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The AAPG/Datapages Combined Publications Database
AAPG Bulletin
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Research on earthquake prediction prior to 1978 depended largely on the gathering of geophysical observations prior to earthquakes. These include measurements of changes in seismicity, strain, magnetic field etc. Recently, however, in an attempt to relate these observations to a physical model of the pre-earthquake failure process, the U.S. Geological Survey has begun a program of direct measurement of the properties of faults through drilling holes in and near the active fault trace.
Laboratory data on failure of rocks show that nonlinear strains develop just prior to rupture, but the character of the failure process is strongly dependent on material properties, pore-fluid pressure, and applied stresses. These parameters are virtually unknown in faults at depths where earthquakes occur. The drilling and in-situ measurements program was designed to determine these quantities and provide samples of the actual fault-zone materials.
Preliminary results from holes near the San Andreas fault can be tentatively interpreted to show that the level of shear stress on the fault is surprisingly low, about 100 bars. The fault-zone rocks at a depth of 600 m are a low-permeability clay-rich gouge in the first hole drilled along the creeping part of the fault. The hole is presently being deepened to allow measurements of stress and pore pressure at depths of 1 km.
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