The Distribution of Magnetic Dip in Ooshima (Oo-sima) Island and its Change that Accompanied the Eruption of Volcano Mihara, 1950.

Comparing the results of the first and second surveys over Ooshima Island, marked changes in the earth's magnetic field were found out during the period from beginning to end of the eruption of Volcano Mihara, 1950. The magnetic dip-angle decreased all over the island. The largest value of the decrease amounted to as much as 30 minutes of arc. Applying the method devised by the writer for finding out the position, intensity and direction of an underground magnetic dipole directly from the distribution on the earth's surface, it was found that the changes obtained here may be almost explained as due to a dipole situated just beneath the centre of the island at a depth of 5.5 km, the direction being approximately opposite to the earth's magnetic field. Taking into account the intensity of magnetization of rocks composing the volcano, the generation of the dipole may be attributed to the demagnetization of a roughly spherical region having a radius of a few kilometers, while the demagnetization seems to be due to the rise in temperature in the said region associated with the eruption. It is of interest that the general mode of the change closely resembles that observed in the case of Volcano Miyake-sima where the rocks composing the volcano is also basalt. Comparison with the survey carried out in 1936 was also made. However, no marked changes were to be found. The writer also studied the local anomaly in magnetic dip from which the mean intensity of magnetization was estimated to be about 0.03 emu. In the course of the study, the writer was encouraged by his senior colleagues. The writer wishes to express his hearty thanks to Professor H. Tsuya, Professor R. Takahasi, Professor T. Hagiwara, Professor T. Minakami, and Dr. T. Nagata for their helpful advices. To Dr. S. Imamiti and Mr. M. Hirayama of the Kakioka Magnetic Observatory who placed magnetic data at the writer's disposal, the writer's cordial thanks are also due. The writer also wishes to express his appreciations to Mr. K. Kasahara, Mr. S. Saito, and Miss Y. Hishiyama who assisted the writer in the field-measurement. The study was done with the financial aid of the research grant from the Deparment of Education, and the writer wishes to express his cordial thanks for the grant received