Mapping the D" Discontinuity Beneath Northern Pacific and Aleutians With USArray and California Seismic Networks

Mapping the distribution the seismic D" discontinuity as well as its lateral depth and strength variations is essential to the understanding of its origin, which is still unclear. Recent developments in passive seismic observations make it possible to apply imaging techniques developed for petroleum exploration to investigate deep mantle structures. We have collected several thousand transverse seismograms that sample the northern Pacific and Aleutian region recorded by broadband seismic networks in California and the expanding USArray stations from more than 60 deep earthquakes (>300 km) occurring in the western Pacific subduction zone. Following previous studies, we are looking for S-wave energy reflected from the D" discontinuity (SdS) that arrives in the time window between the S and ScS phases. We manually selected seismograms with clean S and ScS waveforms and with source-receiver distances between 60° and 80°. When sS and sScS are clearly observed, sSdS was also included. At distances greater than 75°, we observed a strong SKS phase in the transverse component because of the existence of seismic anisotropy, which could interfere with the targeted SdS phase. In principle, as the two phases have quite different slownesses, they can be separated in a vespagram. To investigate the lateral variations of the D" discontinuity, we have gathered seismograms based on the geographic location of reflection points. To isolate the source from the seismograms, for each event we first constructed the source time function by stacking the S wave recorded at distances less than 65° and used it to deconvolve individual seismograms. The deconvolved seismograms were then stacked with a moveout calculated from ray tracing the iasp91 model at depths up to 500 km above the core mantle boundary (CMB) with an increment of 1 km. Our preliminary analysis indicates that the D" discontinuity has varying strength and depth within the area 170°-210°E, 40°- 60°N.