Observation and analysis of core and secondary teleseismic phases at broadband stations in Germany using array methods

In its routine analysis the Federal Seismological Survey at BGR evaluates local seismic earthquakes in Germany as well as teleseismic events. For the analysis of teleseismic events we use the seismic stations in Germany and in some cases stations from surrounding countries. Because the stations are far from the events, we apply array methods (f-k analysis) to identify distinct seismic phases and locate events. We use waveforms of the densely spaced Gräfenberg array sites (GRF) with an interstation distance of 10 to 15 km or of the large aperture German Regional Seismic Network (GRSN) stations if coherent phase picking is possible.
The majority of detected and evaluated phases are first-arriving phases, such as P phases and, at more distant epicentre distances, Pdiff and PKP phases. However, in the case of stronger events, we can also detect later phases, such as PP, PS, SS, PcP, ScS, SKS and others. These secondary phases are identified well via slowness, azimuth and travel time.
Here, we show some striking examples with PKP phases as well as later phases.
Events with PKP phases are numerous and mostly located within the subduction zones at Fiji and Tonga. Due to the epicentral distances of around 145 degrees the German stations are close to the caustic of PKP branches. Therefore, PKPdf, PKPbc, and PKPab often show strong amplitudes. In some cases all three branches as well as their corresponding depth phases are visible and can be picked.
Some of the later phases, such as SKP, are rarely observed. We investigate to what extent the magnitude, the focal depth and also the focal mechanism are responsible for the amplitude of the phase and thus for their visibility at German stations. In order to determine the influence of the focal mechanism, we use the solutions of international agencies and calculate radiation coefficients for the respective phase in the direction of the stations under consideration. This approach may help to decide whether an observed depth phase is a pP or sP phase, for example, and thus enables a better depth determination.
Another special feature are very late phases, such as PKKP. They run on the long path around the Earth to the station and have an azimuth shifted by 180 degrees to the short path. These phases may be misinterpreted as independent events. Here, we show an example.