Linking double seismic zones to oceanic lithosphere rheological layering: the role of mid-lithospheric discontinuities

Hu Tianyu, wang Zhensheng, Fabio Capitanio, Junfeng Zhang, Feng Shi, Xinnuo Li, Sierd Cloetingh, István J. Kovács, Alessio Lavecchia, Alexander Koptev

The origin of the ubiquitous lower seismic layer (LSL) in double seismic zones (DSZs) within subducting oceanic lithosphere remains one of the most persistent unresolved problems in subduction-zone seismicity. Analysis of recent geophysical observations reveals a close spatial association between the LSL and the oceanic mid-lithospheric discontinuity (MLD), a feature attributed to the accumulation and crystallization of residual or intrusive melts within oceanic lithosphere, suggesting a genetic link between the two structures. Here, combining observational constraints with geodynamic numerical modeling, we show that melt crystallization and subsequent metamorphic reactions within the MLD promote grain-size reduction and rheological layering, which localize differential stress in the mantle section above the MLD. These processes create favorable conditions for earthquake generation and naturally explain the development and features of the LSL. Importantly, this mechanism does not require a hydrated lower seismic layer, consistent with seismic observations from numerous subduction zones worldwide.