17:15 〜 19:15
[SIT19-P01] Distribution of the disturbed incoming sediments of the Japan Trench and its potential relation to the outer rise faulting
キーワード：日本海溝、浅部地殻構造、反射法地震探査、チャート層
Numerous seismic reflection surveys have been conducted to investigate the crustal structure in the outer rise region of the Japan Trench. Previous studies have reported cases where chert layers appear disturbed and unidentifiable in seismic reflection profiles. Such anomalous structures suggest the presence of geoscientific processes that can affect normal sedimentation within the sedimentary layer, potentially influencing the occurrence of outer rise earthquakes in the vicinity. Furthermore, these structures may impact the formation of décollements after subduction, along with the oceanic crust, potentially affecting earthquakes in subduction zones. To study these effects, it is necessary to elucidate the nature of the anomalies occurring in the chert layers.
This study classified sedimentary layers into three types (A, B, and C) and mapped their distribution based on seismic reflection profiles. Type A represents the typical sedimentary structure, comprising thick semi-pelagic sediments overlying thin pelagic clay and chert layers. Type B is characterized by high-amplitude reflective surfaces within or on the chert layers. Type C exhibits highly disturbed chert layers, where the original chert layer is replaced by a multi-layered acoustic basement with a polarity reversal at a certain point. Types B and C are generally associated with faults. Type B anomalies are more prevalent in areas distal to the trench, whereas Type C becomes increasingly prominent closer to the trench, particularly around petit-spot volcanoes. These anomalous reflective surfaces suggest material transformation within the sediments, potentially due to metamorphism. One hypothesis posits that, in the distal ocean, away from the trench axis, hydrothermal fluids ascend along faults, altering portions of the chert layers, which are sediments located near the oceanic crust. Closer to the trench axis or in proximity to volcanic areas, more intense faulting and magmatic activity may result in more severe disturbance and alteration of the sedimentary layers.
This study classified sedimentary layers into three types (A, B, and C) and mapped their distribution based on seismic reflection profiles. Type A represents the typical sedimentary structure, comprising thick semi-pelagic sediments overlying thin pelagic clay and chert layers. Type B is characterized by high-amplitude reflective surfaces within or on the chert layers. Type C exhibits highly disturbed chert layers, where the original chert layer is replaced by a multi-layered acoustic basement with a polarity reversal at a certain point. Types B and C are generally associated with faults. Type B anomalies are more prevalent in areas distal to the trench, whereas Type C becomes increasingly prominent closer to the trench, particularly around petit-spot volcanoes. These anomalous reflective surfaces suggest material transformation within the sediments, potentially due to metamorphism. One hypothesis posits that, in the distal ocean, away from the trench axis, hydrothermal fluids ascend along faults, altering portions of the chert layers, which are sediments located near the oceanic crust. Closer to the trench axis or in proximity to volcanic areas, more intense faulting and magmatic activity may result in more severe disturbance and alteration of the sedimentary layers.