Submarine crustal movement and magnetic change in the environs of the Awa Sima (island), Japan Sea associated with Niigata Earthquake of 16 June 1964.
On 16 June 1964, a great earthquake of magnitude 7.5 took place, the epicenter of which was located below the sea floor about 6 km south of Awa Sima (island). Since the Hydrographic Department had dispatched its survey ship the TAKUYO to engage in sounding, dredging and magnetic survey in the environs of Sado Sima and Awa Sima from 6 to 15 June, just before the earthquake occorred, the Department could obtain a great deal of valuable data immediately ahead of the earthquake occurrence. Shortly after the earthquake, from 26 June to 7 July, the TAKUYO again engaged in sounding, magnetic and other surveys in the waters around Awa Sima to reveal any change in the depths and geomagnetism which might be caused by occurrence of the earthquake in the epicentral area. The results of the survey showed that a remarkable upheaval had occurred in the whole area of Awa Sima Kaikyo (strait) including the island, and that magnetic anomalies existed along submarine faults thereabouts. Later,from 18 August to 16 September,another precise survey with higher positioning accuracy using Decca Navigator System was carried out by the survey ship the KAIYO of the Hydrographic Department in the area from Awa Sima approaches to the coast between Iwahune and Nezugaseki. The survey was conducted mainly as to sounding and bottom sampling. It was then confirmed that the upheaval had occurred in the vicinity of Awa Sima and the subsidence and submarine faults had took place along the coast between Iwahune and Nezugaseki. Afterward,the Hydrographic Department started to carry out tidal observation at Awa Sima from September 1965, and also conducted levelling of the island in March and October 1965 in August and September1966 and in September 1967 respectively. Furthermore, in July and August 1966, the survey ship the MEIYO of the Hydrographic Department carried out sounding, magnetic survey and investigation of the bottom with a sonic layer detector, to reveal the subsequent possible changes. From the above mentioned survey, the following results were obtained: (1) Due to the earthquake Awa Sima was uplifted 80 to 100 cm. The island,however, was gradually resuming, its original topographic situation, with such a tendency in which higher speed in sinking was observer in the area where upheaval amount had been larger and vice versa. (2) The earthquake caused an upheaval zone with dual axes running NNE-SSW in the middle of Awa Sima Kaikyo, the maximum upheaval amounting 5 m土 1.2 m. Two years after the earthquake, the zone as a whole showed a subsiding phenomenon and its width became narrow. (3) Five submarine faults were confirmed in the environs of Awa Sima. It is considered that one of them was created by the earthquake. All of the faults were running in parallel with the tectonic line of the region, and have the characteristic .feature of 1 to 6 meter scarps facing east. (4) From the distribution of the magnetic total intensity, it was found that lines of equal total intensity show about NE-SW direction, and that anomalous anomalies were observed around the epicenter as well as over the submarine faults. (5) Changes of the magnetic values at the sea caused by the earthquake are : the anomalous secular variation immediately before the earthquake had been about 50 γ : right after the earthquake the total intensity was increased by 20 γ than before the earthquake: two years later the total intensity was considered restored to the normal value. Based on the above results, the consideration on the change in the geomagnetism due to the crustal movement caused by the Niigata Earthquake can be concluded as follows; The magnetic anomalies in the epicentral area as well as in the areas where large crustal movements occurred indicate that the whole region has an anomalous tectonic aspect. For that reason, at the sea-bed in approaches to Awa Sima some distortion was caused and developed by the compressive stress in the crust, and in parallel with this phenomena the anomalous secular variation in geomagnetism also progressed. The earthquake occurred when the pressure in the earth crust and geomagnetic anomaly reached the critical value, and separate upheaval movements took place block by block in the base. After the earthquake, a rapid reactionary change was observed in the total intensity, showing the tendency to resume its normal value. During these two years, the upheaval zone was tending to gradually return to its old state, and the total intensity almost recovered to normal.
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| Format: | Journal Contribution biblioteca |
| Language: | Japanese |
| Published: |
1971
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| Online Access: | http://hdl.handle.net/1834/16344 |
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