Polarization anomalies of surface waves suggest the existence of lateral variations of isotropic and azimuthally anisotropic velocity structures in the upper mantle. We investigate the polarization anomalies of fundamental-mode Rayleigh and Love waves (37 earthquakes, 128 paths) at periods of 5-30 s as recorded by a local four-station network of broadband seismometers in Hokkaido, Japan. The network has been operated by the Research Center for Earthquake Prediction of Hokkaido University since December 1988. Rayleigh waves coming from many back-azimuthal ranges show three types of particle motion anomalies, which are usually called inclined, tilted, and sloping motions. The Rayleigh anomalies observed in the data for the Vanuatu region are mainly caused by the azimuthally anisotropic structure beneath the northwestern Pacific, because the effects of the lateral eterogeneities on the inclined motions are considered to be negligible. The Love waves coming from the earthquakes located near Oregon and California, USA, show anomalous waves in the vertical and radial components. It was expected that the waves were higher-mode Rayleigh waves. We calculate synthetic waveforms with normal modes for an oceanic spherically symmetric Earth model for the August 17, 1991, earthquake off the coast of northern California, which shows significant anomalous Love waves. A comparison of the synthetic and observed waveforms suggests that the anomalous waves are not higher-mode Rayleigh waves and require the Love to Rayleigh conversion. The conversion locations concentrate in and around the Kuril trench region. The Love wave anomalies may be caused by lateral variation in the isotropic or anisotropic structures beneath the Kuril trench region.
The process of the collapse of the dacitic lava dome and the development of pyroclastic flows at Unzen volcano, Japan, were studied using infrasonic, seismic and video records. Characteristic infrasonic and seismic signals were recorded corresponding to the collapse of lava blocks from the dome, the drop of blocks on the slope and the migration of pyroclastic flow on the mountain slope. Small infrasonic and seismic waves are excited when the lava dome starts to collapse. When the lava blocks fall onto the mountain slope and are fragmented, larger waves are excited. This suggests that the seismic waves are generated by the collision of pyroclastics on the mountain slope and that the infrasonic waves are excited by small fractures of the dome and the fragmentation of pyroclastics. Some of the infrasonic signals show an obvious Doppler effect, indicating that the pyroclastic flows emit infrasonic signals during their propagation. The location of dome collapse and the path of pyroclastic flows can be identified and traced by a network of low-frequency microphones. The migrating source of infrasonic signals and probably seismic signals is inferred to be located near the front of pyroclastic flows by comparison with video images. This suggests that the fragmentation of pyroclastics occurs mainly near the front of pyroclastic flows. The speed of pyroclastic flows is estimated as 10-30 m/s from the infrasonic records. The excitation of infrasonic and seismic signals is affected by the topography of the mountain slope. The infrasonic energy is almost the same order as the seismic energy but the ratio of infrasonic to seismic energies increases for larger and more mobile pyroclastic flows. This means that the development of pyroclastic flows is controlled not only by the volume of lava and gravitational force, but also by the explosivity related to the pore gases in the lava.
We observed three explosions along a 60 km profile in the central part of the Kitakami massif, northeastern Honshu, Japan. Explosion sites and most of the observation sites were located on hard-rock outcrops. Using P-wave first arrivals, the average QP along ray paths and site amplification factors were determined for the frequency range between 6 and 30 Hz based on the amplitude spectra decay with distance. QP increased proportional to fn (n_??_0.9). The difference in amplification factors among hard-rock sites was as much as a factor of five for frequencies lower than 12 Hz and became large at higher frequencies. QS and S-wave site amplification factors were obtained only for 5 and 7.5 Hz. QS was slightly larger than QP, but the difference was not significant. S-wave site amplification factors were more variable than those of P-wave among the stations.
The detailed P-wave velocity structure of the crust in the southern Kanto-Tokai region was analyzed using the tomographic method for seismic refraction survey in this paper. A total of 332 P-wave arrival times received from 13 seismic explosion surveys were used in the analysis. The results indicate that analyses of travel-time curves are probably useful for the evaluation of inverted structures. The lateral heterogeneity of the velocity structure is obviously related to tectonics. The crust in the eastern region is thinner than that in the western region. The Conrad discontinuity obviously fluctuates. The granitic layer is thinner beneath the oceanic region to the east of Oshima. The layer becomes about 16 km thick beneath Suruga Bay. The Conrad discontinuity drops nearly 17 km in depth beneath Suruga Bay, and velocity is relatively low there. The Conrad discontinuity rises 6 km beneath MTL and its vicinity. The Moho discontinuity is located at a depth of around 34 km beneath the region to the west of ISTL and roughly coincides with the upper boundary of the seismic zone due to subduction of the Philippine Sea Plate under the Eurasian Plate. It becomes shallow across the Suruga trough toward the eastern region. The discontinuity is located about 27 km in depth beneath the oceanic region east of Oshima.