A shallow prominent reflector in seawater identified using the seismic oceanographic method off the east coast of the Noto Peninsula, Japan Sea

We conducted a seawater imaging study using multichannel seismic reflection (MCS) data acquired off the eastern coast of the Noto Peninsula in the Japan Sea following the 2024 Noto Peninsula earthquake ( M J 7.6). A shallow prominent reflector (SPR) was identified within the water column of the Toyama Trough. The SPR exhibited significant spatial variations, appearing as a continuous, high-amplitude reflection in select areas while appearing weak and discontinuous in others. To investigate the mechanisms underlying the formation of this reflector, we compared MCS images with concurrently collected conductivity, temperature, and depth (CTD) data. The results suggest that the SPR was generated by a notable change in the acoustic impedance associated with a high vertical density gradient within the pycnocline, which functions as a transition layer between the surface mixed layer and the underlying Japan Sea Proper Water. We conclude that the spatial variations in the characteristics of the SPR could be controlled primarily by changes in the thickness of the surface mixed layer driven by wind-induced mixing, eventually affecting the magnitude of the vertical density gradient. These findings demonstrate the potential of MCS data for visualizing upper-ocean stratification and mixed-layer variability in the Japan Sea.