VII. Surface reflexion of earthquake waves

Abstract

A seismogram, or instrumental time record of the earth movement produced by an earthquake at some distance, may be divided into three main parts, known as the first phase, the second phase, and the long-wave phase respectively. As a rule, the experienced observer has little difficulty in distinguishing these phases, and although difficult cases do arise, there is general agreement that the three divisions are characteristic and correspond to definite properties of the earth. The beginning of the-first phase is denoted by P and the beginning of the second phase by S, and these are interpreted as representing the arrival of longitudinal and transversal waves which start simultaneously at the focus of the earthquake and reach the observing station at different times. By general convention P and S are also used to represent the time interval between the occurrence of the earthquake and the arrival of the corresponding waves at the observing station.