What Is an Earthquake?

What Is an Earthquake?

Earlier today, a 2.3 magnitude earthquake occurred near Ferntree Gully in Melbourne’s eastern suburbs. It got us thinking, what actually causes an earthquake and how do they happen? What is an earthquake?

The Australian Academy of Science says that to find out, we need to go deep underground.

At the centre of our planet—around 5,000 to 6,000 kilometres beneath the surface—is an extremely hot, solid core, made mostly of iron and possibly nickel surrounded by a molten outer core. Further out we find another high-temperature layer, called the convecting mantle, which isn’t exactly molten, but some movement is still possible given sufficient time. This convecting mantle enables convection currents, driven by heat from the planet’s core, to ‘flow’ slowly within it.

With us so far? The uppermost section of the mantle is relatively cool and brittle, and deforms elastically, and above this layer is the crust, the outermost layer of the planet (the bit we stand on). This coupled layer of crust and uppermost mantle is called the lithosphere.

“Although it feels solid as we walk around on it, the lithosphere is actually very thin compared to the other layers of Earth—it ranges from less than 20 kilometres to more than 200 kilometres thick in different areas,” the Australian Academy of Science explains.

The lithosphere is made up of a number of pieces called tectonic plates. They actually rest on the Earth’s mantle: a deeper layer of rock which is neither solid nor liquid but somewhere in between. You can think of them like chunks of ice on an almost-frozen lake — they happily sit on the surface, but it’s easy for them to move around.

The problem is that, because the plates are free to move, they rub against each other. Because their edges aren’t smooth, the plates can snag and edges get stuck. That doesn’t stop the rest of the plate moving, though, and as it does, the plate deforms slightly and stores up energy – like a spring stretching, except this spring weighs as much as a continent. Even the slightest deformation generates massive amounts of energy. When the relative movement becomes too much, and there’s excess energy stored in the plate – BOOM – it’s suddenly released: the “spring” snaps.

When all that energy is released, it takes the form of heat, rupturing and seismic waves. The two big problems thrown up by ‘quakes are ground rupture and vibration. Ground rupture is just what it sounds like: huge cracks, appearing seemingly out of nowhere, as the tectonic plates shift and wrench the upper surface of the Earth apart. More problematic, however, are the seismic waves that carry about 10 per cent of the energy through the Earth’s surface for miles — causing the ground to shake. Pictures fall off the wall, buildings collapse, and so on: seismic waves can wreak havoc.

To describe how severe earthquakes are, scientists use the Moment magnitude scale — it replaced the Richter scale decades ago. It can be thought of as a measure of how much energy a quake releases. Magnitude is logarithmic, which means that at the same distance from the earthquake, the shaking will be 10 times as large during a magnitude 5 earthquake as during a magnitude 4 earthquake.

But the numbers don’t relate directly to the impact of the quakes — that’s down to chance, as well as development and population density. To give some perspective, the Haiti earthquake of 2010 was of magnitude 7.0, while 2011’s Tōhoku 9.0 quake in Japan killed fewer but caused more expensive damage. Back in September 2021, the earthquake that shook east of Melbourne was a magnitude 5.8. Today’s 2.3 was just a baby in comparison.

It’s estimated that 500,000 earthquakes occur every year — of which about 100,00 can be felt. The rest are too small, too deep or too remote to register. But the big, nasty quakes occur far less often, and scientists can predict that roughly 10 times as many earthquakes larger than magnitude 4 occur in a particular time period than earthquakes larger than magnitude 5. But that’s just an approximation: some years are worse than others.

Fortunately, we don’t get bad earthquakes too often. The trouble is, when we’re unlucky enough to have a serious earthquake hit a densely populated city, it’s nearly impossible to avoid its destruction.

This article has been updated since it was first published.

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