We present a new method to obtain in-situ seismic wave velocities in the source region of shallow earthquakes. Travel times of initial P- and S-waves of earthquakes occurring in a small area are analyzed. First, assuming a certain P velocity model, hypocenters of earthquakes are determined using first P arrival times only at near stations. Second, the velocity in the source region is calculated by the regression analysis of travel times of earthquakes observed at each distant station. Third, we compare it with the assumed velocity there. The above two steps are repeated until they agree with each other. We have applied this method to subcrustal earthquakes with depths of 23-38 km in two small regions in the Tokai district, and P and S velocities of 6.9-7.0 and 4.0 km/s were obtained, respectively. Since these values are quite similar to those in the main layer of the oceanic crust, we can conclude that subcrustal earthquakes in these regions occurred within the subducted oceanic crust of the Philippine Sea plate. Moreover, apparent P and S velocities of 7.2-7.3 and 4.1-4.2 km/s were also obtained. Thus we can also conclude that no mantle substance exists above the subducted oceanic crust there.

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The effect of rainfall on crustal strain at the Mikawa Crustal Movement Observatory, central Japan, was studied in detail with a data set from January 1974 through December 1984. Changes in crustal strain caused by rainfall were successfully simulated by a tank model, in which the crustal strain is given as a nonlinear function of precipitation and time. The simulation error was usually less than 10%, but occasionally it became larger. There were anomalies of two kinds: 1) a change in spatial distribution of strains in the observation tunnel; and 2) an increase in simulation error for the rainfall effect. These anomalous responses seem to be related to the occurrence of nearby earthquakes within a particular area around the observatory. The logarithmic duration time of anomaly was roughly proportional to the earthquake magnitude. Data from the past eleven years indicate that about 50% of M ≥ 3.5 earthquakes in the area were preceded by anomalous strain responses if they occurred after rain. The change in strain response to rainfall at this observatory could be a precursor of a nearby earthquake.

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Effects of the strain rate and pressure on various brittle properties of granite under compressional loading are studied experimentally. Granite specimens were tested to failure under various constant strain rates at confining pressures of 0.1 to 200 MPa. The strain rates in these tests varied from 10^-4 to 10^-8s^-1. The results show that the strength of granite decreases linearly as the logarithm of the strain rate decreases, and that the strain-rate dependence on the strength is enhanced at high confining pressures. The dilatant strain and elastic wave velocity variations with stress were found to be independent of the strain rate if the stress is normalized by the strength. The dilatant-strain-versus-stress curve is significantly affected by confining pressure but the confining pressure effect on the dilatant strain versus normalized stress is small. The acoustic emission rate is accelerated at a stress level closer to the failure strength as the strain rate decreases. Some of the timedependent properties are explained by a theory based on the concept of subcritical stress-corrosion cracking. But it is also clear that the stress-corrosion cracking theory does not provide reasonable explanations of the variation of acoustic emission rate with stress and the variation of strain with stress at the stage immediately before fracture.

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As the first application of the method of Hashida and Shimazaki (J. Phys. Earth, 32, 299-316, 1984), a three-dimensional seismic attenuation structure and source strengths are estimated by inversion of seismic intensity data for earthquakes that occurred in the Tohoku district, Japan. We carefully selected 1, 630 intensity data from 101 earthquakes so that the intensities are consistent with accelerations converted from Kawasumi's relation and so that a well-resolved attenuation structure is obtained. A reasonable fit of the formula proposed in our preceding paper to the actual intensity data guarantees that a systematically biased attenuation structure is not obtained. A comparison of the obtained attenuation structure with velocity structures estimated by previous studies shows that high (low) Q nearly corresponds to high (low) V. The correlation of both structures indicates that the attenuation structure estimated by the proposed method is reliable. The resultant attenuation structure shows a remarkable contrast in the attenuation coefficient and two prominent features. The first feature is low-Q zones down to a depth of 90 km, which corresponds to the distribution of volcanoes. The second is high-Q zones that correspond to the subducting Pacific slab. The high-Q slab is in contact with the high-Q zone in a depth range of 30-60 km, which lies on the east side of the volcanic front. The presence of high-stress earthquakes in this depth range, such as the 1978 Miyagi-ken-oki earthquake, is explained by a model in which the contact of the underthrusting Pacific plate with the surface high-Q zone accumulates higher stress and thus causes stronger seismic coupling. The estimated source strength, which is expressed as a point source acceleration, correlates well with earthquake magnitude. Normalized source acceleration, which is an average acceleration over a source area, is estimated. The acceleration suggests that the stress drop of an earthquake becomes higher with magnitude and with depth. The relation between JMA magnitude (M_J) and seismic intensity at a hypocentral distance of 100 km (I₁₀₀) is found to be I₁₀₀=1.5 M_J-6.5 for crustal events. This I₁₀₀ is in agreement with the value reported by Utsu (Bull. Earthq. Res. Inst., 59, 219-233, 1984) which was determined from events excluding those which show anomalous distributions of intensity data. This agreement suggests that our method of estimating earthquake magnitude from intensity data is effective for removing the effect of structure.

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The dynamics of mantle convection below the oceanic lithosphere have been studied numerically, using a layer of a Newtonian constant viscosity fluid below a rigid lid. The rigid lid corresponds to the uppermost part of the lithosphere in which viscosity is extremely high owing to low temperatures. The thickness of the lid is allowed to vary to simulate the growth of the oceanic lithosphere; the bottom of the lid is determined by anisotherm. We find that the thickening of the rigid lid suppresses the growth of cold plumes in the thermal boundary layer just below the lid when the thickening rate L exceeds a certain critical value L_c. The value of L_c is found to be a function of viscosity through numerical experiments. Using the observed values of L for the oceanic lithosphere and the estimated value for viscosity in the lower part of the lithosphere, it is found that L > L_c for most cases. That is, the growth of cold plumes is suppressed in the lower lithosphere. Because growth of plumes in the lithosphere is thought to be one of the mechanisms for the initiation of subduction, the above estimates suggest that thickening of the oceanic lithosphere is an agent that suppresses initiation of subduction, and hence allows the existence of large oceanic plates.

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