A detailed analysis was made of over 100 earthquake P signals that were recorded at the short-period tripartite seismic array near London, Ontario. Deviations of measured values of slowness and azimuth from the theoretical values for a spherically symmetric Earth form a well defined anomaly pattern around the array. The pattern to the north of the array is consistent and significantly different from the weak pattern to the south and is a direct result of the relative station travel-time residuals, which were found to be as large as 0.3 s. Although a unique interpretation is not possible, the consistency of the pattern for travel paths that cover large regions of the Earth suggests that its cause is localized in the crust near the array. Numerical experiments also indicate that the residuals are not caused by slopes on any major seismic discontinuity such as the Precambrian or Mohorovičić, but are more likely to be the result of path lengthening effects that occur as the seismic energy is transmitted through a crust with small lateral and vertical velocity variations.

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