| Budget Amount *help |
¥3,600,000 (Direct Cost: ¥3,600,000)
Fiscal Year 2004: ¥500,000 (Direct Cost: ¥500,000)
Fiscal Year 2003: ¥700,000 (Direct Cost: ¥700,000)
Fiscal Year 2002: ¥600,000 (Direct Cost: ¥600,000)
Fiscal Year 2001: ¥1,800,000 (Direct Cost: ¥1,800,000)
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| Research Abstract |
In order to elucidate the manner of river response or adjustment to the strike-slip fault displacement, I measured planform geometries and longitudinal profiles of offset rivers crossing strike-slip faults in the field. The San Andreas fault offsets some small gullies incising the relatively flat surfaces in the Carrizo Plain, California. Among these gullies, those with larger drainage basins have larger offsets, probably because gullies that have larger floods can keep the channels entrenched and subjected to the fault displacement longer. On the other hand, older offset channels in the area, which have much larger offsets, indicate that the gradual shift to straighter courses seems probable only for offset channels able to keep and accumulate the offset for long time. No above relations are clear for Japanese offset rivers; however, old offset rivers with longer offset length show a negative correlation between angle of deflection and offset length. This can be explained by wider fau
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lt zones and less erodible bank material except for fault crush zones in Japan.
The strike-slip fault displacement offsets a channel as much as the channel can follow with some adjustment until overbank floods at the upper bend, where the channel offset promotes aggradation, induces the channel avulsion to a straight course, or until the channel is captured by the beheaded and abandoned downstream reach of the adjacent channel moved with the fault displacement. The older offset channels are confined in valleys, and this condition is apparently preventive of the channel avulsion or capture. The strike-slip fault displacement also deforms longitudinal profiles of channels, which is considered to reflect the effects of fluvial processes better. Assuming that the elimination of discontinuities in the longitudinal profile is the main response of fluvial processes to the deformation by fault displacement, nearly continuous longitudinal profiles of old offset channels seem to indicate the full adjustment, the condition in which any more changes will occur as a response to fault displacement until the next fault slip deforms them. The irregularities in longitudinal profiles of other offset channels indicate that the channel adjustment by fluvial processes may be on the way but not enough to eliminate them. If an offset river has enough time to adjust its longitudinal profile by itself after the fault displacement, it can attain a continuous longitudinal profile. Otherwise the discontinuity reflecting a various degree of adjustment would remain. in the longitudinal profile until channel avulsion or capture occurs. Less
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