Earth's surface is made up of tectonic plates, which are large slabs of solid rock that fit together like puzzle pieces. These plates meet at plate boundaries and are in constant motion.
Where the surfaces of these plates catch and lock along their boundary, stress builds up as the rest of the plates continue to move. This takes place over long periods of time.
When the stress becomes too great, the plates suddenly slip past one another, releasing tremendous amounts of energy, which causes the earth to shake and crack.
These cracks are called faults. Plate boundaries are made up of many faults. Most earthquakes occur on these faults.
Faults can also occur far from plate boundaries but earthquakes on these faults (intraplate earthquakes) are much less frequent. The scientific community is working to better understand these faults.
One of the many tectonic plates that make up Earth's outer shell descends, or "subducts," under an adjacent plate. This kind of boundary between plates is called a "subduction zone." When the plates move suddenly in an area where they are usually stuck, an earthquake happens.
Stuck to the subducting plate, the overriding plate gets squeezed. Its leading edge is dragged down, while an area behind bulges upward. This movement goes on for decades or centuries, slowly building up stress.
An earthquake along a subduction zone happens when the leading edge of the overriding plate breaks free and springs seaward, raising the sea floor and the water above it. This uplift starts a tsunami. Meanwhile, the bulge behind the leading edge collapses, thinning the plate and lowering coastal areas.
Part of the tsunami races toward nearby land, growing taller as it comes in to shore. Another part heads across the ocean toward distant shores. Source: U.S. Geological Survey
Earthquakes generally occur on three types of faults: normal, strike-slip, and reverse (or thrust).
Tsunamis can be generated by earthquakes on all of these faults, but most tsunamis, and the largest, result from earthquakes on reverse faults. These tsunami-generating earthquakes originate mainly in subduction zones, where tectonic plates collide and one is forced under the other.
Take it to the MAX! Plate Tectonics and Earthquakes
If big enough and close enough to the ocean floor, the energy from such an earthquake can cause the ocean floor to suddenly rise (uplift) or fall (subside). It is this sudden vertical displacement of the ocean floor that typically sets a tsunami in motion.
As the ocean floor rises or falls, so too does the water above it. As the water moves up and down, seeking to regain its balance, a tsunami is born.
Not all earthquakes cause tsunamis. Key earthquake characteristics that contribute to tsunami generation are location, magnitude (size), and depth.
Most tsunamis are generated by earthquakes with magnitudes over 7.0 that occur under or very near the ocean and less than 100 kilometers (62 miles) below Earth's surface (earthquakes deeper than this are unlikely to displace the ocean floor).
Generally, an earthquake must exceed magnitude 8.0 to generate a dangerous distant tsunami. The amount of movement of the ocean floor, the size of the area over which an earthquake occurs, and the depth of the water above the earthquake are also important factors in the size of a resulting tsunami.
Most of the tsunamis (89%) in the Global Historical Tsunami Database were generated by large earthquakes or landslides caused by earthquakes.
In addition to the previously highlighted tsunamis (2004 and 2011), other examples of earthquake-generated tsunamis include:
- November 1, 1755 Lisbon, Portugal - A magnitude 8.5 (estimated) earthquake in the Atlantic Ocean generated a tsunami that affected the coasts of Portugal, Spain, North Africa, and the Caribbean. The earthquake and tsunami killed an estimated 50,000 people and caused widespread destruction.
- January 26, 1700 Cascadia Subduction Zone - A magnitude 9.0 (estimated) earthquake generated a tsunami that inundated the coasts of Cascadia (a region that includes northern California, Oregon, Washington, and southern British Columbia) as well as coastal villages across the Pacific Ocean in Japan. Today, the Cascadia subduction zone is considered one of the largest U.S. tsunami threats.
To learn more about earthquakes, visit the U.S. Geological Survey's Earthquake Hazards Program.