THE CAPE ANN, MASSACHUSETTS EARTHQUAKE OF 1755: A 250TH ANNIVERSARY PERSPECTIVE

The earthquake of November 18, 1755 was experienced over a wide area along the east cost of North America. It caused considerable damage to masonry chimneys and walls in eastern Massachusetts, eastern New Hampshire and southern Maine. A reexamination of the felt reports, immediate aftershocks and modern seismicity indicate that the most likely epicenter was about 40 km ENE of Cape Ann, Massachusetts within a cluster of recent earthquake epicenters. Based on the attenuation of MMI with epicentral distance, the Lg-wave magnitude of the earthquake was about MLg6.2, corresponding to a moment magnitude of M5.9. From analyses using modern ground-motion attenuation relations, conversions from MMI, and estimations from the number and severity of damaged chimneys, it is estimated that in Boston this earthquake caused peak ground accelerations of about 0.08g-0.12g on soil sites, while the 5%-damped response spectral value at a period of 0.3 sec in Boston may have been as small as 0.09g or as large as 0.21g. The Boston ground motion estimates in this study correspond approximately to the 5% in 50 yr ground motions on the 1996 and 2002 USGS National Seismic Hazard Maps.