Earth Inner Core Anisotropy as Observed by PKiKP and PKIIKP Reflected Waves

The idea of Earth inner core anisotropy is well-established, and it is a very important topic in understanding the planet’s core growth, magnetic dynamo, and evolution. Most data analysis techniques used to measure the anisotropy use the differential traveltimes between transmitted wave pairs, which mostly sample only shallow parts of the inner core or may be affected by mantle heterogeneities. We introduce a new method to study the effects of solely the whole inner core using a pair of reflected PKiKP and PKIIKP waves at small epicentral distances (up to 40 degrees). We take the differential traveltime between the PKiKP and PKIIKP arrivals. This differential time measures the two-way traveltime of the P-wave, which samples both the shallow and deep parts of the inner core. This observation geometry yields a clean time window in which these phases can be identified. The small epicentral distance is key to minimizing effects of small mantle heterogeneities. Data for this project include seismograms obtained from the Incorporated Research Institutions for Seismology (IRIS) database for global earthquakes from 1990 to 2016 of moment magnitudes 6.1-6.5. Seismic data were loaded into a MATLAB® GUI program to identify PKiKP-PKIIKP phase pairs at the same station. I have identified an initial set of 99 traveltime pairs from the single-trace seismograms and plotted them against latitude and longitude. No systematic change of differential time with respect to latitude was seen, which implies an anisotropy structure that is in sharp contrast to the commonly observed polar dependence anisotropy. In order to increase the signal-to-noise ratio (SNR), I stacked signals from an array of seismometers for each earthquake. From these cleaner stacked seismograms, I found 59 traveltime pairs that suggest a different view on Earth’s inner core anisotropy, which was similar to the single-trace seismogram observations.