A Study of the Energy Contained in the Seismic Waves P and pP

Citation

Mooney, Harold Morton (1950) A Study of the Energy Contained in the Seismic Waves P and pP. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/T26Z-PD62. https://resolver.caltech.edu/CaltechTHESIS:05242018-145241388

Abstract

The energy observed in the seismic waves P and pP in a large number of earthquakes is compared with the theoretical energy calculated from a standard equation, using two partly independent methods. The results are analyzed using as variables depth of focus, distance from epicenter to recording station, geographical location of epicenter and azimuth from station. When compared with the theoretical value, the ratio of the energy in pP to the energy in P averaged over a distance range 60°-90° is observed to decrease with depth, by 0.5 on a logarithmic scale of energy between 100 and 600 km. depth of focus. The results for the two waves are compared separately with theoretical values and the observed effect appears to be about equally due to an increase of P energy and a decrease of pP energy with depth. The theoretical formula is re-examined to determine if permissible changes in the assumptions or numerical values can account for the results. No such changes are found; certain assumptions as to increased absorption of energy near the surface provide a partial qualitative explanation, but quantitatively they cannot be reconciled with data from other sources. The variation with distance between observed and calculated energies is not large enough to be treated quantitatively; slight changes in the accepted change of velocity vs. depth curve are tentatively suggested on the basis of it. The energy ratio pP/P is significantly too large in shallow earthquakes occurring in the Aleutian region, and can be attributed to pP rather than P. For shallow shocks in the New Hebrides region and very deep shocks of the Southwest Pacific, there is some indication that the energy ratio is smaller than for comparable shocks elsewhere. An attempted correlation of regional anomalies with the dip of large scale fault systems is successful in the Aleutians, inconsistent elsewhere. The effect of differences between instruments is considered.

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