Characterization of the D″ beneath the Galapagos Islands using SKKS and SKS waveforms

SKS and SKKS waveforms from 16 events occurring between 2003 and 2005 in the Tonga Trench that were recorded by the BOLIVAR array are analyzed to determine the structure of the D″ layer beneath the Galapagos Islands. 248 differential travel-time residuals of SKKS-SKS are measured and reveal a region of positive residuals of differential travel times in the northeast portion of the sampled region. Analyzing correlation statistics between the measured SKKS-SKS residuals and the observed absolute travel time delay of the individual SKS and SKKS phases for two events with high data quality, we determine that the residual differential travel time is due to excess low velocity along the SKKS raypaths. First order modeling of three potential low velocity structures, ultra-low velocity zones (ULVZ), plume conduit, D″ structure, indicates that the observed SKKS-SKS residuals can be best explained by a low velocity anomaly within the D″ layer. To determine the presence of lower mantle anisotropy, amplitude ratios of the radial and transverse component of SKS and SKKS waveform are calculated and compared. Regions with significant presence of seismic anisotropy are interpreted as the edge of the flow field associated with a hypothetical mantle upwelling.