 Seismogram of the magnitude 3.5 Hardwick, MA earthquake of October 2, 1994, recorded at Weston Observatory, Boston College |
Revised: January 3, 2004
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Seismogram of the magnitude 3.5 Hardwick, MA earthquake of October 2, 1994, recorded at Weston Observatory, Boston College |
In the early morning of October 2, 1994, residents of central Massachusetts were awakened by ground vibrations that appeared to come from beneath their homes. At first many thought it was their furnace malfunctioning or a large truck passing by, but within a few minutes it became clear that it had been an earthquake.Yes, it's true. Earthquakes 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 when they hear the word "earthquake." While Californians have learned to expect earthquakes, residents of New England 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, earthquake activity is quite common in many areas of the eastern United States, including New England.On December 20, 1940, residents of Springfield, MA felt vibrations from an earthquake centered near Ossipee, NH. The Springfield Republican newspaper reported that "dishes toppled from the shelves in many kitchens and wall pictures swung pendulum fashion on their fastenings."
On November 20, 1755, the Boston Weekly News-Letter reported: "Last Tuesday Morning about half an Hour past Four o'Clock, the Weather being serene, the Air clear, the Moon shining very bright, we were surpriz'd with 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 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.
Earthquakes in the northeastern United States. The data for 1924-1974 is from the U.S. Geological Survey, and the data for 1975-1999 is from the Weston Observatory archives of earthquakes recorded by the Northeastern United States Seismic Network.
There is a general correlation between the distribution of earthquake epicenters determined from the 1975-1999 network data and that of the epicenters of earthquakes that occurred between 1924 and 1974. Nonetheless, during the more recent decades the pattern of seismicity has changed, to some extent, from that of the earlier part of the 20th century. In both the 1924-1974 and the 1975-1999 records, clusters of seismic activity occur in northern New York State, and extend from there along a northwestern trend into Canada. However, the activity in western New York State, seen in the 1924-1974 map, is not observed in the more recent map. During both periods of time there was earthquake activity in New Jersey, eastern Pennsylvania, and the greater New York City area, as well as along coastal New England. Also, in both maps, we see intense activity in the vicinity of the St. Lawrence River. In addition to these clusters of activity, diffuse seismicity has occurred throughout most of the Northeast during the 20th century; however, very few epicenters are located in central New York State or in western Pennsylvania. An interesting feature of the pattern of earthquake activity in the Northeast is that between 1975 and 1999, some relatively large earthquakes occurred in areas that were not particularly active in the earlier part of the century.
The occurrence of earthquakes in the northeastern United States apparently violates the plate tectonic model. The past several decades of research on this topic, however, have demonstrated that it may be possible to explain the occurrence of earthquakes in the Northeast within the framework of plate tectonics. The challenge in figuring out why the Earth quakes in New England is that the earthquake process in plate interiors is 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-1970's 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 1980's and 1990's.
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 United States 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 to affect 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.
Perhaps the most successful attempt to explain the occurrence of intraplate earthquakes on a global scale was a study by Arch C. Johnston of the University of Memphis. Johnston's research suggests that "on a global scale, Mesozoic rifts and continental margins are of premier importance as crustal features that together account for nearly three-fourths of all [stable continental interior] seismicity."
The past few decades of research on the relationship between earthquakes and geological features in southern New England and the adjacent greater New York City area illustrate the kinds of difficulties and frustrations that seismologists in this area face. I have investigated the extent to which the concept of reactivation of Mesozoic rifts can be applied on a smaller scale to the two prominent Mesozoic basins in the Northeast, the Newark and Hartford rift basins. There is a concentration of earthquakes in the area surrounding the Newark basin, which would be expected if Mesozoic basins are the active features in this intraplate environment. It is less obvious, however, whether such a concentration of earthquakes exists in the area surrounding the Hartford basin. Such an ambiguous relationship between geological features and earthquakes is typical of seismological studies in intraplate areas.
Regardless of how optimistic or pessimistic a view one takes, it is clear that, 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 unexpected 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, our knowledge of the earthquake process in New England does make it possible to estimate the odds of a damaging earthquake striking your town.
- Larry Gonick and Woollcott Smith, The Cartoon Guide to Statistics
Based on the type of research described in this document and other seismological input, the U.S. Geological Survey (USGS) has produced a series of earthquake hazard maps for the United States. These 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, many earthquake hazard maps show contours of the percentage of the force of gravity that has a 10% 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 "5% g", then there would be 1 chance in 10 that (at some point during those 50 years) an earthquake would shake your house at a level of at least 5% of the force of gravity.
A 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 features 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.
National Seismic Hazard Map developed by the United States Geological Survey in 1996. This map shows contours of the percentage of the force of gravity that is predicted to have a 10% chance of being exceeded in 50 years.
To appreciate why these values of ground shaking are expressed as a
percentage of the force of gravity, note that it requires more than
100% of the force of gravity to throw objects up in the air. In terms
of felt effects and damage, ground motion at the level of several
percent of gravity corresponds to the threshold of damage to buildings
and houses (an earthquake intensity of approximately V). For
comparison, reports of "dishes, windows and doors disturbed"
corresponds to an intensity of about IV, or about 2% of gravity.
Reports of "some chimneys broken" corresponds to an intensity of about
VII, or about 10% to 20% of gravity.
Although 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 1 chance in 10 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.
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