Field measurements of compressional wave velocities in common crystalline rocks |
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| Authors: | Scott B Smithson Peter N Shive |
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| Institution: | Department of Geology, University of Wyoming, Laramie, Wyo. USA |
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| Abstract: | Field measurements of seismic P-wave velocities have been carried out by surface to surface refraction measurements for five different rock types. Subweathering velocities are surprisingly high and range from 5.0 km/sec in quartz monzonite to 6.2 km/sec in amphibolite in agreement with the proposal of Nur and Simmons for water-saturated low-porosity rocks. Maximum velocities found are 6.1 km/sec for quartz monzonite, 5.8 km/sec for granite gneiss, 6.4 km/sec for syenite, 6.4 km/sec for anorthosite, and 7.0 km/sec for amphibolite, and maximum depth for these velocities is 1.3 km. We conclude as follows: (1) field measurements correspond well with laboratory measurements but true velocity may be slightly higher than laboratory measurements, (2) shallow velocities are much higher than older laboratory measurements but agree well with recent laboratory measurements on water-saturated samples, (3) velocities much less than 6 km/sec seem unlikely in the crust unless in an area of extreme tectonic activity, (4) velocities of 6.5–6.7 km/sec commonly reported for the lower continental crust probably correspond to rocks less mafic than basalt or gabbro. |
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