The January 17, 1995 Hyogoken-Nanbu Earthquake caused catastrophic disaster, especially on the eastern part of Kobe area. I measured natural radioactivity from the ground surface of Higashinada, Ashiya and the south-west part of Nishinomiya for the purpose of detection of the buried faults in this region. I found several sites where the total gamma-ray dose rate was anomalously high compared to the surrounding area. A part of these sites delinates the hypothetical line which can be considered the southwest extension of Koyo Fault. Another hypothetical line parallel to this line can be obtained by anomalous points. These two imaginary lines are considered as buried fault lines.
As a result of nuclide analysis, the activity of either daughters of U- and Th-series from the high radioactivity points is stronger than that from their surrounding normal level radioactivity points. The excess of both daughters of U- and Th-series on the high radioactivity points cannot be elucidated by the phenomena of up-welling Rn. One possibility of explanation on this excess is by the immigration of dissolved Ra through the fracture of surrounding rocks along faults.

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We determine the upper boundaries of the Philippine Sea (PHS) plate in the Tokai District, Japan, based on a total of 11, 340 relocated hypocenters and recalculated mechanism solutions of about twenty percent of hypocenters, for earthquakes observed during recent about 14 years. Variously oriented cross sections of hypocenters and P and T axes distributions are used to delineate the configuration of the PHS slab. The thickness of the Wadati-Benioff zone which nearly corresponds to the oceanic crust is about 5 to 7km. The resultant overall geometry of the upper boundary of the slab is convex seaward and forms NW-to-NNW trending ‘slab valley’ from the point of about 20km depth beneath Pt. Omaezaki to the deepest tapered end of 70km depth beneath central Honshu. Regional sinus shapes of the slab reasonably explain the features of seismicity and focal mechanisms along the plate boundary, in terms of the conservation of the subducted spherical shell of the PHS plate, and the change in the direction of relative plate motion between the PHS and the overriding plates in the past. A strong coupling is suggested along gently-dipping plate boundary in the direction of relative plate motion extending from the point beneath Lake Hamana to the point of 40km depth beneath the southern end of the source area of the great 1891 Nobi intraplate earthquake. The slab subducting at the Suruga trough has rather steeply-dipping angle perpendicular to the trough axis. A remarkable landward-dipping cluster overlies the Wadati-Benioff zone at the mid to lower crustal depths beneath the western coast of Suruga Bay. The focal mechanisms of the clustered events show reverse-slip faultings with subhorizontal NW-SE oriented P axes and near-vertical T axes. A thin aseismic layer is intercalated between the cluster and the underlying Wadati-Benioff zone. From the history of accretion tectonics and the other geophysical and geological evidences in the South Fossa Magna, we infer the cluster is composed of underplated oceanic materials presenting an ocean-to-continent transitional seismic layer. The underplated materials may probably be dragging downward coupled with the underlying oceanic plate through the thin aseismic layer, in accordance with the large crustal subsidence just above the dipping cluster continuing since the 1854 Ansei Tokai earthquake. We interpret that the thin and probably partially-strong intercalated aseismic layer is the site of nucleation of the next ‘Tokai earthquake’. Another important feature is that the deeper Wadati-Benioff zone seems to continue to the shallower gently dipping seismic zone beneath the Suruga trough and the Izu Peninsula. This suggests an alternative tectonic process that a new convergent plate boundary is creating and extending seaward underneath the Suruga trough and the Izu Peninsula.

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Seismic reflection survey was conducted in Kushibiki area, Saitama Prefecture, in the northwest part of the Kanto Plain, central Japan. The 12-kilometers long seismic line spans the northeast edge of the Kanto mountains, a major part of the Kushibiki terrace and a part of the Tone river lowland. It crosses the Hirai fault and two active faults, the Kushibiki fault and the Fukaya falut. Almost all reflectors dip homoclinically to northeast between the two active faults. The angle of dip is between 20° and 30°. The dipping reflectors are traceable from the near surface down to 1.5s in two-way time or 2km in depth, but the bottom of them is unclear. Some of them are very continuous. The dipping reflectors correspond to homoclinic sedimentary rocks of Neogene age and continuous ones probably to acidic tuff layers. All reflectors change dips at the Fukaya fault and the reflectors are almost horizontal on the northeast of the Fukaya fault. Few events are seen on the southwest of the Kushibiki fault. The survey reveals that the deep structure changes obviously both at the Fukaya fault and at the Kushibiki fault, although fault planes or large gaps of reflectors are not recognized at these faults. The survey area can be divided into several blocks bounded by these faults. The boundaries of these blocks are characterized by the change of seismic velocities at the Hirai fault, by the disappearance of reflectors at the Kushibiki fault and by the change of reflectors' dip at the Fukaya fault, respectively.

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The epicenter of the 1670 West-Kanbara earthquake with M63/4 is re-examined on the basis of the the damage documents of the old village. The misunderstanding of the previous estimation of the epicenter was due to the confusion of the old feudal domains named by Kamikawa-Yonmangoku with the present village name of Kamikawa located by the up-stream of the Agano River. The old domains of Kamikawa-Yonmangoku meant the separate territories of the Murakami clan in the central and eastern parts of the Echigo plains. The corrected epicenter of the 1670 West-Kanbara Earthquake was at Lat. 37.8°N and Lon. 139.0°E, 50km west to the present village of Kamikawa. The revised epicenter locates at the northern extension of the Shinano River seismic zone, which is the inland portion of the collision plate boundary between the Okhotsk plate and the Eurasia plate.

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The May 23 1995 Sorachi-Chubu earthquake (M_JMA=5.7) occurred at 18:01:29 JST in northwest Hokkaido at the western end of the Okhotsk plate. This is the largest of the earthquakes which occurred during the last 160 years in the shallow seismic zone with a length of about 250km, which can be considered to be a plate boundary between the Okhotsk and Eurasian plates in the Late Mesozoic. This is also the first significant shallow event in this inland region recorded by a modern digital broadband seismic network, thus providing a first opportunity to examine the focal mechanism of a felt earthquake occurring along this old collision zone. Double-couple mechanism solutions were obtained from three kinds of data: P-wave first motions, broadband waveforms recorded by STS 1 and STS2 seismographs, and acceleration seismograms recorded by strong motion seismographs (JMA Model-87). The source parameters obtained by centroid moment tensor (CMT) inversion are: the centroid depth=7km; (strike, dip, rake)=(187.7°, 23.2°, 115.4°; 339.3°, 69.2°, 79.6°); seismic moment M_o=8.6×10¹⁷N·m(M_W=5.8). The waveform inversion of the strong motion seismograms for a single total source indicates: centroid depth=5km; (strike, dip, rake)=(192.6°, 44.2°, 110.9°; 344.6°, 49.3°, 70.9°); seismic moment M_o=3.0×10¹⁷N·m(M_w=5.5). The seismic moment is about one third as large as that obtained by the CMT inversion. The discrepancy between the two methods may be caused by the large later phases generated within the travel paths used for the CMT inversion. We made the waveform inversion excluding these phases for the strong motion seismograms. The long period CMT solution gives an information on the integrated view of the overall faulting regardless of a higher frequency information on the crustal structure as well as on the fault geometry during the earthquake. As far the fault plane solution, P-wave first motions gives the similar result as those inferred from both of the waveform inversions. As supplementary evidence, the disaster distribution, including tumbled tomb stones, suggests thrust faulting. The principal horizontal stresses obtained are also consistent with those predicted from the active faults ascertained by the geographical surveys. They suggest a reverse fault in which the western hanging wall moves upward. The results of the present study reflect the plate tectonic driving forces. They may be localized at reactivated sites of prior deformation, or at sites of stress concentration resulting from sutured structure, such as along the margin of the Okhotsk plate in the Late Mesozoic. It is reasonable to suspect that part of the present Okhotsk plate movement towards the Eurasian plate is absorbed in the weak zone created by the Late Mesozoic tectonic processes.

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We propose a new idea for the crustal movements associated with past great Nankai earthquakes in Shikoku and the surrounding region, Southwest Japan. This study makes clear that the movements are not due to the oblique subduction of the Philippine sea plate, but due to superposition of strong seismic shaking to the uppermost crust in a compressional stress state in the E-W direction. The ground of this idea is as follows: at the time of the 1707, 1854 and 1946 Nankai earthquakes the Kochi plain subsided and the Muroto and Ashizuri peninsulas uplifted, whereas old documents show that the Kochi plain did not subside at the time of the 1605 Nankai earthquake and therefore no uplifting of the peninsulas is inferred because it is based on a set of subsidence of the Kochi plain and uplifting of the Muroto peninsula appearing at the time of the 1707, 1854 and 1946 Nankai earthquakes. This is explained by the reason why the 1605 (Keicho) earthquake was not accompanied with strong shaking of the ground owing to the tsunami earthquake. Next, because the uplifted peninsulas have anticline axes of the N-S direction, from unconsistency in stress direction it is difficult to attribute the uplifting to the subduction in the NW direction of the Philippine sea plate. On the other hand, it is easily explained that the uplifting was caused by the stress in the EW direction enhanced by strong seismic shaking. Tosa bay, which spreads between the Muroto and Ashizuri peninsulas, is characterized by depression. To be able to explain this depression is not by the elastic rebound theory, but by our idea. The undulation in the forearc zone composed of anticlines (peninsulas) and wide depressions (bays), which range alternately along the Japan island arc, can not also explained by the elastic rebound theory. In addition, we consider that the compressional strain variation in the NW direction of the ground surface observed at present in Shikoku does not result from the oblique subduction in the NW direction of the Philippine sea plate, but it is recovering the overdisplacement of the ground surface caused by the coseismic movement (2-3m at the ground surface) in the SE direction of the Muroto promontory by reverse faulting of the 1946 earthquake. Moreover, this study shows that unconsistency in directions of P axes between the 1946 Nankai earthquake and mantle earthquakes presently occurring in Shikoku and its vicinity is succesfully explained by taking account of constraint of the displacement in the direction along the Japan island arc.

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Seismograms from the 1923 Kanto earthquake (M=7.9) and its aftershocks at the Yamagata observatory of JMA (The Japan Meteorological Agency) in Tohoku district, Japan, are examined. They were recorded by the Imamura-type strong motion seismograph. Horizontal-component records from the main shock and the 1924 Tanzawa earthquake (M=7.3), one of the largest aftershocks, are digitized and the instrumental characteristics of the seismographs are examined. Natural period T_o and damping ratio v of the instrument are evaluated to be 4.5s and 1.5 for both the NS and EW components from the free oscillation records and documents for the results of testing the instrumental response. The maximum displacement in EW component of 11.2cm is obtained for the main shock in the period range from 2 to 20s, after the instrument correction.
On the other hand, uncertainties of the instrumental characteristics remain for the seismograms from the 1923 Kanto earthquake observed at the Mukaiyama observatory of the Tohoku Imperial University in Sendai, [TAKEMURA et al. (1995)]. The Sendai city is located about 40km east from the Yamagata city. The epicentral distance and azimuth of the Mukaiyama observatory is not so different from those of the Yamagata observatory for the 1923 Kanto earthquake. It is found that the displacement records at Sendai and Yamagata have mostly the same amplitude for the recent moderately large earthquakes with almost the same location of epicenter as the 1923 Kanto earthquake. All the records were observed by the strong motion displacement seismographs of T_o=6s and v=8 both at the Yamagata observatory and at the Sendai district meteorological observatory of JMA. This fact indicates that the displacement at the Mukaiyama observatory in Sendai ought to show almost the same amplitude as one at the Yamagata observatory during the 1923 Kanto earthquake. Then, we redetermined T_o of the instrument at Mukaiyama observatory so that the amplitude of the displacement after the instrument correction is the same as that at the Yamagata observatory. Redetermined T_o is 5s in EW component, being meaningfully longer than the results estimated by TAKEMURA et al. (1995).

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We develop a method for imaging subsurface faults using teleseismic waveform data. The target of our method is not an entire velocity structure, but location and size of a subsurface fault. The method consists of the following steps:
(1) pre-processing of observed records,
(2) selection of trial model (initial model of structure),
(3) estimation of incident wavefield from the bottom of the studied region,
(4) forward calculation,
(5) calculation of the steepest descent direction of the model parameters,
(6) image analysis and identification of faults.
The idea of the method has been originally proposed by TAKENAKA et al. (1996). In the present paper we examine the feasibility of the approach using the synthetic data and several trial models in the two dimensional SH cases. We also here propose a practical technique to estimate the incident wavefield from the observed data. In the numerical examination, we could recover the image of the subsurface fault for all trial models we used, which indicates our method has potentiality for sensing real subsurface faults and encourages further development of the method.

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A Bouguer gravity anomaly map was made for the study of subsurface structure in the southeastern part of Japan Sea, offshore Niigata to Aomori area. The gravity data were obtained during five R/V Hakurei-mare cruises, GH89-2, GH89-4, GH90, GH91, and GH92 by the Geological Survey of Japan. This area is characterized by many small ridges and troughs parallel to the Northeastern Japan arc. Geological studies suggested that these small ridges constituting the Sado Ridge, and the Dewa Bank Chain, were formed by the basin inversion of former extensional grabens. The three dimensional relief of the basement has been obtained from the Bouguer anomalies assuming two uniform layers with the density contrast 0.4g/cm³. The result clearly shows the structure by the basin inversion in the Sado Ridge. The basement underlain the Dewa Bank Chain is shallower than that of the Sado Ridge although it have been formed by basin inversion. The basement of the Okushiri Ridge shows different structure from basin inversion. The basement structures of the Dewa Bank Chain is estimated that the extensional grabens before the inversion was smaller than that of the Sado Ridge.

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