The acceleration of the ground surface, such as indicated by the plot shown in Fig. 2.14, is
due to various seismic waves generated by the fault rupture. There are two basic types of
seismic waves: body waves and surface waves. P and S waves are both called body waves
because they can pass through the interior of the earth. Surface waves are only observed
close to the surface of the earth, and they are subdivided into Love waves and Rayleigh
waves. Surface waves result from the interaction between body waves and the surficial
earth materials. The four types of seismic waves are further discussed below:
1. P wave (body wave): The P wave is also known as the primary wave, compressional
wave, or longitudinal wave. It is a seismic wave that causes a series of compressions
and dilations of the materials through which it travels. The P wave is the fastest wave and
is the first to arrive at a site. Being a compression-dilation type of wave, P waves can travel
through both solids and liquids. Because soil and rock are relatively resistant to compression-
dilation effects, the P wave usually has the least impact on ground surface movements.