報告書番号

U03021

タイトル（和文）

横ずれ断層系の発達過程ならびに変位地形の形成過程 --断層模型実験による検討--

タイトル（英文）

Evolution of Strike-slip Fault Systems and Associated Geomorphic Structures: Model Test

概要 （図表や脚注は「報告書全文」に掲載しております）

断層活動性調査の効率化を目的とし，横ずれ断層系の3次元発達過程および変位地形の形成過程を解明するため，断層模型実験を行い，活断層系の形態および変位地形と比較・検討を行った．その結果，横ずれ断層系の発達過程ならびに変位地形の形成過程は次のとおりと考えられる．1）リーデル剪断群の雁行状配列，2）Lower-angle shearの雁行状配列，3）Lower-angle shear間におけるバルジ群の形成，4）バルジを切るY-shearの形成ならびにバルジ間のおける溝状地の形成，5）連続的な溝状地の発達とバルジの沈下．断層調査の対象箇所としては，断層系の発達過程が初期の段階にあるものについてはリーデル剪断よりLower-angle shearに相当する断層を，成熟度が高い断層系においては溝状地もしくはバルジを切る断層を，優先して調査すべきと考えられる．

概要 （英文）

Sandbox experiments were performed to investigate evolution of fault systems and its associated geomorphic structures caused by strike-slip motion on basement faults. A 200 cm long, 40 cm wide, 25 cm high sandbox was used in a strike-slip fault model test. Computerized X-ray tomography applied to the sandbox experiments made it possible to analyze the kinematic evolution, as well as the three-dimensional geometry, of the faults. The deformation of the sandpack surface was analyzed by use of a laser method 3D scanner, which is a three-dimensional noncontact surface profiling instrument. A comparison of the experimental results with natural cases of active faults reveals the following: In the left-lateral strike-slip fault experiments, the deformation of the sandpack with increasing basement displacement is observed as follows. 1) In three dimensions, the right-stepping shears that have a "cirque" / "shell" / "ship body" shape develop on both sides of the basement fault. The shears on one side of the basement fault join those on the other side, resulting in helicoidal shaped shear surfaces. Shears reach the surface of the sand near or above the basement fault and en echelon Riedel shears are observed at the surface of the sand. The region between two Riedels is always an up-squeezed block. 2) Lower-angle shears generally branch off from the first Riedel shears. 3) Pressure ridges develop within the zone defined by the right-stepping helicoidal shaped lower-angle shears. 4) Grabens develop between the pressure ridges. 5) Y-shears offset the pressure ridges. 6) With displacement concentrated on the central throughgoing fault zone, a liner trough developed directly above the basement fault. R1 shears and P foliation are observed in the liner trough. Such evolution of the shears and its associated structures in the fault model tests agrees well with that of strike-slip fault systems and its associated geomorphic structures.

報告者

キーワード