R.D. Oldham

R.D. Oldham was a British seismologist who discovered that earthquakes release three types of waves:

• P-waves, or primary waves, which are compression waves that propogate beneath the surface of the earth. They are the fastest seismic waves and are the reason animals and sensitive humans can detect earthquakes before they seem to have arrived.
• S-waves, or secondary waves, which are shear waves travelling beneath the earth's surface. Although these waves are slower than P-waves, they are much stronger. However, since liquids cannot transfer shear stress, S-waves cannot propogate through liquids.
  
• Surface waves, which occur when P-waves and S-waves reach the earth's surface. These waves are further divided into vertical Rayleigh waves and horizontal Love waves. Although surface waves are the last to arrive, they are the strongest and most destructive seismic waves. These are the waves that are felt during an earthquake.

While studying the Assam earthquake in 1897, Oldham observed that seismic waves that travel along a direct path through the center of the earth arrive after waves that travel obliquely, as shown in Figure 1. In 1906, to explain this phenomenon, he proposed that the core of the Earth was a low-velocity region that produced a shadow zone.