Boston's Earthquake Risk: New England's Surprising Seismicity

In the early morning hours of November 18, 1755 — less than three weeks after the great Lisbon earthquake had devastated Portugal and sent a tsunami across the Atlantic — the residents of colonial Boston were startled awake by violent shaking. Chimneys collapsed across the city. Plaster fell from walls. The shaking was so severe that accounts described people being thrown from their beds, objects flying from shelves, and church bells spontaneously ringing in steeples throughout the region. The earthquake — centered approximately 24 miles northeast of Boston off Cape Ann — was estimated at approximately M6.0–6.2 based on the extent and intensity of the felt reports. It was felt from Nova Scotia to South Carolina and as far west as Lake Ontario — a geographic footprint of nearly 2 million square kilometers, consistent with the remarkably low seismic attenuation of the ancient, cold crystalline crust of the northeastern United States. Contemporary accounts record approximately 100 chimneys fallen or cracked in Boston, 1,500 in the surrounding region, and virtually no building left structurally undamaged in the most heavily shaken areas near Cape Ann.

No one died. Boston in 1755 had a population of perhaps 15,000 — a small colonial town of wooden frame houses, brick buildings, and stone churches — and the shaking, though severe, was not sufficient to collapse the relatively simple structures of the era in large numbers. Had the same earthquake struck the Boston of today — a metropolitan area of 4.9 million people, with a downtown of concrete high-rises, an extensive stock of century-old unreinforced brick buildings, and an infrastructure of tunnels, bridges, elevated highways, and buried utilities running through the soft marine clay deposits that amplify seismic shaking throughout the basin — the consequences would be categorically different. Not catastrophic in the sense of Cascadia or a San Andreas rupture, but potentially among the most economically disruptive earthquakes in American history — striking a high-value, high-density urban economy with building codes and preparedness standards calibrated for a region that most residents do not think of as earthquake country.

That perception — that New England is too far from plate boundaries to face meaningful earthquake risk — is the central misconception this post addresses. New England experiences hundreds of earthquakes per year. The region's geological record contains multiple events of M5–6 in the historical era. The eastern US transmits seismic shaking to greater distances than California for equivalent magnitude events. And Boston specifically sits on some of the most seismically amplifying soft sediments on the East Coast — the marine Boston Blue Clay that underlies the filled tidal flats and the Back Bay, materials with seismic properties that would concentrate and amplify shaking in exactly the areas most densely developed with the region's most expensive real estate.

The Tectonic Setting: Why New England Has Earthquakes at All

New England sits firmly in the stable continental interior of the North American plate — hundreds of kilometers from the nearest active plate boundary. The Mid-Atlantic Ridge is 2,000+ kilometers to the east; the Cascadia subduction zone is 4,500 kilometers to the west; the Caribbean plate boundary is 2,000+ kilometers to the south. By the simple logic of plate tectonics, New England should be seismically quiet — and by the standards of California or the Pacific Northwest, it is. But seismic quiet and seismically inert are not the same thing, and New England's intraplate seismicity arises from a combination of geological factors that are now reasonably well understood.

Ancient Geological Structures as Modern Seismic Hosts

New England's crystalline basement is composed of ancient Precambrian and Paleozoic rocks — the roots of multiple ancient mountain belts that were built and eroded long before the Atlantic Ocean opened. These rocks are cut by numerous ancient fault zones — northeast-trending structural boundaries, Paleozoic thrust faults, and Mesozoic extensional faults associated with the opening of the Atlantic — that are geologically old but not mechanically dead. The contemporary stress field of eastern North America, driven by ridge push from the Mid-Atlantic Ridge and gravitational potential energy from the elevated Appalachians, imposes a northeast-directed maximum compressive stress throughout the region that preferentially reactivates favorably oriented ancient fault zones as seismogenic structures.

Unlike the active faults of the American West, which have surface expressions visible in topography and geology and can be mapped in detail from aerial photographs and satellite imagery, the faults generating New England's seismicity are typically buried under thick glacial deposits — till, outwash, lake sediments — that were deposited during and after the Pleistocene glaciations that covered all of New England under a mile or more of ice. The glacial cover obscures fault surface traces, making paleoseismic investigation far more difficult than in unglaciated regions, and the glacial and post-glacial isostatic rebound — the ongoing adjustment of the crust to the removal of the ice load — introduces additional stresses that may contribute to New England's seismicity independently of plate-tectonic driving forces.

New England's Seismic Zones: The Geography of Risk

The seismicity of New England is not uniformly distributed — it is concentrated in several distinct zones associated with specific geological structures or regions of anomalously elevated background seismicity rates.

The Cape Ann Seismic Zone

The Cape Ann seismic zone — centered on the offshore area northeast of Boston near Cape Ann and the Isles of Shoals — is the most historically productive seismic zone in New England and the source of the 1755 M6.0–6.2 earthquake. The zone produces a background seismicity rate that is elevated above the surrounding region, with small earthquakes (M1–3) occurring several times per year and occasional larger events (M3–4) felt in coastal communities of Massachusetts and New Hampshire. The geological structures hosting the Cape Ann zone have not been definitively identified — seismic reflection and refraction surveys have imaged northeast-trending fault-like reflectors in the offshore basement beneath the Massachusetts Bay, but the specific structure responsible for the 1755 mainshock and the ongoing microseismicity remains uncertain.

The Moodus Noises: Connecticut's Seismic Curiosity

South-central Connecticut hosts one of the most unusual seismicity clusters in the northeastern US: the Moodus microearthquake zone in the East Haddam area of Connecticut, which has produced nearly continuous small earthquakes (typically M0–2) for at least 300 years of historical observation and presumably much longer. The "Moodus Noises" — named for the audible rumbling and detonation sounds heard by local residents when the small earthquakes occur — are among the longest-documented seismic phenomena in North American history, described in Native American oral traditions and in European colonial records from the 1600s.

The Moodus zone is associated with a cluster of hydrothermal alteration in the underlying Precambrian crystalline basement — the small earthquakes are thought to represent fluid-triggered microseismicity on pre-existing fractures in the basement rock, maintained by the circulation of slightly acidic groundwater through the fracture network. While the individual events are too small to be felt by most residents, the zone occasionally produces larger events — a M3.3 in 1981 caused minor damage in East Haddam — and has the potential to generate M4–5 earthquakes that would be felt across a wide area of Connecticut.

The Northern New England Zone

Maine, New Hampshire, and Vermont experience elevated seismicity relative to southern New England, with the most active areas in the White Mountains of New Hampshire and in coastal Maine. The 1940 M5.6 Ossipee, New Hampshire earthquake — which cracked chimneys, damaged buildings, and was felt across New England and into New York — is the largest historical event in the northern New England zone and provides a calibration of what a moderate earthquake in this region can do to the building stock. The Ossipee zone has been continuously seismically active since at least 1940, producing background microseismicity at rates 5–10 times higher than the surrounding crystalline basement terrain.

The St. Lawrence Seismic Zone: The Overlooked Northern Threat

Extending northward across the border into Quebec and the St. Lawrence River valley, the Charlevoix seismic zone — associated with the impact crater of a large meteorite that struck approximately 342 million years ago, creating a structural disruption in the crust that has been reactivated as a seismogenic zone — is one of the most historically active intraplate seismic zones in eastern North America. Major earthquakes in this zone — estimated at M7+ in 1663, M6.5 in 1791, M6.5 in 1860, and M6.2 in 1925 — have been felt throughout New England and as far south as Boston and New York. The 1663 earthquake is among the largest known earthquakes in eastern Canada and eastern North America, and its shaking effects were documented across a felt area of several million square kilometers. A repeat of the 1663 event would generate strong shaking in Vermont, New Hampshire, and Maine and moderate shaking throughout the remainder of New England.

Why Eastern Earthquakes Affect Boston from Greater Distances

One of the defining characteristics of seismic hazard in New England — and throughout the eastern United States — is the remarkably low seismic attenuation of the ancient Appalachian and Laurentian crystalline crust. The same cold, intact, unfractured rocks that make New England's forests and granite quarries possible transmit seismic waves with far less energy loss per unit distance than the hot, fractured, fluid-saturated crust of California and the western United States.

The quantitative difference is striking. In California, a M5.0 earthquake typically produces felt shaking (MMI III+) within approximately 80–120 km of the epicenter. The same M5.0 in New England or the central US produces felt shaking to distances of 200–400 km — roughly 3–4 times the felt area. For the 2011 M5.8 Mineral, Virginia earthquake — which ruptured a previously unmapped reverse fault in the central Virginia seismic zone — felt reports came from as far as Georgia to the south and Quebec to the north, spanning a felt area of approximately 3 million square kilometers. The earthquake shook the Washington Monument so vigorously that several of its stones cracked and the monument was closed for more than a year for repairs — a $15 million repair bill from a moderate earthquake in a region that most planners do not consider high-seismicity. The shaking reached Boston with enough intensity to be widely felt — a reminder that for New England, the relevant hazard is not only local.

Boston's Geological Vulnerability: The Marine Blue Clay Problem

Boston's urban geology presents a specific seismic vulnerability that is closely analogous to the amplification problems that produced catastrophic earthquake consequences in Mexico City, Bogotá, and Quito — but in a North American setting where the hazard is less commonly discussed.

The Boston Basin and Its Fill

The historic city of Boston occupies a peninsula and series of filled tidal flats at the mouth of the Charles River — a geography that reflects centuries of land reclamation that began in the colonial era and continued through the 20th century. The Back Bay, the South End, the Fenway, East Boston, and large portions of the Financial District and the Seaport District are built on artificial fill — materials ranging from colonial-era rubble and garbage to modern engineered fill — placed directly on the soft marine clay deposits of Boston Harbor and the former tidal flats.

Beneath the fill, and extending to depths of 10–30 meters in the most deeply filled areas, lies the Boston Blue Clay — the Presumpscot Formation, a glaciomarine clay deposited when seawater flooded the depressed glacially-compressed crust of New England after the ice sheets melted. This clay is characteristically soft, water-saturated, and highly compressible — properties that make it a challenging engineering material for any type of loading and that give it seismic properties comparable to the soft clays of San Francisco Bay and the Houston Ship Channel: S-wave velocities of 70–150 m/s, near-zero undrained shear strength in the most remolded sections, and amplification factors of 3–8 at periods of 0.5–3 seconds compared to the bedrock outcrops of Boston's drumlins and the Roxbury Conglomerate that forms Boston's geologically firm foundation in some neighborhoods.

Infrastructure in the Clay: The Big Dig and Boston's Tunnels

Boston's infrastructure is unusually concentrated in or through the soft Boston Blue Clay. The Central Artery / Third Harbor Tunnel project — the "Big Dig," completed in 2006 — placed Interstate 90 and Interstate 93 in tunnels running directly through the soft clay and filled tidal flat terrain of downtown Boston. The Ted Williams Tunnel, the Thomas P. O'Neill Jr. Tunnel, and the Storrow Drive Connector are all partly or entirely in the soft clay zone. The MBTA's Red, Orange, Blue, and Green Lines include tunnel sections through the soft clay in the Back Bay and waterfront areas. The seismic performance of these buried structures in the soft clay — particularly the settlement and deformation of the surrounding clay during strong shaking — is a specific concern that has not been systematically evaluated against the design earthquake for the Cape Ann scenario.

The 2011 Mineral, Virginia Earthquake: A Regional Calibration

The August 23, 2011 M5.8 Mineral, Virginia earthquake provided the most recent regional calibration of how eastern US earthquakes are felt in New England — and how unprepared the region's infrastructure is for even moderate shaking. The earthquake was widely felt in Boston (MMI III–IV across most of the metro area), caused panic in downtown Boston office buildings as workers who had never felt an earthquake evacuated, and produced minor cracking in a few older unreinforced masonry buildings. More significantly for the infrastructure case, the earthquake cracked the Washington Monument (600 km from the epicenter), displaced a finial on the National Cathedral in Washington, and caused structural damage to buildings throughout the DC-Maryland-Virginia corridor that had been designed to the local building code — a code that had not been updated to reflect the 2011 earthquake's demonstration that the region's seismic hazard was underestimated by earlier national hazard maps.

Boston's experience of the Mineral earthquake — felt shaking from a M5.8 at 800 km distance — directly illustrates the low-attenuation amplification effect described earlier. A M6.0 Cape Ann earthquake at 40 km would produce shaking 25–50 times more intense than what Boston felt from the Virginia event. That proportional scaling translates directly into the building damage and infrastructure disruption that the scenario analysis projects.

Boston's Building Stock: The Oldest in the United States

The eastern United States, and New England in particular, has the oldest urban building stock in the country — a legacy of being settled in the 17th and 18th centuries and continuously occupied since. Boston has more pre-1900 buildings than almost any other major American city, and its building stock includes concentrations of unreinforced masonry that are as extensive as those of any western US city of comparable size, but which have received almost no systematic seismic assessment.

Unreinforced Masonry in New England

The 19th and early 20th century building stock of Greater Boston — the three-deckers of Dorchester, Roxbury, and Jamaica Plain; the brick row houses of the South End and Back Bay; the Victorian commercial buildings of downtown; the university buildings of Harvard, MIT, and dozens of colleges throughout the metropolitan area — is predominantly unreinforced masonry construction. Brick walls, stone foundations, and minimal structural steel interconnection are the rule rather than the exception. Unlike California, which passed its first statewide URM hazard identification law in 1986 (requiring cities to inventory URM buildings) and where cities including Los Angeles have mandatory retrofit programs, Massachusetts has no statewide URM inventory requirement, no mandatory retrofit law, and no systematic hazard assessment program for the pre-code building stock.

The result is a situation in many ways analogous to the Charleston, South Carolina profile covered earlier in this series — a city with significant seismic hazard, an old building stock with high vulnerability, and an absence of the periodic damaging earthquakes that have driven seismic policy reform in California. The 1755 Cape Ann earthquake provides the only major historical calibration event — and it occurred when Boston's building stock consisted of simple colonial structures, not the multi-story masonry buildings of the Victorian and Edwardian eras that now dominate the urban landscape.

Massachusetts Building Code and Seismic Provisions

Massachusetts adopted the International Building Code (IBC) seismic provisions — replacing the older Massachusetts State Building Code's more limited seismic requirements — progressively from the early 2000s. The current Massachusetts building code classifies most of eastern Massachusetts in Seismic Design Category C (moderate hazard), with some areas adjacent to the Cape Ann zone reaching Seismic Design Category D. New construction must meet the seismic design requirements of these categories, which include basic ductility requirements for structural systems and foundation design that accounts for site class amplification.

The critical limitation — as everywhere in this series — is that these requirements apply only to new construction. The vast majority of Boston's building stock was built to the Massachusetts State Building Code provisions that preceded the IBC adoption, or to earlier codes with minimal seismic provisions, or with no engineering oversight at all (most residential construction in Massachusetts, including the three-deckers and most two-family homes, historically has not required engineer-stamped structural drawings). There is no Massachusetts equivalent of California's statewide URM program, no mandatory hazard identification, no retrofit incentive or requirement for existing buildings.

The USGS Hazard Maps and Boston's Design Earthquake

The USGS National Seismic Hazard Maps classify Greater Boston in a moderate-hazard zone — lower than the San Andreas or Cascadia areas of California and the Pacific Northwest, but comparable to portions of the Central US in the New Madrid hazard zone at similar return periods. The 2% probability of exceedance in 50 years (approximately 2,500-year return period) PGA for eastern Massachusetts is approximately 0.12–0.18g — lower than the 0.4–0.6g in the highest-hazard California zones but substantially above the near-zero values that most Bostonians would guess for their city if asked.

For a site on Boston Blue Clay (Site Class E) in the Back Bay or the Seaport, the site amplification factor at short periods is approximately 2.5–3.5× the bedrock reference — bringing the design ground motion for the most vulnerable soil sites in Boston to approximately 0.30–0.60g at short periods, a range that would cause significant damage to unreinforced masonry and non-ductile concrete frame buildings that were not designed to resist lateral loads.

Scenario Analysis: What a Repeat of 1755 Would Do

The most directly relevant earthquake scenario for Boston is a repeat of the 1755 Cape Ann event — an M6.0–6.2 earthquake at approximately 40 km northeast of the city. Ground motion simulations for this scenario project peak ground accelerations of 0.10–0.20g on bedrock throughout Greater Boston, amplified to 0.25–0.60g on the soft sediment sites of the filled tidal flats and the inner harbor shoreline.

HAZUS loss estimates for this scenario project approximately 200–400 deaths, 3,000–6,000 injuries, and $5–10 billion in direct losses for a daytime occurrence. Nighttime losses would be roughly 60% lower in casualties but comparable in structural damage. The deaths would be concentrated in unreinforced masonry buildings — older residential buildings in Roxbury, Dorchester, and Jamaica Plain; historic commercial buildings in the South End and the older downtown; and institutional buildings on university campuses. The economic losses would be concentrated in the highly valued soft-sediment areas — the Back Bay, the Seaport District, East Boston — where the combination of high land values and amplification-sensitive construction creates the highest loss density per unit area.

For a larger event — an M6.5 at the same location, consistent with the upper end of estimates for Cape Ann — the casualties approximately double and the economic losses increase by a factor of 3–4, driven primarily by the increased number of URM collapses and by more extensive damage to the infrastructure in the soft sediment zones. At M7.0 — not unprecedented in the Charlevoix zone to the north but substantially larger than anything in New England's modern instrumental record — the scenario approaches a regional catastrophe, with the combination of the low-attenuation eastern crust, soft sediment amplification, and the unprepared building stock producing consequences comparable to the 1906 San Francisco earthquake on a metropolitan area with three times the population.

The Preparedness Gap in Context

New England's earthquake preparedness situation is in some ways the starkest illustration in this series of the gap between scientific understanding and institutional action. Unlike California — where the preparedness gap is large but is being actively narrowed by state laws, mandatory programs, and the institutional memory of repeated damaging earthquakes — New England has never had a damaging modern earthquake, has no mandatory URM identification or retrofit programs, has essentially no public earthquake awareness culture, and has not systematically applied the scientific knowledge that USGS and regional researchers have developed about the Cape Ann zone, the Boston Blue Clay amplification, and the building stock vulnerability to the regulatory and investment decisions that govern what buildings are occupied by how many people under what seismic design conditions.

This is not a problem unique to New England. Charleston, South Carolina; the Central Mississippi Valley; and much of the eastern United States face comparable combinations of measurable scientific hazard and minimal preparedness investment. The common thread is the absence of damaging earthquakes in living memory — the very absence that allows a comforting fiction of seismic immunity to persist despite the clear evidence of the historical and geological record.

Conclusion

Boston and New England are not earthquake country in the popular imagination. They are, in the scientific and historical record, earthquake country of a particular kind: intraplate seismicity, low annual probability events, geological structures that are genuinely difficult to characterize, and a physical environment — the cold, intact Precambrian crust — that transmits whatever shaking does occur to remarkably large distances without the attenuation that moderates California's risk at equivalent magnitudes.

The combination of Boston Blue Clay amplification in the most developed areas of the city, a building stock dominated by 19th and early 20th century unreinforced masonry that has never been systematically assessed for seismic vulnerability, no mandatory retrofit programs, and a public completely unaware that the city faces any seismic hazard creates a preparedness gap that a repeat of the 1755 Cape Ann earthquake would expose dramatically — and that a somewhat larger event from the Charlevoix zone to the north would expose catastrophically. The 1755 earthquake cracked chimneys in a city of wooden colonial structures. The 2026 earthquake, if it comes at a comparable magnitude from a comparable location, will crack the unreinforced masonry walls of a city of four million people in buildings that have never been asked to resist lateral earthquake loading. The difference is measurable. The gap is closeable. Whether it will be closed before the question becomes operational is the relevant question for New England's earthquake future.