How Earthquakes Happen: Plate Tectonics and Seismic Waves Explained

Our planet may feel solid and immovable underfoot, but it is anything but static. Earthquakes are the jolting reminder that the ground beneath us is part of a dynamic system. From the mild tremors that rattle dishes to the devastating quakes that topple cities and unleash tsunamis, these events originate deep within the Earth. To understand why they happen, we need to explore how our planet’s crust is constructed and how energy travels through rock.

Earth’s outer shell consists of several massive slabs of rock called tectonic plates. These plates vary in size and thickness and include both oceanic crust (made of dense basalt) and continental crust (made of lighter granitic rocks). They float atop the mantle—a layer of hot, slowly convecting rock—and move a few centimetres a year, driven by heat from Earth’s interior. Tectonic plates interact along boundaries, and the type of interaction determines the geological activity we see. At divergent boundaries, plates pull apart and magma wells up to create new crust, as at the Mid-Atlantic Ridge. At convergent boundaries, one plate dives beneath another in a process called subduction, generating deep ocean trenches, mountain ranges and volcanic arcs like the Andes and Japan. At transform boundaries, plates grind past each other laterally, forming long faults such as California’s San Andreas Fault. It is along these boundaries that most earthquakes occur.

As plates move, their edges become locked by friction. Stress builds as the motion continues, deforming the rocks. Eventually the accumulated strain exceeds the strength of the fault and the rocks snap or slip suddenly. This release of energy sends out vibrations that we feel as an earthquake. The point within the Earth where the rupture initiates is called the hypocentre or focus, and the point directly above it on the surface is the epicentre. If the rupture reaches the surface, it can produce visible offsets in roads, fences and river channels; if it remains at depth, the deformation is hidden but the shaking is still felt at the surface.

The energy unleashed by an earthquake travels through the Earth in the form of seismic waves. The fastest are primary or P-waves, which compress and expand rock in the same direction as they travel, much like sound waves in air. Because they involve compression, P-waves can propagate through solids, liquids and gases. Secondary or S-waves arrive next; they shear the ground perpendicular to their direction of travel, shaking particles side to side or up and down. S-waves travel only through solids because liquids cannot support shear stresses. When these body waves reach the surface, they are transformed into surface waves that travel along Earth’s outer layers. Love waves move the ground horizontally, while Rayleigh waves roll the ground like ocean swell. Surface waves typically have longer wavelengths and cause the strongest shaking and most damage.

Seismologists measure and record these waves with sensitive instruments called seismometers. Early in the 20th century, the American seismologist Charles Richter devised a logarithmic scale to estimate earthquake size based on the maximum amplitude of seismic waves on a standard instrument at a fixed distance. The Richter scale was superseded by the moment magnitude scale (Mw), which calculates the energy released from the area of the fault that slipped, the amount of slip and the strength of the rocks involved. Because the scale is logarithmic, an increase from magnitude 5 to magnitude 6 corresponds to roughly 32 times more energy; a difference of two magnitude units implies about 1 000 times more energy. While magnitude measures how much energy was released, intensity describes the effects of an earthquake at a particular location. The Modified Mercalli Intensity scale ranges from I (not felt) to XII (total destruction) and depends on factors like distance from the epicentre, local geology and building design.

Earthquakes can trigger a host of hazards. Ground shaking can topple unreinforced buildings and bridges. In places where the rupture breaks the surface, the ground can offset by metres, shredding roads, pipelines and railways. Landslides and rockfalls may be triggered on steep slopes, while liquefaction can cause water-saturated soils to behave like quicksand, undermining structures. Undersea earthquakes that cause sudden vertical displacement of the seafloor can generate tsunamis—series of long, powerful waves that race across oceans and inundate coastlines. The 2004 Indian Ocean tsunami and the 2011 Tōhoku earthquake in Japan resulted from megathrust ruptures along subduction zones and led to massive loss of life.

Despite decades of research, precise prediction of when and where a particular earthquake will strike remains beyond our grasp. What scientists can do is estimate probabilities over longer time frames based on the rate of tectonic motion and the history of past earthquakes, and issue earthquake forecasts that indicate increased likelihood following a large event. Engineering and planning play crucial roles in reducing risk: enforcing strict building codes, retrofitting older structures, and using base isolation systems that allow buildings to sway without collapsing can save lives. Early warning systems in countries like Japan and Mexico detect the first-arriving P-waves and automatically send alerts via sirens, phone notifications or train controls, giving people seconds to drop, cover and hold on before the more destructive S-waves arrive.

Preparedness at the individual and community level also makes a difference. In earthquake-prone regions, securing heavy furniture and appliances, creating an emergency kit with water, food, flashlights and first-aid supplies, and having a family communication plan are simple but effective steps. During shaking, the recommended action is to drop to the ground, take cover under a sturdy table or desk and hold on. After the shaking stops, be prepared for aftershocks and avoid damaged structures. If you are near the coast and feel strong shaking, a tsunami may follow; move to higher ground immediately without waiting for an official warning.

Earthquakes are dramatic expressions of our planet’s restless nature. They occur because heat from Earth’s interior drives the slow dance of tectonic plates, building stress that is released suddenly along faults. The seismic waves that emanate from these breaks tell seismologists where and how big an earthquake was, and they provide clues about Earth’s inner structure. While we cannot stop earthquakes, understanding their causes and behaviours helps us build safer communities and respond effectively when they occur. By respecting the power of plate tectonics and preparing for the inevitable, we can reduce the harm these natural phenomena pose and appreciate the dynamic planet we call home.