PARKFIELD, Calif. — Geologists drilling a borehole into the San Andreas Fault to better understand the physics of earthquakes have hit a seismically active section of the fault for the first time.

The scientists, who began drilling in 2004, finally entered the fault zone about two miles below the surface of the Earth on Tuesday. The entire borehole will be covered with steel and cement at the end of the month so scientists can later install instruments to measure future temblors.

"It's the first time we've been inside the earthquake machine," said Bill Ellsworth, a geophysicist with the U.S. Geological Survey in Menlo Park.

The drilling is part of a project, known as the San Andreas Fault Observatory at Depth, to rig a patch of private ranchland near Parkfield with instruments to observe earthquakes up close and to better understand what happens during stress buildup of the ground. Parkfield is one of the world's most seismically active areas.

Parkfield -- located halfway between San Francisco and Los Angeles -- straddles the San Andreas Fault, which is the meeting of the Pacific and North American plates as they grind along 800 miles through the state.

Source: Associated Press