Breaking rules (and rocks) to understand earthquakes
In order to create a realistic earthquake in a laboratory in Cornell’s School of Civil and Environmental Engineering, assistant professor Greg McLaskey has to build an apparatus that does not yet exist. McLaskey is assembling an instrument that, when complete, will be able to apply seven millions pounds of force to a slab of granite three meters long. His machine, located in the Bovay Lab in Thurston Hall, will simulate sequences of earthquakes, including foreshocks and aftershocks.
“Man-made earthquakes happen all the time,” says McLaskey. “When a dam is built and a reservoir fills, that added weight often causes small earthquakes. When fracking waste fluids are pumped deep in the ground for sequestration, that has been shown to cause earthquakes. The earthquakes I am creating are limited to my one slab of granite, but they can tell me about what goes on when ‘real’ earthquakes happen deep in the ground.”
McLaskey’s apparatus will add a new tool to the Bovay Lab’s state-of-the-art earthquake simulation equipment. The lab already has a custom-fabricated "strong wall" for large-scale civil infrastructure research, special hydraulic systems for large displacement testing, several specialized load frames, and a variety of data acquisition and electronic control systems.
Humans have been measuring the waves produced by earthquakes ever since the first seismometer was invented in 1841. Most earthquakes happen between seven and fifteen kilometers below the surface of the Earth. There are no instruments that far below ground. Due to the depth and inaccessibility of most earthquakes, human are forced to view them indirectly. We see the results on the surface and we measure the strength, duration, and speed of travel of the vibrations set off by the quake. But for all of this observation, says McLaskey, “we still don’t know what an earthquake is, mechanically. They initiate in a fraction of a second and last a very short time.”
McLaskey’s research is an excellent example of a scientist taking what has been learned in the field and bringing it into the lab so that he can then make more sense of what is happening out in the world. “Seismologists can measure seismic waves at stations all around the globe,” says McLaskey, “but they have never been able to measure the amount of slip that happens in real time during an actual earthquake.” When McLaskey sets his 3-meter slab of granite in his new device, he will be able to measure both the seismic waves passing through the rock and the amount of slippage that happens along the fault. “This is something that has never really been done before and it has the potential to tell us a lot.”
One reason that creating small-scale earthquakes in the lab has the potential to tell us a lot is that there are scaling relationships in earthquakes. “If you can figure out what is happening in a small earthquake,” says McLaskey, “then you can figure out what is happening in a big earthquake as well.” McLaskey will attach sensors to his slab to measure motion and he will also use optical sensors to measure slip—or the distance the two faces of a fault have moved relative to each other.
After describing his work, McLaskey looks thoughtful for a long moment and adds, “Really, the two things I study are sound and friction. Sound is vibrations, and a seismometer measures the strength and duration of vibrations passing through the Earth as a result of an earthquake. And friction—friction is really very poorly understood, but it is what underlies earthquakes.”
When asked if his ultimate goal is to predict earthquakes, McLaskey looks a bit dubious and says carefully, “We don’t really talk about ‘predicting’ earthquakes any more. Instead, we like to talk about being better able to ‘forecast’ the possibility of an earthquake.” More than predicting or forecasting earthquakes, it is obvious after listening to Greg McLaskey that his real goal is simply to understand earthquakes better. Or, maybe even more basically than that, to start to gain an understanding of friction and its vital role in the physical sphere we inhabit.