Estimating the thickness of a faulted mechanical layer based on fault spacing: An example from Miyako‐jima Island, southern Ryukyu Arc, Japan. (19th December 2019)

• Record Type: Journal Article
• Authors: Maeda, Sumire; Otsubo, Makoto; Asahina, Daisuke; Furuno, Yuki
• Is Part Of: Island arc. Volume 29:Number 1(2020)
• Page Start: n/a
• Page End: n/a
• Publication Date: 2019-12-19
• Subjects: arc‐parallel extension -- fault branching -- normal fault -- south Ryukyu Arc; Plate tectonics -- Periodicals; Island arcs -- Periodicals; Geodynamics -- Periodicals; 551.136
• DOI: 10.1111/iar.12336
• Languages: English
• ISSNs: 1038-4871

Abstract

Abstract: The thickness of the mechanical layer that hosts a group of faults can be estimated from the spacing of saturated faults (i.e. the constant spacing between faults when the fault system is fully developed and has attained its final fault density). We measured fault spacing for a group of saturated active normal faults on Miyako‐jima Island (southern Ryukyu Arc, Japan) and estimated the thickness of the faulted mechanical layer. The measured fault spacing is 1.30 ±0.14 km, and the thickness of the mechanical layer is < 3 km, which is the upper limit of the seismogenic zone. This faulted mechanical layer corresponds to a sedimentary layer in which earthquakes cannot occur. Results indicate that the shallow (< 3 km depth) normal faults on Miyako‐jima Island do not have the potential to cause medium‐size earthquakes as individual faults. The origin of the shallow normal faults might be related to the presence of a larger‐scale, deeper fault. The results indicate that fault spacing provides important information on the potential magnitude of earthquakes associated with active faults. Abstract : We measured fault spacing for a group of saturated active normal faults on Miyako‐jima Island (southern Ryukyu Arc, Japan) and estimated the thickness of the faulted mechanical layer. Results indicate that the shallow (<3 km depth) normal faults on Miyako‐jima Island do not have the potential to cause medium‐size earthquakes as individual faults. The origin of the shallow normal faults might be related to the presence of a larger‐scale, deeper fault. Abstract : 断層の成長過程を理解し​てその規模を推定することは，断層の長期評価を行う上で重要である.正断層群の成長過程の初期段階では個々の断層はランダムに分布するが，断層群の成長が飽和状態になると，断層同士が等間隔の規則的な分布をする.この断層間隔は断層群の存在する層(メカニカルレイヤー)の厚さによって規制されており，正断層群の断層間隔とメカニカルレイヤーの厚さには経験式が構築されている.本論文では，琉球弧南部に属する宮古島の正断層群の断層間隔を計測して断層群の存在するメカニカルレイヤーの厚さを推定した.計測した断層間隔から，これらの正断層群が存在するメカニカルレイヤーの厚さは3 km以下であった.このメカニカルレイヤーは，本地域の震源分布，コアデータ，および，P波速度構造から，非地震発生層に対応していることがわかった.そのため，これらの断層群は個別の活断層として断層運動を引き起こす可能性は低く，地震発生層に存在する大規模断層の分岐断層である可能性が高いことが示唆された.