Determination of local phase velocity by intercomparison of seismograms from strain and pendulum instruments

An attempt was made to determine the local phase velocity of seismic waves by a combined analysis of records obtained by the strain and pendulum seismographs set up at a single station. The record of a strain seismograph may be reducible to the space derivaative of the ground displacement with respect to the direction of a strain rod, and the time derivative of the same displacement may be obtained from a pendulum record. The ratio of the two derivatives should then be the apparent local phase velocity of the waves along the direction of the instruments. An amplitude and phase compensation technique was applied to the two kinds of seismograms over an appropriate frequency range in order to get the phase velocities of seismic waves for five earthquakes recorded at Pasadena. The results obtained by this method show good agreement with the theoretically predicted velocities for body waves and, in some cases, for surface waves. This suggests that the present method may be useful for the identification of many unknown phases, especially of body waves and higher-mode surface waves. It is expected that much better results could be obtained from a system of two-component pendulums and strain seismometers especially designed and calibrated for this purpose.