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Pore pressure changes during creep events on the San Andreas Fault

1973; American Geophysical Union; Volume: 78; Issue: 5 Linguagem: Inglês

10.1029/jb078i005p00851

2156-2202

Ansel G. Johnson, Robert L. Kovach, Amos Nur, J. R. Booker,

Seismic Imaging and Inversion Techniques

A 152-meter-deep well was drilled into the San Andreas fault zone near Hollister, California, to examine the possibility of any variations in fluid pore pressure during fault creep movement. Anomalous water level changes were recorded that coincide, within hours, with the only creep episodes recorded at a nearby site. The ratio of maximum water level change to total creep offset observed is 14 mm/mm. The calculated change in pore pressure for creep events is about 20 mb. Assuming 1.2 cm of total creep displacement per year along this section of the San Andreas fault, we obtain a total stress drop of 60 mb/yr associated with creep at shallow depths. The results are most encouraging, indicating the possibility that deeper wells could be used to monitor stress changes along active fault zones.