講演情報
[T4-O-8]［招待講演］スロー地震の地質学的痕跡を探る
*氏家 恒太郎1、矢部 優2、山﨑 悠翔1、Yeo Thomas1、駒木野 照太1 (1. 筑波大学、2. 産業技術総合研究所)
【ハイライト講演】 近年、スロー地震の発生源を経験したと考えられる付加体および高圧変成岩露出域における露頭観察により、スロー地震の発生像解明に関する研究が行われている．本発表では、上記研究の先駆者である講演者らによって明らかとなった、地震学的に観測される低周波微動の地質学的痕跡や、測地観測で観測できない速度での変形の痕跡などについて紹介していただく。 ※ハイライト講演とは．．．
キーワード：
微動、スロースリップ、石英脈、緑泥石ーアクチノ閃石片岩、交代作用
Slow earthquakes, which occur downdip of the seismogenic zone, are commonly observed in subduction zones under warm-slab environments. Despite their significance, the source processes, mechanisms, and environmental conditions responsible for slow earthquakes remain poorly understood. Here, we investigate accretionary complexes and low-temperature, high-pressure metamorphic rocks exhumed from the source regions of slow earthquakes, focusing on areas analogous to the Nankai and Cascadia subduction zones.
We observed that quartz-filled shear and extension veins in subduction or mélange shear zones often form clusters or networks within viscously sheared argillaceous or blueschist matrix, typically tens to hundreds of meters thick (Ujiie et al., 2018; Ujiie et al., 2024). When preserved, these quartz veins exhibit crack-seal texture, indicating repeated brittle failure under near-lithostatic fluid pressure. Shear veins within the clustered quartz vein zones display low-angle thrusting, slip increments of 0.1–0.2 mm, and low stress drops on the order of tens to hundreds of kilopascals, which are consistent with the source properties of low-frequency earthquake (Fagereng et al, 2011; Ujiie et al., 2018). Using a probabilistic cell automaton model, we modeled seismic wave radiation from clustered quartz veins. The results indicate that synthesized seismic waves radiating from successive ruptures of clustered quartz veins can reproduce the seismologically observed tremor (Yabe and Ujiie, 2025).
We examined the deformation mechanisms and rheological properties of blueschist and chlorite-actinolite schist (CAS). Blueschist deforms through dissolution-precipitation creep (Ujiie et al., 2024) or diffusion creep. At the estimated shear stress in the source region of deep slow earthquakes (e.g., ~10–30 MPa in Nankai and Cascadia), blueschist deformed at strain rates of ~10-13 to 10-12 s-1, consistent with aseismic creep. CAS, on the other hand, forms multiple shear localization zones in the subduction mélange. These zones are often accompanied by metasomatic reaction zones that supply fluids to the CAS (Ujiie et al., 2022). In comparison to blueschist, CAS deformed at lower shear stresses but at one to two orders of magnitude higher strain rates. The lower shear stresses are consistent with shear localization along the CAS layers, while the higher strain rates are likely due to metasomatic dehydration-enhanced viscous shear. The lateral extent of high-strain rate zones is less than a few hundred meters, much smaller than the length scale of geodetically detected slow slip events. Our findings highlight the importance of considering increased slip rates associated with metasomatic reactions at a finer scale than detectable by geodetic methods, which may be referred to as “mini slow slip” or “dark slow slip”.
References
Fagereng et al., 2011, Tectonophysics 510, 381–386, doi:10.1016/j.tecto.2011.08.015
Ujiie et al., 2018, Geophysical Research Letters 45, 5371–5379, doi:10.1029/2018GL078374
Ujiie et al., 2022, Geochemistry, Geophysics, Geosystems 23, e2022GC010569, doi:10.1029/2022GC010569
Ujiie et al., 2024, Journal of Geophysical Research 129, e2023JB027901, doi:10.1029/2023JB027901
Yabe and Ujiie, 2025, Geophysical Research Letters 52, e2025GL115447, doi:10.1029/2025GL115447