Shaking Things Up
Earthquake engineering is a discipline that uses a wide range of techniques: There's forensics, for diagnosing collapsed structures. There's 3D dynamic computer simulations, to test building designs in silico. And there's the mechanical joy of giving things a good hard shake. The last part, clearly, is the sugar that draws the news flies. It certainly brings out the little boy in me.
There are all kinds of ways to subject things to seismic-style shaking. At a scientific meeting not long ago I watched a contest that took student-designed model buildings and put them on a shake table the size of a large microwave oven. A shake table is outfitted with actuators—pistons pushing in all directions—that "play" a seismogram, the record of an actual earthquake. The students and judges were serious, but somehow gleeful too, as the models began to shed pieces onto the floor.
At the same meeting we could ride in a much larger apparatus as it played the 1989 Loma Prieta earthquake for us. As a veteran of that quake, I found this an uncanny experience but still couldn't keep a smile off my face.
The Pacific Earthquake Engineering Research Center, in Richmond, is a leading institute for this stuff. (If you can get a tour, don't miss PEER's 4 Million Pound Universal Testing Machine, a steel behemoth built in 1932, and the boneyard of broken stuff out back.) It has the biggest shake table in the Bay Area, 20 feet square. That's big enough to subject a full-sized cottage or model house to a realistic earthquake experience.
The University of Buffalo used two of these at once to test a full-sized two-story townhouse in 2006 at its Structural Engineering and Earthquake Simulation Lab. (Both PEER and SEESL share resources as part of the nationwide George Brown Network for Earthquake Engineering Simulation or NEES.) In that experiment, the building danced to the tune of the 1994 Northridge earthquake. . . a break dance, you might say. Videos from the project are uncanny, period.
But when something is too big to put on a shake table—like the Earth itself—we have to use a different approach. There's the equivalent of a submarine's sonar or the doctor's tap on your chest (a technique called auscultation, you should know) called active-source seismology. This has a long history and is best developed by oil companies and geotechnical consultants. The actuator that sends out the seismic signal can be as small as a sledgehammer blow or as large as blowing up a ton of dynamite, but I think that a fleet of Vibroseis trucks, pushing their thick steel baseplates against the ground in unison, may be the most impressive.
This week a California research project went to the Sacramento Delta, a water source for some 23 million people among other things, and did some shaking experiments to help get a handle on what a Big One might do there. The news people made a point of getting out to Sherman Island, because what could be cooler? There was a big shiny machine with whirling weights, used to shake nuclear plants, mounted on a segment of simulated levee on top of pure Delta peat. It was easy to visualize mayhem.
The test was not a realistic one: the actuator didn't play a seismogram, just a straight eyeball-rattling vibration. The model levee wasn't twice as tall, a hundred years old, or holding back 20 feet of water like the real levees. The point was to do a basic test of the underlying peat soil—something simple, fundamental and not too dangerous. It was just the start of the thorough science we need, but the experiment made great video. Three newspapers gave it coverage and included footage; links below. In its own way it was as cool as Burning Man.
- San Jose Mercury News, "Earthquake simulator gives model levee a big shake"
- Sacramento Bee, "UCLA researchers shake model levee, for peat's sake"
- Stockton Record, "Ground rumbles for sake of research"