伊豆半島東方沖の群発地震の特性と1989年の手石海丘の噴火

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Earthquake swarms which have frequently occurred off the east coast of the Izu Peninsula since 1978 are investigated, and the relationships between the swarms and the submarine volcanic eruption at Teisi Knoll in 1989 are discussed. The results are summarized as follows: 1) Shapes of epicentral areas of the earthquake swarms can be classified into the following two types, Type I and Type II. The former is an ellipse elongated NE-SW; the latler, an eilipse elongated approximately NW-SE. The swarms in the early years are of Type I, and those in the later stage (in particular, since 1984), for the most part, are of Type II.2) The largest earthquakes in the respective swarms for Type I are located along a straight line in the NE-SW direction, and those for Type II, along a curve line in the E-W to NW-SE directions. These two lines are probably indicative of active tectonic lines. 3) The epicentral areas for Type I and Type II, migrate periodically with a recurrece time of 6-7 years along the tectonic line in the NE-SW direction, and with a recurrence time of about 3.7 years along the tectonic line in the E-W to NW-SE directions, respectively. 4) Teisi Knoll where the submarine volcanic eruption took place on July 13, 1989, is situated in the northwestern part of the tectonic line trending E-W to NW-SE. The eruption was triggerd by the largest shock in the swarm which occurred around the northwestern end of the line.
1992-04-01