Upper Mantle Velocity Structure of the East Antarctica Derived from P, PP, S, and SS Waveforms

We examined P, PP, S, and SS waveforms at epicentral distances of 25 to 60 degree, primarily sampling the East Antarctica. Broadband recordings on the Antarctica were used, with shallow earthquakes which occurred southwest of New Zealand. At the South Pole (SPA), significant phases arrive after the P waves, about 30 seconds before the IASP91 travel times of PP wave. The phases were clearly identified in a range of the epicentral distance from 28 to 34 degree. The apparent velocity is slower than that of the P wave. Beyond 35 degree, the phases are insignificant. The arrivals of the P waves, on the other hand, do not significantly deviate from the IASP91 travel times. In the transverse components of the recordings, clear phases were also observed between the IASP91 travel times of S and SS waves. The apparent velocity is slower than that of the S wave. In a farther range of the epicentral distance, we saw that the PP waveforms vary with the distance, based on the recordings at the Showa station (69.0S, 39.6E). At distances around 42 degree, the PP waveforms are simple with a short duration, and the amplitude is a few times larger than that of the P waves. The duration tends to be longer at distances around 45 degree. At distances beyond 46 degree, the waveforms are complicated and the amplitude is comparable to that of the P waves. A similar variation of the waveforms was observed at Station SNAA. These features of the P, PP, S, and SS waveforms we observed is likely to be attributed to the upper mantle velocity structure of the East Antarctica. Whereas synthetic P waveforms calculated from oceanic upper mantle velocity models or models of velocity smoothly increasing with depth cannot generate seismic energy between the P and PP waves at distances around 30 degree, upper mantle models with a lid and deep low-velocity zone can do. Beneath the East Antarctica, the region of high velocity, down to a depth below 200 km, was found in previous surface-wave analyses (e.g., Danesi and Morelli, 2001). Thus, our observations of the P, PP, S, and SS waveforms can be the result of velocity structure with a lid and deep low-velocity zone.