Ship Seeks Deep Secrets of Bay Area Faults
Every night for the next two weeks, the U.S. Geological Survey ship S. P. Lee will be plying the waters of San Francisco Bay, towing an array of acoustic devices that could help unlock the deepest secrets of the region’s earthquake faults.
The voyages are the centerpiece of an ambitious study that aims to test a novel and potentially revolutionary hypothesis: that seemingly vertical faults such as the San Andreas are actually just the surface manifestation of a much deeper, larger horizontal fault system.
If that is true, scientists would gain a far clearer picture of the relationship between earthquakes along neighboring fault lines, a key step in trying to predict earthquakes. And such a horizontal fault structure could even lead to a broader re-evaluation of the earthquake-prone geological formations throughout California.
“We need to understand the geometry of the faults to better understand what the earthquake hazards are,” said Jill McCarthy, the USGS scientist heading the study. “We know that the San Andreas fault and the Hayward fault (east of San Francisco Bay) are vertical for the first eight miles, but we don’t know what happens at depth.”
Funded by the National Earthquake Hazards Reduction Program, the study is a joint effort of scientists at the USGS, Stanford, UC Berkeley and the Lawrence Berkeley Laboratory, and will be the first effort to attempt such deep mapping of fault lines.
Much like a trolling fishing boat, the 208-foot S.P. Lee will tow an ungainly array of 4- to 6-foot air guns. About every 30 seconds, the guns will release a jolt of compressed air, creating a powerful low-frequency sound signal. The signal will travel through the water and penetrate the floor of the bay.
A network of 120 sound buoys, deployed by motorboats at specified locations every day, will listen to the sound waves bounce off rock layers in the Earth’s crust and transmit what is heard to a shipboard computer. An elaborate network of land-based sensors will also monitor and record the sound pulses.
By comparing the rebounding sound pulses with their knowledge of how fast sound moves through differing rock layers, scientists will be able to reconstruct a map of the deep rock structures.
Although this kind of acoustic imaging has long been used by the oil industry, it has never been employed for a comprehensive study of earthquake geology, McCarthy said Monday aboard the S. P. Lee.
Although the bay provides an ideal environment for this type of study, the technique could in principle work using only land-based sensors and sound sources, she said, making a similar study feasible in Southern California.
David Cacchione, head of the marine geology branch of the U.S. Geological Survey, said there will be a Fish and Game biologist on board the ship at all times to monitor the impact on fish and other wildlife. The USGS took pains to meet with interested groups--including environmentalists, fishermen, and local officials--to let them know of their plans.