This article says that the S waves travel at about one-half the speed of the P waves. Then, in “Seismic waves resolve continental debate” on page 285 of the same issue (Available to subscribers at Seismic waves resolve continental debate), it would appear that the P waves travel at one-half the speed of the S waves. Am I not interpreting the data correctly, or are the labeling and definitions different for the two articles?

Nicholas L. Reuter
Raymond Walters College
Blue Ash, Ohio

The difference in speed mentioned in the second article is the slight difference between two types of S, or shear, waves. Horizontally polarized S waves travel somewhat faster than vertically polarized ones. Both types of S waves travel about one-half the speed of P waves .–S. Perkins