What causes earthquakes and how do they measured

Earthquakes are vibrations of the earth's surface as a result of tremors caused by constant geological processes inside it. In other words, the earth moves all the time — often with such force that it leads to negative consequences for people — destruction of buildings and roads, and other infrastructure. Although, it is sometimes said that humanity understands the processes inside the Sun better than inside our planet, in this lesson of the Windy.app Meteorological Textbook (WMT) and newsletter for better weather forecasting you will learn what causes earthquakes, as well as how do they measured.

The Earth's crust is not what it seems

We should remind you, that the red-hot metallic core is at the center of the Earth; above it are thousands of kilometers of raging molten masses of the outer core and mantle, and only then it is topped by the thin, rocky shell, the crust, and we live on its surface.

The crust's thickness varies from 6 kilometers under the ocean to 70 kilometers under dry land. This is enough for us not to fall into the magma, and intuitively, it seems that the crust is something monolithic and immobile, "the ground beneath our feet."

But the reality is more complicated. The "sphere" of the crust is broken, much like a vase or a jigsaw puzzle, where each piece is moving relative to the others every second and nonstop–even right now, as you read these lines.

That's because the mantle is in constant motion, pressing on the plates from below, and keeping the crust from merging into a single structure.

The rate of movement of these pieces of the planetary puzzle, called tectonic plates, is from 1 to 20 centimeters per year. Because of this movement, the continents are constantly rearranging themselves throughout history.

It is also the reason that earthquakes happen.

Structure of the Earth. Valerya Milovanova / Windy.app

Shock and friction

As the plates move, they inevitably run into and rub against each other. Sometimes there is liquid, i.e., molten rock from the mantle, between them, and the friction happens without significant consequences. And the rock of the plates themselves, depending on its composition, can "give way" in response to pressure.

But because the mantle under the plates does not stop this pressure for a second, they sometimes–not entirely, but in the problematic contact areas called fractures—"get stuck" and accumulate mechanical stress.

If you press two objects against each other with all your strength with their short side– for example, books–one of them will sooner or later rip. As a result, you can hurt yourself. The same thing happens at the epicenter of an earthquake: the tension between the plates is so great that one of them tears away, instantly slipping a couple of meters to the side.

The disruption, in turn, generates seismic waves–the released energy spreads in all directions around the epicenter as waves of tension and compression, deforming the rock, and, if the energy is enough, destroying the buildings on the surface. If there is water near the epicenter, a tsunami can be formed.

Because the rock near the epicenter needs to adapt to the new plate location, weaker shocks can recur for some time after the initial disruption. Such secondary waves are called aftershocks, and minor shocks just before the most powerful ones are called foreshocks.

Earthquakes can have other causes, such as volcanic eruptions, well development, or underground explosions. The earth can shake even from concerts, but most often, people encounter earthquakes due to natural plate friction.

Seismic magnitude scales

Seismic magnitude scales are used to estimate the physical strength of an earthquake, the first of which was invented by Charles Richter. Various versions of this metric are often referred to as the "Richter scale " in the media.

Each integer value on the magnitude scale is a 10-fold increase in the vibration strength of the earthquake at the epicenter, and a 32-fold increase in the energy released. That is, an earthquake of magnitude 7.0 is 32 times "more powerful" than a magnitude 6.0 or 1024 times more powerful than a magnitude 5.0.

Separately, the intensity of an earthquake, that is, the degree of destructiveness, can be distinguished. Other scales are used for this–primarily 12-point scales, where 12 means a maximally destructive earthquake, like the one in Turkey on February 6, 2023.

It seems more or less straightforward: watch for foreshocks and evacuate the population before the main tremors. But there's a problem: despite the vast array of data and thousands of seismic stations worldwide, we still need to learn how to predict earthquakes reliably. So, what's the problem?

It will happen again (future earthquakes)

On average, there are about 500,000 earthquakes annually; 100,000 can be felt, and only 100 lead to noticeable destruction. Events of magnitude 8.0 are recorded a little less than once a year.

At greatest risk are cities around the perimeter of the so-called "Ring of Fire," an area where the edges of the enormous Pacific Ocean plate are retreating under the continental plates around them (a process called subduction). This happens because of spreading--expansion of the plates under the pressure of magma and the mantle from below, which cools down and joins the plate as it reaches the bottom of the ocean.

We are talking about the west coast of the Americas, Japan, and the Philippines. Turkey and Indonesia, which are located at the junction of several plates, are also at high risk.

We know all this from historical experience. Until recently, it seemed that since the velocity of plates and mantle movement is more or less constant, it is possible to predict destructive earthquakes in this or that region simply by relying on their periodicity in the past.

Map of earthquake epicenters in 2000-2008–the curves approximately coincide with the boundaries of tectonic plates. Lisa Christiansen, Caltech Tectonics Observatory

This method, while allowing us to know the approximate time of the next large earthquake at the junction of plates, could be more suitable for adequate warning and timely evacuation of the population just before the catastrophe.

In China in 1975, it was possible to predict and evacuate the population before an earthquake of magnitude 7.3. Still, it remains the only successful example so far: the following year's major earthquake (Tangshan) was not predicted, and killed about 250 thousand people.

The fact is that foreshocks are often impossible to record and correctly classify–each earthquake is unique. Some scientists have suggested tracking concentrations of particular isotopes in the air, which grow when the earth fractures after foreshocks. But this is still considered an unreliable method, since the radon concentration, for example, can rise and fall.

Apparently, in addition to seismic stations, deep (really deep) detectors are needed, which will monitor suspicious vibrations a couple of kilometers underground and complement the current models of processes in the Earth's crust.

But this is still awaited in the near future. In the meantime, you should know and be attentive to the safety recommendations if you live in an earthquake-prone zone. City planners should also ensure that when constructing buildings in these places, all the measures prescribed for their protection by seismologists are followed.

Text: Jason Bright, a journalist and a traveller

Cover photo: Dave Goudreau / Unsplash