The authors have attepted a measurement of the potential of Albizzia plants in the field since June 1977. Generally, the potential of the Albizzia shows certain types of curves under the normal condition. Then, saw-toothed anomalous potential was recorded intermittently since June 10 to June 22, 1978. In the meantime the authors had the Miyagi-Ken-Oki earthquake (1978. 6.12, 17h 14m, M=7.4). The anomalous changes of the potential seemed to have some connections to the earthquake. Thus, the authors would like to pay a close attention to anomalous potential which appeared on the record of June 10 and 11 (55 to 30 hours before the earthquake). It is advisable herewith to perform a long term observation of the potential of the Albizzia plant and/or earth-current itself in the wide area of Japan, especially along the coast of the Pacific-Ocean.

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In-situ stress measurements were made in a 450m deep well in mudstone (Cretaceous) by the hydrofracturing technique. Stress was measured at six points between 60 and 391m. The results show that the maximum horizontal compressive stress increases with depth from 32 to 130bars, and that the minimum stress increases from 26 to 92bars over the same depth range. The maximum and minimum stresses at a depth of 400m were determined to be 136 and 100 bars, respectively. A borehole televiewer was used to detect new fractures created by hydrofracturing. However, the directions of fractures were not detected because of bad condition of the borehole wall for the televiewer.

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Two local earthquakes on April 11 and 21, 1979, which occurred 10km south off Hakodate, recorded seismic intensities III in JMA scale in and around Hakodate city.
This study was designed to confirm that a difference on the patterns of seismic zoning maps exists between at a remarkable event and such a local one. So far as is known, comparatively little work has been done about questionnaires for local events with magnitudes 4.0 or less. Immediately after the earthquakes, the questionnaires were distributed to the persons living in and around Hakodate city. Answers to the questionnaires were retured from about 3000 persons a few days after, and seismic microzoning maps were constructed. The maps show that seismic intensities depend strongly on the subsurface structures at sites; for example, the intensities in unstable places of alluvial formations are, on an average, about 0.3 less than those in relatively stable places of diluvial formations. On the contrary, the former places show about 0.3 greater than the later for the cose of the remarkable event of the 1968 Tokachi oki earthquake (M=7.9).
In addition to the difference of the seismic intensities related to the geological age, the local intensities vary with S-wave velocities in the subsurface structures. The seismic exploration data previously obtained provide with no interface of S-wave velocity discontinuity down to a depth of 40m at least in the region, Nanaehama, while those provide with an clear S-wave velocity interface at a depth of about 15m in the region, Kamiyunokawa. The local intensities in Nanaehama are not so large for the present events though it places in unstable alluvial formations, while, the local intensities around Kamiyunokawa show nearly the maximum in the whole Hakodate. These facts strongly suggest that the subsurface structures act as band-pass or high-cut filters owing to the existances of the clear boundaries of S-wave velocity discontinuities.
Moreover, the predominat periods of incoming seismic waves should also be connected with the response of the subsurface structures. The predominant periods of particle velocities of these local earthquakes were compared with those of the 1968 Tokachi oki eartquake using the empirical relation after SATO (1979). The comparison shows that there exists a considerable difference among these periods; the longer the period, the deeper structure is reflected on it. This shows that the contribution of the inherent characteristics of the incident wave to the seismic intensity is not negligible.
Nevertheless, the local intensities is, on the average, related simply to epicentral distances as
I=-0.1r+3.9
where r and I represent the epicentral distance in km and the average intensity at the distance r, respectively.

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Many precursory phenomena, both geophysical and biological, were reported in relation to the Haicheng (M=7.3, 1975) and Tangshan (M=7.8, 1975) earthquakes in China. Precursor times and epicentral distances of these premonitory effects are analysed by making use of the Weibull distribution which was extensively utilized by RIKITAKE (1975, 1978a, 1978b, 1979a, 1979b) and RIKITAKE and SUZUKI (1979).
Three kinds of precursor, each having a mean precursor time amounting to 418 days, 2.8 hours and 4.2 days respectively are found from the analysis for the geophysical precursors of the Haicheng earthquake. These precursors seem to coincide with the first, second and third precursors as defined by RIKITAKE (1979a). On the other hand, the second or imminent precursor seems to have been missing for the Tangshan earthquake although the other two precursors having mean precursor times respectively amounting to 497 and 21 days seem to have been observed.
Mean precursor times for anomalous animal behavior are estimated as 7.8 and 0.6 days for the Haicheng earthquake, while only one peak of precursor time is obtained at 0.4 days for the Tangshan earthquake.
Maximum frequency of the epicentral distance of precursors seems to occur within a range 120-140km for both the earthquakes.
The results of the present analysis do not differ much from those so far obtained for the distribution of precursor times for world-wide data and a few particular earthquakes. It may be that we may accomplish an earthquake prediction provided careful detection of possible precursors can be performed with a dense array of observation instruments covering an extensive area.

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Explosive seismic waves generated from the 1st Ogishima explosion, western Tokyo explosions, and the 8th Yumenoshima explosion were observed at Koganei, Tokyo. The observations were done by quadripartite or tripartite array in the campus of Tokyo Gakugei University except for the 1st Ogishima explosion which was done by a single station. Apparent velocities and directions of approach of the first arrivals are derived for the respective shots by the conventional analysis of the tripartite method. They are compared with the true velocity of the base rock of Tokyo and with the direction of the great circle azimuth between the station and the shot points. Preliminary results are as follows.
(1) Velocity anomalies, which are defined by the differences between the apparent velocities and the true velocity of the base rock, are positive for waves that approach from S-E region and are negative for those from W region. This result is interpreted in terms of dipping interface of the base rock of about several degrees toward E to SE under Koganei. This interpretation is consistent with rather wide-range three dimensional structure, which is the contour map of the time terms, of the base rock of Tokyo.
(2) Azimuth anomaly, which is the angle between the observed direction of the first arrival and the great circle azimuth, measured counterclockwise from the latter, does not have such a systematic pattern as the velocity anomaly has. It is considered that the azimuth anomaly depends strongly on heterogeneity along the paths of wave propagations. The Bouguer's gravity anomaly at the western Tokyo, which may reflect the heterogeneity of sedimentary layer over the base rock and/or the irregular surface of the base rock, is the azimuth anomaly.
Time terms at shot points and at Koganei for the first arrivals are also obtained. They are consistent with the time term contour except for the B-shot of the western Tokyo explosion.

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An active earthquake swarm have been occurring in and near Iwasaki, Aomori Prefecture, the northeast of Honshu, Japan from September 16, 1978 with precursory earthquake swarm before 3 and 9 days. The magnitude of the largest event was 4.3. A temporal seismic network using wireless telemetry was developed to precisely determine the distribution of hypocenter locations. Hypocenter locations of 2000 events were determined for the period of the observation over 80 days. Most of hypocenters were located at a distance less than 2km from the coast line and in a small region beneath the Sea of Japan of water depth shallower than 50m. An extent of the earthquake swarm region was about 2km in the north-south and 4km in the east-west direction. We found hypocenter distribution of shallow earthquakes having planed structure, which had not been found in the Tohoku district so far. Hypocenter locations projected on the vertical section in the northwest-southeast direction clearly showed that the events occured in a region of planed structure dipping about 40 degrees in the direction of the northwest. The events were at depths ranging from 4km to 7km. A high seismic activity was found at deeper focal depth from 6km to 7km in the first stage of the swarm and decreased rapidly from the latter half of October. However, the seismic activity of shallower depth did not decrease till the end of November.

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The structure of the uppermost mantle in Shikoku, Japan, is examined by using S waves from deep earthquakes (H>60km). The most predominant phase of S waves has nearly the same travel time as derived from the Ichikawa-Mochizuki Travel Time Table. This suggests that the uppermost mantle in Shikoku has the velocity distribution assumed by Jeffreys and Bullen. Therefore, the general supposition that a plate-like layer with a P wave velocity of over 8.0km/sec exists with a thickness of 30 to 50km beneath Shikoku is not adequate. We consider that the mantle at depths greater than 50-60km is so ductile that it cannot accumulate stress enough to cause earthquakes in it. S waves propagating along the intermediate-depth seismic zone beneath Kyushu are observed prior to the arrival of the above mentioned S phases at the observation stations of the Kochi Earthquake Observatory. The amplitude of the earlier phase is smaller than that of the later S phase with a wave path in the ductile mantle. From these observations it is to be inferred that the mantle becomes ductile from just beneath the intermediate-depthseismic zone.

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Some features on earthquake sequences occurred in the Oshima peninsula, southwestern part of Hokkaido are appreciated in this paper. First data on rather remarkable earthquake sequences for 1900-1980 were briefly summarized, then recent microearthquake series observed by a new seismic network of the Research Center for Earthquake Prediction (RCEP), Hokkaido University, were studied.
Seismicity near the ESH station during the period from July 1976 to October 1978 was investigated in detail. While seismic activity was low in the first one year, it became high and 8 series of earthquakes were recognized in the last two years. A premonitory seismic activity which may be closely associated with the forthcoming main earthquake sequence was found in some cases. It has been pointed out that the pattern of earthquake sequences is classified into three types: main shock-aftershock series, foreshock-main shock-aftershock series and swarm, and that high degree of fracturing in mechanical structure of the uppercrust in the Oshima peninsula is deduced from the frequent occurrence of earthquakes preceded by foreshocks and earthquake swarms. However, the 8 sequences investigated showed not only typical swarm type but also other types, including transient or complex ones. Time series pattern of events in the sequences was examined with relation to variation of locations, magnitudes, amplitude ratios of P and S waves, waveforms and so on. The examination shows that there is no common characteristic between the above parameters and the sequence type. Complicated features of earthquake sequences may imply that the degree of stress concentration which may be attributed to heterogeneity in mechanical structure of the uppercrust is different even in a limited region in the Oshima peninsula.
It has been proposed that there is a tectonic line along the northeastern margin of the Oshima peninsula, connecting two active volcanoes, Komagatake and Esan. Earthquakes occurring in the part from Komagatake to ESH may by related to this tectonic line, while no seismic evidence was found in the part from ESH to Esan.

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An observation system for routine recording of the ground's resistivity change was established on May 14, 1968 by the writer at the Aburatsubo Crustal Deformation Observatory, University of Tokyo, on a coast of the Miura Penninsula, Kanagawa Prefecture.
During the observation period from May 14, 1968 to Feb. 28, 1975, 30 records of clear coseismic resistivity change were observed at remarkable earthquakes in and around Japan. Among 30 earthquakes, there are several cases accompanying some sort of preseismic resistivity changes of which the duration times are of the order of a few hours.
The most typical records of preseismic resistivity change are those for two earthquakes of M=7.9 (Tokachi-Oki) at Δ=936km on May 16, 1968, and M=6.9 (Izu-Hanto-Oki) at Δ=144km on May 9, 1974. The duration times of preseismic resistivity change for two earthquakes are 2 and 4 hours, respectively. In both cases, resistivity decreased. Among 30 coseismic resistivity change records, 17 cases clearly indicate preceding resistivity changes about a few hours prior to each earthquake occurence.
The patterns of these 17 preseismic resistivity changes are classified into three types, i. e., I, II and III. Type I is a gradual change of the ground's resistivity leading to a step-like coseismic change. The recorded pattern shows a half dome type or its upside down. Type II is characterized by more or less linear decrease in resistivity mostly followed by step-like coseismic resistivity increase. Type III is a step-like change preceding the coseismic change.
A new type of precursory resistivity change was observed associated with the Izu-Hanto-Toho-Oki Earthquake of M=6.7, June 29, 1980. This type of change, possibly classified as type IV, is characterized by an intermittent pulse-like change toghther with a box-car type change.

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Shear wave velocity, soil density and standard penetration test value in depth at 35 boring points in Nagoya area were measured by means of P and S wave-velocity logging up to about 100m in depth. The empirical equations for estimating shear wave velocity and soil density were derived on the basis of these underground data as well as soil characteristics. By making use of these equations and boring data (depth, standard penetration test value, soil age and soil facies), shear wave velocity and soil density in depth were estimated. By means of multi-reflection method of shear wave, transfer functions at about 280 mesh points in Nagoya area were calculated, and distributions of maximum amplification factors and predominant periods were obtained. It was found that maximum amplification factors were large and predominant periods were long in the western part of Nagoya area. The relation between the maximum amplification factor, and the damage ratio in the Nobi and the Tonankai Earthquakes was studied, and it was clarified that the larger maximum amplification factor the greater damage ratio bacame.

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