An earthquake is the sudden shaking of Earth’s surface. This occurs when energy stored within the Earth’s crust is abruptly released along faults. The movement generates seismic waves that cause the vibrations we feel.
Earth’s Shifting Plates
Earth’s outermost layer, the lithosphere, is broken into several large, rigid pieces called tectonic plates. These plates are constantly in motion, gliding slowly over the asthenosphere beneath them. This movement, averaging a few centimeters per year, is driven by heat from Earth’s deep interior. Forces like convection currents within the mantle and the pulling of dense oceanic plates contribute to this movement. As these plates interact, they generate immense stress along their edges. When this stress exceeds the strength of the rocks, they suddenly break and slip, releasing energy as an earthquake.
Global Earthquake Zones
Most earthquakes occur along the boundaries where tectonic plates meet. The Pacific Ring of Fire, a horseshoe-shaped belt encircling the Pacific Ocean, is the most active seismic region globally. This zone accounts for approximately 81% of the world’s largest earthquakes and is also characterized by significant volcanic activity. Here, oceanic plates are frequently forced beneath other plates (subduction), leading to intense seismic activity.
The Alpide Belt is another major earthquake belt, extending over 15,000 kilometers from Java and Sumatra, through the Himalayas, and across the Mediterranean region to the Atlantic Ocean. This belt is the second most seismically active region worldwide, responsible for about 17% of the planet’s largest earthquakes. Its high seismic activity results from the ongoing collision between the African, Arabian, and Indian plates with the Eurasian plate, creating extensive mountain ranges and numerous fault lines.
The Mid-Atlantic Ridge is an extensive underwater mountain range where two tectonic plates are pulling apart. Earthquakes along this ridge are generally frequent but tend to be smaller in magnitude and shallow, typically less than 30 kilometers deep.
Plate Boundary Interactions
The types of interactions between tectonic plates determine the characteristics of the earthquakes they produce. At convergent boundaries, plates move towards each other, with one often sliding beneath the other (subduction) or two continental plates colliding. These interactions cause about 80% of all earthquakes, including the deepest and most powerful, occurring up to 700 kilometers deep. The immense pressure and friction from converging plates lead to significant seismic events.
At divergent boundaries, plates move away from each other, forming mid-ocean ridges where new crust is generated. Earthquakes in these zones are shallow, less than 30 kilometers deep. While common, they are less powerful than those at convergent zones, with most seismic activity concentrated along transform faults that connect spreading ridge segments.
Transform boundaries occur where plates slide horizontally past each other. Friction between these plates builds substantial stress, leading to shallow-focus earthquakes that can be powerful and destructive. The San Andreas Fault in California, where the Pacific Plate slides past the North American Plate, is a well-known example.
Earthquakes Beyond Plate Edges
While most earthquakes occur at plate boundaries, some also happen within the interior of tectonic plates. These are known as intraplate earthquakes and are relatively rare, accounting for about 5% of all earthquakes. Despite their infrequent occurrence, intraplate earthquakes can still cause substantial damage, as structures in these regions are often not built to withstand significant seismic shaking.
These earthquakes are caused by stresses within the plates, often reactivating ancient fault lines or zones of weakness. These weaknesses can be remnants of past geological activity, such as failed rifts or ancient plate collisions. The New Madrid Seismic Zone in the central Mississippi Valley is an example in the United States, which experienced powerful earthquakes in 1811-1812. This zone remains seismically active, with numerous smaller tremors occurring annually.