What is the Hazard?
An earthquake is a sudden, rapid shaking of the earth as plates shift, rock cracks beneath its surface, and large plates either collide or try to push past one other. As rocks and the earth’s plates are strained by these tremendous geological processes, energy builds up under the earth’s surface. Eventually, accumulated energy deep underground becomes so great that it is abruptly released in seismic waves.
From this source, or “focus,” deep underground, the waves travel away and shake the earth’s surface. An earthquake’s epicenter is the point on the earth’s surface that lies directly above the focus. Seismologists and engineers measure the shaking that occurs as “ground acceleration.”
The intensity of ground shaking depends on several factors, including the amount of released energy, the depth of the earthquake beneath earth’s surface, the distance from the fault, and the type of underlying soil or bedrock.
How intensely a built structure responds to shaking during an earthquake depends on the building’s height, weight, and design.
An earthquake has the potential to damage and destroy buildings and a city’s infrastructure and take lives. Under certain conditions, earthquakes can trigger landslides and cause soil liquefaction. The latter occurs when shaking and ground vibration during an earthquake cause unconsolidated, water-saturated soils to soften and turn fluid. Ground shaking, landslides, and liquefaction together can damage or destroy buildings, disrupt utilities, trigger fires, and endanger general public safety.
Aftershocks are part of the earthquake’s sequence that follows the largest, initial earthquake shock. Aftershocks are typically less intense than the main shock, and may occur for weeks, months, or years after the initial earthquake event.
The 2011 Virginia earthquake, which rattled the ground as far away as New York City, was a magnitude 5.8. By comparison, the 2011 earthquake that created such damage to the eastern coast of Japan was a magnitude 9.0 and considered catastrophic. In theory, these earthquake magnitude scales do not have an upper limit, but no earthquake event has yet reached a magnitude of 9.5. The Modified Mercalli Intensity (MMI) scale is a measurement based upon what has been observed in seismic shaking during earthquakes. The MMI reflects twelve categories of intensity based on people’s reactions, their observations, and building damage during seismic events.
MMI Scale Rating
Approximate Relationship between MMI and PGA
|MMI||Acceleration (%g) (PGA)||Perceived Shaking||Potential Damage|
|I||< .17||Not felt||None|
|VIII||34–65||Severe||Moderate to heavy|
|X||> 124||Extreme||Very heavy|
HOW TO MODEL A BUILDING’S SPECTRAL ACCELERATION (SA)
In a simplified manner a building is represented by an inverted pendulum of a certain mass on a mass-less vertical rod that replicates the building’s natural period of vibration and the mechanical damping.
A very approximate rule for the natural spectral period Tb (seconds) of a building as a function of the number of stories n in the building is as follows: Tb (sec) = 0.1n.
For example, a two-story building tends to have a natural period of about 0.2 second (frequency of 5Hz), whereas a ten-story building tends to have a natural period near Tb=1 second (frequency of 1 Hz).
PGA is also used to understand more about the types of earthquake hazards that are likely. The U.S. Geological Survey (USGS), which studies seismic conditions nationally, produces maps that indicate where future earthquakes are most likely to occur, how frequently they might occur, and how hard the ground may shake (PGA).
These maps estimate the probability that ground shaking, or ground motion, will exceed a certain level in 50 years.
In comparison to the previous map, the latest USGS maps, released in July 2014, show that larger, more damaging East Coast earthquakes are more likely to occur in the NYC area. The USGS map here shows that New York City has a moderate seismic hazard.
Frequency of Damaging Earthquake Shaking in the United States
Source: USGS Earthquakes Hazard Program, 2014 Long-term Model
Strong earthquakes in New York City have not been registered, but moderate-magnitude earthquakes are possible. Even if an earthquake’s epicenter is far from New York City, the geology underlying the Northeast United States can cause some ground shaking to be felt right here.
When an earthquake occurs, the older, harder bedrock of the Northeast generates high-frequency motions that can travel long distances before they subside. For example, tremors from the 2011 earthquake in east-central Virginia and the 2013 earthquake along Canada’s Ottawa River were felt by many people in the eastern United States, including New York City. The 2011 Virginia earthquake, which had Moment Magnitude of 5.8, was felt more than 500 miles from its epicenter, making it the most-felt earthquake in modern U.S. history.
If an earthquake occurs in New York City, the unique geologic characteristics of the metropolitan area could result in significant effects due to soil amplification. The two main factors contributing to soil amplification here are the sharp contrast between softer soils and very hard bedrock, and the bedrock motions, which are expected to be relatively short and shake with high frequency.
High-frequency shaking is more common in the bedrock of Eastern United States and typically affects short, two- to five-story masonry buildings. A shallow layer of soft soil (less than 100 feet in depth) sits atop hard bedrock, so shaking is amplified but only for a relatively short period of time. By contrast, a high-rise building atop deep soil deposits will shake longer and shake more slowly during an earthquake.
Subsurface conditions in New York City, which vary widely across the five boroughs, can affect the degree to which an earthquake’s ground motion is amplified. As shown on this map, geologic conditions range from solid bedrock at ground surface (green) to artificial fill (blue).
New York and Eastern New Jersey Geological Map
Source: Mueser Rutledge Consulting Engineers
For centuries, large areas of the New York City have been filled to cover soft sediments and marshes to create new space for building development. For example, Manhattan’s present-day Chinatown is on land created by filling in a large pond; the World’s Fair park site in Flushing, Queens was built on an ash dump; and JFK Airport on Brooklyn’s south shore was built atop a hydraulic sand fill.
Map of Collect Pond – City of New York, 1783
Source: Local Geology of New York City and Its Effect on Seismic Ground Motions
Between 1700 and 1986, over 400 earthquakes with a magnitude of 2.0 and above have been recorded in New York State.
Between 1973 and 2012, New York State had only two damaging earthquakes with magnitude of 5.0 and above. Historically, larger earthquakes have a longer “return period” in New York City. That is, they happen much less frequently than smaller earthquakes.
On August 10, 1884, one of the strongest earthquakes happened near New York City somewhere between Brooklyn and Sandy Hook, New Jersey. Based on contemporary reports of its damage, scientists today estimate it to have been a magnitude 5.2 earthquake. Although considered moderate by today’s magnitude scales, the shaking from this earthquake event was felt from Virginia to Maine, damaging chimneys and brick buildings in New Jersey and New York City. Considering the amount of building and development along the Hudson and in New York City since 1884, if the same magnitude earthquake occurred today, the amount of damage to people and property would be far worse.
The historical earthquakes map shows the distribution of earthquake epicenters throughout the tri-state area from 1737 to 2014. Note that this map shows only approximate locations of epicenters for pre-1973 events; also, not every pre-1973 earthquake event is included on the map.
Historical Earthquake Epicenters in the New York City Metropolitan Area (1737 – 2018)
Source: USGS; NYS DHSES 2018
For more information on past earthquake events, use the Hazard History and Consequence Database, an interactive tool developed for this website.