Studies of the travel times, periods, and energy of seismic waves SKP and related phases

Citation

Forester, Robert D. (1953) Studies of the travel times, periods, and energy of seismic waves SKP and related phases. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/XPMD-7118. https://resolver.caltech.edu/CaltechETD:etd-05062003-085500

Abstract

The travel times, periods, and amplitudes of SKP core waves have been investigated for shocks of normal, 60 km, and 100 km depths. For shallow shocks, multiple SKP phases are observed as long as 54 seconds after the initial SKP motion on short period instruments, and as long as 87 seconds on long period instruments. Amplitude data indicate that each multiple phase has a focal point similar to that of the initial SKP phase. The focal point for waves having periods of 1 to 5 seconds occurs at 131 1/2[degrees] and that for waves having periods of 5 to 10 seconds is broadly defined between 130 and 131[degrees]. Short period SKP phases extend from 129 to at least 140[degrees]; and long period SKP phases, from 125 to 145[degrees]. The long period waves are believed to be diffracted from the caustic in accordance with Airy's hypothesis. Multiple reflections and refractions in the outer layers of the mantle seems more plausible as an explanation of multiplicity than dispersion. In order to analyze the energy content of the SKP phases, a system was developed for determining the magnifications of the ground motion of seismometers from their records of earthquakes. As an experiment, theoretical travel times for PKP, SKP, and SKS were computed by integration along the travel paths. For this purpose, the velocity distribution within the earth was broken into segments which were represented by continuous functions. The resultant travel times are not valid for rays of grazing incidence to the core. The times computed for PKP and SKP are intermediate between the smoothed times given for them by Jeffreys and the times for them based upon recent observed data. The theoretical travel times for SKP provided a basis for the analysis of the energy content of the SKP phases. For all types of SKP phases, the energy content of short period waves is a small fraction of that of long period waves. For vertical instruments, the agreement between theoretical and observed values of energy is good. For horizontal instruments the observation of too little energy is not satisfactorily explained. SKP", the SKP phase associated with the inner core, is observed between 114[degrees] and 125[degrees]. It records with periods of 1 to 3 seconds on both long and short period instruments. The observations of SKP" present additional support for the hypothesis of a large, but continuous increase of velocity at the transitional boundary of the inner core. Evidence for a difference between the travel paths of short and long period waves is less apparent for SKP than for PKP.

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