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Mapping from Space
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Measurement of Topographic variation by Hyogo-Nanbu Earthquake
with JERS 1STAR Interferometry
3.Interpretation of the Imageries
The double difference interferogram imagery was interpreted on the view point of geological study. The results are simply shown in Figure 5 and the following items are summarized.
Figure 5: Geological interpretation of double difference interferogram image
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Total variation of the area
A rotation of colors indicates the amount of topographic variation by the earthquake. The color rotation, red green -blue -red in the double difference interferogram (Figure 3), repeats six times in this case which means that vertical is recognized in 70 cm approximately in the northern Awaji island after the earthquake.
- Coinciding of the heavily displaced points and center of fringe pattern
The villages of Nojima-Hirabayashi and Najima-Okhawa are locating on the central part of the fringes (circular/ oval pattern ). Some of the biggest vertical displacement on the fault scrap was observed in those villages , Nojima hirabayasi and Najima Okhawa. The displacement was approximately 1.8m as maximum ,however such big displacement was only at the limited part of the fault.
- The fault and related deformation
Location of the fault can be interpreted in discontinuity of the color, and they are making a lineament. The pattern of fringes does not cross the Nojima fault but there distribute many block patterns in the fault zone. Those blocks could be generated by the fault activity, right lateral strike-slip.
Most of the blocks developed along the Nojima fault have an elongated shape with axis from North to South. It seems that ground surface was undulated by the right lateral strike-slip of the Nojima fault (Figure 4and 5). This undulated might be in centimeter order, because the color of this area repeats in red and green, lacking blue color in most part(see Figure3).
4. Conclusion
The result of image interpretation seems to be Consistent to general idea of structural geology. In many cases, strike -slip structure involves en echelon folds with them.
Those folds have axis that cross the shear zone with narrow angles to moving direction. The swelling blocks in the strike-slip zone of Nozima fault are in small scale, however the direction of their axis is harmonious to en echelon folds model. It is considered that the undulation in the fault zone could be involved by the fault activity. From these view points it can be concluded that double difference interferometry with JERS-1 SAR contain enough preciseness and useful for geological purposes in this area.
There, however ,are still some problems to be solved before generalization. The topographic variation has not proved yet by field survey in a vast area. Back scatter of SAR come from ground surface ,forest body by volume scatter and other artificial works. Seasonal charge of forest condition affect on the phase generation . Wave length of microwave should be considered with relation to the vegetation condition .Massonnet,et al., succeeded to get topographical variation by Landers earthquake in California. This study indicates that C-band of ERS-1 SAR Adapt to semi-arid zone where vegetation is sparse.
The mesh size and accuracy of the DEM data also have to be checked. JERS-1 is not designed for interferogram analysis, so data of satellite position is estimation.
Nevertheless, this technique is expected as a new accurate method to measure the topographical variation over the area in a short period.
ERSDAC/MITI is planning the next generation SAR program (SAR2), currently scheduled to launch early 2000s. As new SAR parameters, multi frequency, multi polarization and off-nadir angles are considered so far. Interferometry is also recognized as an important parameter for new SAR Mission.
Reference
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Boulter, C.A., 1989, Four Dimensional analysis of geological maps. Wiley Massonet, D., Rossel M., Carmona, C., Adragna, F.,Petzer,G., Ferigl,K., Rabaute,T.,1993,The displacement field of the landers earthquake mapped by radar interferometry.nature,vol.364
- Lin,A.,Imiya H.,Uda,S., misawa,T.,1995,Morphological characteristic of the Nojima earthquake fault. Japan Society of Engineering Geology 36-1,p39-50
- Haraguchi, T.,Okamura,M.,Tsuyuguchi,K.,1995,Report on the Nojima-Earthquake fault caused by the 1995 Hyougoken-Nandu Eatrhquake. Japan Society of Engineering Geology 36-1,p51-61
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