Basic Knowledge on human behaviors under the circumstances of a large earthquake is of great importance to find out a better way of mitigating earthquake disasters, especially such serious calamity as loss of human lives and injuries.
For this purpose, by means of questionnaire and interview, a field survey was performed when the 1978 Izu-Oshima-Kinkai Earthquake attacked and then continued to a detailed one at the 1978 Miyagi-ken-oki Earthquake.
In this paper, using the data by the questionnaire survey, the relation between frequency in occurrence of various responses and the seismic intensity is investigated so as to disclose an average or general feature of human response during and after a large earthquake.
A major result obtained is a remarkable correlation of human responses to seismic intensities; when I_JMA increases from III to VI, (i) Psychological responses such as fearfulness become severer and passive or unautonomic behaviors increase in number, (ii) Behaviors immediately after the end of a quake become active in order to cope with damage and to prepare against aftershocks, and (iii) Undesirable effects on daily life tend to be greater and last longer.

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A new concept “seismic diversity” (SD) is proposed as a new measure of magnitude-frequency distribution. SD is defined as
H=1/nlog(n!/n₁!n₂!…n_s!)
where n is the total number of earthquakes in a given set of observations, and n_i′s (i=1, 2, …, s) are the number of events in evenly devided magnitude ranges. This formulation assumes neither specific form of distribution for the population nor random samplings from a population for the data set. Thus it is more appropriate for treating actual seismic data than the conventional constant b value formulation. The concept of SD is extended as a practical measure to represent the spacial distribution of earthquakes. It is shown that variations of both magnitude and spacial distributions of earthquakes are adequately represented by using this new measure.

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The source parameters of the Shizuoka earthquake (M=6.3) of July 11, 1935, are determined mainly on the basis of the close-in long-period seismograms. The epicenter and focal depth are redetermined at 35.0°N, 138.4°E and 27km. This earthquake represents a left-lateral strike-slip faulting on a plane dipping 70° toward 15°SE with a dimension of 11km(length)×6km(width). The average dislocation, rise time and stress drop are determined to be 1m, 1sec and 70 bars, respectively. The faulting at the depth of about 20km is very rare in Japan; because most of the major strike-slip events in Japan occurred near the ground surface. The theoretical ground motions expected from the above dislocation parameters are consistent with the leveling data and with the field data on the collapsed structures in the epicentral area.

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Application of a track etch method, for the mapping and detection of active faults and the evaluation of their activities has been carried out at 10 sites for the purpose of the earthquake research.
The method is applied conventionally for the measurement of relative random concentration in the soil gas by counting the number of tracks per cm²·day on a small piece of plastic film (cellulose nitrate) which is sensitive to α-ray radiation.
As the results of the track measurements on many survey lines crossing 10 active faults including earthquake faults in the areas of the Izu Peninsula and those in the lower reachs of the Fuji river, it was clarified that:
1. The peak of the track number appears just above the fault line generally, and sometimes shifts from it slightly.
2. The line connecting peaks on the several survey lines is parallel to the strike of of fault.
3. Relative position between the peak and the fault line on the surface suggests the type of the fault, normal or reverse.
4. The number of tracks counted on the Inatori-Omineyama and Sengenyama earthquake faults, which occured by Izu-Oshima-Kinkai Earthquake in January 1978, was anomalously high as much as several times of that on the existing active faults. This fact indicates that the supply of radon contained gas from the deep crust through fault plane would be activated by the crustal disturbance.
In addition to the above, weekly observation to monitor changes in radon content in soil gas using a track etch method has been carried on across the several faults such as Inatori-Omineyama, Himenoyu and Fujikawa faults for the purpose of the earthquake prediction, by present authers.

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In order to investigate the seismicity in and around Suruga Bay, several temporal observation stations with high sensitive seismometers have been set up since June, 1975.
The seismicity is not so simple as to suggest the existence of the sinking Philippine Sea plate in this region. The seismic activity in the northern half of Suruga Bay has been very low throughout the observation period, whereas the activity in the southern half had been low until September, 1976 and then turned out high.
On October 26, 1976, a small earthquake (M≤4) and its aftershocks occurred near Shizuoka City. The foci of these shocks distributed along the western part of the boundary between the seismic and aseismic areas in Suruga Bay, suggesting the existence of a tectonic line. An east-westward principal compressional axis has been suggested from the first motion data of the main shock. This orientation is different from those of major earthquakes in the earlier periods in this region, which show north-southward principal compressional axes.
According to the data by JMA, the Suruga Bay region was active before the Shizuoka earthquake of April, 1965 (M=6.1). The following inactive period lasted until summer in 1976. The subsequent seismicity according to JMA is consistent with our observation. These activity changes seem to be associated with focal mechanism changes of earthquakes in the central Shizuoka prefecture.

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The negative binomial distribution is found to show a good fit to the distribution of total number of aftershocks whose magnitudes differ by no more than d from the magnitude of the main shock M₀. On the basis of this observation, the probability is derived that the difference D_i in magnitude between the main shock and its i-th largest aftershock becomes larger than d.
Pr{D_i≥d}=∑^i-1_k=0(α+k-1k)(αbln10)^α{a(10^db-1)}^k/{αbln10+a(10^bd-1)}^α+k
where a and α denote the mean of a and the power parameter of the negative binomial distribution, respectively, and a and b indicate thee parameters of the Gutenberg-Richter's magnitude-frequency relation. It is assumed that the Gutenberrg-Richter's relation n(M)dM=a10^-b(M-M₀)dM holds for aftershocks and that b is constant for every aftershock sequence. we studied the aftershock sequences in the area of Japan and Greece, which have been compiled by Utsu and Papazachos etc., respectively. The distribution of the difference D₁ in magnitude between the main shock and its largest aftershock deduced from “negative binomial model” is agreeable with the observation. The value of b deduced from D₁ distribution is consistent with the one of aftershock sequences given by Utsu and Papazachos. The observed distributions of D₂ and D₃ show slight discrepancy from those expected from the model.

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Simple dislocation fault models of the San Fernando, California, earthquake of 1971 as deduced from leveling data have been presented with emphasis on an abrupt change in dip angle of fault plane at a shallow depth, and geologic and geomorphic implications of the models have been discussed.
Model-1 is a basic fault model ignoring the effects of sporadically distributed surface breaks. This model consists of two rectangular fault segments: the deeper (F₁) and the shallower (F₂). The geometry of F₁ was fixed with the constraints that it should fit the focal mechanism and that the hypocenter should lie on F₁. The geometry of F₂ and the offsets on F₁ and F₂ were determined by trial and error. In Model-2, a subsidiary fault segment (F₃) that represents the surface breaks was added in order to make displacement pattern near the surface breaks better fit.
The results obtained are as follows:
(1) The dip angle of F₁ and F₂ are 52° and 13° respectively. The dip of F₂ is very low angle, so that the fault plane bends abruptly at a depth of about 2.5km.
(2) The offsets on F₁ and F₂ are 1.5m and 5.5m respectively. The offset on F₁ is notably smaller, so that no noticeable uplift appears in the San Gabriel Mountains which has been uplifted due to thrust faulting during late Cenozoic time as suggested by geological evidence. If we assume uniform offset on entire depths of fault plane, it becomes easier to avoid this contradiction. Therefore it seems likely that the deeper part of the fault had slipped seismically or possibly aseismically prior to the earthquake of 1971.
(3) The geometry of fault models, together with some geological evidence, permits us to describe a probable history of structural evolution in the San Fernando Valley-San Gabriel Mountains border region: The Santa Susana fault, which is the projection of F₁, had been active and thick basin deposit had been deposited on the down-thrown side of the fault until early Pleistocene time. In middle Pleistocene time, F₂ was formed in, or on the base of, the basin deposit, and the Santa Susana fault became inactive. Consequently the location of surface faulting shifted several km south of the Santa Susana fault, and the up-thrown side of F₂ has been uplifted and intensely deformed scince middle Pleistocene time.
(4) Although the mechanism is unknown, the bending of fault plane can be explained in terms of an effect of “accretion” by the analogy of that in subduction zones. Such a bending model of fault plane may be applicable to some thrust fault systems in Japan which have very similar geologic and geomorphic settings to the San Fernando Valley-San Gabriel Mountains border region.

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