Student data show unexpected picture of California earthquake


The six students who signed up for IAP course 111 knew they'd get hands-on experience in studying the surface of the Earth around earthquake faults, but none could have guessed that they would be joining the scientists who flocked to Los Angeles after the recent 6.6 temblor.

Or that measurements they would make would disprove a model of the quake distributed by other researchers.

Further, the MIT group's own model of the quake is contributing to scientists' understanding of the event. Among the group's preliminary findings: "it's fairly clear now that there were two fault planes," said Tom Herring, associate professor of geophysics and one of two faculty members who taught the course. In other words, he said, "the earthquake started in one direction then midway through changed direction down another fault plane."

For the last four years Professor Herring and Professor Brad Hager, both of the Department of Earth, Atmospheric and Planetary Sciences, have taught IAP 111. The course introduces students to the Global Positioning System (GPS), a tool that scientists can use to study movements of the Earth's crust around fault lines.

In GPS, radio signals from a network of satellites are picked up by portable antennas and receivers located at sites around the area of interest. By analyzing those signals, scientists can determine the position of each site with an accuracy of 2 millimeters. They can then track any movement of a given site by measuring its position over time. Cumulatively, the movements of sites around, say, an active fault reveal information about that fault and how the Earth's crust is responding to it.

In this year's IAP course, which ran from January 12-26, Professors Hager and Herring gave several days of lectures on GPS, then planned to take the class on a field trip to southern California (this was the first year that IAP 111 featured a trip to California; last year the class went to New York). Once in California they were going to use GPS to survey the area around the Lander's fault, which caused an earthquake in 1992.

Their plans changed. On Monday, January 17, the L.A. quake hit-four days before their intended departure to the Lander's area. The MIT group still flew out to southern California-complete with two bright yellow GPS receivers brought as carry-on luggage-but when the researchers arrived they headed instead for the Northridge area near the epicenter of the L.A. temblor.

The two professors knew that the group could make some valuable contributions to modeling the quake. Only last November, Principal Research Scientist Bob King, Professor Hager and a colleague at the Jet Propulsion Lab (JPL) had done a GPS survey of several sites in the area of the L.A. fault. As a result, by remeasuring the positions of those sites they knew they could learn a great deal about how the Earth's surface stretched, shrank, or rose in response to the quake.

Further, by monitoring each site even after they got there, they could catch any further movements of the Earth's crust.

"The Earth has mechanical properties like Silly Putty, so if you apply a stress to it, it will eventually relax," said Dr. Hager, who is the Cecil and Ida Green Professor of Earth Sciences.

The six students who went on the trip were freshman George Candea; Asantha Cooray, a sophomore in physics; Philip Tracadas, a junior in management; Elaine Gregory, a senior in physics; Amy Mattinson, a senior in mathematics with computer science, and Stephen Karner, a graduate student in EAPS.

Once in the area, the MIT researchers rented two vehicles (including a Jeep) and drove to six sites spread out over an area of about 1,500 square miles. At each they set up and monitored a GPS receiver and antenna.

The group logged data for three full days, then downloaded it onto diskettes and returned to Boston the Monday following the quake. Back at MIT the team began to analyze the data and quickly realized that a model of the quake that had just been published electronically by the United States Geological Survey was wrong. The USGS model couldn't account for the MIT observations. For example, said Professor Hager, "the movement we measured at one of the sites was a factor of 10 larger than what their model would have predicted."

The MIT group sent off their conclusions to the USGS but have not yet heard back from the agency. "It's kind of like David and Goliath-these MIT kids versus the USGS," said Professor Hager, who went on to explain that the USGS model was wrong because it was based on a limited set of data that only included GPS measurements from a small area around the quake. The MIT data represented a number of outlying areas.

The MIT group is continuing work on its own model of the quake based on the MIT information and data from some of the USGS receivers. They're also working to get the data from the remaining USGS receivers and from receivers run by JPL.

What were some of the most exciting parts of the trip? Aftershocks certainly made an impression. Mr. Cooray remembers sitting in a Jeep with Mr. Karner while Professor Hager placed a call from a phone booth, "and all the trees started to shake-you could hear the sounds of their leaves. Then everything started shaking, and I had to hold on to one of the seats." That particular aftershock was a magnitude 4.5 jolt, and was quickly followed by another that was 4.2, Mr. Cooray said.

Ms. Gregory commented that it was interesting to be able to compare media reports of the earthquake with personal observations. For example, she said, "I had no idea that only a relatively small section of L.A. was really damaged, and that the quake was very concentrated."

Above all, however, both Ms. Gregory and Mr. Cooray emphasized the experience of actually measuring how much the Earth moved. "The students got to see real-time science going on," Professor Hager said, "complete with the excitement of everybody running around making measurements."

IAP 111 was funded by the Department of Earth, Atmospheric, and Planetary Sciences.

A version of this
article appeared in the
February 9, 1994

issue of MIT Tech Talk (Volume
38, Number
22).


Topics: Earth and atmospheric sciences, Students

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