Velocities of long period surface waves and microseisms and their use in structural studies. I. Mantle Love and mantle Rayleigh waves and the structure of the earth's upper mantle. II. Microseisms and their application to seismic exploration

Abstract

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 Fourier analyzed, and the phase spectra are used in determining the phase velocities. Where the great circle paths are close, the phase velocities over these paths are found to be in very good agreement with each other indicating that the measured velocities are accurate and reliable. Phase velocities of Love waves over paths that 1ie far from each other are different, and this difference is consistent and much greater than the experimental error. From this it is concluded that there are lateral variations in the structure of the earth's mantle. The phase velocity data are compared with theoretical dispersion curves of seven different earth models. None of these models fit the data. Two new upper mantle models, one to fit the data over an almost completely oceanic path and the other over a mixed oceanic and continental path, are designed. The significant features of these models are correlated with the body wave observations and with the hypothesized thermal model and the mineralogical structure in the mantle. Part II: A study of microseisms is made to determine some of their statistical properties and to investigate the feasibility of their use in determining the shallow structures of the earth's crust by the phase velocity method. It is found that the microseisms in the period range of to 6 seconds arrive from several directions with comparable strength and at the same time. There are occasional short intervals of 10 - 40 seconds during which microseisms are mostly unidirectional. It is also found that these relatively short period microseisms are not stationary in the wide sense over time intervals longer than 5 or 10 minutes. The phase velocities of microseisms recorded with an array of 8 instruments are measured In four different locations. The velocities, although scattered, are found to be in agreement with the theoretical dispersion curve for the fundamental Rayleigh mode, computed using the available seismic velocity information. An error analysis is made and the confidence limits are placed within ±20 percent of the measured velocities.