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

4W_2E-P26

Date/Time: July 24 Poster

Stability assessment of Unzen lava dome by long and short range displacementmonitoring
Masaaki Mantoku1 , Mitsunori Sugiyama1 , Tomonori Todaka1 , Takao Yamakoshi2 , Senro Kuraoka3 ,Yasuhisa Tajima3 , Yuichi Nakashima3 , Takanori Kamijo3
1

Ministry of Land, Infrastructure, Transport and Tourism, Unzen Restoration Work Office, Japan, 2 Public
Works Research Institute, Japan, 3 Nippon Koei Co., LTD, Japan
E-mail: a4982@n-koei.co.jp

The eruptions of Unzen volcano occurred in 1990, leading to the recurrent formation and collapse of lava domethat almost ceased by 1996. Although these hazardous failures of the dome have ceased, the large remaining lava
dome(100 million cubic m) that exist along the steep slope, pose threat to the public. Hence, Unzen Restoration
Work Office (Ministry of Land, Infrastructure, Transport and Tourism) has been monitoring the lava dome, by
surveillance cameras, vibration sensors, and electronic distance measurements (EDMs), in order to ensure thesafety of the public. This presentation mainly reports the recent results of monitoring the movements of the lava
dome.
One of the ongoing main monitoring methods for detecting the movements of lava dome is the automatic long
distance EDM of the eight reflecting prisms placed at near the end of the dome. The results, obtained with the two
EDM stations at 3 km and 3.5 km from the dome, suggest that the eight prisms are continuously moving at a rateof 50 mm to 100 mm per year. The vertical component of the displacement, however, is not clear due to limitation
with the accuracy in measuring the change in elevation of the eight prisms. On the other hand, the manual shortrange EDM, performed at a distance of approximately 500 m from the end of the dome, was able to measure thevertical displacement component and indicated that the dome may be moving downward at an angle close to thatof the original slope surface before eruption.
While the results of automatic EDM provide valuable data in help revealing the mechanisms of the movements of
the lava dome, the noise due to climatic variation is relatively large such that it is difficult to examine whether
the increase in the displacements are associated with the rainfalls or earthquakes. Moreover, the measured
displacements are limited to those obtained by the eight prism points on the large lava dome and therefore it is
not clear whether the whole lava dome is deforming or there are areas more active than other areas.
Recently, ground based synthetic aperture radar has become available for monitoring of landslides. This radar has
been scanning the lava dome of Unzen volcano at distance of approximately 3.5 km since October 2011. The
scanned area of roughly 800 m by 400 m is resolved into 20,000-30,000 pixels, allowing coverage of wide areasthat cannot be measured with EDM. The displacement maps, corresponding to 1 year of measurement, show that
there are areas with larger movements than the other areas. It was also found that the variation in the GBSAR, due
to the climatic effects, is smaller than that of the EDM and permits detection of change the displacement velocities,
which were not easily performed with the EDM. Thus, GBSAR is expected to be a viable method to assess thedeformation mechanisms of the lava dome in more detail and also to detect precursor of instability for early warning
purpose.

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