THE NEAR-FIELD LOVE WAVES BY THE EXACT RAY METHOD

抄録

We have synthesized near-field Love waves by the exact ray method. The near-field Love waves are defined, in the present paper, as long-period seismic SH waves, which consist of waves reflected and/or refracted a small number of times at shallow boundaries in the crust such as the Mohorovicic and Conrad discontinuities.
The long-period records obtained at the Abuyama Seismological Observatory of Kyoto University (φ-N85°W, Δ-301km) at the time of the Off-Izu-Peninsula earthquake of May 9, 1974 (M=6.8 after JMA) have been analyzed. As the Abuyama station is located almost west of the epicenter, the NS component seismogram of the records scarcely includes any seismic waves other than Love waves (SH waves). By comparing the NS record with the synthesized near-field Love waves, we have identified the first three longperiod (15-20 seconds) predominant phases on the record with head S waves, direct S waves and the S waves totally reflected once at the Moho.
A method is proposed for determining an average S wave velocity structure by fitting peak-arrivals of some of the characterized phases of the synthesized near-field Love waves to those of the record. By applying this method, we have obtained the structure along the south coast of the Tokai district in central Japan with a satisfactory accuracy: the thickness of the crust is 30±5km, the S wave velocity in the crust and upper mantle are 3.5±0.05km/sec and 4.6-4.7km/sec, respectively. The lowest crustal layer with P wave velocities of 6.6-6.8km/sec may not exist or may be thin if it exists.
We have also synthesized the normal mode Love waves and compared them with the near-field Love waves synthesized by the exact ray method. The agreement is excellent except for the head wave portion.