Constraints On The S Velocity In The Uppermost Inner Core From PKIIKP/PKIKP Amplitude Ratios

Studying the S velocity structure at the top of the inner core is important to understand the growth of the inner core.PKIIKP waves(reflected from the underside of the inner core boundary)are sensitive to the S velocity in the uppermost inner core,suggesting that we could use PKIIKP/PKIKP amplitude ratios to study it.With few antipodal observations,the four anomalous PKIIKP amplitudes found at seismic station TAM in North Africa are very important.These anomalous PKIIKP have been used to infer anomalous structure(s)of the inner core boundary(including a sharp drop of P velocity in the bottommost of the outer core or a near-zero S velocity in the uppermost inner core).However,observations of 12 earthquakes located antipodal to TAM suggest,for three reasons,that these anomalous PKIIKP might be PKIKP coda waves scattering off from the strong heterogeneities in the mantle wedge above the subducting Tonga slab.These anomalous PKIIKP,appear at distance less than 179.6°,seem unlikely to be modeled by the anomalous structure(s)of the inner core boundary.These anomalous PKIIKP appear consistently from antipode to distances less than 160° and has horizontal slowness similar to the PKIKP.The travel times of these anomalous PKIIKP seem unlikely to be modeled by that of PKIIKP.We search antipodal observations from IRIS Data Management System and China Digital Seismic Network without the seismograms recorded at station TAM.We find 140 earthquakes which including 556 seismograms in the 178° to 180° distance range.We develop a method to estimate PKIIKP/PKIKP amplitude ratios aiming at reducing the interference of PKIKP coda waves.Measuring the PKIIKP/PKIKP amplitude ratios from 556 seismograms using root mean square(RMS),we find these ratios varying between 0.05 and 0.65.We find 15 high quality earthquakes including 234 seismograms.They have a clear PKIIKP,in normalized slant stacks,without the interference of PKIKP coda waves.These ratios measured from the 234 seismograms varying between 0.05 and 0.5.We segment the epicentral distance range by a step of 0.1° and find that the minimun ratio in each segment suggest a reduction in S velocity of 10% in the uppermost 10 km of the inner core.Measuring the average amplitude ratios of the 15 earthquakes in the 175° to 180° distance range using beam power,we find that these ratios are consistent with a similar anomalous S structure in the uppermost inner core.As the thickness of the anomalous S structure decreases gradually from 10 km,the reduction of it will increase gradually from 10%.