Eastern Quakes: Real Risk, Few Precautions
1989 San Francisco Earthquake
By WILLIAM K. STEVENS
Published: October 24, 1989
AN EARTHQUAKE as powerful
as the one that struck northern California last week could occur almost
anywhere along the East Coast, experts say. And if it did, it would
probably cause far more destruction than the West Coast quake.
The chances of such an occurrence are much less in the East than on
the West Coast. Geologic stresses in the East build up only a hundredth
to a thousandth as fast as in California, and this means that big
Eastern quakes are far less frequent. Scientists do not really know what
the interval between them might be, nor are the deeper-lying geologic
faults that cause them as accessible to study. So seismologists are at a
loss to predict when or where they will strike.
But they do know that a
temblor with a magnitude estimated at 7 on the Richter scale – about the
same magnitude as last week’s California quake – devastated Charleston,
S.C., in 1886. And after more than a decade of study, they also know
that geologic structures similar to those that caused the Charleston
quake exist all along the Eastern Seaboard.
For this reason, ”we can’t preclude that a Charleston-sized
earthquake might occur anywhere along the East Coast,” said David Russ,
the assistant chief geologist of the United States Geological Survey in
Reston, Va. ”It could occur in Washington.
It could occur in New York.”
If that happens, many experts agree, the impact will probably be much greater than in California.
Easterners, unlike Californians, have paid very little attention to
making buildings and other structures earthquake-proof or
earthquake-resistant. ”We don’t have that mentality here on the East
Coast,” said Robert Silman, a New York structural engineer whose firm
has worked on 3,800 buildings in the metropolitan area.
Moreover, buildings,
highways, bridges, water and sewer systems and communications networks
in the East are all older than in the West and consequently more
vulnerable to damage. Even under normal conditions, for instance, water mains routinely rupture in New York City.
The result, said Dr. John Ebel, a geophysicist who is the assistant
director of Boston College’s Weston Observatory, is that damage in the
East would probably be more widespread, more people could be hurt and
killed, depending on circumstances like time of day, and
”it would probably take a lot longer to get these cities back to useful operating levels.”
On top of this, scientists
say, an earthquake in the East can shake an area 100 times larger than a
quake of the same magnitude in California. This is because the earth’s
crust is older, colder and more brittle in the East and tends to
transmit seismic energy more efficiently. ”If you had a magnitude 7
earthquake and you put it halfway between New York City and Boston,” Dr.
Ebel said, ”you would have the potential of doing damage in both
places,” not to mention cities like Hartford and Providence.
Few studies have been done of Eastern cities’ vulnerability to
earthquakes. But one, published last June in The Annals of the New York
Academy of Sciences, calculated the effects on New York City of a
magnitude 6 earthquake. That is one-tenth the magnitude of last week’s
California quake, but about the same as the Whittier, Calif., quake two
years ago.
The study found that such an earthquake centered 17 miles southeast
of City Hall, off Rockaway Beach, would cause $11 billion in damage to
buildings and start 130 fires. By comparison, preliminary estimates
place the damage in last week’s California disaster at $4 billion to $10
billion. If the quake’s epicenter were 11 miles southeast of City Hall,
the study found, there would be about $18 billion in damage; if 5
miles, about $25 billion.
No estimates on injuries or loss of life were made.
But a magnitude 6 earthquake ”would probably be a disaster unparalleled in New York history,” wrote the authors of the study, Charles Scawthorn and Stephen K. Harris of EQE Engineering in San Francisco.
The study was financed by the National Center for Earthquake
Engineering Research at the State University of New York at Buffalo. The
research and education center, supported by the National Science
Foundation and New York State, was established in 1986 to help reduce
damage and loss of life from earthquakes.
The study’s postulated epicenter of 17 miles southeast of City Hall
was the location of the strongest quake to strike New York since it has
been settled, a magnitude 5 temblor on Aug. 10, 1884. That 1884 quake
rattled bottles and crockery in Manhattan and frightened New Yorkers,
but caused little damage. Seismologists say a quake of that order is
likely to occur within 50 miles of New York City every 300 years. Quakes
of magnitude 5 are not rare in the East. The major earthquake zone in
the eastern half of the country is the central Mississippi Valley, where
a huge underground rift causes frequent geologic dislocations and small
temblors. The most powerful quake ever known to strike the United
States occurred at New Madrid, Mo., in 1812. It was later estimated at
magnitude 8.7 and was one of three quakes to strike that area in
1811-12, all of them stronger than magnitude 8. They were felt as far
away as Washington, where they rattled chandeliers, Boston and Quebec.
Because the New Madrid rift is so active, it has been well studied,
and scientists have been able to come up with predictions for the
central Mississippi valley, which includes St. Louis and Memphis.
According to Dr. Russ, there is a 40 to 63 percent chance that a quake
of magnitude 6 will strike that area between now and the year 2000, and
an 86 to 97 percent chance that it will do so by 2035. The Federal
geologists say there is a 1 percent chance or less of a quake greater
than magnitude 7 by 2000, and a 4 percent chance or less by 2035.
Elsewhere in the East, scientists are limited in their knowledge of
probabilities partly because faults that could cause big earthquakes are
buried deeper in the earth’s crust. In contrast to California, where
the boundary between two major tectonic plates creates the San Andreas
and related faults,
the
eastern United States lies in the middle of a major tectonic plate. Its
faults are far less obvious, their activity far more subtle, and their
slippage far slower.
Any large earthquake would be ”vastly more serious” in the older cities of the East than in California,
said Dr. Tsu T. Soong, a professor of civil engineering at the State
University of New York at Buffalo who is a researcher in
earthquake-mitigation technology at the National Center for Earthquake
Engineering Research.
First,
he said, many buildings are simply older, and therefore weaker and more
vulnerable to collapse. Second, there is no seismic construction code in
most of the East as there is in California, where such codes have been
in place for decades.
The vulnerability is evident in many ways. ”I’m sitting here looking
out my window,” said Mr. Silman, the structural engineer in New York,
”and I see a bunch of water tanks all over the place” on rooftops. ”They
are not anchored down at all, and it’s very possible they would fall in
an earthquake.”
Many
brownstones, he said, constructed as they are of unreinforced masonry
walls with wood joists between, ”would just go like a house of cards.” Unreinforced
masonry, in fact, is the single most vulnerable structure, engineers
say. Such buildings are abundant, even predominant, in many older
cities.
The Scawthorn-Harris
study reviewed inventories of all buildings in Manhattan as of 1972 and
found that 28,884, or more than half, were built of unreinforced
masonry. Of those, 23,064 were three to five stories high.
Buildings of reinforced masonry, reinforced concrete and steel would
hold up much better, engineers say, and wooden structures are considered
intrinsically tough in ordinary circumstances. The best performers,
they say, would probably be skyscrapers built in the last 20 years. As
Mr. Silman explained, they have been built to withstand high winds, and
the same structural features that enable them to do so also help them
resist an earthquake’s force. But even these new towers have not been
provided with the seismic protections required in California and so are
more vulnerable than similar structures on the West Coast.
Buildings in New York are not generally constructed with such seismic
protections as base-isolated structures, in which the building is
allowed to shift with the ground movement; or with flexible frames that
absorb and distribute energy through columns and beams so that floors
can flex from side to side, or with reinforced frames that help resist
distortion.
”If you’re trying to make a building ductile – able to absorb energy – we’re not geared to think that way,” said Mr. Silman.
New York buildings also
contain a lot of decorative stonework, which can be dislodged and turned
into lethal missiles by an earthquake. In California, building codes strictly regulate such architectural details.
Manhattan does, however, have at least one mitigating factor: ”We are
blessed with this bedrock island,” said Mr. Silman. ”That should work
to our benefit; we don’t have shifting soils. But there are plenty of
places that are problem areas, particularly the shoreline areas,” where
landfills make the ground soft and unstable.
As scientists have learned more about geologic faults in the
Northeast, the nation’s uniform building code – the basic, minimum code
followed throughout the country – has been revised accordingly. Until
recently, the code required newly constructed buildings in New York City
to withstand at least 19 percent of the side-to-side seismic force that
a comparable building in the seismically active areas of California
must handle. Now the threshold has been raised to 25 percent.
New York City, for the first time, is moving to adopt seismic
standards as part of its own building code. Local and state building
codes can and do go beyond the national code. Charles M. Smith Jr., the
city Building Commissioner, last spring formed a committee of
scientists, engineers, architects and government officials to recommend
the changes.
”They all agree that New York City should anticipate an earthquake,”
Mr. Smith said. As to how big an earthquake, ”I don’t think anybody
would bet on a magnitude greater than 6.5,” he said. ”I don’t know,” he
added, ”that our committee will go so far as to acknowledge” the damage
levels in the Scawthorn-Harris study, characterizing it as ”not without
controversy.”
For the most part, neither New York nor any other Eastern city has
done a detailed survey of just how individual buildings and other
structures would be affected, and how or whether to modify them.
”The thing I think is needed in the East is a program to investigate
all the bridges” to see how they would stand up to various magnitudes of
earthquake,” said Bill Geyer, the executive vice president of the New
York engineering firm of Steinman, Boynton, Gronquist and Birdsall,
which is rehabilitating the cable on the Williamsburg Bridge. ”No one
has gone through and done any analysis of the existing bridges.”
In general, he said, the large suspension bridges, by their nature,
”are not susceptible to the magnitude of earthquake you’d expect in the
East.” But the approaches and side spans of some of them might be, he
said, and only a bridge-by-bridge analysis would tell. Nor, experts say,
are some elevated highways in New York designed with the flexibility
and ability to accommodate motion that would enable them to withstand a
big temblor.
Tunnels Vulnerable
The underground tunnels
that carry travelers under the rivers into Manhattan, those that contain
the subways and those that carry water, sewers and natural gas would
all be vulnerable to rupture, engineers say. The Lincoln, Holland,
PATH and Amtrak tunnels, for instance, go from bedrock in Manhattan to
soft soil under the Hudson River to bedrock again in New Jersey, said
Mark Carter, a partner in Raamot Associates, geotechnical engineers
specializing in soils and foundations.
Likewise, he said, subway tunnels between Manhattan and Queens go
from hard rock to soft soil to hard rock on Roosevelt Island, to soft
soil again and back to rock. The boundaries between soft soil and rock
are points of weakness, he said.
”These structures are old,” he said, ”and as far as I know they have not been designed for earthquake loadings.”
Even if it is possible to survey all major buildings and facilities
to determine what corrections can be made, cities like New York would
then face a major decision: Is it worth spending the money to modify
buildings and other structures to cope with a quake that might or might
not come in 100, or 200 300 years or more?
”That is a classical problem” in risk-benefit analysis, said Dr.
George Lee, the acting director of the Earthquake Engineering Research
Center in Buffalo. As more is learned about Eastern earthquakes, he
said, it should become ”possible to talk about decision-making.” But for
now, he said, ”I think it’s premature for us to consider that
question.”
