Earthquakes and Stations of the Southern Hemisphere: Keys to the Inner Core Anisotropy Puzzle?

Inadequate spatial sampling of the inner core by PKP waves in all directions makes further advances in understanding anisotropic properties (especially anisotropy's radial dependence and hemispherical pattern) very difficult. One of the reasons for this incomplete sampling lies in the fact that, in order to pass through the deeper portion of the inner core, PKP waves must be nearly antipodal. With the spatial distribution of large earthquakes and current configuration of the seismographic stations worldwide, this is difficult to achieve, except for the paths nearly parallel to the equatorial plane. An exception is a group of ray-paths from the South Sandwich Islands (SSI) region earthquakes recorded at the northernmost stations, whose corresponding PKP differential travel times account for a large percentage of the most anomalous travel time data. Three major approaches could be pursued to achieve better spatial sampling of the core by body waves: 1) Observation of seismic phases with more complex geometry, such as PKPPKP or PnKP, as a necessary supplement to PKP travel time data because of their unique sampling of the core that cannot be achieved by PKP waves; 2) Installation of seismic stations at extreme geographic latitudes and oceanic islands, and 3) Development and application of new techniques (e.g. array signal processing techniques, or travel time measurements by a non-linear inversion waveform matching), which will allow picking of PKP data that were previously discarded. I present recent results from all three of the above-mentioned strategies with a particular focus on the SSI earthquakes and the core-sensitive travel time data recorded in Antarctica. A new dataset of PcP-P travel time measurements is collected from detected arrivals and travel times of PcP and P waves from the set of 132 moderate and large earthquakes originated in the SSI region in the time period form 1990 to the present day. Differential PcP-P travel times recorded on all available Antarctic stations, augmented by all available stations in South America for the same time period are predominantly negative with respect to the ak135 model, with some travel time residuals exceeding 2 seconds for both the Antarctic and the South American stations. This points to relatively sharp lateral gradients in the mantle in the vicinity of the SSI region resulting in advanced PcP waves. The importance of this result is that the same mechanism could also account for a significant part of anomaly observed in the PKP differential travel times. The PKP travel time data originating from the SSI earthquakes were also used in a past study to demonstrate that the heterogeneity in the outer core within the tangent cylinder could explain the observed patterns in PKP differential travel times. The SSI- related PKP waves sample, however, only the northern cap of the tangent cylinder in the outer core. A newly collected and analyzed dataset of PKP differential travel times from the Antarctic stations in the time period from 2001 to the present day samples the southern cap of the tangent cylinder. I demonstrate that the observed travel times of PKP waves do not support significant heterogeneity within the southern half of the tangent cylinder, making the tangent cylinder hypothesis less likely.