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.
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. SR