Cascadia's Fault by Jerry Thompson Read Free Book Online Page B

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 line of dark shadows that ran for miles and miles in the early morning light. There it was, plain as day—two tectonic plates grinding past each other at the blistering speed of two inches (5 cm) per year, roughly the same speed as your fingernails grow.
    Far above the valley floor, it was also easier to see why the prediction experiment was being conducted along this particular segment of the fault. The San Andreas is not a straight line. Far from it, especially in Parkfield, where the shadow line zigzags ever so slightly and in a stretch northwest of town actually kinks. There’s a five-degree bend along a segment 1.2 miles (2 km) long that was the epicenter of the 1966 quake.
    In their paper, Bakun and Lindh referred to this as a “geometric discontinuity” and suggested the bend probably controls how much of the fault moves when it ruptures. I imagined it as a kind of plug or doorstop jammed in the crack, causing the fault to slow down or temporarily stop moving. Not only that, with all this zigzagging, there are rough patches in the rocks—seismologists refer to these as asperities—that create friction and also slow the creeping motion along the fault. While the rest of the San Andreas is ripping along at almost 2 inches (5 cm) per year, the Parkfield segment is lagging behind at only 1.4 inches (3.5 cm) per year.
    With tectonic plates as big as these, however, it’s obvious the little snags—those Parkfield asperities—can’t hold up progress forever. The stress builds to a point where the rocks fail. The rough spots finally shear away. In the span of less than a minute, roughly twenty-two years of “lost motion” along the fault is recovered as the Parkfield segment catches up with the rest of the San Andreas in a shuddering leap to the north. An earthquake.
    Another important reason for clustering so many instruments here is that just before the 1966 main shock, the earth may have given off subtle warning signs. Twelve days before the temblor, fresh cracks
appeared in the ground near the center of the rupture zone. Nine hours before the main shock, an irrigation pipe that crossed the fault broke and separated. Were these true