2002 Volume 2002 Issue 22 Pages 33-44
A boulder on the excavated trench wall just upper part of the Makigahora active fault, a part of the Takayama Oppara active fault zone, in Gifu Prefecture, Japan, is observed. The existence of boulders just upper part of, or several meters apart from, lateral active faults is sometimes observed on the wall of trench. When the sediments are composed of terrace deposit or fan deposit or debris flow, boulders are naturally found. In case of swamp deposit how boulders are derived?
The author hypothesize that boulders a re concentrated by uprhrow in near field strong ground motion in earthquakes. The field survey is conducted along the part of the Makigahora active fault.
As a result of the field survey, boulders are not accidentally concentrated n e a r the fault line, especially several tens meters near the fault line.
Many surveys along surfacer uptures of earthquakes clarified that the distribution of upthrow of boulders corresponds to the area of near field strong ground motion.
When boulders appear just upper part of, or s e v eral meters from, the active fault, especially within swamp deposit, we should treat them carefully as an evidence of the occurrence of strong earthquake. Plants under the boulders show the time of the strong earthquake. The distribution of boulders along active faults might show the strong ground motion area.
