Measurements Of An Exposed Earthquake Fault Helps Scientists Understand Subsurface Faults' Behavior, Improve Hazard Forecasts

Ali Sayed, a master’s degree student in geological sciences at Virginia Tech, will present the new information during a session on structural geology and tectonics at the 50th annual meeting of the Southeastern Section of the Geological Society of America April 5-6 in Raleigh, N.C.

Rocks near a fault are created by the deformation of surrounding rocks and may comprise a zone anywhere from zero width to more than a mile wide. Major faults, such as the San Andreas, are thought to have a fault zone of a few hundred meters (yards).

Sayed explains that exposed faults are often not representative of deep faults because faults at the surface of the earth have often been changed by weathering. The rock has been broken down, says John Hole professor of geological sciences at Virginia Tech, who is a partner in the research, along with Harold Tobin, professor of earth and environmental sciences at New Mexico Tech.

The researchers have located a fault that is at the surface and is unweathered as a result of coastal erosion constantly uncovering deeper rocks -- the San Gragorio fault where it intersects the coast in Moss Beach, Calif. The site is located in a community park and is only available for research at low tide.

In order to measure seismic reflection and refraction across the fault, Sayed and Hole, who are seismologists, place seismometers on one side of the fault and discharge blanks in a special gun buried on the other side of the fault.

"We record the vibration to measure how it travels from the source to the receiver, says Hole. "We are most interested in the speed of the wave. We determine the way the speed changes inside and outside the fault zone – how the speed of a seismic wave changes with the rock type and fracturing and between the types of minerals."

Rocks are remineralized at the fault face and within the zone, where there is fracturing, he explains. "Because we have analyzed the type of material around this fault, our findings will help us understand measurements from other faults that we cannot see. It’s similar to a medical ultrasound, but on a larger scale."

Sayed reports that the seismic waves slow in the ‘gouge’ area, which is the material about 30 meters or 100 feet wide, next to the fault. The gouge is an area of remineralized sand and clay. "Originally it was sand and silt with a little clay. These minerals are squeezed so that they change structure to form new minerals – usually clay. The fault has actually done that," says Hole. "The damage area that surrounds this fault is actually 130 meters (about 425 feet) wide -- as wide as the length of a football field plus end zones."

Sayed’s presentation will emphasize the velocity contrast. "The speed of the primary wave decreases by a factor of two from the outside of the gouge to the fault face," he says. The paper, "In situ measurements of seismic velocity across an exposed brittle fault zone," by Sayed, Hole, and Tobin, will be presented at 4:40 p.m. April 5 at the Sheraton Capital Center Hotel in Raleigh.

The mystery? "The velocity is actually slower than we expected," says Hole. "It is slower than it would be in water and these rocks are still filled with water, which usually results in a wave as fast as or faster than if it traveled through water."

By the time he completes his thesis this summer, Sayed will have also studied a secondary seismology wave, which is a measure of the ability of the material surrounding the fault to vibrate in different directions.

Hole says the research is helping him to better understand and interpret readings from subsurface faults. "It helps us to find them at depth and to relate to the geological properties. By better understanding physical properties within a fault, we can better understand fault behavior, including the earthquake process. We will not be able to predict earthquakes any time soon, but we are contributing to improving our ability to forecast long-term earthquake hazards," says Hole.

This is the first study of the seismology of an active fault that is not weathered at the surface. In addition, this research offers high spatial resolution of the readings, says Hole.

Tobin, a geologist, has taken samples of the rocks from various locations and is doing similar seismic measurements in the lab, then is pulling the rocks apart to study their properties. "Thus, we get a continuous profile of the geology of the site and he gets spot measurements. That combination of study has not been done before," Hole says.