abstract

This paper describes the travel-time relations for the upper mantle P-wave phases in the epicentral distance range of 700 to 4500 km. The short-period data analyzed were from underground nuclear explosions in central Aleutian and Nevada regions and from earthquakes located along the north Pacific seismic belts; the data were recorded by the central Alaskan seismic network.

The observations could be explained by three linear relations: the first extends from 700 to 1900 km with apparent velocity of 8.29 km/sec, the second is between 1900 and 2850 km with apparent velocity of 10.39 km/sec and the third extends from 2850 to 4500 km with apparent velocity 12.58 km/sec.

In the epicentral distance range of 1900 to 2600 km, the first P-wave arrivals from earthquakes in the Aleutian area were found systematically late by 3 to 4 sec compared with those from central Aleutian underground nuclear explosions. This discrepancy in travel times has been interpreted as due to mislocations and errors in origin time for earthquakes in the above area. Also, the well-known 400- and 600-km discontinuities in the upper mantle have been correlated with the sharp changes in travel-time gradients at 1900 and 2850 km, respectively.

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