Because deformation in the lower crust immediately below a seismogenic fault can accumulate stress on the fault, understanding the dominant deformation mechanism and the formation of ductile shear zones in the lower crust is crucial to reveal occurrences of intraplate earthquakes. Plagioclase-rich rocks (e.g., gabbro and gabbronorite) are major constituents of the lower crust. A change in the deformation mechanism from grain-size-insensitive creep to grain-size-sensitive creep would result in significant rheological weakening. The transition potentially increases the strain rate more than an order of magnitude at a constant stress, and the process of grain size reduction in plagioclase-rich rocks is particularly important. In addition to dynamic recrystallization, fracturing is another dominant mechanism of grain-size reduction at high temperatures. The most likely explanation for the occurrence of fracturing in the lower crust is the deeper penetration of large earthquakes, which nucleate in the seismogenic zone and propagate into the underlying ductile region at the stage of coseismic displacement. Zones with very fine grains that result from fracturing would be deformed by grain-size-sensitive creep. Fracturing also facilitates fluid flow due to the increased dilatancy and connectivity of pore spaces. An influx of fluid enhances hydration reactions, which form small, strain-free recrystallized grains. This process promotes grain-size-sensitive creep in narrow zones, and result in the development of ductile shear zones in the lower crust.
