In the evening of October 16, 2012 residents of Hollis Center, Maine felt the ground shake from a magnitude 4.0 earthquake centered in southern Maine. This earthquake was widely felt across New England and into adjacent New York, New Jersey and Canada. Near the epicenter people reported that their whole house shook and that it felt like a train going right through their house.Yes, it's true. Earthquakes really do occur on a regular basis in New England. Notable examples of earthquakes that caused damage in New England and adjacent areas are: the earthquake off the coast of Cape Ann, MA in 1755; two earthquakes near Ossipee, NH in 1940; and an earthquake near New York City in 1884. In spite of this regular occurrence of earthquakes in the northeastern United States, most people living in New England probably think of places like California or Japan, rather than New England, when they hear the word "earthquake." While Californians have learned to expect earthquakes, residents of New England more likely consider the ground beneath their feet to be "solid as a rock." Nonetheless, the record of earthquake activity in the United States shows that, while the highest level of activity is, of course, in the western part of the country, earthquakes are quite common in many areas of the eastern United States, including New England.
On the morning of December 20, 1940, the Earth suddenly quaked near Lake Ossipee, New Hampshire. An earthquake with a magnitude of about 5.5 had occurred. Damage was reported in a broad region across New England, including chimneys thrown down, cracked plaster, broken dishes, and overturned objects.
On November 20, 1755, the Boston Weekly News-Letter reported "a most terrible Shock of an Earthquake: The conditions were so extreme as to wreck the Houses in this Town to such a degree that the Tops of many Chimnies, and some of them quite down to the Roofs, were thrown down. . . ." This earthquake, with a magnitude of about 6.0, was centered off the coast of Cape Ann, MA.
Why then do earthquakes occur in New England? What causes earthquakes in major seismic zones like California and Japan? Why are earthquakes more common in California than in the northeastern United States, and how does earthquake activity in California differ from that in the Northeast?
Of course the real world is more complicated than this model suggests. While the two largest historically-documented earthquakes to occur in California (1906, San Francisco and 1857, Fort Tejon) did rupture a plate boundary along the well-known San Andreas fault, when looked at in detail, earthquakes in California are rather diffusely distributed in the vicinity of the plate boundary. Furthermore, earthquakes are scattered throughout the interior of the North American plate (as well as throughout the interiors of all plates).
With this perspective on plate boundary earthquakes, let us look at earthquakes in the northeastern United States.
in New England and surrounding areas. Left:
June 1638 to December 1974.
Right: January 1975 to February
2014. From the Weston Observatory archives
of historical and instrumentally recorded
a general correlation between the distribution of
earthquake epicenters determined from the 1975-2014
network data and that of the epicenters of earthquakes
that occurred between 1638 and 1974. In both the 1638-1974
and the 1975-2014 records, clusters of seismic activity
occur in northern New York State, and extend from there
along an approximately northwestern trend into Canada.
During both periods of time there was earthquake activity
in New Jersey and the greater New York City area, as well
as along coastal New England and in most of Maine and New
Hampshire. Also, we see intense activity in the vicinity
of the St. Lawrence River in both maps. In addition to
these clusters of activity, diffuse seismicity has
occurred throughout most of the region during both periods
The occurrence of earthquakes in the northeastern United States apparently violates the plate tectonic model, and a major research challenge is how to explain earthquakes in the Northeast within the framework of plate tectonics. What stymies seismologists' attempts to figure out why the Earth quakes in New England is that the earthquake process in plate interiors seems to be more complex than at plate boundaries. Unlike the situation in California, there is no obvious relationship between earthquakes and geologically mapped faults in most intraplate areas.
With plate tectonics under their belt and a seemingly endless supply of research funds flowing in, a number of geologists and geophysicists in the mid-1970s turned their attention to a (presumably) straightforward problem: What causes intraplate earthquakes? These were heady times for seismologists, and some were quite confident that the problem would be easily resolved. Hypotheses abounded. With a minimum of data available, it sometimes seemed like there was more fanciful hypothesis-generating than hypothesis testing. Two well-known hypotheses to explain earthquakes in the Northeast were the supposed existence of a "Boston-Ottawa seismic zone" and the presumed activity of the Ramapo fault (an ancient geological fault in northern New Jersey and southeastern New York State). In spite of many such attempts to explain the cause of earthquakes in the Northeast, however, none of these specific hypotheses have fared very well in light of new data gathered in the past few decades.
At the present time, a commonly accepted explanation for the cause of earthquakes in the Northeast is that "ancient zones of weakness" are being reactivated in the present-day stress field. In this model, preexisting faults and/or other geological features formed during ancient geological episodes persist in the intraplate crust, and, by way of analogy with plate boundary seismicity, earthquakes occur when the present-day stress is released along these zones of weakness. Using this model as a guide, much of the research on northeastern US earthquakes has involved attempts to identify preexisting faults and other geological features that might be reactivated by the present-day stress field. While this concept of reactivation of old zones of weakness is commonly assumed to be valid, in reality the identification of individual active geologic features has proven to be quite difficult. Unlike the situation for many plate boundary earthquakes, it is not at all clear whether faults mapped at the Earth's surface in the Northeast are the same faults along which the earthquakes are occurring.
During the past half billion years, the Earth's crust underlying New England has been the site of two major geological episodes, each of which has left its imprint on the New England bedrock. Between about 450 million years ago and about 250 million years ago, this area was the site of a "continental collision," in which the ancient African continent collided with the ancient North American continent to form the supercontinent known as Pangaea. Beginning about 200 million years ago, the present-day Atlantic ocean began to form as plate tectonic forces rifted the continent of Pangaea. The last major episode of geological activity affecting the bedrock in New England occurred about 100 million years ago, during the Mesozoic era, when "continental rifting" led to the opening of the present-day Atlantic ocean.
At least for the time being, the future time and place of any particular earthquake is hard to pin down. We can of course say with certainty that future earthquakes will occur in New England. But as for when and where . . . the scientific study of these New England earthquakes has a long way to go.
"We muddle through life making choices based on incomplete information..."Life is uncertain. No matter what we do, there will always be risks that we accept as part of our daily lives. No matter how advanced our scientific knowledge of the Earth becomes, it is unlikely that there will ever come a time when all natural disasters are predictable. Nonetheless, although we can't predict when specific earthquakes will strike, our knowledge of the earthquake process in New England does make it possible to estimate the odds of damaging earthquake vibrations striking your town.
- Larry Gonick and Woollcott Smith, The Cartoon Guide to Statistics
Recognizing that there are many unknowns and uncertainties regarding the cause of earthquakes and where they are likely to strike, the U.S. Geological Survey (USGS) has produced a series of National Seismic Hazard Maps based on their assessment of the current state of knowledge regarding earthquake activity across the United States. These maps were developed in the hope of providing the most accurate information possible regarding ground shaking hazard, and are updated as the knowledge base increases and new scientific findings on earthquake processes become available. The maps show the amount of earthquake generated ground shaking that, over a specified period of time, is predicted to have a specified chance of being exceeded. Ground shaking caused by earthquakes is often expressed as a percentage of the force of gravity. For example, the earthquake hazard map shown below gives contours of the percentage of the force of gravity that has a 2% chance of being exceeded in 50 years. Based on such a map, if you were living in the same house for 50 years, and that house was in a zone labeled "0.10 g", then there would be 1 chance in 50 (i.e., 2% chance) that (at some point during those 50 years) an earthquake would shake your house at a level of at least 10% of the force of gravity.
recent version of the National Seismic Hazard Maps
developed by the USGS is shown below. Because of the
difficulty of identifying specific seismically active
geological faults in the Northeast, the level of seismic
hazard in the northeastern part of the USGS map is
based primarily on the past record of seismic activity.
Earlier versions of the National Seismic Hazard Maps
placed more emphasis on the locations of geologically
mapped faults (and other types of geological features) in
the Northeast. These maps must be understood as a work in
National Seismic Hazard Map developed by the U.S. Geological Survey in 2008. This map shows contours of the percentage of the force of gravity that is predicted to have a 2% chance of being exceeded in 50 years.
seismologists have not yet found an unequivocal answer to the question,
"Why Does the Earth Quake in New England?", earthquake
hazard maps generally show that in most parts of New
England, there is about a 2% chance that in any given 50
year period of time, earthquake vibrations that are
potentially damaging will strike. This type of information
can be used as input for the design and construction of
buildings and for public policy decisions requiring an
assessment of earthquake risk. The better we can
understand the cause of earthquakes in the northeastern
United States and the nature of ground motion generated by
those earthquakes, the better we will be able to provide
the necessary information to estimate (and plan for) the
earthquake risk in New England.
Bolt, B.A., Earthquakes and Geological Discovery, Scientific American Library, 1993.
Ebel, J.E., and A.L. Kafka, Earthquake Activity in the Northeastern United States, in Neotectonics of North America, D.B. Slemmons, E.R. Engdahl, M.D. Zoback, and D.D. Blackwell, eds., Geological Society of America, 1991.
Johnston, A.C., and L.R. Kanter, Earthquakes in Stable Continental Crust, Scientific American, March 1990.
Kafka, A.L., Musings on Earthquakes and Related Matters, akafka.wordpress.com, 2014.
Kafka, A.L., "Cellular Seismology": Does Past Seismicity Delineate Zones Where Future Earthquakes are Likely to Occur, Musings on Earthquakes and Related Matters, akafka.wordpress.com, August, 2012.
Kafka, A.L., Faults and Earthquakes in the Greater New York City Area: Reflections at the Intersection of Science, the Media, and the Public, Musings on Earthquakes and Related Matters, akafka.wordpress.com, September, 2008.
Seismicity Delineate Zones Where Future Large Earthquakes will
Occur in Intraplate Environments?, In Continental Intraplate
Earthquakes: Science, Hazard, and Policy Issues,
Geological Society of
America Special Paper 425, Edited by S. Stein and
S. Mazzotti, 2007.
Public Misconceptions About Faults and
Earthquakes in the Eastern United States: Is it our own
Research Letters, 71(3), 311-312, 2000.
Kafka, A.L. and Walcott, J.R., How Well Does the Spatial Distribution of Smaller Earthquakes Forecast the Locations of Larger Earthquakes in the Northeastern United States?, Seismological Research Letters, 69(5), 428-439, 1998.
Kafka, A.L. and P.E. Miller, Seismicity in the Area Surrounding two Mesozoic Rift Basins in the Northeastern United States, Seismological Research Letters, 67(3), 69-86, 1996.
Kafka, A.L., M.A. Winslow, and N.L. Barstow, Earthquake Activity in the Greater New York City Area: A Faultfinder's Guide, in Field Trip Guidebook, 61st Annual Meeting, N.Y. State Geological Association, D. Weiss, ed., p. 177-205, 1989.
Kafka, A.L., E.A. Schlesinger-Miller and N.L. Barstow, Earthquake Activity in the Greater New York City Area: Magnitudes, Seismicity and Geologic Structures, Bulletin of the Seismological Society of America, 75(5), 1285-1300, 1985.
Raymo, C. and M.E. Raymo, Written in Stone: A Geological and Natural History of the Northeastern United States, The Globe Pequot Press, 1989.
Reiter, L., Earthquake Hazard Analysis: Issues and Insights, Columbia University Press, 1994.
and S. LeVay, The Earth in Turmoil: Earthquakes,
Volcanoes, and Their Impact on Humankind, W.H. Freeman and Company, 1998.