abstract

Travel-time residuals of teleseismic P waves have been widely used to derive the spatial variation in crustal and upper mantle structure near the recording network. This is not possible in Atlantic Canada, due to the low number of seismograph stations. Using similar principles to those employed in the analysis of P residuals, PP travel-time anomalies may be used to give information about the reflection point. The method given here relies on the use of many published data, rather than relatively few but more accurately processed readings such as have been used in similar studies elsewhere. Errors may arise due to mislocated hypocenters, travel-time table errors, lateral variations in deep mantle structure, and station corrections. The effects of these may be minimized by suitable source-receiver combinations, and by careful selection of data sets.

Taking data from Central American earthquakes recorded in Europe, it is shown that the travel-time anomalies due to reflection points in eastern Canada may be consistent with the near-surface structure known from refraction profiles, although the data do not correlate well with the regional gravity anomalies. It is unlikely that the larger travel-time residuals observed (up to 6 sec) can be entirely accounted for by effects other than structure near the reflection point. The PP-wave method may be used for both oceanic and onshore reflection points, whereas P-wave residuals may usually only be studied on land.

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