What Causes Earthquakes

Ancient cultures all had their own beliefs about what caused earthquakes. These ranged from the elephants holding up the world were getting tired (India), a flea-infected dog that was pulling a sled under the Earth was stopping to scratch (Russia) or even a pranking-catfish was controlled by a magical rock (Japan). You’re welcome to indulge in the same mystical beliefs, but we’re here to tell you what really gets the world shaking.

Earthquakes are the vibrations caused by rocks breaking under stress, usually when tectonic plates rub up against each other. (tectonic plates). But basically, these tectonic plates are always on the move….but veeeerry slooowly.

Tectonic plates don’t have smooth boundary lines that can gently slide over each other; they’re rough, so when each end moves against each other, they get ‘stuck’ for a while against the jagged edges, the pressure builds up until the rocks break under tremedous stress, causing an earthquake.

The place where two tectonic plates slip against each other is called the fault. The place where the rock breaks underground is called the focus or hypercentre. The area directly above at the surface is the epicentre.

Sometimes foreshocks occur before an earthquake; these are smaller earthquakes that happen before the mainshock and can be followed by aftershocks. Aftershocks can continue for days to years after the mainshock, but aren’t as strong.

How are Earthquakes measured?

If you think the answer is ‘using the Richter scale’, then you’re wrong. Just like the Mercali scale, the Richter has had its day. It just wasn’t accurate.

Earthquakes are now measured using the Moment Magnitude Scale (MMS).

The MMS measures the seismic moment which is essentially the amount of energy released (magnitude). The exact MMS is measured by

(1) The distance the rock face slips at the fault, this is called the, wait for it….,  fault slip. (2) the area of the fault surface which is broken by the earthquake and (3) the type of rock at the fault. Rocks like granite are rigid and need more energy to break, whereas sandstone is soft and easily broken.

Therefore: Fault Slip + Fault Area + Rigidity + a whole lot of complicated maths = MMS

The level of magnitude is calculated using a scientific instrument called a seisomograph which detects the amplitude (level and frequency) of the seismic waves. Seismic waves are produced when the energy is released at the fault, they radiate out just like the ripples you see when you throw a stone in a pond.

There are two types of seismic waves:
1. Body waves – these move underground and are divided into ‘P’ (fast waves) and ‘S’ (slower waves). The move through the earth and to the surface dislodging rock and stones.
2. Surface waves – also known as ‘L’ waves, have the slowest frequency. They roll along the surface causing most of the destruction.

Seismographs are extremely sensitive, they can detect earthquakes from around the world, and as these waves all have a different frequency, the results from several seismographs can be put together, using a process called trilateration to accurately calculate the location of the epicentre, fault slip etc to provide an MMS.

Images: pbs learning media


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