When you feel the earth shaking, it might feel like the earthquake is right under you, but it’s likely that the earthquake started dozens of kilometres away.
For larger earthquakes you can sometimes sense two distinct “bangs” or two different levels of shaking, separated by some number of seconds. When a fault ruptures, it sends out energy in all directions, but some energy waves travel faster than others.
The fastest P (primary) waves travel at around 6km/s and oscillate in the direction that they are traveling, so you’ll initially often feeling the ground shaking mainly vertically as those waves reach the surface. The slower S (secondary) waves travel at around 4km/s and oscillate across the direction of travel, so you’ll feel this shaking mainly horizontally. S waves are usually stronger, and tend to be those that cause damage to structures – because most structures are primarily designed to resist gravity (i.e. forces in the vertical direction) and are more susceptible to being shaken sideways.
When there’s a storm coming count the number of seconds between the lightning and the thunder and divide by 3 to estimate how many kilometres away that lightning struck. Similarly, you can roughly tell how far an earthquake is away by counting the number of seconds between the initial P-wave shake and the stronger S-wave shaking. Because earthquake energy waves travel much faster through rock than sound waves travel through air, the number of seconds between the P and S multiplied by 8 gives you the distance to the earthquake’s origin in kilometres. This is a very rough estimate, and doesn’t tell you what direction the earthquake approached you.
By using multiple time-synchronised seismographs, we can work out the distance of the earthquake from multiple locations, and with 3 or more stations we can determine the location of the hypocentre – the point below the epicentre where the fault rupture started, which is usually many kilometres below the surface.
The earth is much more complicated than this, of course. The crust is made up of many layers of rock, each with their own density which affects the speed of the earthquake waves. Seismologists have developed models that compensate for the wave speed through different layers, and the more earthquakes we record the more that we can refine these velocity models.
The SRC’s free earthquake analysis software, Waves, includes a simple layered velocity model that can be adapted for different parts of the world, and by marking P and S wave arrival times on seismograms anyone can easily locate earthquakes with just a point and a click.
You can watch a one-minute video on this topic on this TikTok channel.