GROUP VELOCITIES, WAVE FORMS, AND PARTICLE ORBITS OF THE FIRST HIGHER MODE OF OCEANIC RAYLEIGH WAVES EXCITED BY A DEEP EARTHQUAKE ON OCTOBER 7, 1966, NEW HEBRIDES ISLANDS

抄録

The first higher mode of oceanic Rayleigh waves from a deep earthquake which occurred at the New Hebrides Islands on October 7, 1966, has been investigated with respect to group velocities, wave forms, and particle orbits, the waves propagating through the normal oceanic basin in the Pacific. Observed and theoretical particle orbits of the wavetrains of the first higher mode showed predominantly retrograde elliptic motions for periods around 30 s, though a pure progressive motion was partly observed. Group velocities of the first higher mode showed 4.32 km/s for periods from 20 to 50 s, and are close to those of the fundamental mode of Love waves, though Love waves contain longer period components than the first higher mode of Rayleigh waves. The observed group velocity 4.32 km/s of the first higher mode for periods mentioned above significantly exceeds a theoretical value of 4.28 km/s calculated for the model PC-MAX given for the highest-velocity-area in the Pacific. Those observed high group velocities and simulated seismograms of the first higher mode suggest that the shear velocity of 4.35 km/s is dominant in the low velocity zone under the normal oceanic basin, particularly in regions of the ocean-floor age of more than 90 m.y.