Foci of Volcanoes Page: 2 of 43
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Foci of Volcanoes
By Izumi YOKOYAMA
Department of Geophysics, Faculty of Science,
One may assume a centre of volcanic activities beneath the
edifice of an active volcano, which is here called the focus of
the volcano. Sometimes it may be a " magma reservoir ". Its
depth may differ with types of magma and change with time. In
this paper, foci of volcanoes are discussed from the viewpoints of
1. Geomagnetic changes related with volcanic activities
The researches of the changes in geomagnetic field related
with volcanic eruptions as well as those with earthquakes have
been one of the aims of the geomagnetic surveys throughout Japan
which were commenced by A. TANAKADATE in 1893. Many observational
data accumulated since that time have given a large effect on the
images of physical mechanisms and energetics of volcanic activ-
ities in this country. At the time of the 1940 great eruption of
Miyake Volcano, a few geophysicists observed the anomalous changes
in geomagnetic field independently each other. These observations
convinced Japanese geophysicists that the geomagnetic changes ac-
companied by volcanic activities might be expected on basaltic
volcanoes such as Miyake and Oosima Volcanoes, Izu.
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Yokoyama, I. Foci of Volcanoes, article, January 1, 1974; United States. (digital.library.unt.edu/ark:/67531/metadc875635/m1/2/: accessed February 20, 2019), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.