An earthquake is a sudden, rapid shaking of the Earth caused by the release of stored energy in the crust. This energy is released when two blocks of rock on either side of a fault abruptly slip past one another, generating seismic waves. The global distribution of these events follows distinct, predictable patterns related to the movement and interaction of the planet’s outermost shell.
Plate Tectonics and Fault Systems
The Earth’s rigid outer layer, the lithosphere, is fractured into segments called tectonic plates. The vast majority of seismic activity is concentrated at the boundaries where these plates meet and interact. As plates slowly drift, friction causes their edges to lock up, preventing smooth movement. Stress builds up along these locked sections until the accumulated force overcomes the friction, resulting in a sudden slip and an earthquake.
These plate interactions produce three main types of boundaries, each associated with specific fault systems. At convergent boundaries, where plates collide, one slab often slides beneath the other, creating thrust faults and megathrust earthquakes. Divergent boundaries, where plates pull apart, are characterized by normal faults and generally produce smaller, shallower earthquakes. Transform boundaries involve plates sliding horizontally past each other, leading to strike-slip faults where shearing stress is released.
The High Activity Zone of the Pacific Ring of Fire
The primary area of seismic activity on Earth is the Pacific Ring of Fire, a horseshoe-shaped zone tracing the edges of the Pacific Ocean basin. Stretching approximately 40,000 kilometers, this belt is responsible for an estimated 90% of the world’s earthquakes. It is defined by a nearly continuous series of subduction zones, where dense oceanic plates are forced downward beneath lighter continental or oceanic plates.
This subduction process generates both deep and powerful earthquakes, including all recorded events greater than magnitude 9.0. The friction and pressure at these convergent boundaries also produce chains of volcanoes parallel to the deep ocean trenches. Key segments of this active zone include the western coasts of South and North America, the Aleutian Islands off Alaska, and the island arcs of Japan and the Philippines.
Secondary Global Seismic Belts
Outside of the Pacific basin, two other major zones account for the remaining global seismicity. The second most active region is the Alpine-Himalayan Orogenic Belt, which extends for over 15,000 kilometers from Indonesia, through the Himalayas and the Mediterranean, and into the Atlantic Ocean. This belt generates approximately 17% of the world’s largest earthquakes and is defined by continent-continent collision.
The mountain ranges of the Himalayas are the direct result of the Indian plate pushing northward into the Eurasian plate. This collision zone creates shallow, destructive earthquakes as the crust is compressed and thrust upward. The Mid-Atlantic Ridge represents a major divergent boundary running down the center of the Atlantic Ocean. Here, plates are pulling apart, allowing molten material to rise and form new crust, resulting in a narrow belt of frequent but relatively shallow, lower-magnitude seismic events.
Intraplate Earthquakes and Stable Regions
While most global seismicity occurs at plate boundaries, earthquakes occasionally occur far from these edges, within the interior of a tectonic plate. These are known as intraplate earthquakes, accounting for only about 5% of global seismic activity. Such events are less frequent, but they can still be destructive because the built environment in these areas is often not designed for significant ground shaking.
The causes of these interior quakes are often linked to the reactivation of ancient, buried faults or rift zones. Tectonic stresses originating from distant plate boundaries can be transmitted across the rigid plate, exploiting these pre-existing weaknesses. A notable example is the New Madrid Seismic Zone in the central United States, where powerful earthquakes occurred in 1811 and 1812 along a failed ancient rift system. Additionally, some localized events can be human-induced, often related to activities like deep wastewater injection, which increases fluid pressure on dormant faults.