The P and S velocities along three mutually perpendicular directions are measured as a function of the axial load on specimens of Indiana limestone. The specimens are rectangular prisms, and variable axial load is applied in the vertical direction parallel to the long dimension (Z axis). A clamp is used to hold the specimen firmly along a transverse direction (Y axis). Piezoelectric transducers of the compressional and shear type are used for both transmitting and receiving the seismic signal and are mounted on opposite faces of the specimen so that seismic velocities can be measured along the three orthogonal directions (Z, Y, and X axes). Seismograms showing the arrivals of P and S waves are obtained while the axial load is increased in small increments until shear fracture takes place. An examination of seismograms shows that prior to rupture both P and S velocities decrease in all three directions but by quite different amounts. The ratio of P to S velocities, ξ, remains nearly constant along the Z axis, decreases somewhat along the Y axis, and drops considerably along the X axis. These results, interpreted as being due to prefailure dilatancy, provide qualitative information on the anisotropic behavior of ξ, an important premonitory indicator of earthquakes.
Application of nonhydrostatic stress to rock induces velocity anisotropy, causing the S wave to split into two components traveling with somewhat different velocities. Large premonitory changes in the extent of S-wave splitting have been observed for two earthquakes in Nevada. Observations of the difference between the two S-wave velocities may provide a simple method for pedicting earthquakes.
F‐k analyses of short‐period recordings of both U.S. and Soviet underground nuclear explosions at the Eskdalemuir (EKA), Scotland array indicate arrivals from a local source about 15 km due northwest of the array. Use of residual seismograms derived by subtracting the beamed record from each array channel confirmed the presence of the same scatterer. The secondary seismic source appears to be short‐period surface waves due to the scattering of incident P waves at a nearby deep valley known as Moffat Water, in agreement with Key's (1967) results. Similar analyses of the NORESS (Norway) array data from U.S. and Soviet nuclear shots also indicate secondary arrivals suggesting a local near‐surface scatterer about 25–30 km southwest of the array, in the region of Lake Mjosa. Thus, short‐aperture array data can be useful in identifying and locating sources of near‐receiver scattering.
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