Myanmar-Thailand tremors: Why earthquakes happen and what makes them so devastating

Earthquakes are among nature’s most powerful and sudden geological events, causing the ground to shake violently as energy stored within Earth’s crust is released. These seismic events have shaped our planet’s surface for billions of years and continue to remind us of the dynamic forces at work beneath our feet.

We thought of explaining the mechanism of earthquakes against the backdrop of devasting tremors in Myanmar and Thailand, killing hundreds of people. This Reuters LIVE report gives you the latest on the casualty updates. Here, we try to explains the mechanisms behind earthquakes with detailed descriptions of what the relevant sketches would look like:

The foundation: Where Earth decides to shake up the world

Earth’s outermost layer, called the crust, is not a single continuous shell but is broken into large sections of rock known as tectonic plates. These plates float on top of a fluid-like layer of convection currents deep underground, fitting together like a giant puzzle to create Earth’s surface1.

Sketch 1: Tectonic plates on Earth

These tectonic plates are constantly in motion, though this movement typically occurs very slowly—only a few centimeters per year2. The movement is driven by convection currents in the semi-fluid mantle beneath the crust, causing the plates to drift, collide, separate, or slide past one another1.

The mechanism: Elastic rebound theory

The primary cause of most earthquakes is explained by the elastic rebound theory. When tectonic plates move against each other, they don’t slide smoothly. Instead, friction along their boundaries causes them to become stuck while pressure continues to build1.

Sketch 2: Elastic deformation and rupture

This illustration is divided into three panels showing a sequence:
Panel 1: Two blocks of rock on either side of a fault line, perfectly aligned
Panel 2: The blocks becoming increasingly deformed as stress builds up, with arrows showing the direction of force while the fault line remains locked
Panel 3: The sudden rupture and shift of the blocks to new positions, with wavy lines emanating from the fault representing seismic waves

Think of it like a tug-of-war game: as both sides pull on the rope (analogous to tectonic pressure), tension builds in the rope (the fault zone). When one side suddenly lets go (the fault slips), all that stored energy is released at once, sending the other person tumbling (the earthquake shaking)1.

The rocks on either side of a fault may move only a few centimeters per year, but this movement produces enormous stress and strain in the rocks. As movement continues over years, or even centuries, stress and strain increase, storing more and more elastic energy in the rocks2. When the accumulated stress finally exceeds the frictional forces keeping the plates locked together, the fault suddenly slips, releasing the stored energy in the form of seismic waves that radiate outward and cause the ground to shake4.

Types of faults, how they move, and also move you

Different types of earthquakes occur based on how tectonic plates interact at their boundaries. There are three main types of faults that cause earthquakes, each with distinctive movements3.

Sketch 3: The three main fault types

This drawing would be divided into three sections showing:
Section 1: Normal fault – Two blocks with one sliding downward relative to the other, with arrows showing tension forces pulling apart
Section 2: Reverse fault – Two blocks with one pushing up and over the other, with arrows showing compression forces pushing together
Section 3: Strike-slip fault – Two blocks sliding horizontally past each other, with arrows showing shearing forces in opposite directions

Normal faults

Normal faults occur when tension forces pull the crust apart, causing one block to slide downward relative to the other along a sloping fault line. On geologic maps, normal faults are typically shown by a line with a small perpendicular line indicating the direction of the downward-moving block3.

Reverse faults

Reverse faults form when compression forces push rocks together, forcing one block to move upward and over the adjacent block. These are drawn on maps as lines with “teeth” symbols pointing in the direction of the overriding block3.

Strike-slip faults

Strike-slip faults result from shear stress, where blocks move horizontally past each other with little vertical motion. The San Andreas Fault in California is a famous example of a strike-slip fault. On maps, these are represented by lines with half-arrows on each side showing the direction of movement23.

When an earthquake occurs, it originates at a specific point underground known as the focus (or hypocentre), and the energy radiates outward from this point in all directions as seismic waves12.

Sketch 4: Focus and epicentre

This cross-sectional view shows:
– The Earth’s surface with buildings on top
– Underground layers with a fault line
– A star marking the focus (earthquake origin point) along the fault
– A point directly above on the surface marked as the epicentre
– Concentric circles or wavy lines emanating from the focus representing seismic waves

The epicentre is the point on Earth’s surface directly above the focus. Maximum damage typically occurs at or near the epicentre, although this can vary depending on local geological conditions, the depth of the focus, and the magnitude of the earthquake2. As seismic waves travel from the focus, they cause the ground to move in different ways. Body waves travel through Earth’s interior, while surface waves travel along the ground surface and typically cause the most damage1.

Earthquakes follow some principles

Scientists and educators often use models to demonstrate how earthquakes work. One popular classroom model is the “Earthquake Machine,” which uses simple materials to show how energy builds up and is suddenly released in an earthquake5.

Sketch 5: A model of earthquake

In this model, various types of waves are shown in the way they contribute to creating an earthquake kilometres below the surface of Earth.

Let’s rew/mind

Earthquakes are the result of a complex interplay between Earth’s tectonic plates, the buildup of elastic energy in rocks, and the sudden release of that energy when the stress overcomes the friction holding rocks in place. The movement along faults—whether normal, reverse, or strike-slip—determines the type of earthquake that occurs, while the location of the focus and epicentre influences how the seismic energy propagates to the surface.

Understanding these mechanisms not only helps us comprehend one of nature’s most powerful phenomena but also assists in developing better prediction methods and designing structures that can withstand these seismic events. While we cannot stop earthquakes from happening, our increasing knowledge about their causes and behaviours continues to improve our resilience in the face of these geological events.