A Fault Runs Through It

Amid a flurry of smaller quakes, geophysicists drill deep in anticipation of the next Big One

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Aerial view of the San Andreas fault in the Carrizo Plain, South-Central California.

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.

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