IAVCEI 2013 Scientific Assembly - July 20 - 24, Kagoshima, Japan
Forecasting Volcanic Activity - Reading and translating the messages of nature for society

0P1_2F-O3

Room B2

Date/Time: July 20 15:15-15:30

Volocano-tectonic earthquakes correlated to stress rate in Izu-Oshima volcano
Yuichi MORITA
ERI, university of Tokyo, Japan
E-mail: morita@eri.u-tokyo.ac.jp

It is well known that the seismicity around a volcano is one of the well-established indicators of volcanic activities.
Many evidences show that increasing seismicity is followed by the volcanic eruptions or magma intrusion because
magma migration makes large stress change. On the other hand, the increasing seismicity does not always result tovolcanic eruptions. Therefore, seismicity around volcano is generally treated as a powerful but not definite indicatorof volcanic eruption. It is partly because the stress change that generates earthquakes has not been estimatedquantitatively and the cause of the stress change has not been studied systematically. Aim of this presentation isto reveal the effect of stress as well as stress rate on volcano-tectonic earthquakes occurring at Izu-Oshima volcano.

Izu-Oshima is located at central Japan and is a volcano island. The ambient tectonic stress affects largely
eruptive activities in this volcano. In the latest eruption in 1986, fissure eruptions and large scale dike intrusion
followed a summit eruption. The activity ceased in early 1990s. At present, the volcanic activity is low but distinct
volcano-tectonic seismic activities synchronized with the ground deformations are observed every 2-3 years. The
present activities are characterized as follows. 1) Seismic activities are clearly categorized in two groups: One isan earthquake swarm occurring at the depth of 1-2km beneath the summit caldera (caldera swarm, hereafter). The
other is earthquake swarms that occur off the north and west coast of the island at the depth of 3 to 8km (off-coastswarms). 2) Hypocenters in the swarms are aligned on several sub-vertical planes. Each seismic swarm occurs
on almost same plane repeatedly. 3) The pressure source of the ground inflation is located beneath the summit
caldera at the depth of several kilometers.

We evaluate temporal change of Coulomb stress generated by magma intrusion and ambient tectonic stress on thesupposed planes where earthquake swarms occur. For the off-coast swarms, burst-type of swarms are generated
when Coulomb stress exceed 100-500 KPa from the level of the previous swarm. On the other hand, the calderaswarms are activated when stress changes rapidly, that is to say stress rate is high. The different stress responsein the two swarms can be explained by rock property where earthquake occur. Seismic structure deduced fromlarge scale seismic exploration shows that the layer in which the off-coast swarms occur is similar to normal upper
crust structure otherwise the layer in the caldera swarm has more week medium composed by maybe volcanicdeposits. We propose that there are two kinds of earthquake generating mechanisms around the volcano and therock property control the stress response to the volcano-tectonic earthquakes. Further study at other volcanoes
will be helpful to understand the volcano-tectonic earthquake systematically.

©IAVCEI 2013 Scientific Assembly. All rights reserved.

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