Earthquake

Earthquakes are sudden geological disturbances characterised by the abrupt release of energy within the Earth’s lithosphere, generating seismic waves that propagate through the crust. These events occur across a spectrum of intensities, ranging from imperceptible microseisms to destructive shocks capable of levelling cities, triggering landslides and producing tsunamis. The distribution, frequency, and magnitude of earthquakes are closely linked to tectonic processes, particularly along plate boundaries such as those of the Pacific Ring of Fire. While earthquakes are mainly natural geological phenomena, certain human activities have also been shown to induce seismicity.

Geological Background and Causes

The principal cause of most natural earthquakes is the rupture of geological faults—fractures within the Earth’s crust where blocks of rock move relative to one another. Tectonic plates, driven by mantle convection, continuously interact through divergence, convergence and lateral shearing, storing elastic strain energy in the surrounding rock. This energy accumulates when faults become locked by asperities, or irregularities on fault surfaces, preventing smooth movement. When accumulated stress exceeds the strength of the rocks, the fault ruptures suddenly in a process known as the elastic-rebound mechanism, releasing energy in the form of seismic waves.
While tectonic processes account for the majority of earthquakes, other natural triggers include volcanic activity, gravitational collapse leading to landslides, and occasionally even rapid sediment compaction. In addition to natural drivers, anthropogenic activities such as deep mining, hydraulic fracturing, reservoir-induced seismicity, underground blasting and nuclear weapons testing have the potential to generate seismic disturbances.
The point at which rupture initiates is termed the hypocentre or focus, and the point directly above it on the Earth’s surface is the epicentre. The nature and severity of earthquake impacts depend on several factors, including focal depth, distance from the epicentre, geological conditions, and the structural resilience of the built environment.

Characteristics and Measurement

Earthquake strength is commonly expressed through magnitude and intensity scales. Magnitude, measured using systems such as the moment magnitude scale (Mw), represents the total energy released during an event. Intensity, meanwhile, refers to the observed effects and damage on the Earth’s surface, often measured by descriptors such as the Modified Mercalli Intensity scale.
Seismic activity within a region—its seismicity—includes not only magnitude but also the frequency and type of earthquakes over time. High-seismicity areas tend to coincide with active boundaries where strain accumulation and release occur most frequently, although destructive earthquakes can also strike in regions where major events are comparatively rare, often with more devastating results due to inadequate preparedness or weak structural resilience.

Major Historical Earthquakes

Several major earthquakes in recorded history illustrate the catastrophic potential of seismic events. One of the most devastating was the 1556 Shaanxi earthquake in China, which caused large-scale destruction, with fatalities exceeding hundreds of thousands. Many victims lived in yaodong dwellings—cave-like structures carved into loess slopes—which collapsed during the violent shaking.
Another significant event was the 1976 Tangshan earthquake in China, regarded as the deadliest earthquake of the twentieth century, with casualties estimated between 240,000 and 655,000. In South America, the 1960 Valdivia earthquake in Chile stands as the largest instrumentally recorded earthquake, with a magnitude of 9.5. This megathrust event released roughly twice the energy of the next most powerful recorded earthquake and triggered widespread tsunamis across the Pacific.
Patterns of devastating historical events highlight that earthquakes causing the greatest loss of life tend to occur either near densely populated centres or beneath ocean basins, where displacement of the sea floor generates tsunamis impacting coastlines thousands of kilometres away. Poor regions with inadequate or improperly enforced construction standards remain at especially high risk.

Tectonic Settings and the Elastic-Rebound Theory

Earthquakes can occur wherever sufficient elastic strain energy accumulates within the crust. Plate boundary interactions provide the most favourable environments for this process, with the Pacific Ring of Fire representing the most seismically active zone on Earth. Here, subduction of oceanic plates beneath continental or other oceanic plates produces frequent megathrust earthquakes.
Faults generally move aseismically—known as creep—when they are smooth and free of asperities. However, most faults possess irregularities that cause frictional locking. Progressive tectonic motion increases stress near these locked segments until rupture occurs, releasing stored elastic strain. Although only a small proportion of the released energy propagates as seismic waves, the remainder contributes to fault friction heating and fracturing of rock. This process lowers the Earth’s available elastic potential energy but produces negligible overall thermal change relative to larger geothermal processes.

Fault Types and Associated Earthquakes

Faults responsible for seismic events occur in three principal types:

• Normal faults, associated with extensional tectonics, develop where the crust is pulled apart. These faults commonly occur near divergent boundaries, such as mid-ocean ridges. Earthquakes along normal faults tend to be of moderate magnitude, generally below 7, and are limited by the thinner brittle crust in such regions.
• Reverse (thrust) faults form where the crust is compressed, particularly at convergent boundaries. These faults generate the planet’s most powerful earthquakes, known as megathrust events, which account for about 90 per cent of the global seismic moment. Subduction zones are especially prone to these large ruptures.
• Strike-slip faults occur where crustal blocks slide laterally past one another, characteristic of transform boundaries. Steeply dipping structures such as the San Andreas Fault have produced several major earthquakes, although their rupture lengths are typically shorter than those observed in subduction zones. Some faults exhibit oblique slip, involving a combination of vertical and horizontal displacement.

The brittle upper crust and the cool descending slabs of subducting plates are the only regions able to store significant elastic energy. At depths where temperatures exceed thresholds for ductile flow, rocks deform plastically and therefore do not generate earthquakes.

Impacts and Secondary Hazards

Earthquakes produce a range of hazards, both primary and secondary. Ground shaking is the most widespread effect, capable of collapsing structures, rupturing pipelines and damaging critical infrastructure. Soil liquefaction, in which water-saturated sediments lose strength, can cause buildings to sink or tilt. Earthquakes may also trigger landslides on unstable slopes, intensifying damage in mountainous areas.
When seismic events occur offshore, abrupt displacement of the seabed can generate tsunamis, producing destructive waves that propagate across ocean basins. The 2004 Indian Ocean earthquake is a notable example, causing widespread devastation in multiple countries.

Prediction, Preparedness and Engineering

While precise prediction of earthquake timing remains scientifically unattainable, forecasting models can estimate long-term probabilities based on fault behaviour and regional seismic history. Mitigation efforts therefore focus on preparedness and reducing vulnerability.
Key strategies include:

• Seismic retrofitting to strengthen existing structures.
• Earthquake engineering, employing building designs that absorb or dissipate seismic energy.
• Early warning systems, which detect initial seismic waves and provide seconds of notice before strong shaking begins.
• Public education and emergency planning.

Regions with strong building codes and rigorous enforcement exhibit significantly reduced mortality and damage compared with those lacking such measures.

Broader Significance and Cultural Influence

Earthquakes have long influenced human societies, shaping myths, religious traditions and cultural narratives. Modern media and scientific exploration continue to examine their causes and consequences, highlighting the persistent need for research and mitigation planning. Beyond Earth, similar seismic phenomena—marsquakes and moonquakes—have been recorded, reinforcing the universality of tectonic and thermal stresses on planetary bodies.