Reading the Deep Faults

The earthquake that caused the tsunami that recently devastated coastal areas and islands of the Bay of Bengal points to a real need for accurate seismic data on fault zones.

Unfortunately, little data has been collected on the deep underlying structures of fault lines because it’s very expensive to drill deep wells and install instruments that far below ground. But a new use for classified technology developed by the military during the Cold War may give geoscientists a way to gather information critical to understanding unstable areas of the Earth’s crust.

Last fall, a team of geoscientists led by Rice’s Manik Talwani conducted a first-of-its-kind experiment on California’s famed San Andreas Fault. The researchers gathered data that could give scientists a much clearer picture of the fault’s “gouge zone,” a region two to three kilometers beneath the surface and consisting of gravel-sized rocks created when continental plates grind against one another.

The experiments took advantage of the fact that gravity varies slightly over Earth’s surface due to small changes in the mass of subsurface rock and sediments. Talwani and his colleagues flew over a 100-square-kilometer region near the town of Parkfield in central California and used a gravity gradiometer to measure the rate at which gravity changes from place to place along the fault. The gradiometer originally was developed by Lockheed Martin as a silent navigation system for nuclear submarines. The flights were conducted near Parkfield because that is the site of the International Continental Drilling Program, a scientific mission that is taking core samples, and the physical evidence gathered by the drilling will help Talwani’s team analyze its own data.

Ultimately, Talwani hopes the technology will change the economics of studying fault lines by making it affordable to conduct baseline and follow-up surveys of significant portions of fault lines—something that just isn’t cost-effective with land-based instruments. “If this technique works, it will open the door for geoscientists to affordably gather information about fault lines and other subsurface areas of interest,” says Talwani, the Schlumberger Professor of Advanced Studies and Research in Earth Science. “Moreover, these flights will give us a baseline measurement that we can compare with future surveys to find out how things are changing in the shallow crust beneath the surface of the fault.”

Houston-based Bell Geospace Inc. helped carry out the airborne gradiometer survey, and the project is supported by the National Science Foundation, the National Imagery and Mapping Agency, and several industrial firms.

—Jade Boyd