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1. Analysis objective
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In order to estimate the position and quantity of magma and to estimate the magma movement mechanism, we attempted to obtain an understanding of the areal distribution of crustal deformation accompanying a volcanic eruption from high-precision matching of SAR images.
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2. Analysis procedure Analysis flow chart
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1) Satellite data used: RADARSAT SAR images
Descending (Fine4N): April 3 and April 27, 2000
Ascending (Fine3F): April 5 and April 29, 2000
2) Summary of analysis
On a previously-taken image we established reference pixels at every 25th pixel from top to bottom and from left to right, then performed high-accuracy matching of the image with a second image taken at a later date, and measured the differences between the images.
We carried out the measurements for both descending data (observations of Mt. Usu from the east) and ascending data (observations of Mt. Usu from the west). By synthesizing the vectors of each movement, we separated the crustal deformation into component in a vertical direction, component in the east and west direction and component in the north-south direction.
Next, we assumed the speed of the crustal deformation to be proportional to the amount of crustal deformation, and divided the crustal deformation into that which occurred from April 3 to April 5 and that which occurred from April 5 until April 27.
In addition, we determined the cumulative crustal deformation from the areal distribution of the amount of movement.
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3.Analysis results
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1) The maximum amount of uplift between April 3 and April 27 was 22m. The maximum movement to the north was 12m, and the maximum to the south was 7m. While we did recognize a movement of several meters in the east-west direction as well, the amount of the change was small compared to that in the north-south direction.
2) The speed of the uplift between April 3 and April 5 was 3.3m/day in some places; the speed opened to the north-south was 2.7m/ a day.
3) The cumulative increase in size between April 3 and April 27 was 23,000,000m3, with an average volume change rate from April 3 through April 5 of 3,400,000m3/day.
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4. Usefulness of the analysis results
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The advantages of detecting crustal deformation by using highly accurate matching of SAR images are the ability to make areal observations of crustal deformation over a wide area without being affected by factors such as clouds, fog or volcanic smoke, and the ability to use satellite observation data taken repeatedly without having to physically enter danger zones. Compared with the interferometry, this technique is particularly effective when the crust undergoes substantial deformation that cannot be verified by interferometry.
This technique also makes it easy to obtain all of the three-dimensional elements of crustal deformation and simultaneously estimate the speed of deformation. Negative aspects of this technique include an accuracy level of about 2m (compared to several millimeters to several centimeters using the interferometry), time resolution of 2 or 3 days at best, and the fact that full analysis normally requires 2-3 weeks.
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5. Analysts and sources
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Analysts: Tobita Mikio, Murakami Makoto, Nakagawa Hiroyuki and Yarai Hiroshi
Tobita M., Murakami M., Nakagawa H., Yarai H. and Fujiwara S.: Mapping of Three-dimensional Components of Movement and Velocities around Usu Volcano Associated with the 2000 Eruption by Matching of SAR Amplitude Images, Abstracts of the 94th Meeting of the Geodetic Society of Japan, pp. 67-68, 2000
Tobita, M., Mak. Murakami, H. Nakagawa, H. Yarai, S. Fujiwara, P. A. Rosen (2001), 3-D surface deformation of the 2000 Usu eruption measured by matching of SAR images, Geophys. Res. Lett. Vol. 28, No. 22, 4291-4294.
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