Now that we know the basics of seismology, what seismic activity is and how seismographs work, we can dive deeper into understanding seismic data.
What are waves?
To understand how seismologists interpret seismic waves, we must first understand what waves are.
Waves are the transfer of energy through a medium. Seismic waves are a certain type of wave that result from earthquakes and other large disturbances in the earth’s lithosphere. All waves have certain characteristics that distinguish them:
- Wavelength: The distance between consecutive peaks (tops) in a wave train.
- Amplitude: How “high” the wave is from peak to trough.
- Frequency: How fast the wave is repeating from peak to peak.
The different types of seismic waves
When an earthquake occurs, it releases waves of energy that spread out from the focus point. There are two main types of waves:
- Body waves travel through the earth’s interior.
- Surface waves travel along the surface of Earth’s sphere.
Body waves include primary waves, or P-waves, and secondary, or S-waves. P-waves are also called compressional waves, because they push and pull material as they propagate. They are the fastest moving waves, and therefore are the first waves detected by seismographs. The slower S-waves, also called shear waves because of the side-to-side movement they create as they pass through material, are picked up second by the seismographs. S-waves have a much lower frequency and therefore appear much larger on the seismograph.
Surface waves are the last to arrive, because they only travel over the less direct path of the earth’s crust. They are the most destructive type of earthquake wave, because of the distinctive orbital motion they cause. At a molecular level, surface waves behave similarly to ocean waves. In fact, ocean waves are a type of surface wave.
How do seismologists interpret seismic readings?
With the information that seismologists record on seismographs, they can determine the location, magnitude, and type of earthquake. One of the most important actions that seismologists must do when a new earthquake is recorded is identify the epicenter through triangulation. Triangulation is the use of seismic data from three or more seismographs to identify the epicenter, using the time interval between the arrival of the P and S-waves. The distance away from the seismograph where the waves originated can be deduced from the signals, and with three or more seismographs, it is possible to identify the approximate location of the epicenter.