Velocities of mantle Love and Rayleigh waves over multiple paths

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 lie 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. One interpretation of this variation is that the mantle under the continents is different from that under the oceans, since the path with the highest phase velocities is almost completely oceanic. This interpretation, however, is not unique and variations under the oceans and continents are also possible. Group velocities are computed from the phase velocities and are also directly measured from the seismograms. The group-velocity curve of Love waves has a plateau between periods of 100 and 300 seconds with a shallow minimum at about 290 seconds. The sources of error in both Fourier analysis and direct time domain methods of phase velocity measurement are discussed.

[1]  D. L. Anderson,et al.  Long‐period love waves in a heterogeneous, spherical Earth , 1962 .

[2]  M. Båth,et al.  Attenuation and dispersion of G waves , 1962 .

[3]  M. Toksöz,et al.  Source‐mechanism from spectra of long‐period seismic surface‐waves: 1. The Mongolian earthquake of December 4, 1957 , 1962 .

[4]  B. Bolt,et al.  Eigenvibrations of the Earth observed at Trieste , 1962 .

[5]  J. Brune,et al.  Rayleigh wave dispersion in the Pacific Ocean for the period range 20 to 140 seconds , 1962 .

[6]  F. Press,et al.  Upper Mantle Structure under Oceans and Continents from Rayleigh Waves , 1961 .

[7]  F. Press,et al.  Experimental determination of earthquake fault length and rupture velocity , 1961 .

[8]  J. Brune,et al.  Long-period surface waves from the Chilean Earthquake of May 22, 1960, recorded on linear strain seismographs , 1961 .

[9]  Ari Ben-Menahem,et al.  Radiation of seismic surface-waves from finite moving sources , 1961 .

[10]  James N. Brune,et al.  The polar phase shift of surface waves on a sphere , 1961 .

[11]  J. Brune,et al.  Group and Phase Velocities for Rayleigh Waves of Period Greater than 380 Seconds , 1961, Science.

[12]  J. Brune,et al.  Observations of phase velocity for Rayleigh waves in the period range 100 to 400 seconds , 1960 .

[13]  M. Ewing,et al.  Study of shear-velocity distribution in the upper mantle by mantle Rayleigh waves , 1960 .

[14]  Jack Oliver,et al.  A simplified method for the analysis and synthesis of dispersed wave trains , 1960 .

[15]  Yasuo Satô,et al.  Attenuation, dispersion, and the wave guide of the G wave , 1958 .

[16]  F. Press,et al.  An investigation of mantle Rayleigh waves , 1954 .

[17]  P. Valle.  SULLA MISURA DELLA VELOCITA' DI GRUPPO DELLE ONDE SISMICHE SUPERFICIALI , 1949 .

[18]  C. Pekeris.  THEORY OF PROPAGATION OF EXPLOSIVE SOUND IN SHALLOW WATER , 1948 .