Boston: Another Bay
Area where quakes mean big trouble
BY JANET BASU
The Boston region is not
renowned as earthquake country. It's that other Bay Area,
on the West Coast, that rattles and rolls every few
months. But quakes do occur in Massachusetts, including
two in the 1700s with magnitudes greater than 6.0. In a
Dec. 8, report to the American Geophysical Union, a
Stanford graduate student says that when such a quake
strikes again, Bostonians have reason to worry about
their tallest buildings and some of their most treasured
neighborhoods.
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Information:
Eva Zanzerkia, now a
graduate student in geophysics at Stanford, found in
research done for her undergraduate honors thesis at
Harvard that downtown high-rises such as the Hancock
Tower and historic neighborhoods such as South Boston and
the Back Bay stand on land that is likely to amplify the
seismic waves from an earthquake.
Zanzerkia worked with her
adviser, Harvard Associate Professor Jeroen Tromp, a
seismologist, to develop a three-dimensional seismic
analysis of the soil types in the Boston Basin.
Surrounded on three sides by bedrock, the basin is an
area that seismologists would expect to trap and amplify
seismic waves, so they resonate back and forth like
echoes in a canyon. But when Zanzerkia analyzed rock
types in the basin she found some comfort for the
residents: "Most of the sedimentary rock has been
there so long and has gone through so much metamorphism,
it's almost as hard and strong as the bedrock," she
said. Seismic waves would not be slowed and trapped as
they passed from bedrock to the hard sediment.
However, Boston also is
built along river basins lined with sediment over glacial
till. And many occupied areas are built on man-made fill.
Zanzerkia mapped these softer areas, then used a computer
model developed by Tromp and former Harvard postdoctoral
fellow Zheng Wang, to analyze what would happen to
seismic waves from an earthquake, as the waves passed
from hard rock into the softer areas.
The model shows that no
matter where an earthquake originates in the region near
Boston, its seismic waves should travel quickly and
relatively non-destructively until they hit the small
basins of soft rock. There the waves are slowed, from 4
kilometers per second in bedrock to 2 kilometers per
second in glacial till, and to 1.5 kilometers per second
or slower in soft sediment and man-made fill.
The slowed waves become
trapped, and bounce back and forth. "When the earth
resonates in that way, the energy is amplified,"
Zanzerkia said. In glacial till, the waves are two to
three times larger than in hard rock; in man-made fill
they are two to three times bigger still. "It's much
more dangerous for buildings," Zanzerkia said.
"There's a greater intensity of shaking, and the
shaking lasts longer. And the ground can be subject to
liquefaction, a condition in which filled land mixes with
water and turns to a consistency like quicksand."
In addition, Zanzerkia and
Tromp found that the seismic waves resonate at
frequencies close to the natural frequency of tall and
mid-sized buildings. This finding could be of concern to
engineers, because it is thought that buildings collapse
more readily if they are shaken at their natural
frequencies.
To obtain "ground
truth" on their computer model, Zanzerkia and Tromp
took seismograms during a giant explosion, set off by
construction engineers to start the "Big Dig"
a construction project for a highway tunnel near
Boston. "We found that the resonance frequencies
from those seismic waves were very similar to those in
our models," Zanzerkia said.
Last August, Stanford
civil engineering doctoral student Rachel Davidson showed
with her new Earthquake Disaster Risk Index that
Bostonians face an overall risk of damage from
earthquakes similar to San Franciscans. While quakes are
less frequent in Boston, more buildings there were built
before the introduction of modern seismic safety codes in
1975. Zanzerkia's study indicates that the hazard from
quakes in Boston is more than a theoretical possibility.
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