Local Effects on Strain Seismogram at Matsushiro Seismological Observatory - 1. Love Waves

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We proposed a simple method to evaluate local effects on a strain seismogram, and applied the method to investigation of the effects at Matsushiro Seismological Observatory, Japan Meteorological Agency, central Japan, for a Love wave in a period range of 170-400 sec. First, we took a ratio of EW to NS component strain for a Love wave on a time-frequency plane. Although NS and EW component strain for a Love wave travelling in any direction have the same amplitude theoretically, the ratio of observed EW to NS component strain was 0.7. Next, we compared the strain seismogram with a velocity seismogram recorded with STS-1 broadband seismometer. A ratio of a partial derivative of a displacement field with respect to time to that with respect to wave propagation direction equals a phase velocity of the wave theoretically. Utilizing the fact, we estimated the phase velocity of a Love wave using the observed velocity and strain seismogram, after NS component strain had been multiplied by 0.7. The result was 24% smaller than the theoretical phase velocity. Finally, we compared the velocity seismogram with other velocity seismograms recorded by STS-1 seismometers at F-net observation stations near Matsushiro and by STS-2 seismometer at Matsushiro, and found that the every deviation from Matsushiro was less than 10%. From these results, we conclude that the EW and NS component strain seismograms have been amplified by factors of 1.32 and 1.88 for a Love wave, respectively. This fact, which may be because of local geology and/or topography effects, must be taken into account when the strain seismogram is used for seismological applications such as the CMT inversion and research on earth's free oscillations.