Abstract

Phases have been observed in the S-wave coda of local earthquakes recorded at the Fuchu strong-motion array, located in the Kanto basin in the western suburbs of Tokyo, Japan. These phases are often larger than the direct S-wave at a period of 1 sec. Because these phases are observed only on horizontal components at surface sites and are weakly observed at only the shallowest (150 m) of a number of borehole sites, they are interpreted as Love waves. Particle motions indicate significantly off-azimuth travel directions. The Fuchu array is too sparse to allow the application of standard array analysis to determine the origin of these phases. Instead, we use a combination of three-component polarization, enveloping, and slant-stack techniques that allows propagation directions to be reasonably well determined (± 10°); however, group slownesses are more difficult to obtain precisely (1 to 2 sec/km). Using these results and assuming that the Love waves are generated by the scattering of S-waves at the Earth's surface, we estimate locations of scattering points. We find that the scattering occurs along a short segment of the Kanto basin boundary for widely distributed earthquake source locations. We believe that this segment is important because it is the closest part of the basin boundary to the observation sites. The existence of these basin-induced Love waves violates backscattering models for the generation of the coda of local earthquakes. This must be kept in mind when applying coda techniques to sediment basin sites at periods of 1 sec or more.

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