Crustal phases in earthquake location

For crustal earthquakes of moderate or large size, the first reported phases at distant stations are usually the first predicted by the crustal model used. For smaller events, however, or for stations at larger distances, the first phase detected is often a later crustal phase of larger amplitude. This may be eitherPg which travels entirely in the upper crustal layer with a velocity of about 5.6 km/s, orP ^* with a path mainly in the lower crustal layer at a velocity of about 6.7 km/s. Many earthquake location programs do not take account of these phases, and treat their arrivals as if they were the earlier phasePn. At the International Seismological Centre we re-identify up to 200 crustal phases each month. This often results in significant improvement in position, the direct determination of depth or even the obtaining of a formal solution where none was possible before. We find that these re-identifications are needed in most continental areas of the world, and that the original crustal model of Jeffreys and Bullen still remains a useful standard.On leave from the Seismological Laboratory, Institute for Earth Physics, P. O. Box MG-2, 76900 Bucharest-Magurele, Romania