10:45 AM - 11:00 AM
[SCG46-01] Albitization due to interaction between seafloor basalt and fluid increases the potential of megathrust earthquakes in subduction zones
★Invited Papers
Keywords:Basalt, Friction, Earthquake
Megathrust earthquakes have been considered to be closely related to diagenetic processes during subduction of a plate and overlying sediments. The seismogenic zone at temperatures of approximately 150-300°C where megathrust earthquakes nucleate has been discussed as a result of changes in the frictional properties of plate boundary faults with depth due to the illitization of smectite with increasing temperature and the lithification of sediments by compaction. On the other hand, in on-land accretionary complexes such as the Shimanto Belt, which has experienced seismogenic zones in the past, deformation appears to be concentrated in basaltic rocks of oceanic crust. This implies that earthquakes in seismogenic zones can occur not only in sediments but also in oceanic crust. However, the frictional properties of subducting basalt and their influence on the seismogenic process have not been well understood.
In this study, we conducted hydrothermal friction experiments using basalt collected from the Mugi mélange in Tokushima, Japan, which is thought to have experienced the seismogenic zone depth. For details, see Okuda, Niemeijer et al. (2023, JGR Solid Earth). The (a-b) values, the velocity dependence of friction coefficient, were negative between 100°C and 400°C. When (a-b) is negative, there is a potential for earthquakes to occur. This suggests that subducting basalt can generate earthquakes over a wide temperature range including the seismogenic zone in subduction zones.
Microstructural observations of the post-experiment samples showed that the albite in the basalt undergoes pressure-solution creep, while the pyroxene appears to deform in a brittle manner. Based on the results of microstructural and frictional properties at multiple temperatures and velocities, pressure-solution creep of albite plays a dominant role in the observed frictional properties of altered basalts. Albite grains in altered basalts is formed from anorthite at about 150°C by the reaction of unaltered basalt with seawater-origin fluids during the subduction process, also accompanied by grain size reduction and the formation of fine-grained chlorite. In general, pressure-solution creep occurs more effectively with smaller grain sizes, and the sheet-like crystal of chlorite leads to a low friction coefficient. This indicates that alteration processes by fluid reactions during subduction control the frictional properties of subducting oceanic crust and increase its seismogenic potential in the seismogenic zone.
There are a lot of unanswered questions on such chemical effects due to fluid-rock interactions on mechanical properties of rocks. Future deformation experiments using samples with controlled fluid reactions and/or long-term chemical reactions would be beneficial for the more comprehensive understanding of the seismogenesis in subduction zones.
In this study, we conducted hydrothermal friction experiments using basalt collected from the Mugi mélange in Tokushima, Japan, which is thought to have experienced the seismogenic zone depth. For details, see Okuda, Niemeijer et al. (2023, JGR Solid Earth). The (a-b) values, the velocity dependence of friction coefficient, were negative between 100°C and 400°C. When (a-b) is negative, there is a potential for earthquakes to occur. This suggests that subducting basalt can generate earthquakes over a wide temperature range including the seismogenic zone in subduction zones.
Microstructural observations of the post-experiment samples showed that the albite in the basalt undergoes pressure-solution creep, while the pyroxene appears to deform in a brittle manner. Based on the results of microstructural and frictional properties at multiple temperatures and velocities, pressure-solution creep of albite plays a dominant role in the observed frictional properties of altered basalts. Albite grains in altered basalts is formed from anorthite at about 150°C by the reaction of unaltered basalt with seawater-origin fluids during the subduction process, also accompanied by grain size reduction and the formation of fine-grained chlorite. In general, pressure-solution creep occurs more effectively with smaller grain sizes, and the sheet-like crystal of chlorite leads to a low friction coefficient. This indicates that alteration processes by fluid reactions during subduction control the frictional properties of subducting oceanic crust and increase its seismogenic potential in the seismogenic zone.
There are a lot of unanswered questions on such chemical effects due to fluid-rock interactions on mechanical properties of rocks. Future deformation experiments using samples with controlled fluid reactions and/or long-term chemical reactions would be beneficial for the more comprehensive understanding of the seismogenesis in subduction zones.