Travelling at a speed typically around 60% that of P waves, S waves always arrive at a location after them -- the "S" stands for secondary. S waves are transverse shear waves. This means that they create a shearing, side-to-side motion transverse (perpendicular) to their direction of propagation (but in any possible orientation, unlike Love waves). Because of this, they can only travel through a substance that has shear strength -- the ability to elastically resist this kind of motion. Liquids and gases have no shear strength, meaning S waves cannot travel through water, air, or even molten rock. It may thus help to think of the "S" as meaning "shear", in addition to "secondary".
Identifying the exact time of the initial S-wave arrival can be
challenging, even for experienced seismic analysts. Usually, it
is accomplished by noting two features of the waveform trace:
amplitude and wavelength. S-waves, in addition to being slower
than P-waves, also tend to be lower in frequency and longer in
wavelength. A sudden increase in wavelength is one way to
recognize the arrival of an S-wave on a seismogram.
Typically, the more obvious indicator of an S-wave's arrival
is a sudden increase in the amplitude of deflection. In cases
where the earthquake is large and the source is nearby, however,
this method is often not feasible, because the P-wave shaking has
not yet decayed to the point where the S-wave arrival "overpowers" it.
The image at right, like that on the previous page, is a link
to an example of an identified S-wave arrival time, on the same
seismogram used before. Can you see why that point was chosen?
In addition to producing potentially damaging shaking at the surface, both types of body waves have allowed seismologists to "see" structures far beneath the surface. When body waves encounter a boundary across which their velocity changes -- a contact between two different types of rock, for example, or the ground surface, where rock ends and air begins -- they will reflect and refract, sometimes spawning other body waves, or even surface waves. Large earthquakes can produce body waves detectable all across the globe. Their reflections and refractions as they pass through the planet produce identifiable phases that allow researchers to detect structural and compositional boundaries deep within the Earth. This phenomenon is also responsible for one of the most eerie of earthquake effects: "earth noises". These are thought to be the result of a transfer of P-wave motion from the ground to the air.
