated MMI values has shown that observed MMI is a function of response spectral amplitudes, magnitude, and distance (Atkinson and Sonley, 2000). Comparison of strong-motion and MMI data from the 1988 Saguenay, Québec, earthquake confirms applicability of the California relations to ENA, at least for epicentral distances of 150 km or less. I therefore apply the relationships of Atkinson and Sonley (2000) to predict the MMI for historical ENA earthquakes as a function of distance, assuming that the response spectral amplitudes follow those of the ENA ground-motion relations of Atkinson and Boore (1995). The ground motions are adjusted to show the expected MMI for a range of soil conditions, including hard rock, firm ground, and soft soil.

The predicted MMI values for firm-ground to soft-soil conditions are consistent with historical observations from most ENA earthquakes, with typical discrepancies representing overprediction or underprediction of observations by about 1 MMI unit. Specifically, observed intensities from the 1886 Charleston earthquake (M 7.3) and the 1925 Charlevoix earthquake (M 6.4) are weaker than predicted by about one intensity unit, while observed intensities from the 1811-1812 New Madrid earthquakes (M 8) are larger than predicted by about one unit. Intensities from the 1968 Illinois earthquake (M 5.4) and the 1988 Saguenay earthquake (M 5.8) are larger than predicted by about 1.5 intensity units. On the whole, it is concluded that the historical MMI data are consistent with currently used ENA ground-motion relationships.

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