Characteristics of PKP-Cdiff coda revealed by small-aperture seismic arrays: Implications for the study of the inner core boundary

PKP-Cdiff is a compressional wave diffracting around the inner core boundary (ICB), which may contain much information on the structure of the ICB. Here I have determined the slowness of PKP-Cdiff and its coda with small-aperture arrays to approach the unsettled issues on the origin of PKP-Cdiff coda. Short-period seismic array data were analyzed using beam forming as function of slowness and back azimuths. The analysis was applied to band-pass filtered array records for 16 frequency bands with central values of 0.25–4.0 Hz. The difference of array response functions were adjusted by using the phase weighted stack with large power indexes of the phase stack. The stacking analysis resulted in enough slowness resolution in the higher frequency range of 1–4 Hz, PKP-Cdiff coda was identified as an assembly of coherent signals. PKP-Cdiff coda lasted longer than the duration of PKP-AB, suggesting that PKP-Cdiff coda is independent of source time function and/or site effects. The slowness of PKP-Cdiff coda was distributed from 1 to 5 s/deg with blunt peaks at 2, 3 and 4 s/deg, whereas those of PKP-DF and PKP-AB were relatively concentrated in narrow ranges around 1–2 and 4–5 s/deg, respectively. The slowness larger than 2 s/deg and travel times of the PKP-Cdiff coda can be explained by scattering of PKP waves near the caustic at the core-mantle boundary (CMB). Especially, the slowness of the scattering waves from the source side CMB is close to that from the ICB and it is difficult to distinguish them. In order to obtain secure observations of PKP-Cdiff coda from the ICB, we better avoid analyzing an event that is located over strong heterogeneities at the CMB.