Propagation Characteristics of Intermediate-Period (1-10 Seconds) Surface Waves in the Kanto Plain, Japan

Abstract

Propagation characteristics of the intermediate-period (1-10 s) surface waves predominated in strong motion records are analyzed on the basis of an array observation deployed in metropolitan Tokyo area, Japan. Seismic energy source is the main shock of the 1984 western Nagano earthquake (M 6.8) with a focal depth of 2 km; epicentral distances (Δ) for the stations are about 157-205 km. Phase analyses using the sub-array at KOG (Δ_??_7175 km) indicate that dominant phases in the period range 1-10 s are Love and Rayleigh waves with clear dispersion. Apparent velocities of their phases are lying in the range of 800-2, 100 m/s. They are matched with the fundamental mode of Love waves for transverse motion but are stretching over the two modes of Rayleigh waves (M₁₁ and M₂₁) for radial component; the crustal model, on which the normal mode solution is based, includes thick (-52, 000 m) sedimentary layers lying on the Pre-Tertiary bedrock. The above results in conjunction with direction of wave propagation and particle orbit of the ground motion suggest that the intermediate-period surface waves are perturbed by other phases such as diffracted surface waves which are owed to lateral heterogeneities in the uppermost crust. Correspondence of major phases in the strong motion records of the long-span array gives rise to "inter-station group velocities" of the surface waves of the intermediate-period. Thus derived group velocities enable us to interpret that the well-dispersed later phases are on a branch near the Airy phase of which propagation velocity is 300-800 m/s.