[1ch113-20] T3.［トピック］スロー地震に関する地質学的・実験的・地震学的研究の連携と進展

[T3-O-11] Spatial changes in inclusion band spacing as an indicator of temporal changes in slow slip and tremor recurrence intervals

*西山 直毅¹、氏家 恒太郎¹、加納 将行² (1. 筑波大学、2. 東北大学)

キーワード：クラックシール脈、スロー地震、再来周期、流体圧、石英沈殿

Slow slip and tremor (SST) represent transient plate boundary slip that lasts from days to years with recurrence intervals of months to years [1]. Slow earthquakes such as SST downdip of the seismogenic zones may trigger megathrust earthquakes due to frequent stress transfer to seismogenic zones [2]. Geodetic observations have suggested that the recurrence intervals of slow slip decrease toward the next megathrust earthquake [3]. However, temporal variations in the recurrence intervals of SST during megathrust earthquake cycles remain poorly understood because of the limited duration of geodetic and seismological monitoring of slow earthquakes. Repeated brittle thrusting near the downdip limit of the seismogenic zone was recorded in quartz-filled, crack-seal shear veins in the Makimine mélange of the Late Cretaceous Shimanto accretionary complex, Japan [4]. The measurement of spacing of inclusion bands in shear veins shows cyclic changes in the inclusion band spacing in the range from 4 to 65 μm. The two-phase primary fluid inclusions in quartz between inclusion bands exhibit varying vapor/liquid ratios regardless of inclusion band spacing, suggesting a common occurrence of fast quartz sealing due to a rapid decrease in quartz solubility associated with a large fluid pressure reduction. A kinetic model of quartz precipitation, considering a large fluid pressure change and inclusion band spacing, indicates that the sealing time during a single crack-seal event cyclically decreased and increased in the range from 0.16 to 2.7 years, with one cycle lasting at least 27 to 93 years [5]. The ranges of sealing time and duration of a cycle may be comparable to the recurrence intervals of SST and megathrust earthquakes, respectively. We suggest that the spatial change in inclusion band spacing is a potential geological indicator of temporal changes in SST recurrence intervals, particularly when large fluid pressure reduction occurs by brittle fracturing.

References
[1] Behr WM, Bürgmann R (2021) Phil Trans R Soc A 379: 20200218.
[2] Obara K, Kato A (2016) Science 353: 253–257.
[3] Kano M, Kano Y (2019) Earth Planets Space 71:95.
[4] Ujiie K. et al. (2018) Geophys Res Lett 45:5371–5379.
[5] Nishiyama N., Ujiie K., Kano M. (2021) Earth Planets Space 73:126.