Cascadia's Fault - Jerry Thompson [17]
Of all the places he might have arranged to meet us, he had chosen this bridge for a reason and wanted us to see it from below. So we unpacked our gear and trudged down into the gully for a closer look. That’s when Lichtie explained that this big ditch was part of the San Andreas, that the bridge literally crossed the fault, and that the last Parkfield event, back in 1966, had torn the old bridge off its foundations.
We were looking at a replacement span that already showed signs of stress. Doug, my cameraman, got a telling close-up of one big bolt holding two heavy steel girders together by no more than a few threads. The two main sections of bridge deck had already been pried apart far enough for sunshine to burn through a gap between the beams. It was a crude yet graphic display of creep along the fault.
Lichtie took us up the road to a cow pasture, where we hiked across the dun-colored grass toward a dry gulch with a storm culvert dug vertically into the earth. When he removed the cover, we could see a metal platform bolted to the corrugated wall of the culvert with a cable-anddrum contraption that looked like something a kid might build with an erector set.
Halfway down the culvert was a circular hole cut into the earth several feet below the surface, where a horizontal plastic drainpipe extended toward the far side of the gulch. Inside the pipe was a pencilthick braided steel cable that looked like a buried trip wire. Lichtie called it a creepmeter and explained that it was pretty much what it looked like—a wire sixty-five feet (20 m) long, stretched across the fault and connected to a strain gauge (in a toolbox at the bottom of the culvert) capable of measuring even a few fractions of an inch of slip along the plates.
Next Lichtie took us to a nondescript shed in a grove of walnut trees, halfway up the side of the gulch. When he unlocked the door we saw what looked like a high-end amateur telescope, with a white steel barrel the size of a small cannon, mounted on a high-tech tripod anchored to a concrete pad with a small spotting scope bolted on top like a rifle sight. He switched on the power and a cherry-red laser shot a beam across the valley toward another tiny shack so far in the distance we could see only a smudge through waves of dusty heat rising in the noonday sun.
Lichtie used the rifle scope to line up the laser with a parabolic reflector in the other little shed three miles (5 km) away. “We’re shooting the beam across the fault to a reflector, which brings it back here. And we can measure to within a half a millimeter how far that reflector has changed in relation to this building,” said Lichtie. As expected, the laser device had already documented right-lateral motion along the fault—the Pacific plate creeping north toward Alaska.
As part of Bakun and Lindh’s experiment, the USGS was in the process of installing a cluster of these and other instruments at various points along the fifteen-mile (25 km) rupture zone in Parkfield. The data were being beamed continuously by microwave to a real-time processor in Menlo Park, where members of the research team were keeping constant watch. They even wore pagers that would wake them in the dead of night or ruin a perfectly good dinner if the fault started to creep or warp or bend itself out of shape.
When producer David Kaufman and I realized the fault ran right up the middle of this gulch, it was impossible to resist the temptation to straddle the fracture and take a picture. Not that you could really see a crack or crevice in the ground; there was nothing more to look at than the V-shaped bottom of this little gully, swathed in straw-colored pasture grass in dire need of rain.
A better way to see the fault was from the air. Aerial pictures showed that one side of the fault had been thrust up slightly higher than the other side, enough to cast a distinct