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Volume 5: Number 1: Article 6
Low-Frequency Seismogenic Electromagnetic Emissions as Precursors to Earthquakes and Volcanic Eruptions in Japan
Takeo Yoshino, University of Electro-Communications, 1-5-1 Chofugaoka,
Chofu-shi, Tokyo 182, Japan
A multipoint network was constructed in the Tokyo area for earthquake
prediction using seismogenic electromagnetic emissions. The network
consists of eight observational points within 50 km of each other. Each
point has a digital direction-finding detector with two loop sensors
tuned to 82 kHz. The output signals of the receivers are added into
a digital vector composition circuit to obtain the direction angle of
the source point, and this signal is telemetered to the central computer.
To protect from false alarms caused by local man-made noise interference,
the warning is announced only where there is a high cross-correlation
between almost all detectors pointing to one small area. The mechanism
of these earthquake precursors can be explained as electromagnetic emissions
from the rocks around the focus when they are crushed completely by
the distortion pressure. These emissions propagate along the fault plane
as an EM surface mode and radiate from the slit antenna formed by the
intersection of the fault plane and ground surface.
In the last five years, we have detected impulsive noise bursts of
seismogenic emissions at 82 kHz, 1.525 kHz, and 36 Hz using our multipoint
detection network around the Tokyo region and Izu peninsula. This system
has recorded EM signals prior to the following events: volcanic eruptions
on November 15 and 21, 1986 at Mt. Mihara on Ohshima Island, and on
July 12, 1989 in Itoh Bay in the Izu peninsula region, and also a minor
earthquake on October 14, 1989 at Ohshima Island.
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