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 are the different types waves.
Waves are the transfer of energy through a medium, like air, water, or any material really! Seismic waves transfer of energy as waves from earthquakes. All waves have certain characteristics:
- They all have a wavelength, amplitude, and frequency that distinguish them
- The wavelength is the distance between consecutive peaks (tops) in a wave
- The amplitude is how “high” the wave is from peak to troug
- The frequency is how fast the wave is repeating from peak to peak during 1 second
The different types of seismic waves
When an earthquake occurs, it releases waves of energy that spread out from the epicenter. There are two main types of waves: body waves and surface waves. Body waves travel through the earth’s interior, and surface waves travel along the surface of the Earth’s sphere.
Body waves include primary waves (also called 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 detected later by the seismographs. S-waves have a much lower frequency than P-waves.
Surface waves are the last to arrive because they travel over the less-direct path of the Earth’s crust. They are the most destructive type of earthquake wave because of the way they move ground. Surface waves move similarly to ocean waves.
How do seismologists interpret seismic readings?
One of the most important actions that seismologists must do when a new earthquake is recorded is identify the epicenter, or the point on Earth’s surface where the earthquake occurred. Triangulation uses seismic data from three or more seismograms to identify the epicenter by estimating the time interval between the arrival of the P and S-waves. The distance away from a seismograph can be deduced from measuring the P-S wave time intervals and estimating the epicentral distance – this is calculated for you on the “EQ Locator” app on the Raspberry Shake website. If this is done for three or more seismograms, then it is possible to identify the approximate location of the epicenter by finding the intersection of these three distances (the red star in the image below).