Terrestrial phenomena disasters/Earthquakes/Geotechnical disasters. Geotechnical disasters/Crustal movement
Disaster name
1998 Iwate-ken Seihokubu Earthquake (September 3, 1998, M6.1)
Authors of WEB conversion
Une Hiroshi and Nishimura Takuya

Case Study

No. 21

1. Analysis objective

Understand the spatial distribution of crustal deformation using GPS and SAR interferometry, infer a crustal deformation model and earthquake mechanisms, and investigate the causal relationship between the volcanic activity and the occurrence of the earthquake.

2. Analysis procedure Analysis flow chart

1) Satellite data used: SAR image from JERS-1 (Fuyô No. 1)

Descending orbits: April 12, 1996, November 5, 1997, April 30, 1998, June 13, 1998, July 27, 1998, September 9, 1998

Ascending orbits: April 14, 1998, July 11, 1998, August 24, 1998, October 17, 1998

2) Summary of the analysis

After making SAR interferometry images of various combinations, we investigated the time-series crustal deformation patterns. Remarkable crustal deformation accompanying the earthquake was evident in the SAR interferometry images for the period including the earthquake.

The displacement obtained from SAR interferometry are one-dimensional in the direction of the line of sight to the satellite. By using the difference between directions of the lines of sight for the ascending and descending orbits, we combined two interferograms from both ascending and descending orbits to separate the displacement into east-west component and net up-down component and calculated the spatial distribution of displacement in the two components. We used these results and a time-series crustal displacement obtained from GPS observations to develop three models of crustal deformation for the period until April 29, 1998, the period from April 29 through September 2, and the eatrhquake fault on September 3. We also used the models to estimate the change in stress.

3. Analysis results

1) Although we were unable to see any remarkable deformation from the SAR interferometry images until April 30, 1998, the deformation was observed from about February by GPS. For this reason we believe there was no significant component of deformation in the direction of the satellite's line of sight until April 30, and that most of the crustal deformation was in a north-south direction caused by the opening fissures running nearly east-west.

2) The crustal deformation from April 30 until September 2 can be explained by assuming a point inflation source (Mogi source) 13km west of the summit of Mt. Iwate at a depth of 7.9km.

3) We learned from the crustal deformation map calculated using the SAR interference image for the period including the earthquake that the two surface ruptures of fault run in parallel in a northeast-southwest direction, separated by a distance of approximately 1.5km.

4. Usefulness of the analysis results

The ability to clarify crustal deformation in time-series and its spatial distribution using surveys such as GPS and synthetic aperature radar, and the ability to model and explain the crustal deformation accompanying volcanic activity and/or an earthquake, open up new possibilities for research of crustal deformation. We believe that continuing this research in the future and accumulating data will make it possible to provide information useful for activities such as predicting volcano eruptions.

5. Analysts and sources

Analysts: Fujiwara Satoshi, Murakami Makoto and Nishimura Takuya

Nishimura T., et al.: Crustal deformation and causal relation of the volcanic activity of Mt. Iwate and the northern Iwate earthquake (M6.1), Journal of the Geographical Survey Institute, Vol. 94, pp. 21-30, 2000

Nishimura T., et al: The M6.1 Earthquake triggered by volcanic inflation of Iwate volcano, northern Japan, observed by satellite radar interferometry, Geophysycal Research Letters, Vol 28, pp 635-638, 2001