Characterization of D” Anisotropy beneath Western Pacific using SKS-SKKS Splitting

Abstract:

The D’’ layer beneath the western Pacific is characterized by obvious lateral heterogeneity in seismic velocity structure, variable seismic discontinuity at its top and existence of ultra-low velocity zones above core mantle boundary. Details on the geometry and strength of anisotropy, especially those near the edge of LLSVP (large low shear velocity provinces) in the D’’ layer are still debated. The SKS and SKKS phases, for the same event-station pair, sample the similar raypath in the shallow mantle but different raypath in the lowermost mantle. The splitting data of the SKS and SKKS pairs thus can be used to constrain seismic anisotropy at the lowermost mantle. In this study, we use the SKS splitting data from different earthquakes to estimate the anisotropy beneath a seismic station, and then use them to extract D’’ seismic anisotropy from SKKS splitting data. We collected 1700 SKS and 456 SKKS high-quality waveforms for 45 earthquakes from Toga recorded at distances between 85° - 120° by the seismic networks of the China Earthquake Administration (CEA). Most of the SKS-SKKS pairs show similar splitting. However, large differences are observed from pairs that sample the edge of LLSVP, suggesting the presence of seismic anisotropy in the lowermost mantle. The fast polarization direction estimated from the SKKS data after correction of shallow anisotropy is roughly perpendicular to the edge of LLSVP. The observation of significant lowermost mantle anisotropy from the edge of LLSVP beneath western Pacific would shed lights on the nature of LLSVP.