A Fault Runs Through It

California has been doing a lot of shaking of late. In mid-June a sizable quake off its north coast triggered a tsunami warning, a false alarm, fortunately, while far to the south, earthquakes of lesser power knocked stuff off shelves and seriously rattled the composure of those who felt the ground sway beneath them. That’s because the earthquakes that concern Californians most are those that haven’t happened–at least, not yet–along the state’s fault-fractured western edge.

Of all those faults, the most feared is the San Andreas, which slashes its way along the California coast for 750 miles. Many scientists believe that after decades of quietude, the pressure on sections of the San Andreas is reaching the point at which something will have to give. Researchers have been rushing to instrument the fault–“setting out traplines,” as Ken Hudnut, a geophysicist with the U.S. Geological Survey (USGS), puts it–to catch the faintest movements and seismic mutterings.

Nowhere has scientific activity been more intense than near the small town of Parkfield, which sits astride a transitional zone between a segment of the San Andreas that in 1857 produced one of the largest quakes in U.S. history and another segment characterized by snail-like creep and small, quiet microquakes. Here, amid rolling hills and golden pastureland, scientists with a National Science Foundation initiative called EarthScope are building a remarkable underground observatory known as SAFOD, or the San Andreas Fault Observatory at Depth.

Just last week SAFOD’s giant Texas-style drill bored to an inclined depth of 11,000 ft., coming to within 1,000 ft. of the San Andreas. Around July 4, the giant drill’s steel teeth should chatter through to the fault itself, reaching the far side of the San Andreas later this summer. At that point, Stanford University geophysicist Mark Zoback and his colleagues will finish casing the perimeter of their borehole with steel and start packing it with instruments.

The effort seems bound to pay off. Last September, the Parkfield zone gave rise to a magnitude-6 earthquake whose throaty rumblings were recorded by a rich array of seismometers and other instruments, including several nestled inside a mile-deep pilot hole the SAFOD team reamed out just two years earlier. Puzzling to many scientists was the seeming absence of precursory activity, save for subtle signs that strain may have increased ever so slightly the day before.

Do earthquakes have precursors? SAFOD should help answer the question. “This is a new window on the earthquake process,” says Stephen Hickman, a senior scientist at the USGS in Menlo Park, Calif. SAFOD could also help settle a number of long-simmering disputes. Although the basic cause of earthquakes on the San Andreas is well understood–the fault marks the major interface between two sections of the earth’s crust that are grinding past each other–scientists argue endlessly about the details. Among the most pressing questions are whether the rock in the fault zone is intrinsically strong or weak and whether an increase in fluid pressure helps trigger earthquakes by prying apart the fault. “We have lots of ideas, and finally we’re getting a chance to test them,” says William Ellsworth, chief scientist for the USGS Earthquake Hazards Team.

SAFOD’s subterranean spyglass is aimed at a geophysical sweet spot on the San Andreas that is a miniature earthquake machine. The size of a football field, it rattles with microearthquakes–in this case, earthquakes of magnitude 2–with surprising regularity. Right next door, within a 2-mile radius, are more microquake clusters. In the coming years, Ellsworth anticipates, SAFOD will record fine-grained portraits of thousands of tiny temblors, many not much bigger than magnitude 0. By closely examining those portraits, scientists should be able to tell how closely one event resembles another and whether earthquakes, at least in principle, are predictable.

On this point, Parkfield’s seismic history seems suggestive. For more than a century, the area just south of the drill site produced magnitude-6 earthquakes on a roughly 22-year cycle–or so it seemed in the mid-1980s, when a USGS team threw a net of instruments over the area, hoping to catch the next iteration. The last quake occurred in 1966, so scientists figured the next would come around 1988. Instead, the 1966 quake was followed by a 38-year pause. Some speculate that another earthquake, which occurred on a nearby thrust fault in 1983, reset the seismic clock by altering the local stress field.

Now scientists are trying to gauge how last year’s Parkfield quake affected the broader San Andreas system. Stress has been off-loaded to the section of the fault directly south of the rupture, and that has at least the potential to set the stage for a larger upheaval. In 1857, for example, a moderate temblor at Parkfield was followed within hours by a major earthquake that started in the vicinity of Cholame, 15 miles away, and ripped south for 225 miles. In some places the ground moved more than 25 ft.

Most geophysicists don’t believe sufficient stress has accumulated along this section of the fault to power another 1857-style spasm. That one approached a magnitude of 7.9, making it even stronger than the 1906 quake that devastated San Francisco. Still, experts acknowledge, it’s not inconceivable that the next moderately strong shake-up at Parkfield could lead to the unzipping of a longer section of the fault, spawning a quake of, say, magnitude 7. If that happens, SAFOD would provide scientists with more than they bargained for–a near ringside seat at the start of, if not the Big One, something pretty close.