Is Boston overdue?
By Brian Fitzgerald
“I feel the earth move under my feet,” Carole King sang in 1971. Rachel Abercrombie felt that way on the morning of April 20, 2002, when her home in Belmont, Mass., started shaking.
Actually, at 6:50 a.m., when the BU earthquake expert felt the 15-second-long tremor, her exact thoughts were: “Hmm. That lasted too long to be just a big truck going by. It must have been an earthquake.”
Abercrombie, a CAS associate professor of earth sciences, was right. Media reports began filtering in about an earthquake in upstate New York measuring 5.1 on the Richter scale, with the epicenter just west of Lake Champlain. The rumble, which damaged roads, a bridge, and water mains in New York’s Clinton County, was felt across New England and as far away as Baltimore and Toronto.
Little did Abercrombie know, however, that ripples from that earthquake in Au Sable Forks, N.Y., would have a seismic effect on her research. Now she is conducting two federally funded studies on the 2002 quake. One, in conjunction with Columbia University, compares the source processes of the earthquake — along with other recorded earthquakes in the Northeast — to those of earthquakes of a similar magnitude in other tectonic settings with the goal of determining whether they behave in the same way. The other project, in collaboration with Tufts University, looks at the wave propagation, or ground motions, produced by the Au Sable quake. Both studies will lead to a better understanding of the physics of earthquakes, as well as improved estimates of the seismic hazard in Boston and surrounding urban areas.
Helping to earthquake-proof
They could also help determine how best to earthquake-proof existing buildings. Massachusetts is one of the few states in the East whose building code contains a seismic provision. Instated in 1975, it has ensured that all buildings constructed since then can withstand a moderate-sized earthquake. However, Boston and other areas in the state have many unreinforced buildings from the 19th and 20th centuries that were not built to withstand shaking. And a number of structures in Boston are built upon large areas of landfill, which amplifies the effects of the tremors.
Scientists have never been able to predict a major earthquake, but real-time earthquake “alarms” are being developed that might be able to help people take evasive action when there is a seismic event. “Conceivably this could lead to, say, an early warning to some buildings seconds before the quake hits,” Abercrombie says. “Some kind of electronic or radio signal could be sent to the building, and the gas could be shut off. Fires after earthquakes are a huge problem.”
However, the U.S. Geological Survey, which is funding Abercrombie’s studies, is focusing on long-term mitigation of earthquake hazards rather than aiming for short-term predictions.
But New England doesn’t have serious earthquakes, does it? “Not compared to California,” says Abercrombie. “The rock here is much more stable.” While California earthquakes occur along major faults, those in New England do not. New England is near the center of the North American tectonic plate, and earthquakes here are “internal plate quakes” produced by stress that builds up under the continent, rather than near a major boundary separating two of Earth’s tectonic plates, as is the case in California.
Nonetheless, the region has had its share of temblors, and still does, with about 40 micro-earthquakes (less than 3.0 on the Richter scale) annually. “Before seismometers were introduced in the early 1900s, scientists had to rely on people’s descriptions of the damage to determine an earthquake’s magnitude,” says Abercrombie. The earliest accounts in the nation are of New England’s earthquakes. When European settlers first arrived, Native Americans told them about an earthquake in 1568 in Moodus, Conn. In 1638, William Bradford, governor of the Plimoth Colony, wrote that an earthquake “came with a rumbling noise, or low murmur like unto remote thunder. As the noise approached nearer, the earth began to shake and came at length with that violence as caused platters and dishes, and such like things as stood upon shelves, to clatter and fall down; yea, persons were afraid of the houses themselves.”
Although seismologists consider New England to be a moderate earthquake risk zone, earthquakes have wreaked havoc here. In 1755 there was a magnitude 6.2 earthquake on Cape Ann, “the largest earthquake in recorded history in New England,” says Abercrombie. It knocked down or damaged as many as 1,600 chimneys, collapsed brick walls of several buildings, and toppled stone fences throughout the countryside, especially on a line extending from Boston to Montreal. Scientists say that today an earthquake of similar magnitude could kill hundreds of people, injure thousands, and result in as much as $6 billion in property damage.
Fortunately, the earthquake in Au Sable Forks wasn’t near a densely populated area. “I got excited about the Au Sable earthquake because it was the largest to occur in the Northeast since the installation of current broadband networks,” says Abercrombie. “There were some good recordings, and the aftershocks were also well recorded. Hopefully, from these studies we will gain more of an understanding of how waves propagate from the earthquake’s source in New England.”
England to New England
Abercrombie first became interested in earthquakes as an undergraduate at Cambridge University in England, where she specialized in earth sciences. She received her master’s and Ph.D. in seismology at the University of Reading, and worked as an associate research scientist at the University of Southern California. After working as a scientist at the Institute of Geological and Nuclear Sciences in Wellington, New Zealand, she was a research associate at Harvard University. She came to BU in 2001. “There is seismic activity in New England, but at least I can put bottles and knick-knacks on my shelves without fear,” she says with a laugh. “I couldn’t do that in California or New Zealand.”
Is New England due for a strong (intensity 6.0-6.9) or moderate (5.0-5.9) earthquake? Abercrombie says no and yes, respectively: seismologists have determined that earthquakes with a magnitude of six or greater occur in New England on average once every 450 years. Since the last one was in 1755, we may not have one for several centuries. But a magnitude five quake hits the region every 50 or 60 years, and there was such a temblor in 1940 near Ossipee, N.H. So there is a 19 to 28 percent likelihood in New England by 2013, Abercrombie says, and a 41 to 56 percent likelihood by 2043.
Still, it’s impossible to predict the time and location of future earthquakes, in New England or anywhere. “What we are trying to do is better understand the physics of an earthquake’s rupture process,” she says, “and what damage one might do if it hit near Boston.”