The highest earthquake hazard in the continental U.S. is largely associated with California, but all parts of the country experience low-intensity tremors throughout the year, and destructive earthquakes are by no means limited to the West Coast. Across the Rocky Mountains, in the Central and Eastern United States, the hazard drops significantly. While earthquakes in the Central and Eastern U.S. are not as frequent as on the West Coast, the geological conditions east of the Rockies cause earthquakes of a given magnitude to affect a much larger area than earthquakes of the same magnitude that strike the West Coast. Notable earthquakes have occurred in the Central and Eastern U.S.
“The Shock Came on With Redoubled Violence, and Loud Noise”
One of the most severe earthquakes known to have occurred in the northeastern U.S., for example, was the Cape Ann earthquake of November 18, 1755. This quake, which struck at about 4:30 a.m., is estimated to have had a magnitude of about 5.9; its epicenter was located out at sea roughly 24 miles east of Cape Ann, which is about 30 miles northeast of Boston, Massachusetts. There were several aftershocks, some of which were widely felt throughout eastern Massachusetts; aftershocks were noted into December. The mainshock affected a broad area of the East Coast and shaking was felt as far afield as Halifax, Nova Scotia; Lake Champlain in Upstate New York; and Winyah, South Carolina. The earthquake may also have triggered the small tsunami noted in the Leeward Islands, 1,000 miles to the south.
There were no fatalities caused by the Cape Ann earthquake, and while damage was not severe it was reported in locations as widespread as Portland, Maine, and Newhaven, Connecticut. Property damage seems to have consisted mainly of damage to chimneys, ranging from cracks and twisting to total collapse. Bricks falling from chimneys damaged roofs and obstructed streets. In Boston, where 1,300-1,600 chimneys were damaged to some degree, the gable ends of several houses also collapsed and a few church steeples were shifted from the vertical. Liquefaction was also an issue in some locations. Cracks in the ground were reported in Massachusetts near Scituate and Pembroke, south of Boston, and Lancaster, west of Boston; in some places water and sand were seen to emerge from the cracks. Most of the property damage in Boston was reported in areas of infill around the harbor.
Hazard in the Region
The northeastern U.S. has been the site of large intraplate earthquakes. They are rare when compared to the West Coast; most earthquakes in the region have small magnitudes and are not widely felt. However, seismic activity is persistent, which indicates that some crustal deformation is occurring. Prior to the 1755 event, for example, Cape Ann had experienced a powerful earthquake in 1727 and a weaker one in 1744. The most recent significant earthquake in the northeastern U.S. occurred on September 25, 1998, when an M5.2 event struck Pymatuning Reservoir, Pennsylvania. In 1904, an earthquake with an intensity of VII (MMI scale) was recorded in Eastport, Maine.
The northeastern U.S. is affected by the Charlevoix seismic zone in eastern Canada, also known as the Charlevoix-Kamouraska seismic zone, located along the St. Lawrence River, about 62 miles north of Quebec City. Since 1663 this area has experienced five earthquakes of M6.0 or greater. Remote sensing indicates strike-slip to reverse faulting on buried crustal faults, and crustal weakness combined with fluid pressure are believed to contribute to the earthquake activity.
Particularly at Risk
The cause of the Cape Ann earthquake is unclear, but the region’s risk is manifest. Since 1755 the amount and value of exposure in the northeastern U.S. has increased exponentially while the hazard remains undiminished. In Massachusetts, Boston is at particular risk for several reasons. Over the course of two centuries the city expanded from its original 783 acres to become 40 times larger through annexing neighboring towns and infilling large areas at the water’s edge with building rubble, gravel, and soil won by lowering its three hills.
It is estimated that 75% of Boston’s buildings predate the introduction of seismic building codes in the city in 1975. A high proportion of the structures built on Boston’s unstable reclaimed land were constructed without any seismic provisions or requirements; they are consequently vulnerable to both ground shaking and liquefaction. Much of the city’s aging infrastructure is similarly at risk. In the northeastern and southeastern parts of the U.S. earthquake risk is relatively low, but for that reason so too is the general level of preparedness.