Seismic Waves Help
Today’s digital seismic monitoring instruments use waveforms to record and analyze seismic data. They give information not only on local seismic activity, but also earthquakes at far distant locations. When the Chilean earthquake took place in 1960, seismographs recorded the seismic waves that traveled all around the Earth. Seismic waves rattled the Earth for days. This occurrence is known as the free oscillation of the Earth.
There are two types of seismic waves, body waves and surface waves . Body waves travel through the interior of the Earth, while surface waves travel only within the top surface layers. Most earthquakes take place at depths of less than 80km below the Earth’s surface. There are two types of body waves, P waves and S waves . P waves bunch together and then spread apart when they move. It is a bit like the movement of an inch worm or a slinky. S waves are like rolling ocean waves or like when you snap a rope. The oscillations are in a waveform. Figure 12-7 gives you a rough idea of how P waves and S waves look.
P waves are the fastest seismic waves and the first to arrive at any monitoring station. P waves travel at about 5km/s (14 times faster than sound waves travel through air). For this reason, they are called primary waves or P waves of an earthquake.
P waves are longitudinal compressional waves . Like sound waves, they travel through rock or buildings causing squeezing and expanding, parallel to the direction of transmission. The speed that P waves travel depends on the types of matter through which they move. Generally, in the case of P waves, the denser the matter, the faster the wave travels.
Because P waves are the first seismic waves to reach any given location after an earthquake takes place, their arrival can easily be identified on a seismogram. The tracing is flat during zero or background vibration, then jumps into action as the first P waves arrive.
S waves arrive at a monitoring location second, since they only travel around 60% of the speed of P waves. For this reason, they are called secondary or S waves .
S waves are transverse shear waves . They cause a shearing, side-to-side motion transverse (perpendicular) to their direction of travel. Because of this, they can only travel through a substance like rock that has shear strength. Liquids and gases have no shear strength and S waves cannot travel through water, air, or even molten rock.
Since the seismogram is already jumping wildly with the action of the P waves, it can be tricky to figure out when S waves arrive. However, seismologists study the amplitude and wavelength of a seismic recording. S waves are often lower in frequency and longer in wavelength. So a sudden increase in wavelength tips off the arrival of an S wave. A better indicator of an S wave’s arrival is a sudden jump in the amplitude. In cases where the earthquake is large and the source is nearby, however, this doesn’t always work because the P wave movement has not yet slowed to the point where the S wave arrival overwhelms it.