Explosion seismic observations were made for the investigation on the crustal structure of Shikoku along a profile in NE-SW direction crossing the Median Tectonic Line (MTL), one of the most pronounced tectonic lines in Japan. Seismic waves generated by the Iejima explosion of 3500kg in charge amount and the first Torigatayama explosion of 2400kg were observed at sixteen temporary stations in January, 1977 and those by the second Torigatayama explosion of 4100kg, at thirteen temporary stations in February, 1978. A detailed shallow velocity structure in relation to the MTL was obtained from clear refracted waves through the granitic layer. The first layer shows an abrupt change of its thickness at the MTL. The layer is about 4km thick to the northeast of the MTL, but it might almost disappear in the immediate southwest of the MTL. Apart from the MTL towards southwest it tends to thicken. The deeper structure up to the upper mantle was obtained by making use of clear later arrivals such as reflected and refracted waves besides a small number of first arrivals at long distances. The granitic layer with a velocity of 6.1km/s has an average thickness of about 20km and is underlained by the basaltic layer with a velocity of 6.7km/s. The velocity of the upper mantle is 7.8km/s and the depth of the Moho discontinuity is 30-35km.

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Focal mechanism solutions have been determined for 113 shallow earthquakes which occurred from 1968 to 1971 in the east part of Kinki district and Chubu district, Central Honshu, Japan. These mechanisms have been obtained from the first motion of P waves recorded by the stations of Japan Meteorological Agency (J. M. A.) and microearthquake observation stations. Mechanisms have been determined for the earthquakes with magnitude as small as 2.5.
Although the local variation of pressure direction becomes larger, the smaller the earthquakes become, almost of the pressure directions obtained in this study agree well with those of the major shocks analyzed by many seismologists. But there exists a remarkable contrast of the pressure directions in the southwest of Chubu district (Mikawa district and Ise Bay), where pressure lay east-west in the crust but south-north in the upper most mantle. Similar results were obtained in the Kii straight and the south part of Shikoku by SHIONO (1970) and SAWAMURA and KIMURA (1971), respectively. These facts can be interpreted by assuming the pressure which lay perpendicular to the Nankai Trough and the subsedence of the Philippine Sea plate.

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Focal mechanisms of very shallow earthquakes on the west and the southwest of Lake Biwa have been determined by smoothing the first-motion radiation pattern and by individual solutions. Published focal mechanism solutions are also used to examine the tectonic stress which generates earthquakes in the north Kinki district. Along the west coast and the east coast of Lake Biwa seismic active regions of very shallow earthquakes occur and in Lake Biwa seismic activity is very low. Most of mechanism solutions in these active regions are reverse faultings with the maximum pressure axes in the direction of nearly east-west or southeast-northwest. In other regions of north Kinki district and west Chubu district dominant type of faulting is strike-slip with the same pressure direction. These types of faulting from earthquake focal mechanism solutions agree well with those of tectonic active faults developed in this area. The reverse faulting as well as the high seismicity on the both coasts of Lake Biwa is consistent with the subsedence of Lake Biwa which has been continued throuth the Quaternary.

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Many boulders were thrown off out of their former sockets by the Western Nagano Prefecture Earthquake of September 14, 1984. Anomalous high accelerations of 5-30g, in the frequency range of 5-10Hz, are estimated from the displacement of these thrown out boulders.
Almost all of the thrown out boulders were found on the tops, ridges and suddles of the mountains covered with the volcanic ash. The amplification effects by topography and soil sediment of the surface are estimated from the observation of the aftershocks recorded on the mountain-top, -foot and rock. The spectral ratios of seismic waves with the frequencies of 5-10Hz, namely mountain-top/-foot and soil/rock, are 2-7 and 2-10, respectively.
The accelerations on the basement rock are obtained dividing the accelerations estimated on the mountain-tops. The high accelerations exceeding 1g distribute within a small area with a length of 3km and a width of 1km. This small area corresponds to the large dislocation portion of the assumed fault and the low active region of aftershocks.

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An aftershock sequence accompanied by the earthquake occurred at the central part of Gifu Prefecture on Sept. 9, 1969 was studied. Because of short epicentral distances, a number of shocks were recorded at the Inuyama seismological network from the beginning of the main shock to the end of the November of 1970. These data were useful for the long-term investigation of the aftershock sequence. Those data recorded at Takayama seismological station and temporary stations at Hirayu and Okuzumi were also used in this study. The following results were obtained:
1) Epicenters of aftershocks were distributed along NW-SE direction. It was likely that they were represented by a two dimensional normal distribution with standard deviations of 2.2-3.5km and 3.5-5.0km across and along an assumed fault, respectively. The epicentral region was found to spread gradually towards both sides of the fault.
2) With the use of both data recorded at Inuyama and J. M. A. networks, the epicenter of the main shock was recalculated to the point where aftershocks were clustered.
3) The daily frequency of aftershocks for 450 days from two hours later from the main shock was well represented by single formula, n(t)=k·t^-p, with p=0.95.

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Accuracy of master event location method is studied for the microearthquakes recorded by the telemetered network of the Abuyama Seismological Observatory in northern Kinki district, Japan. The range in which the master event method is effective is examined in order to apply the master event technique using several master events to earthquakes in large area. Moreover the variations of hypocenters are estimated for the variations of the velocity and that of the thickness of the surface layer and the velocity of basement layer.
The master event method is effective for the events of which epicenters are within 20km from a master event, in the case the error of relative location is 0.2km for epicenters and 0.5km for focal depths. The errors of the relative epicenters and focal depths are also small for the possible change of structure parameters. Therefore, the regional change of focal depths can be detected clearly by the master event method using several master events. The precise distribution of hypocenters determined by the use of six master events shows that focal depths of all the earthquakes in this district are located in upper part of the crust shallower than 20km. Moreover the depth-frequency distribution of the earthquakes indicates two peaks at depths of 7-9 and 11-13km with abrupt decrease at 15-20km. Furthermore the depth of the seismic-aseismic boundary changes from place to place, such that the boundary dips from southwest to northeast by about 5km. The focal depths determined by this method is accurate enough to discuss the regional variation of seismic-aseismic boundary and its relation to the brittle-ductile boundary in the crust and to the Conrad discontinuity.

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Frequency characteristics of the major aftershocks of the Mid-Japan Sea earthquake of May 26, 1983 (M=7.7) have been studied by use of the waveform records of the telemetered stations of the Abuyama Seismological Observatory and those of the temporary station at Fukaura. The major aftershocks during a month from immediately after the main shock are divided into three groups, type H, L and M by their characteristic frequencies. The shocks of type H are characterized by their anomalously high frequencies of 6-9Hz, whereas the shocks of type L, by comparatively low frequencies of 1-3Hz. The shocks of type M have both low and high frequencies and show rather flat velocity spectra of 1-10Hz. The frequency characteristics of the aftershocks are the same as those recorded at Fukaura where the epicentral distances (50-100km) are much shorter than those (630-810km) of the substations of the Abuyama Seismological Observatory. The aftershock area is divided by the frequency characteristics of the after-shocks into three parts which seem to correspond to the rupture characteristics of the main shock which consists of at least two big events. The aftershocks of type M are frequent in the southern part of the aftershock area where the first event of the main shock propagated from south end towards north-northeast and stopped at the northern end, while the aftershocks of type H occurred only at the central part of the aftershock area where the second event of the main shock began. Furthermore, the shocks of type L are frequent in the northern part where the second event propagated with a slower rupture velocity than the first event of the main shock and with a different rupture direction of south-southeast to north-northwest. This suggests that the frequency characteristics of aftershocks closely correlate with the rupture process of the main shock.

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The crustal structure of Shikoku, Japan was studied by seismic refraction experiments which were carried out in March 1975. Thirteen temporary stations were aligned to the SW direction from off Sakaide to Cape Ashizuri, Shikoku. All of the stations were equipped with magnetic tape recording systems.
Two kinds of refracted waves with the apparent velocity of 6.05km/s and 7.8km/s were observed as first arrivals, although the seismograms were not of good quality. Certain secondary phases can be interpreted as refraction from the layer of 6.6km/s velocity.
Because of small number of data and of lack of reversed profile, only a rough picture on the crustal structure can be discussed in this paper. The superficial layer with a velocity of 5.5km/s becomes thin toward the southwest in Shikoku. This layer shows an abrupt change of its thickness at about 25km south of the Median Tectonic Line. The upper crustal structure with a velocity of 6.0km/s has an average thickness of about 20km. The velocity of the lower crust is assumed as 6.6km/s. The derived crustal structure is concordant with the structure of the eastern part of Shikoku derived from the Miboro and the Toyama explosions.

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A seismic refraction experiment was conducted in the northern part of Lake Biwa, Kinki District, Central Japan, in October, 1978. The purpose of the experiment was to know the thickness of the sedimentary layer beneath Lake Biwa, preparing for the boring project on paleolimnological study.
Four shots were detonated on land and fifteen seismometers were set at the bottom of the lake, along a profile of 34km long. By the analyses of the clear P wave onsets, the underground structure of Lake Biwa was obtained. The P wave velocity of the sedimentary layer varies from 1.5 to 2.0km/s, and it is assumed to be 1.8km/s in this study. The thickness of the sedimentary layer is 0.8km in the western part of the lake, decreasing towards northeast part with an inclination of 6-7 degrees. The basement layer has a velocity gradient, which has the P wave velocity of 5.0-5.2km/s near the upper boundary and the velocity gradient of 0.25km/s per 1km. Comparing this velocity increace with that from laboratory experiment on rocks, this velocity gradient in the basement layer is considered to show the in situ pressure effect.

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The Atotsugawa fault system, which is located in the northern region of Central Honshu, Japan, is composed of the Amidagawara, Atotsugawa, Mannami, Mozumi-Sukenobe, and Ushikubi faults. The Atotsugawa fault in the central part is creeping where seismicity is low. In contrast, the edges of the creeping zone are locked showing relatively high seismicity. In order to study the relationship between the distribution of earthquakes and the fault structure, we analyzed seismic survey data along and across the fault system, and relocated hypocenters using a 1-D velocity structure determined in this study. P-wave velocity structures around the Atotsugawa fault system are determined by comparing first arrival travel time data from explosion surveys with the travel times from forward calculations by a ray tracing method. On the basis of the P-wave velocity structures, we have found that the crust around the Atotsugawa fault system consists of three layers, an upper crust which includes a surface layer, a middle crust and a lower crust. Furthermore, two distinct reflectors are located at depths of about 11km and 20km below the Atotsugawa fault system. The depth of the shallower reflector is close to that of low resistivity layer. Comparing the relocated hypocenters with the depths of the reflectors, the shallower reflector is roughly coincident with the base of the seismogenic layer and the second reflector is several kilometers deeper. In addition, seismicity is concentrated in the upper crust (with velocities of 5.9-6.2km/s) and only a few earthquakes occur at the bottom of the middle crust. The difference in seismic structures between the creeping zone and the other regions is not clear. This is probably because the creeping zone does not have a significantly different velocity structure that is detectable from these data.

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An earthquake of magnitude 6.1 occurred off the Atsumi Peninsula on January 5, 1971. A few foreshocks and many aftershocks were recorded at many seismological stations of both the Wakayama Micro-earthquake and the Inuyama Seismological Observatories. The distribution of hypocenters and nodal plane solutions were examined with the following results:
1) The epicenters of foreshocks, the main and large aftershocks were located along the line N 60°W-S 60°E across the Median Tectonic Line. The foreshocks occurred in the northern side of the Median Tectonic Line were very shallow as deep as 2km. However, the main shock and most of the aftershocks occurred in the southern side of the Median Tectonic Line were rather deep, about 30km in depth. But some of focal depths of aftershocks occurred near the largest foreshock were about 5km.
2) It seems that there were two types of earthquake mechanism in this region. The mechanisms of foreshocks and the aftershocks which occurred in the foreshock region were dip-slip-fault type with the pressure axes of N 90°W. On the contrary, those of the main shock and the deep aftershocks which occurred near the main shock were strike-slip-fault type with the pressure axes of N 30°W.
3) The daily frequency distribution of aftershocks was well represented by the Omori's formula N (t) =k. t^-p for the first fifteen days, where p=1.2.

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Gravity measurement was carried out at more than 200 stations with LaCoste-Romberg gravimeters in the vicinity of the Arima-Takatsuki Tectonic Line, a prominent fault system in the northern Kinki district, central Japan. The areas with low Bouguer anomalies derived from the measurement correspond to sedimentary planes and basins, while areas of high Bouguer anomalies to the mountainous areas. The areas of steep horizontal gradients of the Bouguer anomaly lie along active faults developed in the district. In particular a line of steep gradient of the gravity anomaly is seen along the Arima-Takatsuki tectonic line, which also correspond to the boundary between the sedimentary plane by the Yodo River and the outcropped area of the Tanba zone. Most of changes in the Bouguer anomalies are explained by that of the thickness of the unconsolidated sedimentary layer of about 1km. The seismic activity in the area however is clearly bounded by the Arima-Takatsuki tectonic line; in its northern part the seismicity is very high, while in the southern part a few earthquakes occur. This suggests that the fault movement at the depth of the earthquake occurrence, 5 to 15km is concerned to the variation of the thicknesses of the sedimentary layer. A line of a weak horizontal gradient of the gravity anomaly is detected along a clear boundary of the seismicity which is located along the Inagawa river striking northwest to southeast. This implies a deep embedded fault in the area, though the boundary does not corresponds to any confirmed active faults.

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Attenuation property of lithosphere has been studied in various regions by analyzing decay rate of coda amplitude. In this paper, Q_c values were determined for intra-plate earthquakes recorded by the telemetered network in the middle and northern Kinki district, southwest Japan. All events are shallower than 20km having magnitudes of 1.8-4.0. Q_c was determined by fitting a back scattering model to band-pass filtered coda decay curves in five frequency bands from 2 to 32Hz, with 8 lapse times ranging 20 to 85 s. By fitting Q_c to the power law, Q_c=Q_ofⁿ, average values of Q_c and n in this district were obtained as Q_o=74 and n=0.77. These are typical values in the active tectonic region such as island arcs. Although the Q_c value depends on the lapse time used for the analysis, n is almost independent of the lapse time. The radius (r) of scattering area contributing coda wave is roughly estimated as r=V_st/2, where V_s is S-wave velocity and t is lapse time. Therefore, Q_c values can be derived as a function of depth from the relationship between Q_c and lapse time. Q_c increases rapidly with increasing lapse time and becomes nearly constant at large lapse times. This shows that Q_c increases rapidly in the shallow crust and becomes nearly constant in the upper mantle. The increasing of Q_c with depth is larger in the northern part than in the middle part of Kinki district. This suggests that the depth dependence of Q_c is related to the structure of the lower crust and the upper mantle.

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The source characteristics of the Japan Sea earthquake, May 26, 1983 (M=7.7) is inferred from the seismic observation system with wide-frequency and large-dynamic range at the Abuyama Seismological Observatory. The duration of oscillation of the long-period low-gain seismogram (T₀=25s) is much longer than those of other earthquake with nearly the same magnitude and nearly the same epicentral distance, which implies that the earthquake is a multiple shock. The relationship between a multiple shock and duration of oscillation is more clearly indicated in the figure of double amplitude envelope to eliminate the difference in amplitude by magnitude and focal mechanism. This simple method is applicable to detect multiple shocks in seismograms at one station, especially in historical seismograms with a few instrumental records.
Seismograms of the main shock of the Japan Sea earthquake recorded by Wiechert seismographs and those of middle-period (T₀=10s) low-gain velocity seismographs show a clear onset of the second event at about 22 seconds after the first arrival. Since no such second arrival is seen on the seismograms of the aftershocks at the same station, the phase is not a crustal phase but a P-wave arrival of the second event of the main shock.
The main shock recorded by the middle-period low-gain velocity seismograph contains more complicated high frequency waves than the largest aftershock. This indicates that the rupture process of the main shock is much complicated compared with that of the aftershock. Further, comparing the spectrum of the first event of the main shock with that of the second event, the average amplitude at a low frequency (5-10 s) of the first event is smaller than the second event, while that at high frequency (1-2s) is larger than the second event. This suggests that the main shock is composed of double events of different rupture type; the rupture of the first event is smaller and radiated much high frequency waves than the second event.

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A modified master event method has been used for the pricise determination of hypocenters of small earthquake sequences that occurred within the telemetered station network of the Abuyama Seismological Observatory in the northern Kinki district, central Japan. By use of the accurate P wave arrival times of telemetered stations, this method has decreased the uncertainty of relative location of each event to its master event to be about 0.1km. And the detailed spatial distributions have been obtained for ten earthquake sequences of which magnitudes are from 2.5 to 4.0. The characteristics of the distributions are as follows; (1) The distribution of aftershocks and foreshocks of small earthquake, as well as that of large earthquake, defines a plane which seems to indicate a fault plane. (2) The trend of the aftershock area agrees with the trend of one of the nodal planes of main shock determined from the first motion of P waves. (3) Most of main shocks have been located on one end of aftershock areas and foreshocks have been located in the vicinity of the main shock. (4) The length of aftershock area is about 0.8km for M=4.0 earthquake sequence and 0.5km for M=3.0 sequence. These values roughly agree with the expolated values of UTSU's empirical equation between linear dimension of aftershock area and magnitude. The area of earthquake swarm, however, is two or three times larger than that of ususl main shock-aftershock sequence. (5) The aftershock area enlarged slightly during the activity.

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