National Geographic : 1995 Apr
-'tuct-e crust - p"Ductile crust Thidevils beneath the City of Angels Residents of Los Ange les had long taken com fort in knowing that the San Andreas Fault skirts the city-veering some 30 miles north and east of downtown. They're complacent no more, shaken up by last year's Northridge quake in the city's San Fernando Valley. Measuring 6.7 on the moment-magnitude scale (see page 10), Northridge was the largest quake directly under a major U. S. city since the 1906 San Francisco disaster. Geologists warn that scores of faults, each capable of magnitude 6.5 quakes, may slice through rock under Greater Los Angeles. Many are blind thrust faults, which usually do not break the surface of the earth-and so can elude geologists until a quake occurs. Both the Northridge quake and the Whittier Narrows quake of 1987 occurred on blind thrust faults. Stress buildup In addition to relieving stress, quakes boost it in adjacent areas. A computer model calculates where stress has increased (red areas) as a result of large quakes since 1933 (purple boxes). The next temblors will occur where stress has risen.