[S-CG45] Science of slow-to-fast earthquakes

[SCG45-P19] Origin of fluid responsible for silica precipitation in source area of deep slow earthquakes

山本 久暉¹、*氏家 恒太郎¹、佐野 貴司² (1.筑波大学、2.国立科学博物館)

Geophysical surveys and models have shown that slow earthquakes downdip of the seismogenic megathrust occur in regions of highly overpressured fluid. In northern Cascadia subduction zone, precipitation of silica from fluid was proposed to control tremor recurrence time, and geological observations suggested that crack-seal quartz veins may be geological fingerprints of low-frequency earthquakes (LFEs) and tremor. The fluid responsible for quartz vein formation has been thought to originate from warm slab dehydration, but remains poorly understood. Here, we conducted trace element and Sr-Nd isotope analyses on the quartz veins in the Tomuru metamorphic rocks in Ishigaki Island, Ryukyu Arc, which deformed under epidote-blueschist facies metamorphism at ~450 °C, comparable to the environments of deep slow earthquakes in northern Cascadia. The geological structure of the Tomuru metamorphic rocks is characterized by repetition of metabasite and metapelite, possibly representing a duplex underplating. Metabasite recorded prograde metamorphism from greenschist to blueschist. The samples for trace element and Sr-Nd isotope analyses were taken from quartz veins in metapelite, metabasite, and mixed zone composed of metapelite and metabasite. Rare earth element patterns of greenschist and blueschist are nearly identical to those of normal mid ocean ridge basalt. Trace element patterns show decreased concentration of fluid mobile elements (Pb and Sr) in blueschist relative to greenschist, suggesting dehydration during prograde metamorphism. However, the chemical mass balance analysis demonstrated that silica was not released during dehydration of subducting metabasite. The trace element patterns and the Sr-Nd isotope ratio of quartz veins resemble those of metapelite regardless of quartz vein-bearing host rocks, indicating that quartz veins are derived from dehydration reactions within subducting metapelite. Our results highlight the importance of sediments dehydration that may modulate spatiotemporal distribution of LFEs and tremor. We further suggest that trace element and Sr-Nd isotope analyses are useful methods for identifying the origin of fluid responsible for silica precipitation in source area of deep slow earthquakes.