Millikan Library is the tallest building on the Caltech campus. It is also the most sensitive.
The sleek, gray nine-story building is home to 13 pocket-sized seismometers and a rooftop “shaker” that sends vibrations through the structure. If something shakes Southern California, researchers are checking the frequency of the seismic waves that course through Millikan Library.
“What's interesting about Millikan from my perspective is it belongs to us at Caltech,” said professor Thomas Heaton, director of Caltech Earthquake Engineering Research Laboratory. “We can put instruments in it any time we want. We can shake it and because we own it, we use it as a laboratory.”
Monica Kohler, a Caltech senior research fellow, says the gadgets allow scientists to monitor seismic frequency changes in the building during a large earthquake, a signal that severe damage has occurred.
A small earthquake, such as the magnitude-4.5 temblor in Yorba Linda on Aug. 8, doesn't usually prompt a change in frequency. But Millikan still picked it up. “It was beautifully recorded on every floor where we had a sensor,” Kohler said.
As a small demonstration of the sensor system, last month engineering graduate student Ming Hei Cheng kicked a seismogram on the floor near a computer. Small waves appeared on the computer monitor.
During large earthquakes, lines called “picks” appear on the seismogram, and scientists use the data to determine the epicenter of the temblors. Thunderstorms and strong winds can also produce picks. In the fall of 2011, the hurricane-force windstorm that knocked down trees and left thousands of San Gabriel Valley residents in the dark set off several sensors.
“The system won't peg those events as earthquakes, because for earthquakes the time lines up pretty well,” Cheng said. “The earthquake starts, and the waves hit the building almost at the same time.”
The sensors are linked to Caltech's Community Seismic Network, which includes hundreds of residences in the Pasadena area and a few downtown Los Angeles high rises. Caltech is seeking residents to volunteer to host a device at home, though volunteers must agree to one important rule: They must leave their computers on at all times.
Caltech offers the $100 sensors for free in an effort to reach the goal of monitoring temblors block by block. The initiative could help emergency responders pinpoint affected areas and create more detailed shaking maps.
Kohler appreciates the size of the network, which includes sensors on three buildings on the USC campus. But she wants sensors in more tall structures in the region.
“I would love to get all the tall buildings,” she says. “I would love to instrument every floor of every building, if I could.”
Millikan's rooftop shaker plays a different role, measuring the changes to the 1966 building as time passes.
The machine is nearly as old as the building and consists of two rotating buckets that move weights back and forth. The more weight placed on the buckets, the bigger the force.
Cheng said researchers usually don't use the shaker when the building is busy, as it can generate motions similar to a swaying boat. He prefers to use the device late at night, allowing him to take in the Pasadena panorama from the roof.
“It's extremely useful,” says Kohler. “This shaker is what has allowed us to identify, in part, the permanent changes in the frequencies that have occurred in this building since it was constructed.”
City Editor Bill Kisliuk contributed to this story.