Ramapo Fault: East Coast Earthquake Risk Explained

When a M4.8 earthquake struck northern New Jersey on April 5, 2024, tens of millions of people across the northeastern United States experienced something profoundly unfamiliar: the ground shaking beneath their feet. Social media erupted. New Yorkers poured into streets. Building managers evacuated offices. The United Nations General Assembly paused its session.

The earthquake occurred near the Ramapo Fault system, a roughly 300-kilometer structure that runs through New Jersey, New York, and Pennsylvania. While the event caused no significant structural damage, it served as an abrupt reminder that the East Coast is not immune to seismic hazard — and that the region's enormous population density, aging infrastructure, and near-total lack of earthquake preparedness create a fundamentally different risk equation than anything found in California.

This article examines the Ramapo Fault: its geology, its history, and what seismologists actually know about the earthquake risk it poses to the most densely populated region in the United States.

Geography of the Ramapo Fault System

The Ramapo Fault runs along a northeast-southwest trend through the northern Appalachian region. Its surface expression follows the eastern edge of the Appalachian Highlands, forming a geological boundary between the older Precambrian rocks of the Highlands to the west and the younger Mesozoic sedimentary rocks of the Newark Basin to the east.

The fault's principal trace extends from southeastern Pennsylvania, through northern New Jersey's Ramapo Mountains, across the Hudson Highlands of New York, and into southeastern New York state near the Connecticut border. The total length of the system, including related splay faults and associated structures, spans approximately 300 km (185 miles).

Key geographic reference points along the Ramapo Fault system include:

• Southeastern Pennsylvania: The southwestern terminus of the system, where it intersects older Appalachian structures
• Northern New Jersey: The fault passes through the Ramapo Mountains, near communities including Mahwah, Ringwood, Pompton Lakes, and Morristown
• Rockland County, New York: The fault crosses the Hudson River in the vicinity of the Tappan Zee/Mario Cuomo Bridge
• Westchester County and beyond: Splay faults and related structures extend into the lower Hudson Valley

The fault's closest approach to Manhattan is approximately 40 km (25 miles) to the northwest. However, the broader Ramapo seismic zone — the region of diffuse seismicity associated with the fault system — extends considerably closer to the city.

[MAP: Ramapo Fault system trace from southeastern Pennsylvania through northern New Jersey and into southeastern New York, with NYC metropolitan area overlay showing population density, historical earthquake epicenters, and the Indian Point nuclear power plant (decommissioned).] Data source: USGS Quaternary Fault and Fold Database, Lamont-Doherty Earth Observatory, U.S. Census Bureau. Features: Fault trace, Ramapo seismic zone boundary, historical epicenters (scaled by magnitude), NYC metro boundary, population density shading.

Geological Origins and Reactivation

An Ancient Fault

The Ramapo Fault is far older than California's plate-boundary faults. Its origins date to the Proterozoic and Paleozoic eras — roughly 500 million to over 1 billion years ago — when it formed as part of the mountain-building events that created the Appalachian Mountains. The fault has been reactivated multiple times over its long geological history.

During the Mesozoic Era (approximately 200 million years ago), the Ramapo Fault was reactivated as a normal (extensional) fault during the rifting that opened the Atlantic Ocean. This phase of extension created the Newark Basin — a large sedimentary basin filled with Triassic and Jurassic rocks — bounded on its western side by the Ramapo Fault. The distinctive red sandstones and shales of the Newark Basin, visible in road cuts throughout northern New Jersey, were deposited in this rift environment.

Modern Stress and Reactivation

Today, the Ramapo Fault exists in an intraplate tectonic setting. Unlike California's faults, which sit on an active plate boundary, the Ramapo is located in the middle of the North American Plate, far from any plate edge. The stress field in the northeastern United States is dominated by broad-scale tectonic forces: primarily ridge-push from the Mid-Atlantic Ridge and glacial isostatic adjustment (the ongoing rebound of the Earth's crust following the removal of Pleistocene ice sheets).

According to research by scientists at Columbia University's Lamont-Doherty Earth Observatory, these forces generate a compressive stress field oriented roughly east-northeast. This stress can reactivate ancient faults like the Ramapo, even though they are no longer associated with an active plate boundary. The result is a low but non-negligible rate of seismicity scattered across the region.

The mechanism is fundamentally different from California's seismicity. The San Andreas Fault system accommodates approximately 50 mm/year of plate motion and produces thousands of detectable earthquakes annually. The Ramapo Fault has no measurable geodetic slip rate — any movement is too slow to detect with current GPS technology, estimated at less than 0.1 mm/year. Earthquakes occur, but they are infrequent and typically small.

Historical Seismicity

The Seismic Record

The northeastern United States has a longer written historical record than most of the American West, which provides a valuable (if incomplete) window into past earthquake activity near the Ramapo system. The region has experienced several notable earthquakes over the past three centuries.

Table: Significant Historical Earthquakes in the New York–New Jersey Region

The 1884 New York City Earthquake

The August 10, 1884 earthquake remains the largest known seismic event in the immediate New York City area. With an estimated magnitude of approximately 5.2, it occurred near the coast of New Jersey or Brooklyn (the exact epicenter is uncertain due to the era's limited seismic instrumentation). Contemporary newspaper accounts describe chimneys falling in New York and Brooklyn, cracked walls across lower Manhattan, and panic in the streets.

The earthquake was felt across an area of roughly 180,000 square kilometers — from Virginia to Maine and as far inland as Vermont. This enormous felt area illustrates a critical difference between East Coast and West Coast seismicity: because the crust in the eastern United States is older, colder, and more intact, seismic waves propagate much more efficiently. An M5.2 earthquake in the East can be felt over an area 10 to 20 times larger than a comparable event in California.

If the 1884 earthquake were to recur today, its effects would be dramatically different. The New York metropolitan area has grown from roughly 2 million people in 1884 to over 20 million today. The built environment includes millions of unreinforced masonry structures, aging infrastructure, and dense urban canyons that can amplify ground motion.

The 2024 New Jersey Earthquake

On April 5, 2024, a M4.8 earthquake struck near Tewksbury, New Jersey, at a depth of approximately 4.7 km. According to the USGS, the event was felt across multiple states, from Virginia to New Hampshire. The agency estimated that approximately 42 million people experienced the shaking, making it one of the most widely felt earthquakes by population in United States history.

The earthquake's epicenter was located near the Ramapo Fault system, though the precise fault association remains under investigation. The focal mechanism indicated reverse faulting with a northeast-striking plane, consistent with the regional stress field. Multiple aftershocks followed, including a M3.8 event on the same day.

Despite the widespread shaking, no significant structural damage was reported. The earthquake registered at most a Modified Mercalli Intensity of V–VI (moderate to strong) in the epicentral area. However, the event served as a powerful wake-up call for a region where earthquake preparedness is minimal.

[CHART: Bar chart — Estimated Population Exposed to Shaking by Selected U.S. Earthquakes] Data: 2024 NJ M4.8: ~42 million felt; 1994 Northridge M6.7: ~12 million felt; 2010 Haiti M7.0: ~3.5 million in affected zone; 1989 Loma Prieta M6.9: ~7 million felt; 2011 Virginia M5.8: ~100+ million (estimated, broad felt area). Source: USGS ShakeMap, "Did You Feel It?" reports.

How East Coast Earthquake Risk Differs from the West Coast

Understanding earthquake risk on the East Coast requires a fundamentally different framework than what applies in California. The hazard is lower, but the vulnerability is higher in several important dimensions.

Hazard: How Often and How Large

The rate of earthquake occurrence in the northeastern United States is roughly 50 to 200 times lower than in California. The USGS National Seismic Hazard Model reflects this: peak ground acceleration values for the New York City area are a small fraction of those for Los Angeles or San Francisco.

The maximum credible earthquake on or near the Ramapo Fault is a matter of considerable debate. Most seismologists estimate that an earthquake up to approximately M6.0–6.5 is physically possible on the Ramapo or associated structures, though some researchers have argued that the historical and paleoseismic evidence is insufficient to constrain this estimate with confidence. A 2008 study by Sykes et al. at Lamont-Doherty Earth Observatory concluded that M5+ earthquakes near New York City have a recurrence interval on the order of approximately 100 years, and M6+ events may occur on the order of every 670 to 3,400 years.

Vulnerability: Buildings and Infrastructure

The critical difference between East Coast and West Coast seismic risk lies in vulnerability:

• Building codes: Modern seismic building codes in California have been continually strengthened since the 1933 Field Act. In the northeastern United States, seismic provisions in building codes have historically been minimal. Many structures in the NYC metro area — particularly older unreinforced masonry buildings, brownstones, and pre-war apartment buildings — were designed with no consideration of seismic forces.
• Infrastructure age: Much of the New York metropolitan area's critical infrastructure — bridges, tunnels, water mains, subway tunnels — was built in the early to mid-20th century, before seismic design was a consideration for the region.
• Amplification effects: Parts of lower Manhattan, Brooklyn, and other coastal areas are built on soft sediments, fill material, and reclaimed land. These deposits can amplify seismic waves, a phenomenon well-documented in earthquakes worldwide (e.g., the 1985 Mexico City earthquake, the 1989 Loma Prieta damage in San Francisco's Marina District).

Exposure: Population Density

The New York City metropolitan statistical area has a population exceeding 20 million, making it the most populous metro area in the United States. Northern New Jersey alone — the area closest to the Ramapo Fault — is home to approximately 9 million people. The population density in this region is among the highest in the Western Hemisphere.

This density means that even moderate earthquakes affect enormous numbers of people. The 2024 M4.8 earthquake, which would barely merit a news article in California, dominated national headlines for days precisely because of how many people experienced it.

Quantifying the Risk

USGS Seismic Hazard Assessment

The USGS National Seismic Hazard Maps, updated most recently in 2023, incorporate the Ramapo Fault and the broader northeastern seismic zone into their probabilistic models. The maps show that the New York City area has a low but non-trivial seismic hazard:

• 2% probability of exceedance in 50 years (roughly a 2,475-year return period): The NYC area faces peak ground acceleration values of approximately 0.10–0.15g, roughly one-quarter to one-third of values for Los Angeles.
• 10% probability of exceedance in 50 years (roughly a 475-year return period): NYC values are approximately 0.04–0.06g.

These are substantially lower than West Coast values, but they are not zero. For context, the Modified Mercalli Intensity that corresponds to potential damage to vulnerable buildings begins at around 0.10–0.15g — the range that NYC could experience in a low-probability, high-consequence event.

What a Moderate Earthquake Would Mean for New York City

A 2003 study by the New York City Area Consortium for Earthquake Loss Mitigation (NYCEM) modeled the impact of a repeat of the 1884 M~5.2 earthquake on the modern metropolitan area. The study estimated:

• Widespread non-structural damage (fallen objects, broken glass, cracked plaster)
• Significant structural damage to unreinforced masonry buildings
• Possible partial collapses of the most vulnerable structures
• Substantial disruption to transportation, communications, and emergency services
• Economic losses potentially in the billions of dollars

A larger event — M6.0 or above — would produce far more severe consequences, though such events are much rarer. The combination of vulnerable building stock, critical infrastructure dependency, and dense population makes even moderate earthquakes consequential in this region.

Table: Comparison of Seismic Risk Factors — NYC Metro vs. Los Angeles Metro

The Ramapo and Nuclear Power

The Ramapo Fault's proximity to critical infrastructure has been a subject of particular attention regarding nuclear facilities. The Indian Point Energy Center, a two-unit nuclear power plant located in Buchanan, New York — approximately 38 km north of midtown Manhattan and near the Ramapo Fault system — operated from 1962 until its final unit was shut down in April 2021.

Concerns about seismic risk were among the factors cited in debates about Indian Point's continued operation. A 2008 reassessment by the Nuclear Regulatory Commission (NRC) identified a fault crossing the Indian Point site, and subsequent studies examined whether the plant's seismic design basis was adequate given updated understanding of regional seismicity. The plant's closure and ongoing decommissioning have largely mooted the operational seismic risk question, but the episode illustrates how seriously seismic hazard near the Ramapo system has been taken by regulatory authorities.

Current Monitoring and Research

Lamont-Doherty Earth Observatory

Columbia University's Lamont-Doherty Earth Observatory (LDEO) in Palisades, New York, is the primary research institution studying seismicity in the northeastern United States. LDEO operates a network of seismometers across the region and maintains the most comprehensive catalog of earthquake activity in the greater New York City area.

Research at LDEO has focused on characterizing the regional stress field, identifying active structures within the Ramapo system, and improving seismic hazard estimates for the metropolitan area. Key contributors to this work include Won-Young Kim, Leonardo Seeber, and Lynn Sykes, whose publications have shaped the current understanding of East Coast earthquake risk.

Northeast U.S. Seismic Network

The USGS, in cooperation with university partners, operates seismic monitoring stations across the northeastern United States. While the station density is lower than in California's highly instrumented fault zones, the network provides sufficient coverage to detect and locate earthquakes down to approximately M1.0–2.0 in the New York–New Jersey region.

Earthquake Early Warning

As of 2025, the USGS ShakeAlert earthquake early warning system is operational only in the western United States (California, Oregon, and Washington). No comparable system exists for the East Coast. Given the infrequent nature of significant earthquakes in the region, the cost-benefit calculus for an eastern early warning system remains under discussion.

What Residents Should Know

For the approximately 20 million people living in the New York City metropolitan area, earthquake preparedness occupies a different mental space than it does for Californians. Earthquakes are rare, unfamiliar, and easy to dismiss. The 2024 event demonstrated that dismissal is not a sound strategy.

Practical steps recommended by the USGS and FEMA include:

• Know your building: Unreinforced masonry structures (common in older NYC neighborhoods) are the most vulnerable building type in earthquakes. Understanding your building's construction can inform decisions about insurance and safety planning.
• Secure heavy objects: Bookcases, water heaters, and heavy wall-mounted items can topple during shaking.
• Have a plan: Identify safe spots in each room (under sturdy tables, away from windows and heavy objects). Practice "Drop, Cover, and Hold On."
• Earthquake insurance: Standard homeowner's and renter's insurance policies in the Northeast typically do not cover earthquake damage. Separate earthquake insurance is available.

New Jersey earthquake information and New York earthquake information provide additional detail on historical seismicity and preparedness in these states. For a broader overview of earthquake causes, see What Causes Earthquakes.