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

3W_2A-P15

Date/Time: July 23 Poster

Intermittent inflations recorded by broadband seismometers prior to caldera formation at
Miyake-jima volcano in 2000
Tomokazu Kobayashi1 , Takao Ohminato2 , Yoshiaki Ida3 , Eisuke Fujita41

Geospatial Information Authority of Japan, Japan, 2 Earthquake Research Institute, University of Tokyo,Japan, 3 Advance Soft Corporation, Japan, 4 National Research Institute for Earth Science and Disaster
Prevention, Japan
E-mail: tkoba@gsi.go.jp

Preface: Miyake-jima island, which is about 200 km south of Tokyo, is one of the most active volcanoes in Japan.
In most historical events of this volcano, basaltic magma have erupted mainly from part of the flank in a more orless constant time interval, but the 2000 eruption took place in a quite different way. One of the main phases of the2000 activity was a formation of caldera as had happened 2500 years ago. Interesting seismic signals that have apulse width of about 20 s on velocity seismograms, called very-long-period (VLP) seismic pulse events hereafter,are recorded by broadband seismometers deployed in the island. The VLP events were observed frequently 1-2
days prior to the first summit subsidence producing the caldera. The seismic events were hardly recognized after
the subsidence began. Considering this temporal sequence in relation to the summit subsidence event, the VLP
signals likely took place in the preparatory process of the caldera formation. In this presentation, we will report theanalysis results for the VLP signals.

Data analysis: From the displacement records, we clearly find that the VLP signals have smoothed step-likechanges. The vertical seismograms display significant uplifts for all the stations. The particle orbits of the VLP
signals show that all the stations have elliptical or rectilinear particle orbits, the major axes of which point to aregion at the depth of a few km beneath the south 1-2 km of the summit. Each initial motion is oriented outwardly
from the region that the particle orbits point to. This evidence strongly suggests that the VLP event was excited bya volumetric expansion. The waveform inversion we conducted shows that the source time histories have step-likeshapes with a rise time of 20 s. Prominent step-like increases of the diagonal components indicate a significant
dilatational variation of the source. A candidate for the source mechanism is the inflation of an elliptical cylinder withaxis tilted 20-30 degrees from vertical and major axis of the elliptical cross section oriented northeast-southwest.

Speculation: We may interpret that the 20 s-VLP events were produced by falling mass injection into amagma reservoir associated with the caldera formation, whose idea was proposed by Kumagai et al (2001)
to explain a reproducible nature that repeated with almost same duration times synchronously with successive
caldera growth. We assume here that the falling mass events already started in the period when 20 s-VLP eventswere recognized prior to the caldera collapse on the surface. Applying the model for the 50 s-VLP events, weestimate injection of mass having the piston length of 560 m from the duration of 20 s for the 20 s-VLP events withthe other parameter values fixed. We speculate that the repeated occurrences of the rock collapses resultantly
lead to the caldera formation due to the gravitational instability in the crust.

©IAVCEI 2013 Scientific Assembly. All rights reserved.

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