Small scatterers in the lower mantle observed at German broadband arrays

Seismograms of earthquakes from the South Pacific recorded at a German broa dband array and network show precursors to PKPdf. These precursors mainly o riginate from off-path scattering of PKPab or a nearby PKPbc to P (for rece iver-side scattering) or from scattering of P to PKPab or PKPbc on the PKPd f path (for source-side scattering). Standard array processing techniques b ased on plane wave approximations (such as vespagram or frequency-wavenumbe r analysis) are inadequate for investigating these precursors since scatter ed waves cannot be approximated as plane waves for arrays and networks larg er than 300 x 300 km for short-period waves. We therefore develop a migrati on method to estimate the location of scatterers in the mantle, at the core -mantle boundary and at the top of the outer core. With our method we are a ble to find isolated scatterers at the source side and the receiver side, a lthough the depth of the scatterer is not well constrained. However, from l ooking at the first possible arrival time of precursors at different depth and the region where scattering can take place (scattering volume), we beli eve that the location of the scatterers is in the lowermost mantle. Since w e have detected scatterers in regions where ultralow-velocity zones have be en discovered recently, we think that the precursor energy possibly origina tes from scattering at partial melt at the base of the mantle. Comparing re sults from broadband and band-pass-filtered data the detection of small-sca le structure of the ultralow-velocity zones becomes possible.