Seismic interferometry of teleseicmicS-wave coda for retrieval of body waves: an application to the Philippine Sea slab underneath the Japanese Islands

Abstract

SUMMARY The reconstruction of surface waves from the cross-correlation of random wavefields has recently been extensively inspected by theoretical and experimental approaches. However, the reconstruction of body waves has not been extensively studied. In this study, we present a method for extracting body waves, that is, direct P and S waves, and reflected waves from the Philippine Sea slab. We use the cross-correlation of the wavefield generated by the teleseismic S waves observed by Hi-net tiltmeters with a passband of 0.07–0.5 Hz. To enhance the contribution of the S coda, we reduced the source-time function and the deterministic phases sS, ScS and SS, and the surface wave in S coda. The records of the processed S coda observed at every station-pair are cross-correlated for each teleseismic event. The cross-correlation functions (CCFs) of different earthquakes are stacked to retrieve wavefields propagating between the station-pairs. As a result, direct P, S and the reflected waves could be extracted when the stacked CCFs are aligned as a function of distance of separation between two stations. The gradients of the traveltime curves for the direct P and S waves are approximately 5–7 and 3k m s −1 , respectively. These velocities correspond to the seismic velocities of the crust or the P-wave velocity of the uppermost mantle. In order to enhance the reflected waves, we searched for the reflection points by assuming that the later phases in the CCFs are SS reflections and map the amplitudes onto the depth sections. As a result, the negative phases dipping to the north can be traced right below the region of hypocentre distribution. These negative phases probably correspond to the oceanic Moho within the Philippine Sea slab. We also show that the oceanic Moho can also be traced by assuming PP reflections. These results indicate that the CCFs plausibly contain information regarding both P and S waves propagating between the two receivers, and are capable of detecting reflected phases in addition to the direct waves.