Fine Seismic Velocity Structure of the Lowermost Outer Core (F-layer) Beneath the Eastern Pacific

Wednesday, 17 December 2014
Toshiki Ohtaki, Geological Survey of Japan, AIST, Tsukuba, Japan and Satoshi Kaneshima, Kyushu University, Fukuoka, Japan
Solidification or melting at the inner core boundary, the phenomena that have been suggested to occur reflecting the dynamical processes either of the inner core or of the outer core, might cause a Fe-rich or Fe-poor layer in the lowermost outer core (F-layer). Such a compositional anomaly might be detectable by investigating fine seismic structure of the F-layer. In our previous study we determined the overall Vp structure near the inner core boundary beneath Antarctica using differential traveltimes between PKIKP and PKPbc, waveform modeling of PKIKP and PKiKP, and amplitude ratios between PKIKP and PKPbc. But the fine structure of the F-layer remained poorly constrained.
In this presentation, we examined the Vp structure of the F-layer beneath the eastern Pacific using differential traveltimes between PKiKP and PKPbc as well as frequency dependence of differential traveltimes between PKIKP and PKPbc, because these two analyses are particularly sensitive to the F-layer structure. We analyzed broadband seismograms of South American earthquakes observed at HI-NET in Japan. The differential traveltime residuals (observed minus calculated) between PKiKP and PKPbc are sensitive to the Vp excess relative to the reference model that is summed over the F-layer below the turning depth of PKPbc. For between 147 and 150 degrees the observed differential residuals show larger negative values and no noticeable dependence on distance compared to that for AK135. PREM that has larger Vp values in the F-layer than AK135 gives smaller differentials than the observations. On the other hand frequency dependence of differential traveltimes between PKIKP and PKPbc has unique sensitivity to the Vp slope in the F-layer, and low sensitivity to the Vp value on the ICB. We measured differential traveltimes for two different frequency bands for between 150 and 157 degrees, and then calculated the difference of the differentials between the two frequency bands. The observed differences show the dependence on epicentral distance. Calculated dependence of the difference for AK135 is gentler than the observation, suggesting that the Vp slope in the F-layer is steeper than that of AK135.
These two results suggest that Vp in the F-layer beneath the eastern Pacific lies between those of AK135 and PREM, and that the Vp slope is steeper than that of AK135.