What Causes Earthquakes: Plates, Faults, and Released Energy

The Moving Shell of the Earth

Earth's rigid outer layer, the lithosphere, is broken into roughly a dozen large slabs called tectonic plates and several smaller ones. These plates ride atop the hotter, partially molten mantle below, driven by convection currents generated by the planet's internal heat. The continents and ocean floors are simply the surfaces of these slowly moving slabs, drifting at speeds of two to ten centimeters per year, roughly the rate a fingernail grows.

What Happens at Plate Boundaries

Most earthquakes occur where plates meet. At convergent boundaries, two plates collide. When a denser oceanic plate meets a lighter continental plate, the oceanic plate dives beneath in a process called subduction. These zones produce the world's largest earthquakes and most destructive tsunamis; the 2011 Tohoku quake off Japan's coast resulted from this mechanism. At divergent boundaries, plates pull apart and magma wells up to fill the gap, creating new ocean floor. These rifting zones generate frequent but generally moderate quakes. At transform boundaries, plates grind past each other horizontally with neither plate sinking. California's San Andreas Fault is a classic example: sections that are locked rather than freely sliding store enormous elastic stress that releases in sudden jumps.

Types of Earthquakes

Not every earthquake originates at a plate boundary:

• Tectonic earthquakes are caused by fault slip at or near plate boundaries and account for the vast majority of seismic events worldwide, including virtually all large and damaging ones.
• Volcanic earthquakes are triggered by moving magma, gas pressure, or explosive events inside volcanoes. They are typically shallow and confined to the area around the volcanic system.
• Induced earthquakes result from human activities: injecting large volumes of wastewater from oil and gas production deep underground, filling large reservoirs behind dams, or removing rock through deep mining. Oklahoma's dramatic spike in magnitude 3 and larger events after widespread wastewater injection began is a documented example.
• Collapse earthquakes are small tremors produced when underground cavern roofs or mine tunnels give way. They are local and rarely exceed magnitude 3.

How Energy Travels: Seismic Waves

When a fault slips, it releases stored elastic energy in the form of seismic waves that radiate outward from the rupture point. Primary waves, called P waves, are compressional: they alternately push and pull the rock in the direction they are traveling, much like a sound wave in air. P waves move through both solid rock and liquid, and they arrive at any given location first. Secondary waves, called S waves, shake the ground perpendicular to their direction of travel. They move more slowly than P waves and cannot travel through liquid, which is why Earth's liquid outer core blocks S waves entirely. The time gap between P and S wave arrivals at a seismograph station allows scientists to calculate the distance to the earthquake's origin point. With readings from at least three stations, the exact location can be triangulated.

Measuring Magnitude

Scale	Basis for measurement	Practical range	Current status
Richter (ML)	Peak wave amplitude on a specific seismograph at 100 km	M 2–6, local earthquakes only	Historical; largely replaced
Moment Magnitude (Mw)	Total seismic energy: fault area × slip distance × rock rigidity	Full range, all distances	Universal scientific standard

Both scales are logarithmic. Each whole-number increase represents about 32 times more energy released: a magnitude 7 earthquake releases roughly 32 times the energy of a magnitude 6, and about 1,000 times the energy of a magnitude 5.

Why Some Regions Are Earthquake-Prone

The Pacific Ring of Fire, a zone encircling the Pacific Ocean, experiences about 90 percent of the world's earthquakes and produces nearly all of its largest ones. Japan, Indonesia, the Philippines, Chile, and the western coast of North America all sit along this chain of subduction zones and transform boundaries. Continental interiors far from active plate boundaries are not entirely immune, however. Ancient fault systems buried under stable cratons can reactivate when distant tectonic forces transmit stress across large distances, as the 1811 New Madrid earthquakes in the central United States demonstrated.