We report the detection of a large ultra-slow earthquake. Subsequent to the July 18, 1992, Sanriku-Oki, Japan, earthquake (M_s6.9, 39.42°N, 143.33°E), a clear post-seismic phase appeared in the crustal strains recorded by quartz-tube extensometers at Esashi (Δ=174 km, φ = -99°, from the NEIC CMT source). We have decomposed the crustal strains into atmospheric pressure effects, tides and trends by the Bayesian tidal analysis method. The EW component of the trend thus extracted displays a clear quasi-static increase of -4 x 10^-8 with a time constant of about one day. This is about four times greater than the co-seismic strain step. We have investigated the cause of the post-seismic phase to conclude that this was not due to local disturbances. A similar post-seismic phase of -15 x 10^-8 was also observed in the EW component of crustal strain records at Miyako (Δ =117 km, φ = - 80° ), Tohoku University. From the close similarity between the post-seismic waveforms at Esashi and Miyako, we attribute the post-seismic phase to the ultra-slow faulting subsequent to the seismic event. We have inverted the strain steps to obtain a solution of an interplate low-angle thrusting located about 54 km to the east from the NEIC CMT epicenter with a moment release of up to (1-4) x 10²⁰ m, which corresponds to that of a M_W7.3-7.7 seismic event.

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This paper provides a new approach to detect changes in the groundwater radon concentration related to an earthquake. We express changes in radon concentration in a radon-detection chamber by using stochastic linear differential equations. These equations are represented by the state space notation, and then its solution is replaced by an estimation of the state vector at discrete points in time with an assumption that the coefficients describing the stochastic differential equations are constant for a sufficiently small time interval. Since the solubility of radon in water depends strongly on temperature, the separation of radon from liquid water, which is necessary for radon detection, causes fluctuations in the observed radon concentrations due to water temperature changes in the chamber. We applied our procedure to some actual data sets on groundwater radon concentration with those on simultaneously observed water temperature, and found that the temperature effects on the fluctuations in the observed radon concentration can be satisfactorily described by our procedure. Furthermore, we were able to estimate the original radon concentration in groundwater before it was introduced into the radon-detection chamber, which was not affected by water temperature changes. The obtained original radon concentrations are very stable during normal periods, and anomalous changes associated with earthquakes were easily detected. Our new method will be very useful to examine time-variation patterns of changes in groundwater radon and will provide important information about the mechanism of radon changes related to earthquakes.

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Well-developed basin-induced waves with a predominant frequency of about 0.3 Hz were detected as later arrivals of a direct S-wave in seismograms observed in the Osaka basin located in western Honshu, Japan from three deep earthquakes occurring near Japan. The wave type and the secondary source area of the basin-induced waves were examined using a semblance technique and polarization analyses for small aperture array data in the center of the basin. The results well agree with the characteristics of the Love wave such as particle motion perpendicular to a propagation direction and dispersed apparent phase velocities. The arrival direction was evaluated to be almost due east. Seismograms from the east-west linear array crossing the basin show that the observed Love wave propagated from the east to the west after being excited at the eastern edge of the basin by incidence of the direct S-wave. Numerical simulations of two-dimensional SH problems were performed for the east-west cross section models of the Osaka basin subject to incidence of the direct S-wave. The synthetics agree well with the observed Love wave in a predominant frequency and a group velocity.

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The temperature variations of normal mode frequencies of a specimen of α-quartz single-crystal have been measured nearly up to the α-β transition (T₀ 573°C), and all of the independent elastic constants were determined simultaneously by the rectangular parallelepiped resonance method. The results demonstrate that bulk moduli decrease largely toward T₀, while shear moduli show only a slight decrease. The trigonal constant C₁₄ decreases just below T₀, but seems to remain a finite value even at T₀. It is noted that the isotropic aggregate of α-quartz is a curious and scarce material in the sense that its bulk modulus is smaller than the shear modulus, and Poisson's ratio is very small or even negative at temperatures higher than 450°C. In the measured temperature variations of mode frequencies, a characteristic feature was observed when two branches of A_g modes came close, suggesting mode coupling. Except for this coupling between A_g modes themselves, no definite evidence was observed which was indicative of coupling between modes of different symmetry. So it can be said the present sample is uniform in its property enough to suppress the coupling between modes of different symmetry.

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