National Geographic : 1973 Jan
Wrenched askew, streams jog sharply as they cross the fault, here a muted line running horizontally across the picture. As the earth shifts during suc cessive quakes, streams hold fast to their courses across the bleak Carrizo Plain. Much of the offset visible here, about 70 feet, probably occurred during the mighty Fort Tejon quake of 1857, when the earth ruptured for 200 miles and the west side of the fault, at top, shot 30 feet past the east. Diagnosing strain along the fault, the U. S. Geological Survey measures bend ing of surface rock near Hollister. Positioning a laser (bottom) over a geodetic marker, the team bounces the beam off a reflector at another marker ten miles across the fault (upper). The beam's travel time is corrected for atmo spheric distortions read by instruments on the helicopter. By recording the buildup of strain, U. S. and California agencies seek ways to forecast quakes.