報告書番号

N08035

タイトル（和文）

プレート沈み込み境界上盤側に分布する短い活断層の特徴と成因

タイトル（英文）

Geomorphological and geological property of short active fault in fore-arc region of Japan

概要 （図表や脚注は「報告書全文」に掲載しております）

近年，短い活断層に対する地震規模評価手法について議論されており，その高精度化が求められている．本研究ではプレート沈み込み境界上盤側に分布する短い活断層について，地形・地質学的調査により，その成因，特性を明らかにし，震源断層評価における判断基準の明確化を図った．
1923年大正関東地震の際，地表に出現した延命寺断層および室戸半島西岸域に分布する短い活断層を対象に，詳細な地形・地質学的調査をおこない，成因，特性を明らかにした．
調査の結果，両地域でみられた短い活断層の特性は①活断層の長さに対して，1回の変位量が大きい，②活動間隔がプレート境界地震より明らかに長い，③累積性がある，であった．
今後，プレート沈み込み境界上盤側における短い活断層の調査で，以上の特性が得られた場合には，起震断層としてではなく，プレート境界地震，あるいはプレート境界から派生する比較的規模の大きい活断層を評価することが必要となる．

概要 （英文）

The important issue in the earthquake magnitude evaluation method is the classification of short active faults or lineaments. It is necessary to determine the type of active fault to be included in the earthquake magnitude evaluation. The particular group of fault is the surface earthquake faults that are presumed to be branched faults of large interplate earthquakes in subduction zones.
We have classified short lineaments in two fore-arc regions of Japan through geological and geomorphological methods based on field survey and aerial photograph interpretation.
The first survey is conducted at Enmeiji Fault in Boso Peninsula. The fault is known to have been displaced by 1923 Taisho Kanto earthquake. The altitude distributions of marine terrace surfaces are different on both sides of the fault.
In other words, this fault has been displaced repeatedly by the large interplate earthquakes in the past. However, the recurrence interval of this fault is far longer than the large interplate earthquake calculated by the slip rate and the displacement per event.
The second survey is conducted in the western side of Muroto Peninsula, where several short lineaments are distributed. We have found several fault outcrops along the few, particular lineaments. The faults in the region have similar properties to Enmeiji Fault. On the other hand, short lineaments are found to be structural landforms. The comparison of the two groups enables us to classify the short lineaments based on the geomorphological property and geological cause of these faults.
-Displacement per event is far larger than displacement deduced from length of the active fault
-Recurrence interval of the short active fault is far longer than that of large interplate earthquake
-Displacement of the short active fault has cumulative
The earthquake magnitude of the faults have these characters need to be evaluated by the plate boundary fault or the long branched seismogenic fault.

報告者

キーワード