From faults to tremors: the true causes of earthquakes

Ever been caught in an earthquake evacuation drill at school or work and wondered what really goes on beneath the surface when the kingdom of Earthquake comes knocking?

Scientifically speaking, the National Aeronautics and Space Administration (NASA) describes an earthquake as a phenomenon where powerful jolts rock the planet’s surface, triggered by shifts in Earth’s outer shell.

Earthquakes can range from imperceptible tremors to powerful events capable of damaging or destroying entire towns. Most earthquakes are caused by tectonic activity — the movement of rigid plates that make up Earth’s lithosphere, as described by the theory of plate tectonics.

What causes earthquakes? The plate tectonics connection

The causes of earthquakes and the plate tectonics theory are like birds of a feather that usually flock together. The meeting point of two tectonic plates is called “plate boundaries” or “faults.” These faults are the breeding ground for the disaster.

This is where the collision, slip, or spread of the plates happens, building up the pressure. When the build-up increases beyond a point of redemption, they break loose and release energy in the form of seismic waves that shake the ground and cause earthquakes.

Plate boundaries & types

The regions where tectonic plates come into contact are called plate boundaries. There are three main types of plate boundaries, and each one is defined by its own unique movements and geological activities:

1. Convergent boundaries: These are the most common type and are responsible for 80% of all earthquakes. They occur when two tectonic plates collide, pushing against each other with immense pressure. This process can create mountain ranges, trigger volcanic eruptions, and cause powerful earthquakes.
   
2. Divergent boundaries: These happen when two plates drift apart, creating a gap that allows magma to rise to the surface. This process can lead to volcanic activity and, in some cases, earthquakes.
   
3. Transform boundaries: These occur when two tectonic plates move past each other horizontally. The friction and stress that build up between these plates can lead to some seriously powerful earthquakes, like the ones we often see along the well-known San Andreas Fault.

Earthquake hotspots: Where do most earthquakes occur?

Big earthquakes usually happen where Earth’s tectonic plates meet. People began to notice this a long time ago when they felt the ground shaking in the same areas again and again. Today, modern technology confirms what they suspected, showing that most earthquakes do, in fact, occur along the edges of these plates.

One of the most active areas is the Circum-Pacific Belt. It surrounds the Pacific Ocean and affects New Zealand, New Guinea, Japan, the Aleutian Islands, Alaska, and the west coasts of North and South America.

Around 80 percent of all the energy released by earthquakes around the globe comes from events happening within the Circum-Pacific Belt. The seismic activity in this area isn’t uniform. It changes along the belt’s path and has several branches that extend out at different spots. Because volcanic activity often occurs alongside earthquakes in many regions of this belt, it’s often called the “Pacific Ring of Fire.”

Major causes of earthquakes

Now that we’ve covered the why, where, and when of earthquakes, it’s time to explore the what and how. What are the key factors that trigger these powerful natural events, and how do different types of earthquakes originate?

Tectonic reasons

Earthquakes come into being when there is some kind of sudden release of energy along the Earth’s crust. As discussed before, this energy might arise from a number of reasons, some of which include the build-up of gravitational strain, large chemical changes, or even the movement of large amounts of mass.

However, the core reason for powerful earthquakes is the release of elastically strained energy. Only this form of energy can be accumulated to produce adequately large amounts of significant land tremors. When earthquakes originate from such a release of energy, they are called tectonic earthquakes.

When the rock stress exceeds rock strength and sudden fracture results, there are tectonic earthquakes. The elastic rebound theory of Harry Fielding Reed provides further explanation for this.

Such breakdowns, or fractures, result in ground displacement as they spread over hundreds of kilometers, with rapid weak areas. For example, the 1906 San Andreas Fault breakdown included a horizontal displacement of 6 meters (20 ft) and extended 430 kilometers (270 miles).

Rocks move in opposing directions and bounce back when a fault ruptures, creating irregular, step-like motions that produce seismic waves. Asperities, or rough spots, and fault barriers, or places where rupture slows or stops, are examples of these irregularities in rupture. The break may propagate in one or two directions until it is stopped by a barrier. It begins at the earthquake focus, which is typically 5 to 15 km (3 to 9 miles) below ground. The break may stop, resume on the other side of the barrier, or break through the barrier.

Earthquakes are also classified by fault slip type based on the fault’s orientation and movement. Strike-slip faults involve horizontal shifts along the fault line, either right or left lateral. Dip-slip faults have vertical movement; if the hanging wall moves down, it’s a normal fault, while upward movement causes a reverse or thrust fault.

Though some ancient faults are inactive, all faults are thought to have experienced past quakes. Faulting is complex, and energy may not always come from a single plane. Slips during earthquakes range from centimeters to meters, such as the 1 m (3 feet) shift in the 1976 Tangshan quake and the 8 m (20 feet) vertical slip in Taiwan’s 1999 quake.

Volcanic eruptions

Although related to volcanic activity, these earthquakes are caused by the sudden slipping of rock mass with volcanoes, which releases stored stress energy. The sudden release of pressure or magma under the Earth may be partly responsible for this activity.

Major earthquakes and regions with frequent volcanic activity are closely associated, especially in the Circum-Pacific Belt and along oceanic lines.

Nevertheless, epicenters of strong shallow earthquakes are usually hundreds of kilometers away from volcanic vents. In addition, non-volcanic zones are sites where most seismic activities occur. Both earthquakes and volcanic activity are usually caused by the same tectonic forces. So, even if an earthquake happens right beneath a volcano, it doesn’t necessarily mean one caused the other.

Human causes

Some earthquakes aren’t entirely natural as some human activities can cause them too. This phenomenon is known as induced seismicity. These involve deep fluid injection, underground nuclear tests, mining, and large reservoir creation.

Mining changes the stress in the underground, occasionally causing faults or collapsing rocks. Fluid injection may grease faults, which results in premature fault movement, and nuclear testing has initiated fault slips by unloading stored strain.

One of the most important causes is reservoir-induced seismicity, by which major dams increase regional earthquake frequency. The riskiest are large reservoirs 100 m (330 feet) or deeper and holding more than 1 km³ of water (0.24 mi³). Notable examples are Hoover Dam (U.S.), Aswan High Dam (Egypt), and Kariba Dam (Zambia-Zimbabwe). The increased water pressure reduces the nearby faults, which are therefore urged to move under existing tectonic pressure.

In a few instances, the fault type has been ascertained. Koyna Dam (India) experienced strike-slip faulting, while Kremasta Dam (Greece) and Kariba Dam witnessed dip-slip movement. Nurek Dam (Tajikistan) had more than 1 800 earthquakes in nine years since impoundment, four times more than earlier activity. Lucky for us, the majority of reservoirs do not induce hazardous earthquakes, yet.

Earthquakes are inevitable, but disaster doesn’t have to be

Call it fate, call it karma, but earthquakes, like all-natural disasters, are unstoppable. We can’t prevent the ground from shaking, no matter how advanced technology gets. But what we can do is build smarter, stronger structures designed to withstand the tremors and protect lives.

Sadly, in many earthquake-prone regions, cost-cutting takes priority over safety, leaving communities vulnerable when disaster strikes. The price of safety should never be measured in dollars—it should be measured in lives saved.

Harsh Vardhan

My passions include trying my best to save a dying planet, be it through carpooling or by spreading awareness about it. Research comes naturally to me, complemented by a keen interest in writing and journalism. Guided by a curious mind and a drive to look beyond the surface, I strive to bring thoughtful attention and clarity to subjects across Earth, sciences, environment, and everything in between.

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