BS in Physics
John J. Jasbinsek
Seismograms recorded in the western United States from a Guatemalan earthquake are examined for matching pairs of PKiKP and PcP waveforms for use in estimating the density contrast at earth’s inner-core boundary via a PKiKP/PcP amplitude ratio analysis. Examination of waveforms from the SN, SC, TA, US and CI arrays finds 68 visually identifiable PKiKP and PcP pairs from which 17 were judged to be of sufficient quality (low signal-to-noise ratio) for use in the PKiKP/PcP amplitude ratio analysis. The epicentral distance range of the selected data spans epicentral distances from 22 – 37 degrees inside of which stations are generally in narrow epicentral range clusters. The observed amplitude ratios are generally higher than the AK135 Earth Model value of 0.6 g/cm3, although the amplitude ratios do decrease with increasing epicentral distance. On average, the data suggests a density contrast at the inner-core boundary of 0.9 g/cm3. However, substantial variation in the density contrast estimate occurs over narrow epicentral distance ranges. This observation is consistent with a recent model of the inner-core boundary surface as made up of a mosaic of patches where the liquid to solid phase transformation is solidified in some locations, and mushy mixed-phase layer in others.