著者

久保寺 章 岡野 健之助

出版者

公益社団法人 日本地震学会

雑誌

地震 第2輯 (ISSN:00371114)

巻号頁・発行日

vol.13, no.4, pp.210-218, 1960-12-25 (Released:2010-03-11)

参考文献数

5

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The seismic waves due to the great nuclear explosions in the central Pacific Ocean (Bikini Atoll) in June and July 1958 were recorded by routine seismographs at the Abuyama Seismological Observatory of Kyoto University.The remarkable result was that we found a new kind of wave-group which has the following properties on the records of long-period seismograph. The wave in this group is of a period 9 to 1 minute and its propagation velocity is about 300m/s. The arrival time is about 3 hours later than that of seismic P-wave. These properties are similar to those of the microbarographic oscillations produced by the nuclear explosions, and the phenomena can hence be attributed to the effect of atmospheric pressure fluctuations upon the inertia mass of the seismograph.

著者

岡野 健之助 木村 昌三

出版者

公益社団法人 日本地震学会

雑誌

地震. 2輯 (ISSN:00371114)

巻号頁・発行日

vol.49, no.3, pp.361-374, 1996-11-23

参考文献数

45

被引用文献数

5 1

---

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.

著者

木村 昌三 岡野 健之助

出版者

日本地質学会

雑誌

地質学論集 (ISSN:03858545)

巻号頁・発行日

no.40, pp.187-195, 1992-12-15

被引用文献数

6

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四国の中央構造線周辺地域に発生する地震の震源を再検討した上で, その震央ならびに深さ分布を求め, 構造線との関係について考察している。深さが5 kmより浅い地震の多くは構造線の南側にのみ観測されている。震源の深さ分布のずれからも, これは構造線を境にして南側(三波川変成帯)の上昇を示唆する。深さ8〜18 kmの地震では燧灘における活動が目立ち地形との関連が印象的である。下部地殻には地震が観測されないので, 23 kmより深い地震はすべて最上部マントル内のものであるが, この北傾斜で分布する地震は, 四国西部では構造線の制約を受けず構造線を越えて中国地方へ連続した分布を示す。しかし中東部では構造線の制約を受けて北側での活動は極めて低く, しかもその分布は中国地方に想定されるモホ面直下に水平に分布する。これらの地震を発生させる主圧力の方向は, 地殻内地震ではほぼ東西, マントル地震では南北である。

著者

岡野 健之助 木村 昌三

出版者

公益社団法人 日本地震学会

雑誌

地震 第2輯 (ISSN:00371114)

巻号頁・発行日

vol.49, no.3, pp.361-374, 1996-11-23 (Released:2010-03-11)

参考文献数

45

被引用文献数

1

---

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.

著者

木村 昌三 岡野 健之助

出版者

公益社団法人 日本地震学会

雑誌

地震 第2輯 (ISSN:00371114)

巻号頁・発行日

vol.50, no.4, pp.461-470, 1998-03-13 (Released:2010-03-11)

参考文献数

28

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The source region of the 1946 Nankai earthquake is examined based on the recent studies of microearthquakes, focal mechanisms and the crustal structure in Shikoku, southwest Japan as well as the aftershock distribution just after the main shock. Although it is generally known that an aftershock distribution is nearly equal to the fault zone and the source area of tsunami, such an agreement has not been recognized in the case of the 1946 Nankai earthquake. In the present study, we point out the misinterpretation about the aftershock area and the focal plane in the previous works, and give a new interpretation indicating good agreement among the aftershock distribution, source area of tsunami, crustal movements, disaster area and fault region. In the previous works, the defect of the observation network in the west of Shikoku and the east of Kyushu was not taken into consideration on the estimation of the aftershock area. Furthermore, the focal plane was not properly estimated owing to misunderstanding of the crustal movements and the source area of tsunami. Since the focal plane exists in the focal layer in the uppermost mantle, we consider that the aftershocks occurred only in this layer and the crustal earthquakes were induced by stress release due to the occurrence of the main shock. Our microearthquake observations show that this layer has a thickness of about 5km from the west of Shikoku to the western part of the Kii peninsula and from the Median Tectonic Line in the north to the Nankai Trough in the south.

著者

木村 昌三 岡野 健之助

出版者

公益社団法人 日本地震学会

雑誌

地震 第2輯 (ISSN:00371114)

巻号頁・発行日

vol.48, no.2, pp.213-221, 1995-08-25 (Released:2010-03-11)

参考文献数

20

被引用文献数

1

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We examine the decreasing of seismic activity which occurred in Shikoku, and the Wakayama and Tanba districts prior the 1946 Nankai earthquake, southwest Japan. Considering the effect of the Second World War, the seismic observation for that period by the Japan Meteorological Agency was not in sufficient conditions. Thus, the idea of seismicity lowering on the area has not been generally accepted, notwithstanding some studies on the subject were previously presented. This study proves however the lowering was not caused by the insufficient seismic observation. We give an explanation for the cause of lowering of seismicity. We also showed that the crustal seismic activity in Shikoku in terms of earthquake occurrence gradually decreased with time after the 1946 earthquake. The decrease in seismicity could be anticipated to continue till the next Nankai earthquake in the above mentioned districts. Accordingly, the lowering of seismic activity is expected as a precursor of the great Nankai earthquake, in particular in the Wakayama district where the usual seismic activity is comparatively high compared to other districts.