Seismic scatterers within subducting slab revealed from ambient noise autocorrelation

Abstract

We reconstructed new migrated images from autocorrelation functions of ambient noise at seismic stations in northeastern Japan. During the analysis period, the autocorrelation functions, having a frequency range of 0.5–2 Hz, showed some coherent signals at large lapse times of 10–50 s as well as at small lapse times of less than 10 s. These signals were also coherent in space along cross sections across the Japan subduction zone. The migrated images show seismic scatterers within the subducting Pacific Slab at the depth range of 50–150 km. The distribution of the scatterers within the subducting slab is comparable to the double seismic zone in the subduction zone; in particular, the thickness of the distribution of the seismic scatterers is consistent with that of two planes of the double seismic zone. This indicates inhomogeneities in the seismic velocity structure around the upper and lower planes of the seismicity. The new migrated images suggest that seismic scatterers, such as fluid from dehydration reactions of the hydrated metamorphosed mantle, exist in the lower as well as upper plane of the seismicity in the double seismic zone; this fluid may act as a scatterer within the subducting slab.