P-wave attenuation in the Pacific slab beneath northeastern Japan revealed by the spectral ratio of intraslab earthquakes

We investigate P-wave attenuation, Q_p ⁻¹, in the Pacific slab beneath northeastern (NE) Japan, adopting for the first time the spectral ratio technique for intraslab earthquakes. When seismograms of two earthquakes are recorded at a station and their ray paths to the station are largely overlapped, station-dependent amplification and structural effects on the overlapped rays can be canceled out from the ratio of the spectral amplitudes of the seismograms. Therefore, adopting the spectral ratio technique for intraslab earthquakes has a great advantage for the precise evaluation of Q_p ⁻¹ in the slab because the structural effects above the slab, including the high-attenuation mantle wedge, are removed. For estimating the intraslab Q_p ⁻¹, we determined corner frequency of the intraslab earthquakes using the S-coda wave spectral ratio as the first step. Then, we evaluated the inter-event path attenuation, Δt^⁎, from the ratio of the spectral amplitudes of P waves. The obtained result shows that P-wave attenuation in the Pacific slab marks Q_p ⁻¹ of 0.0015 (Q_p of ∼670) at depths of 50–250 km. This indicates that the P-wave attenuation in the Pacific slab is weaker than that in the mantle wedge. The relatively high-Q_p ⁻¹ is correlated with the distributions of intraslab earthquakes, suggesting that the P-wave amplitude is more attenuated around active seismicity zones in the slab. Therefore, our observations likely indicate the presence of fractures, hydrous minerals, and dehydrated fluid around seismogenic zones in the slab at intermediate depths.