Back-analysis of liquefaction ground failures is shown to be a very useful auxiliary in determining approximate earthquake intensity levels for historic events. Available techniques are illustrated by applying them to well documented cases of ground failure due to the November 18, 1755 “Cape Ann” Massachusetts earthquake. Intensity levels obtained are shown to be in substantial agreement with those inferred from historic damage reports. It is further shown that distribution of accelerations and of occurrence and nonoccurrence of liquefaction are more consistent with an epicentral location inland from Boston than with the generally accepted location off Cape Ann. This conclusion has important practical implications, since an event of similar magnitude and location to the “Cape Ann” earthquake has been assumed in earthquake hazard studies for coastal Massachusetts and New Hampshire.