Most earthquakes occur along the edges of Earth’s tectonic plates, where massive slabs of rock grind past, pull apart from, or dive beneath one another. Roughly 90 percent of all earthquakes strike within a single zone called the Ring of Fire, which traces the borders of the Pacific Ocean. But earthquakes also happen far from any plate boundary, and in recent decades, human activity has triggered them in places that rarely shook before.
The Ring of Fire
The Ring of Fire is a horseshoe-shaped belt stretching about 40,000 kilometers around the Pacific basin. It runs from New Zealand up through Indonesia, Japan, and Alaska, then down the western coasts of North and South America. This zone produces nine out of every ten earthquakes recorded worldwide, and it’s home to 75 percent of all active volcanoes on Earth.
What makes the Ring of Fire so active is the sheer number of subduction zones packed along its length. In a subduction zone, one plate slides beneath another and sinks into the Earth’s interior. The sinking plate doesn’t move smoothly. Segments lock in place for years or centuries, building up enormous stress, then lurch forward in a single violent release. The 2011 magnitude-9.1 earthquake off Japan, the 2010 magnitude-8.8 quake in Chile, and virtually every other giant earthquake in recorded history happened at subduction zones within this belt.
The Alpide Belt
The second most seismically active zone on Earth is the Alpide belt, which stretches from Java and Sumatra through the Himalayas and the Mediterranean and out into the Atlantic Ocean. This belt accounts for roughly 5 to 6 percent of the world’s earthquakes. Countries along this zone include Indonesia, Myanmar, India, Nepal, Iran, Turkey, Greece, and Italy.
The Alpide belt is powered mostly by the collision of the African, Arabian, and Indian plates with the Eurasian plate. Unlike subduction zones where one plate dives cleanly beneath another, much of the Alpide belt involves continents crumpling into each other. That collision built the Himalayas and the Alps, and it continues to generate devastating earthquakes. The 2023 magnitude-7.8 earthquake in Turkey and the 2015 magnitude-7.8 earthquake in Nepal both struck within this belt.
Three Types of Plate Boundaries
Subduction Zones
Where one plate dives beneath another, earthquakes can occur at a wide range of depths. The USGS classifies earthquakes as shallow (0 to 70 km deep), intermediate (70 to 300 km), or deep (300 to 700 km). Subduction zones are the only places on Earth that produce intermediate and deep earthquakes, because the descending slab carries brittle rock far into the mantle. In 1994, a magnitude-8.3 earthquake struck 636 km beneath Bolivia, one of the deepest large earthquakes ever recorded in South America. The deepest confirmed earthquake occurred about 680 km below the surface in the Izu-Bonin subduction zone, roughly 1,000 km offshore of Japan.
Transform Faults
At transform boundaries, two plates slide horizontally past each other. The grinding produces shallow earthquakes, typically less than 20 km deep, along with broad zones of crushed and deformed rock. California’s San Andreas Fault is the most famous example: the Pacific Plate moves northward past the North American Plate at roughly 5 cm per year. Other major transform boundaries include the Queen Charlotte Fault off western Canada, the Dead Sea Transform in the Middle East, and the boundary where the Caribbean Plate slides past the North American Plate near the U.S. Virgin Islands. Transform faults rarely produce the very largest earthquakes, but they can still reach magnitude 7 or higher and cause severe damage because they tend to run through or near populated areas.
Divergent Boundaries
Where plates pull apart, hot rock rises from below to fill the gap, creating mid-ocean ridges. These spreading centers produce frequent earthquakes, but they’re almost always shallow and relatively small, typically below magnitude 6. The Mid-Atlantic Ridge, which runs down the center of the Atlantic Ocean, is the longest divergent boundary on Earth. Iceland sits directly on top of it, which is why the island experiences regular seismic activity despite being far from any subduction zone.
Earthquakes Away From Plate Boundaries
Not all earthquakes happen at plate edges. Intraplate earthquakes occur in the middle of tectonic plates, sometimes hundreds or thousands of kilometers from the nearest boundary. They’re less common but can be just as dangerous, partly because the regions they strike are often unprepared.
The most famous example in the United States is the New Madrid Seismic Zone, centered where Missouri, Tennessee, Arkansas, and Kentucky meet. In 1811 and 1812, this zone produced three earthquakes estimated at magnitude 7 or higher, strong enough to ring church bells in Boston and temporarily reverse the flow of the Mississippi River. The seismicity there is driven by ancient faults buried deep beneath the surface. These faults originally formed when the continent tried to rift apart roughly 500 million years ago. Although the rifting failed, the old fractures remain, and the compressive stress field pushing across eastern North America reactivates them. The zone still produces hundreds of small earthquakes each year.
Similar intraplate zones exist elsewhere. Parts of Australia, northern China, and central India experience significant earthquakes along ancient buried faults that are reactivated by regional stress.
Human Activity and Induced Earthquakes
Over the past two decades, human industrial activity has created new earthquake zones in places that were historically quiet. The primary cause is the underground disposal of wastewater from oil and gas production. When large volumes of fluid are pumped deep into the earth, the added pressure can lubricate existing faults and trigger them to slip.
Oklahoma is the clearest example. The state experienced a surge of earthquakes beginning in 2009, going from a handful of magnitude-3 or larger events per year to hundreds. In 2016, a magnitude-5.8 earthquake in central Oklahoma became the largest earthquake ever documented from fluid injection in the scientific literature. A magnitude-5.3 quake hit the Raton Basin in Colorado in 2011, also linked to injection wells. Hydraulic fracturing (fracking) itself is a less common trigger. Only about 2 percent of Oklahoma’s induced earthquakes are linked directly to fracking operations, and the largest fracking-induced earthquake in the U.S. was a magnitude 4.0 in Texas in 2018.
Induced seismicity isn’t limited to the United States. Oil production in Uzbekistan has been linked to earthquakes above magnitude 7, showing that human-triggered events can, in extreme cases, rival natural ones.
How Many Earthquakes Happen Each Year
The USGS National Earthquake Information Center locates about 20,000 earthquakes around the globe each year, roughly 55 per day. That count only includes quakes large enough for seismometers to pinpoint reliably. The actual number, including tiny tremors, is far higher.
Based on records going back to about 1900, the long-term average is around 16 major earthquakes per year: 15 in the magnitude-7 range and one at magnitude 8.0 or greater. Some years see more, some fewer, but the overall rate has stayed relatively stable over the past century. The distribution, however, is uneven. Subduction zones along the Ring of Fire and the Alpide belt absorb the vast majority of this seismic energy, while large stretches of continental interiors may go decades between noticeable tremors.