Newly formed oceanic crust interacts with penetrating seawater, resulting in the formation of secondary minerals. Sediment cover can potentially change the redox conditions of underlying basaltic crusts, significantly affecting the types of secondary minerals and element transfer during alteration. However, previous studies have not revealed the quantitative regional variation and controlling factors of seafloor alteration using altered samples taken from different sites. We present a novel approach for the quantitative analyses of element mobility related to seafloor alteration based on a regional dataset of altered basalt bulk compositions and highlights the effects of the redox state and duration on alteration. The protolith reconstruction models (PRMs), machine learning-based element mobility analyses, were applied to the compositional data of the basaltic crusts from the South/Northwest Pacific region. The analyses revealed that altered basalts with older ages showed higher element mobility, particularly characterized by an enrichment of Rb and K, which were associated by up to 100 times with the formation of secondary minerals. In the oxidative settings of the South Pacific region, enrichment of Ba, U, and Pb and depletion of P were observed in samples with intense alteration. In contrast, under reductive conditions in the Northwest Pacific region, alterations associated with carbonate veins caused U enrichment. Our research suggests that sediment thickness is a key factor in the redox conditions during alteration, which changes the characteristics of element transfer and secondary minerals. Additionally, seafloor alteration likely persisted for at least 30 Myr, irrespective of whether the environment was oxidative or reductive.

An intense earthquake swarm is occurring in the crust of the northeastern Noto Peninsula, Japan. Fluid movement related to volcanic activity is often involved in earthquake swarms in the crust, but the last volcanic activity in this area occurred in the middle Miocene (15.6 Ma), and no volcanic activity has occurred since then. In this study, we investigated the cause of this earthquake swarm using spatiotemporal variation of earthquake hypocenters and seismic reflectors. Hypocenter relocation revealed that earthquakes moved from deep to shallow areas via many planes, similar to earthquake swarms in volcanic regions. The strongest M5.4 earthquake initiated near the migration front of the hypocenters. Moreover, it ruptured the seismic gap between the two different clusters. The initiation of this earthquake swarm occurred at a locally deep depth (z = ~17 km), and we found a distinctive S-wave reflector, suggesting a fluid source in the immediate vicinity. The local hypocenter distribution revealed a characteristic ring-like structure similar to the ring dike that forms just above the magma reservoir and is associated with caldera collapse and/or magma intrusion. These observations suggest that the current seismic activity was impacted by fluids related to ancient or present hidden magmatic activity, although no volcanic activity was reported. Significant crustal deformation was observed during this earthquake swarm, which may also be related to fluid movement and contribute to earthquake occurrences. A seismic gap zone in the center of the swarm region may represent an area with aseismic deformation.