J. LEES & P. MALIN, UC SANTA BARBARA / SCIENCE PHOTO LIBRARY J. LEES & P. MALIN, UC SANTA BARBARA / SCIENCE PHOTO LIBRARY
Seismic tomogram of the San Andreas fault at Parkfield, central California, showing the location of microquakes (white dots). Each image shows a horizontal cross-section (tomogram), 25km by 20km, at different depths. Green squares are seismometers. Blue areas represent brittle rock formations where frequent microquakes relieve strain caused by movement of the crustal plates on either side of the San Andreas fault. But the blue area beneath Middle Mountain (circle) appears unrelieved by microquakes. This was the epicentre of the 1966 Parkfield earthquake & may be the site of the one predicted to occur before 1993.
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