How Earthquakes Form: Faults, Seismic Waves, and Why Some Regions Shake More

What Is an Earthquake?

An earthquake is the sudden release of energy stored in Earth's crust, causing the ground to shake. Most earthquakes occur when stress that has accumulated along a geological fault — a fracture in Earth's crust where two blocks of rock have moved relative to each other — exceeds the frictional resistance holding the rocks in place. The rocks suddenly slip, releasing energy as seismic waves that radiate outward from the point of initial rupture (the hypocenter or focus), causing the ground shaking felt at the surface (the epicenter is the point directly above the focus on the surface).

The Tectonic Cause

Most earthquakes are caused by the movement of tectonic plates — the massive, slowly moving slabs that make up Earth's outer shell. Plates move at rates of 2–10 cm per year, driven by mantle convection. Where plates meet, they interact in several ways:

• Transform boundaries: Plates slide horizontally past each other. The San Andreas Fault in California is a classic example — the Pacific Plate moving northwest relative to the North American Plate. This motion generates frequent earthquakes along the fault, and builds stress for large earthquakes.
• Convergent boundaries: Plates collide. If one is oceanic, it subducts (dives under) the other — the subducting slab generates deep, powerful earthquakes as it descends. The 2011 Tōhoku earthquake (magnitude 9.0) and 2004 Indian Ocean earthquake (magnitude 9.1) were megathrust earthquakes at subduction zones.
• Divergent boundaries: Plates pull apart. Generally produces smaller earthquakes as magma fills the gap. Mid-ocean ridges are divergent boundaries.

The Elastic Rebound Theory

Developed after the 1906 San Francisco earthquake by Harry Fielding Reid, the elastic rebound theory explains the earthquake mechanism: rocks on either side of a fault are continuously being stressed by tectonic forces. Like bending a rubber band, they deform elastically — storing energy. When stress exceeds friction, the rocks suddenly snap to new positions (elastic rebound), releasing stored energy as seismic waves. After the earthquake, stress begins building again for the next event.

Types of Faults

• Strike-slip faults: Horizontal motion (like the San Andreas). Produces earthquakes but little vertical displacement — explains why strike-slip earthquakes rarely generate tsunamis.
• Normal faults: One block drops down relative to the other — extensional tectonics at divergent boundaries.
• Reverse/thrust faults: One block pushed up over the other — compressional tectonics at convergent boundaries. The largest earthquakes (megathrusts) occur here. Generates seafloor displacement that creates tsunamis.

Seismic Waves

Earthquakes generate four types of seismic waves that travel through and around Earth:

• P-waves (primary, compressional): Travel through solids, liquids, and gases. Fastest — arrive first at seismographs. Push-pull motion in the direction of travel.
• S-waves (secondary, shear): Travel only through solids. Slower than P-waves. Side-to-side motion perpendicular to travel direction. More damaging than P-waves.
• Love waves: Surface waves with horizontal shaking. Can cause significant building damage.
• Rayleigh waves: Surface waves with elliptical rolling motion — the "rolling" sensation felt during large earthquakes. Slowest but often most destructive.

Measuring Earthquakes

The Richter scale (magnitude) was the original measure, but is now largely replaced by the moment magnitude scale (Mw) — based on the total seismic energy released and more accurate for large earthquakes. Each whole number increase represents approximately 31.6× more energy released.

• Mw 3.0: Generally not felt
• Mw 4.0: Felt widely; rarely damaging
• Mw 5.0: Can cause moderate damage
• Mw 6.0: Can cause significant damage near epicenter
• Mw 7.0: Major earthquake; severe damage over large area (2010 Haiti: Mw 7.0, ~316,000 deaths)
• Mw 8.0+: Great earthquake; can be felt thousands of km away
• Mw 9.0+: Megathrust earthquake; rare but most powerful natural events

Earthquake-Prone Regions

The "Ring of Fire" — a horseshoe-shaped belt around the Pacific Ocean — accounts for about 90% of the world's earthquakes and 75% of volcanoes. It follows the boundaries of several major tectonic plates: the Pacific, Philippine, Juan de Fuca, Nazca, Cocos, and Caribbean plates interacting with surrounding plates. Japan, Indonesia, the Philippines, Chile, and the Pacific Northwest of the U.S. and Canada are among the most seismically active regions.