Not all earthquakes are created equal, according to new research that has brought earthquakes into the lab. For the first time, researchers physically observed two types of earthquake growth patterns, which until now, were limited to the realm of theory.

Previously, geophysicists constructed models, based on seismic data, to show two ways earthquakes can grow. In the classic “crack mode,” a rupture travels out concentrically from the earthquake’s center, as slip continues to occur over the entire area for the duration of the event — think of dropping multiple pebbles into a placid lake, and the resulting ripple of waves that travel outward. For earthquakes that grow according to the “pulse mode” mode, however, slip occurs only along the rupture front, like the primary wave of a tsunami, “healing” behind itself as it travels. Neither mode, however, has ever been physically observed.

To attempt that feat, George Lykotrafitis, then a mechanical engineering graduate student at Caltech in Pasadena, Calif., and colleagues tried to simulate earthquakes in the lab, to see if they could identify the conditions that lead to either the crack or pulse growth modes predicted by the models. Publishing in the Sept. 22 Science, the team experimentally showed that each earthquake growth mode indeed exists, and is linked to the initial force that triggers the event.

In nature, faults are held together by friction as two slabs of rock — typically continental plates — try to slide past one another. Stresses build up, which occasionally release in an earthquake.

To replicate a fault in the lab, the team stacked together two sheets of a hard, clear polymer, one on top of the other. They applied artificial stresses, scaled down from nature to match the size of the laboratory “fault.” Then, to simulate energy release associated with real earthquakes, and to make the plates slip, the team shot pellets at the plates using an air gun. High-speed cameras took pictures of the plates as they slid, and recorded the resulting wave pattern of stresses visible on the plates.

When struck by a high-speed pellet, the plates slid by one another at a constant speed, implying crack-like propagation. A slower impact speed, however, resulted in the plates sliding with a mixture of crack-like and pulse-like rupture propagation. Finally, further slowing the impact speed resulted in a purely pulse-like slip.

That the experiment showed that various impact speeds can result in crack-like, pulse-like, or a combination of the two earthquake growth modes, confirms in the lab what models had predicted, says Ares Rosakis, an aeronautics and mechanical engineering professor at Caltech. He is “hesitant,” however, to say exactly how the two modes translate into the strength of an earthquake in nature. But the “single most important contribution” of the study, he says, is “showing that pulse-like and crack-like behaviors exist.”

Kim Olsen, a geophysicist at San Diego State University in California, not associated with the study, says that the earthquake lab experiments are “interesting” and supply many possibilities for how an earthquake grows. The lab experiments, however, don’t say anything about which event is more likely to occur in nature, he says, and “cannot include the complexity seen in nature.” If scientists could identify how likely it is that a type of earthquake behavior will occur on the San Andreas Fault, for example, “then I would be very happy with that experiment,” Olsen says.

Rosakis says that he and colleagues will continue working to replicate earthquakes in the lab under increasingly realistic conditions. The next step is to trigger the rupture with an instantaneous plasma explosion, rather than by an air gun, which Rosakis says is “more natural” and better simulates the sudden release of pressure that triggers earthquakes.

Kathryn Hansen

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