A mobile seismic array of seven stations was deployed at 11 sites along the fault trace of the M7.4 Landers earthquake of June 28, 1992, with a maximum offset of 1 km from the trace. We found a distinct wave train with a relatively long period following the S waves that shows up only when both the stations and the events are close to the fault trace. This wave train is interpreted as a seismic guided wave trapped in a low‐velocity fault zone. To study the distribution of amplitude of the guided waves with distance from the fault trace and also their attenuation with travel distance along the fault zone, we eliminated source and recording site effects by the coda normalization method. The normalized amplitudes of guided waves show a spectral peak at 3–4 Hz, which decays sharply with distance from the fault trace. Spectral amplitudes at high frequencies (8–15 Hz) show an opposite trend, increasing with distance from the fault trace. The normalized amplitudes of guided waves at 3–4 Hz also show a systematic decrease with hypocentral distance along the fault zone, from which we infer an apparent Q of 50. In order to confirm the existence of the guided waves, a dense array of 31 stations was deployed at one of the 11 sites. The resultant records revealed unequivocal evidence for the existence of guided waves associated with the fault zone. By modeling the waveforms as S waves trapped in a low‐velocity waveguide sandwiched between two homogeneous half‐spaces with velocity Vs = 3.0 km/s, we infer a waveguide width of about 180 m, a shear velocity of 2.0–2.2 km/s, and a Q of ∼50. Hypocenters of aftershocks with clear guided waves show a systematic distribution both laterally and with depth delineating the extent of the low‐velocity fault zone in three dimensions. We find that the zone extends to a depth of at least 10 km. This zone apparently continues to the south across the Pinto Mountain fault because guided waves are observed at stations north of the Pinto Mountain fault for earthquakes with epicenters south of it. On the other hand, the zone appears to be discontinuous at the fault bend located about 20 km north of the mainshock epicenter; guided waves were observed for stations and epicenters which are located on the same sides of the fault bend but not for those on the opposite sides.

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We determine the three‐dimensional P wave velocity structure beneath the Kanto‐Tokai district, central Japan, and simultaneously relocated by inverting travel time data for 91 events that were observed at 61 stations operated by the National Research Center for Disaster Prevention. The variance of the travel time data is reduced by 75%, as compared to the variance for the routinely used laterally homogeneous model. In general, our velocity model is consistent with tectonic models of the Kanto‐Tokai district. Our velocity model is also in good agreement with seismic explosion data and Bouguer gravity anomalies. The uppermost layer (from 0 to 16 km depth) is characterized by lateral velocity differences of more than 20%. We find low‐velocity zones beneath Tokyo Bay, the middle of the Boso Peninsula, and Omae‐zaki, where there are thick sedimentary layers. A marked 30‐km‐thick high‐velocity zone extends to the northeast and the west of the Izu Peninsula. This zone dips gradually from the surface to a depth of 70 km in the eastern region and to a depth of 40 km in the western region and is interpreted as the Philippine Sea (PHS) plate, which is being subducted at the Sagami and the Suruga troughs. The data unquestionably require a high‐velocity zone beneath Mount Tsukuba that cannot be explained in terms of present tectonic models. We use our three‐dimensional velocity model to relocate about 23,000 earthquakes that took place from 1983 through 1985. The accuracy of the hypocentral parameters improved considerably. As a result, the double seismic zone within the subducting Pacific plate is more clearly delineated than in previous studies. The inclined seismic zone for the subducting PHS plate coincides fairly well with the high‐velocity zone of our three‐dimensional velocity model.

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