Short Note Assessing the Location and Magnitude of the 20 October 1870 Charlevoix, Quebec, Earthquake

The Charlevoix, Quebec, earthquake of 20 October 1870 caused damage to several towns in Quebec and was felt throughout much of southeastern Canada and along the U.S. Atlantic seaboard from Maine to Maryland. Site-specific damage and felt reports from Canadian and U.S. cities and towns were used in analyses of the location and magnitude of the earthquake. The macroseismic center of the earthquake was very close to Baie-St-Paul, where the greatest damage was reported, and the in- tensity magnitude MI was found to be 5.8, with a 95% probability range of 5.5-6.0. After corrections for epicentral-distance differences are applied, the modified Mercalli intensity (MMI) data for the 1870 earthquake and for the moment magnitude M 6.2 Charlevoix earthquake of 1925 at common sites show that on average, the MMI readings are about 0.8 intensity units smaller for the 1870 earthquake than for the 1925 earthquake, suggesting that the 1870 earthquake was MI 5.7. A similar com- parison of the MMI data for the 1870 earthquake with the corresponding data for the M 5.9 1988 Saguenay event suggests that the 1870 earthquake was MI 6.0. These analyses all suggest that the magnitude of the 1870 Charlevoix earthquake is between MI 5.5 and MI 6.0, with a best estimate of MI 5.8.