Part I:
Phase velocities of Love waves from five major earthquakes are measured over six great circle paths in the period range of 50 to 400 seconds. For two of the great circle paths the phase velocities of Rayleigh waves are also obtained. The digitized seismograph traces are Fourieranalyzed, and the phase spectra are used in determiningthe phase velocities. Where the great circle paths areclose, the phase velocities over these paths are found tobe in very good agreement with each other indicating thatthe measured velocities are accurate and reliable. Phasevelocities of Love waves over paths that 1ie far from each other are different, and this difference is consistent andmuch greater than the experimental error. From this it isconcluded that there are lateral variations in the structureof the earth's mantle.
The phase velocity data are compared with theoretical dispersion curves of seven different earth models. None ofthese models fit the data. Two new upper mantle models,one to fit the data over an almost completely oceanic pathand the other over a mixed oceanic and continental path,are designed. The significant features of these modelsare correlated with the body wave observations and with thehypothesized thermal model and the mineralogical structurein the mantle.
Part II:
A study of microseisms is made to determine someof their statistical properties and to investigate thefeasibility of their use in determining the shallowstructures of the earth's crust by the phase velocitymethod. It is found that the microseisms in the periodrange of to 6 seconds arrive from several directions withcomparable strength and at the same time. There are occasionalshort intervals of 10 - 40 seconds during which microseismsare mostly unidirectional. It is also found thatthese relatively short period microseisms are not stationaryin the wide sense over time intervals longer than 5 or 10minutes.
The phase velocities of microseisms recorded with anarray of 8 instruments are measured In four differentlocations. The velocities, although scattered, are foundto be in agreement with the theoretical dispersion curvefor the fundamental Rayleigh mode, computed using theavailable seismic velocity information. An error analysisis made and the confidence limits are placed within ±20percent of the measured velocities.
