Description:

Routine processing of recordings from Sanae IV for the years 1999 to 2003 revealed that Sn-shear waves originating from shallow earthquakes in these regions generally arrive clearly earlier at Sanae IV (and at Neumayer Station) as predicted by modern standard earth models.

1. The South Sandwich Islands Region (SSIR) is the most seismically active region in the regional distance range to Neumayer Station and Sanae IV. Here the South American plate is subducted below the South Sandwich plate, thus resulting in the formation of the deep sea trench and the island chain. Numerous earthquakes are generated by this subduction process. To the East of the SSIR decreasing seismicity is mainly concentrated along the Southwestern Atlantic fracture zones and around Bouvet Island triple junction (see figure 1 below).

2. Observation: The average amount of time shift is approx. 11 seconds. The arrivals of the first compressional Pn-waves, however, are in average delayed by some 2 seconds compared to "ak135". This is a convincing argument that the too early arrivals of Sn-waves are not caused by systematic mis-localizations (see figure 2 below).

3. A convincing explanation for this effect is that Sn-waves, which are travelling in the uppermost mantle, propagate with higher velocities compared to theoretical velocities in modern standard Earth models. This implies that globally averaged Earth models are only true to some extent. As observed arrival times of Pn-waves do not much differ from theoretical arrival times an anomalous low ratio of Pn/Sn-velocities in the uppermost mantle must be assumed.

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