On amplitudes of P near the shadow zone

• Main Author: Inge Lehmann
• Format: Article
• Language: English
• Published: Istituto Nazionale di Geofisica e Vulcanologia (INGV) 2010-02-01
• Series: Annals of Geophysics
• Online Access: http://www.annalsofgeophysics.eu/index.php/annals/article/view/4589

51 short-period vertical North American and Canadian records of the Samoa earthquake 14 April 1957 were examined. Amplitudes of P were very large at the western stations (Δ < 84˚) and small at the stations to the east, 8 of which recorded P at distances greater than 100˚. There was a lack of stations at intermediate distances. At the greatest distance, 108˚.2, the P waves were not diffracted waves, for the phase had the same general character as at shorter distances; the period was 2 sec. From 70˚ to about 90˚ the Samoa P timedistance curve is steeper than the Jeffreys-Bullen curve. It had been found previously that from about 90˚ epicentral distance onwards the variation ot the P amplitude with distance was not always the same and also that the shadow zone seemed to begin at different distances. It is suggested that this be due to small regional differences of velocity in the deepest layer D" of the mantle.