In 1981, the Research Group for Explosion Seismology recorded a seismic-refraction profile across northern and central Nagano Prefecture, central Japan, to study the velocity structure of the upper crust. The 220-km-long profile, which consisted of six shot points and 82 temporary stations, extended northeastward from near Mt. Ontake to the southwestern part of Niigata Prefecture, crossing the Itoigawa-Shizuoka tectonic line. The entire region is characterized by the highly heterogeneous upper crust. In the northeastern part of the profile, three thick folded surface layers with P-wave velocities of 2.0, 3.1, and 4.7 km/s overlay the basement with a velocity of 6.0 km/s, that slopes down northeastward. In the Nagano basin, the basement lies at about 1 km depth below the surface. An abrupt change of the basement depth by 4 km is found on the east of the Matsumoto basin. The associated subsurface fault is inferred. to a reverse type to satisfy the constraints from both the travel time and the Bouguer gravity anomalies. This fault may be a part of the Tsunan-Matsumoto tectonic line, which has been geologically recognized. The Itoigawa-Shizuoka tectonic line, which is one of the major tectonic lines in Japan, strikes from north to south in the Matsumoto basin. It is, however, not marked by any distinctive seismic velocity features. Southwest of the Matsumoto basin, Paleozoic-Mesozoic rocks are recognizable, and a basement with a velocity of 5.9 km/s lies at about 1 km depth.

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Wave propagation in a cut-ground is determined by using the Equivalent Mass-System Model, a type of finite difference method with some modification. A point-concentrated force is applied on a lower surface. Waves observed are concerned with the resultant diffractions at lower and upper corners, and with fraction of step-height H to wavelength λ. Wave characteristics are determined from the inclination of the time-distance curves, and are justified by the orbital motions. The leading part of waves is a direct Rayleigh pulse on a lower surface; a Rayleigh pulse across the lower corner and a Rayleigh pulse reflected from the upper corner on the cliff-face; and an SV pulse diffracted at the lower corner from the direct Rayleigh wave and a Rayleigh pulse travelling through the whole surface on the upper surface. The maximum amplitude of Rayleigh pulse across the step to the step-height is exponentially decaying: exp(-0.1H/λ). Some comments on diffraction at the corner are given.

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A theoretical analysis is performed on the scattering of Rayleigh waves at a variously inclined discontinuity, where a two-dimensional sinusoidal Rayleigh wave is incident. Reflection and transmission coefficients are numerically derived. The calculable extent of the inclination angle of the interface ranges from 74° to 106° to the free surface, though it is reduced to a certain degree, according to the assumption of elasticities in both sides of the discontinuity. Within this range the variations of the coefficients are discussed under various assumptions of elastic constants in the media. The amplitude ratios of reflection and transmission do not greatly change with the inclination of the interface, but the phase shift of reflection varies linearly, and hence, by the use of the latter it is possible to determine the actual inclination of the interface from the observed Fourier spectra of incident and reflected Rayleigh waves. Scattered body waves, especially S waves, from an interface show rather complicated distributions, depending on the properties of the media.

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A theory of SH-wave generation by hammering horizontally a plate placed on the ground is presented. This technique is called the Ita-tataki method. The system consisting of a hammer, plate, and weight for the method is replaced by a system consisting of two point masses, ml corresponding to a hammer and m2 corresponding to a plate subjected to a frictional force, and a spring having an effective' constant k. The motions of the masses are classified into three stages and separate equations of motion are given for each stage. Analytical expressions of the spectra of forces exerted on the ground by this method are derived. Dependence of the ground displacement on the initial velocity of the hammer V₀ and physical constants of the system are as follows: With other constants fixed, there exists a limiting weight at which a maximum amplitude is attained. Any weight beyond this limit has no effect. The limiting weight is the minimum weight under which the plate will not move. In this case, the spectrum is the so-called cosine-type. Its zero-frequency-amplitude, f(0), is determined by 2 km₁ and the first zero-amplitude-frequency, v₀(∞), by (3/2π)√ k/m₁ · In the case of a lighter weight, v₀(∞) is reduced but f(0) remains almost constant. A lower frictional coefficient results in a heavier limiting weight. For a given weight and V0, use of a plate having larger k is effective to obtain a larger ground displacement. Because the ground motion generated by this method is highly frequency dependent, selection of an appropriate observing frequency (in the displacement space) is desirable. The present theory is consistent with almost all experimental results reported.

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Several earthquakes with focal depths of 12 to 27 km occurred in the Kinki, Chugoku, and Shikoku districts of Japan, and their magnitudes were from 3.0 to 5.6. Initial motions of seismic waves from these earthquakes were comparatively clearly recorded at seismic networks for the observation of microearthquakes which are equipped in the surrounding regions of their epicenters. From the comparison between the observed travel times and the travel times calculated under assumed crustal structures, the following conclusions are drawn. 1) No sharp discontinuity in seismic wave velocity is present in the depth range of 10 km to the Moho discontinuity. 2) The P wave velocity in the lower crust has only a little variation and perhaps its value is not more than 6.5 km/s. 3) The P wave velocity in the depth range of 12 to 20 km increases gradually from about 6.0 to 6.4km/s. 4) The Moho discontinuity is very sharp and the transition thickness between the crust and the mantle is a few kilometers at most. The depth of the Moho discontinuity is 36-37 km.
In addition, compared to the propagation velocity of P waves in the north-south direction in the crust, an increase of about 1.5% was found in the east-west direction. It is supposed that this effect is due to the tectonic forces acting on these districts.

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