Continuous imaging of the subducting Pacific plate under Japan using receiver function analysis of the amphibious array data

The interplate seismogenic zone of NE Japan subduction zone locates at offshore, where in situ observation is only possible with ocean bottom stations. The seismogenesis of subduction zones has a close relationship with the hydrous state of the subduction zone system, and water has been observed as a low velocity zone of the subducting plate. Continuous imaging of the subsurface structure from the trench, crossing the whole forearc, is important to understand the relationship between seismicity and seismic structure, as well as to delineate the water transportation in subduction. On land, arrays from different institutes cover Japanese islands densely enough to retrieve good quality receiver function images. On the seafloor, several temporary arrays of pop-up type ocean bottom seismometers were deployed over the years (e.g., Shinohara et al. 2012). Furthermore, the ocean bottom cable network off the coast of NE Japan (S-net) has allowed a real-time and permanent observation on the seafloor from late 2016. In this work, we conduct the teleseismic P receiver function analysis of those amphibious array data to produce continuous receiver function images from ocean to land. To use the ocean bottom seismic data for imaging, special care has to be taken for the effect of the low velocity sediment on top of the seafloor; an extremely slow S-wave velocity (< 0.5 km/s) of the thin sediment causes a large delay in arrival times (e.g., ~2 seconds) to the converted waves, which may result in misplacement of an image if the effect was not corrected. The delay of arrival times is measured using both local and teleseismic event records. Moreover, the strong seismic velocity (elastic property) contrast between the sediment layer and sediment basement is also implied from clear peaks in autocorrelation functions of earthquake and ambient noise records of horizontal components. The common conversion point stacked images show a continuously dipping oceanic Moho along profiles crossing ocean and land. Besides, a coherent negative amplitude above the positive phase (oceanic Moho) is observed along a profile at ~39ºN as land stations do. The timing of negative phase is inexplainable by the sediment reverberation. The result of this work presents the first receiver function image in NE Japan using both ocean and land stations that shows coherent structural signatures of the subducting oceanic plate.