相模湾の海底地形地質構造について

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Sagami Bay is structurally active area through the geological history, and also the epicenter of the Great Kanto Earthquake (1923) was here. To make clear the features of the topography and geological structure of the active area, it seems to be fundamental step for the earthquake predication today. In April to May 1968, the surveying ship Meiyo of the Japanese Hydrographic Office carried out the sea bottom surveys of Sagami Bay with echo-sounder, seismic profiler and proton-geomagnetometer, including the dredge and coring operation, and took the data from about 3000 kilometers of her traverses. The ships position was mainly determined by an auto-tape system with the estimated accuracy of ±2 meters. The simultaneous measurement of magnetic intensity, bathymetry and submarine geological structure by air-gun system allows direct comparison of the measurements. Figure 1 is the track chart of the Meiyo and figures 2 and 3 are some profiles based on the collected data. From the analyses of the collected data mentioned above, the topography and geological structure and magnetic anomaly of Sagami Bay can be summarized as follows. 1. Topography Figure 4 shows the bathymetric chart of Sagami Bay. In the eastern part of the bay, many banks and small basins range from NW to SE. Many submarine canyons cross the topography mentioned above and incise deeply, so that especially small basins do not keep their complete shape. The central part of the bay is called Sagami Trough, which is deeper than 1 kilometer. The floor of the trough generally shows low relief with the exception of the eastern part of the trough, which submarine canyons flow into, and gentle slope with the trend of NS at the central part, facing east and reflecting the fault topographically. In the western part of the bay, continental shelf and slope are monotonous, comparing with its eastern part. There are several conical banks between the Izu Peninsula and O Shima Island. 2. Geological structure Geological structure based on the seismic profiler is shown in Figure 5. The area surrounded Sagami Bay is relatively uplifted zone in contrast with the trough showing the relative subsidence. Acoustic basement rock outcrops on the uplifted zone and may probably be composed of the Hayama group (Lower Miocene) and partly the lower part of the Miura group (Middle Miocene). The large part of the thinly layered sediment at the upper part of sediment body in Sagami Trough is younger than the age of uplifted zone mentioned above. The layered sediment is wholly tilted to the axis of the trough, and is found the folding in the margin of the trough and also the active fault near surface. Its maximum thickness is found at the northern part of the trough, and is over about 1 second in the reflection time approximately. The area along the Izu Peninsula and around O Shima Island is extensively covered with the volcanic products. Small banks between the Izu Peninsula and O Shima Island are volcanic cones of the effusive body which pierced through the layers probably composed of the Shirahama group (Middle to Upper Miocene). 3. Geomagnetism The resultant magnetic chart, contoured at intervals of 100 gammas, was in Figure 6. The results of the magnetic surveys revealed that there were many magnetic anomalies of about 500 to 800 gammas off shore along the Izu Peninsula in Sagami Bay and a pair of remarkable positive and negative magnetic anomalies of about 1800 gammas in the sea area along the west side of O Shima Island. On the other hand, it was found that there was no remarkable magnetic anomaly in the central part and eastern part of Sagami Bay. From the results of magnetic surveys and submarine geological structure, it can be concluded that the sea area off shore along the Izu Peninsula and around O Shima Island are composed of volcanic rocks of higher magnetic susceptibility, such as basalt or andesite.

相模湾の海底地形地質構造について

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