"We have learned a lot since the Landers earthquake in the Mojave Desert in 1992," says Dr. Chris Marone, professor of geosciences. "We learned that earthquake triggering happens a lot more than we thought. The mechanism is not well understood."
Marone is working with Margaret S. Boettcher, a Ph.D. student he coadvises at the Massachusetts Institute of Technology, and Heather M. Savage, his Ph.D student at Penn State, investigating in the laboratory the way triggering of earthquakes works and whether or not a time lag exists between the initial earthquake and the ones that follow.
The researchers use a deformation apparatus that simulates the fault zone between slipping rock masses and the slipping forces on it. Then a force is placed perpendicular to the fault to simulate the perpendicular vibration caused by the energy waves from the initial earthquake on the already stressed "fault." The researchers reported their results in a recent issue of the Journal of Geophysical Research.
"Yes, we do find lags between the changes in the forces and the changes in the strength," says Marone. "There are seconds of delay in the laboratory between the force being applied and the fault moving."
While the delay in the laboratory is in seconds, in the real world the delay can be from minutes to a week after the initial shock. The researchers believe they know why a delay exists between the vibration waves of the initial earthquake and the motion on other faults. The area of interest is the gouge zone, the space between the solid rock filled with everything from sand to pea size gravel to large boulders. This granular fault gouge can be up to a kilometer in width.
"We have known since the 1800s that compacted grains when sheared expand and increase volume," says Marone. "The best example of this phenomenon, known as dilatancy, is on the beach. Your foot, as you step, shears the compacted sand and the beach surface dries momentarily as water drains into the pore space between grains. When you lift your foot, the granules collapse back into their compacted position, leaving a dry footprint."
Within this gouge zone, a competition between compaction and dilation of the granules takes place. The perpendicular force of the periodic waves produced by the initial earthquake changes the steady state density and porosity. The change in porosity is dilation. Through compaction and dilation, an area parallel to the fault in the gouge is set up where the slipping movement of the earthquake actually takes place.
The Lander's earthquake was a shallow earthquake and created many surface waves. Other similar earthquakes have occurred in the Mojave, Denali, the Hector Mine earthquake and in ChiChi, Taiwan. Potential for this type of earthquakes exists worldwide.
"People have been taking laboratory data and trying to model seismic hazard from trigger earthquakes," says Marone. "The lag between the time stresses reaches a fault and, when the strength in the fault gouge changes, must be considered to model this properly." The National Science Foundation and the United States Geological Service funded this research.