Coseismic dislocation creep and recrystallization in a shallow carbonate-hosted active fault

Identifying coseismic deformation along shallow faults is crucial for assessing seismic hazards associated with surface rupture and ground shaking, yet unequivocal evidence of seismic slip in carbonate-hosted faults remains uncertain. Active normal faults on Miyako Island in the southern Ryukyu arc (Japan) are hosted in Middle to Late Pleistocene carbonate rocks derived from uplifted coral reefs and are considered capable of generating magnitude-7-class earthquakes. Field observations of one of these faults show that the ∼50-m-thick fault zone contains multiple ∼2-mm-thick narrow slip zones characterized by mirror-like, striated slip surfaces and limestone fragments embedded in an ultrafine-grained calcite matrix. Microstructural analyses indicate that calcite grains within these zones experienced localized dislocation creep and recrystallization at temperatures >400 °C, substantially higher than the ambient temperature of ∼25 °C. These microstructural features provide compelling evidence of coseismic frictional heating at shallow crustal levels, providing key constraints on paleoseismicity and the seismic hazards of carbonate-hosted faults.