Arrival times of P waves and upper mantle structure

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Abstract
Residuals of arrival times of P waves, as given in the International Seismological Summary for Kiruna, Sweden, Reykjavik, Iceland, and Scoresbysund, Greenland, were studied in order to detect upper mantle anomalies. The Kiruna arrivals were systematically too early, with a mean residual of −1.4 seconds, while the mean Reykjavik residual was +1.3 seconds. The difference in mean residual was 2.7 seconds with a standard error of about 0.5 second. The mean residual at Scoresbysund was −0.4 second. It is assumed that there is a depth D below which the mantle is homogeneous. The difference in mean residuals at the stations is assumed to be caused by different wave velocities at depths less than D in the vicinities of the stations. If it is assumed that the P-wave velocity in the upper mantle is constant down to a depth D below each station, this depth can be computed. This velocity is known from other data to be 8.36 km/sec below Kiruna and 7.4 km./sec. below Reykjavik. If only earthquakes at distances from 20° to 39° were used, D is determined to be 246 ± 36 km. (standard error). Earthquakes at distances 40° to 59° give D = 177 ± 25 km., at distances 60° to 79° give D = 234 ± 14 km., and at distances 80° to 99° give D = 281 ± 20 km. The most probable value of D is thus about 240 km. below the earth's surface, with a standard error of about 40 km. In the vicinity of Scoresbysund the upper mantle velocity is found to be about 8.0 km./sec., using the same assumption.

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