Linking Historical Intensity Observations with Ground-motion Relations for Eastern North America

Abstract

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, Quebec, 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.