The direct method of fuzzy pattern recognition has been applied to researches of earthquake precursors observed in the Tokai area and its vicinity. The precursors discussed are those in radon content in ground water, level, tilt, tide and volume strain. The basic technique of the method is to construct a suitable membership function for several kinds of precursors or for one kind of precursor observed at different stations. Possible precursor patterns for short-term earthquake prediction in the Tokai area are proposed. One is applied to a multi-approach analysis of several kinds of precursors observed at different stations, another is a synthetic analysis of one kind of precursor observed at different stations. By using the method, short-term earthquake precursors can be recognized more clearly. Moreover, some noises such as rainfall influence, seasonal variation and so on can be removed. Then on the basis of fuzzy recognition of earthquake precursors, the temporal and spatial characteristics of precursors before large earthquakes can be studied more quantitatively.
The effect of underground water on the borehole volumetric strainmeter is investigated by conducting continuous measurement of the underground water level in the borehole at Yugawara, Kanagawa Pref., central Japan. The dilatational change soon appears after the rising of the underground water level, but about two days later, the strainmeter begins to show the compressional sense. The former feature may be explained by the effect of underground water that decreases stress in the rock surrounding the strainmeter, which brings about the expansion of volume strain. On the other hand, the latter is explained by the increase of load due to the rising of the underground water level to the rock underneath the water table. Characteristics of strainmeter response to the change of the underground water level exhibit seasonal variation, which may indicate the change of transfer function due to difference of the underground condition.
A series of the volcanic eruptions of Izu-Oshima Volcano began on November 15, 1986. The tremors preceding these eruptions were investigated by using the seismic records obtained at Oshima Weather Station, 0.8km away from the Miharayama crater. The intermittent tremors began to be observed in July, i.e., four months before the first volcanic eruption. The tremors were generated repeatedly with a relatively stable recurrence time. Their duration and amplitude were relatively stable as well. The generation mechanism of these tremors can be considered something like the following. Energy is continuously supplied into the source area; the accumulated energy is released when it exceeds some threshold value; the vibration in the source area which causes the energy release results in the generation of tremors. Detailed observation showed that the maximum amplitudes of intermittent tremors gradually increased with time. The E-W components of the tremors were predominant in the early stage, while the N-S ones became dominant toward the end of October. The trains of the tremors generally recurred at regular intervals of about 2 hours. But trains of tremors were generated at intervals of about 3 hours from July 19 to 21, and from October 21 to 24. The generation of tremors stopped on October 24. Tremors began to be observed again after a quiescence of 3 days, and then not intermittently but continuously. The apparent period of the tremors remained almost constant, i.e., 0.6 seconds, whether the tremors were intermittent or continuous. Some of these characteristics of the tremors may be explained by the source migration and/or by the path effect, but most of them are attributed to the generation mechanism of the tremors. The total energy of the premonitory volcanic tremors were estimated to be 1015 erg. It was found that the energy release rates of the source area increased toward the volcanic eruptions.
The along-wind elongation of the tracer plume was observed in the medium-range atmospheric diffusion experiments with finite release time of air tracer substance. Since the elongated portion of a detached plume pursues the same course in the treatment of along-wind diffusion as an instantaneous puff, the along-wind diffusion parameter σx was estimated from the time variation of concentration in the elongated portion of the detached plume. This was compared with σx from other equivalent range experiments after grouping under stable, near neutral, and unstable classifications of atmospheric stability. In stable condition, a slight tendency to smaller σx values was recognized, but distinct dependency of σx growth on stability was not observed. Furthermore, consideration was taken of the valuation of time average concentration relating to the tracer release time, sampling time, and total time of tracer cloud passage. The method of evaluating the average concentration was considered, and it was made clear that the method was available for any sampling mode.
The relationship between spring snow cover and summer atmospheric circulation has been studied using satellite derived snow cover data and 500mb geopotential height. A good correlation was found between April snow cover over West Siberia and June 500mb height over northern part of East Asia.