Attenuation in the uppermost inner core from PKP recordings at African seismological stations

Abstract

The attenuation factor Q P at the top of the inner core is evaluated by using the amplitude spectral ratio of PKPdf and PKPbc phases observed at African stations (BGCA mostly), from strong deep earthquakes in the Pacific Ocean area. The maximum depth of penetration of the PKPdf phase into the inner core (IC) is roughly 377 km, and the sampled region of IC is centered beneath the Southern Indian Ocean. The derived mean value of Q P is 249 ± 31 (95% confidence level) in the frequency range 0.2 − 2 Hz, where no frequency dependence of attenuation has been reliably observed. By using Student's t-test, we show that the value is statistically significantly different (with a probability greater than 95%) from other mean values of Q derived by using the same method, for both the western (180°W to 40°E) and eastern (40°E to 180°E) hemispheres of the IC. The decrease of Q with the radius of the turning point (denoted by r TP ), according to Q P = 840 − 0.62 r TP , has a moderate statistical support (the R-squared value is 38%).