A simple model of an almost frequency-independent Q is used to estimate the density spectra of the elastic signal recorded at a given distance from the source: the case of a contained underground explosion is treated in some detail. The computed spectra give fairly good fits to the experimental curves of five presumed explosions, each recorded at five teleseismic arrays (FILSON and FRASIER, 1972), and allow an estimation of tentative values of the duration of the pressure in the cavity where the presumed explosion occurred. They fit also the observed spectra of EVERNDEN (1977).
In case the explosion is represented by a pressure of amplitude D and duration b applied on a contained sphere of radius r₁, the theory allows the retrieval of the three source parameters.
The results of the paper can be immediately applied to the case when the signal is originated by an earthquake and may be useful for the reconstruction of the source parameters.

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Quasi-static strains and tilts on the surface are investigated for a dislocation source in a stratified elastic half-space with an intervenient, Maxwellian viscoelastic layer. Integral representations for the strains and tilts are obtained by differentiating those for the displacements derived in a previous paper (MATSU'URA et al., 1981) with respect to space coordinates. Evaluation of the integral is carried out by using a method of approximating a part of the integrand by a simple analytical function.
Numerical examples of the strain and tilt fields are computed for some typical cases to examine general features of the viscoelastic surface deformations. Time-dependent behavior of the tilt fields caused by a point dislocation source in the elastic surface layer of a three-layered model is examined for various cases with different thicknesses of the viscoelastic layer. A common feature to all the layered structure models is that the amount of the viscoelastic deformation increases gradually with time, and the deformed area extends out quite slowly from the source. The change in the thickness of the viscoelastic layer affects the amount and extent of the deformation. Patterns of the shear strain, principal strain, and the ground tilt due to a finite-dimensional fault are presented for a three-layered model (Model III), and compared with those for different types of structure models, such as a viscoelastic half-space model (Model I) and a two-layered model composed of an elastic surface layer overlying a viscoelastic half-space (Model II). Patterns of the viscoelastic deformation fields for Model I are quite different from those for the layered structure models. The difference between Model II and Model III is not so distinct, except that the deformation field for Model III is somewhat restrained in amplitude and extent, compared with that for Model II.

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Subcrustal earthquakes beneath the Lake Biwa in the northeastern Kinki district, Japan have been investigated from data of microearthquake observation networks. Earthquakes, which belong to this group, are different in their occurrence mode from other subcrustal earthquakes in the Kinki and the Chubu districts. Their hypocenters, the depth of the Moho discontinuity and the P_n velocity are determined consistently. The Moho discontinuity beneath the Lake Biwa lies at a depth of 40km which is the largest in the Kinki and the Chubu districts. The P_n velocity beneath the Lake Biwa is about 7.5km/sec and is about 4% lower than the average one, say 7.8km/sec, in the Kinki district. Subcrustal earthquakes beneath the Lake Biwa are different from other subcrustal earthquakes which occur in the uppermost mantle in the neighbouring region due to the subduction of the Philippine Sea plate. Seven earthquakes occurred at the depth of the Moho discontinuity, and others occurred at 3km above and 2km below the discontinuity. The deepest one show the focal mechanism of the reverse faulting with P axis in the direction of NE-SW. Six earthquakes which occurred at the depth of and above the Moho discontinuity show the focal mechanism of the normal faulting with T axes in the direction of NW-SE, which is parallel to the direction of the slip vector of the relative motion between the Philippine Sea plate and the Asian plate and is locally perpendicular to the leading edge of the Philippine Sea plate. The bending of the crust due to the compressional stress by the Philippine Sea plate is one of the possible interpretations for the nature of the source mechanism. This idea is also supported by the focal mechanisms of crustal earthquakes around the Lake Biwa. An idea that accounts for northwest-ward migration of the Lake Biwa is also proposed based upon the occurrence characteristics of earthquakes in the region.

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The total power, the duration, and the starting time of band-pass filtered accelerograms are chosen as model parameters to express amplitude variations of ground acceleration in both the time and frequency domains. In order to estimate model parameters of certain earthquakes and site conditions, a probabilistic source model is used and accelerograms recorded during the Matsushiro earthquake swarm are analyzed statistically. The duration and the starting time of the strong ground motion are found to have close relations to the amplification characteristics of surface layers at the sites. As an example, model parameters are estimated by assuming the source parameters and the site conditions of the Miyagi-Oki earthquake of June 12, 1978. Wave-forms, running spectra, and velocity response spectra of synthetic accelerograms resemble those of observed accelerograms.

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This paper investigates the stress field in the interior of a semi-infinite elastic medium due to internal stress sources. Importance of strain nuclei as sources of the internal stress is emphasized. Patterns of the stress changes induced by introducing a simple strain nucleus are illustrated with the aid of equal area projection. The internal stress field in and around Japan is calculated by distributing double couple point sources on the deep-seismic plane with their compressional axes parallel to the dip of the descending plates, and the results obtained are projected on a Mercator map. Such a model may explain the actual stress field in this region at least to some extent.

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