Identification of different seismic pulses

Although Poisson worked out the theory of transmission of elastic waves in solids in 1831, the relation of this theory to seismic waves was not realized for a long time. This is not surprising considering that the first practical seismograph was not invented by Cecchi until 1875 and that John Milne began recording earthquakes only in 1880. In 1847, William Hopkins pointed out that earthquakes must consist of elastic waves. Robert Mallet (1852, 1861) accepted this idea and even tried to measure their velocity of transmission. His instrumentation, however, was inadequate, and he obtained velocities that were much too low to be other than some phase of what we now know to be surface waves.

The earliest seismograms showed that earthquakes consist of a principal series of oscillations preceded by weaker motions and followed by a gradual dying out of the waves. Little attention was at first given to the preliminary oscillations (called vorlaufer, meaning forerunners) though it was suspected that they might be compressional waves. The principal oscillations were at first thought to be shear waves. Richard D. Oldham (1899, 1900) in studying seismograms of the great 1897 Indian earthquake identified the earliest arrivals as compressional and showed that a later vorlaufer pulse was a shear wave (Figure 5.1). The two vorlaufer pulses were given the names primary and secondary pulses, now abbreviated P and S.