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“Sensing a Vibe: Seismic-alert system could give Los Angeles a few seconds’ warning” (SN: 5/3/03, p. 276: Sensing a Vibe: Seismic-alert system could give Los Angeles a few seconds’ warning) says that the S waves travel at about one-half the speed of the P waves. Then, in this article, 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 this 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