Initial motion of the first longitudinal earthquake wave recorded at Pasadena and Huancayo

Abstract

The initial motion of the first longitudinal earthquake waves (P waves) recorded at a station is (1) upward or away from the epicenter (compression = c) or (2) downward or toward the epicenter (dilatation = d). The P waves do not change their phase during their propagation through the earth, and the first motion, obtained from seismograms, depends only on the earthquake mechanism. The directions of later waves (such as pP, P_cP, PP, etc.) depend both on the earthquake mechanism and the possible phase changes on reflections; also, their directions are generally more difficult to determine from seismic records, owing to the motion already existing when they arrive. When the earthquake mechanism has been established by means of the P waves, the direction of motion of the later waves could give some information about the earth's interior.