Observation of Rayleigh wave dispersion at very long periods

Abstract

The LaCoste-Romberg earth tide gravimeter with a Weber-Larson sensor plus a band-pass filter for rejecting the large semidiurnal and diurnal tidal signals is an excellent ultralong-period seismometer. As an accelerometer its passband extends from 140 to 7200 s at the 3-dB points; as a displacement transducer the peak of the instrumental response is about 100 s. The seismograms display Rayleigh wave signals at periods rarely seen visibly on World-Wide Standard Seismographic Network long-period seismograms. Phase velocities from fundamental mode Rayleigh waves at periods in excess of 400 s have been computed for several moderate-sized earthquakes for globe-circling paths; these phase velocities for the traveling wave spectrum of Rayleigh waves are in good agreement with values obtained from studies of the free oscillations of the earth.