A tsunami is a series of ocean waves caused by the massive and rapid displacement of a large volume of water. While displacement can be triggered by events like volcanic eruptions or underwater landslides, the most common cause is a powerful earthquake beneath the seafloor. The relationship between earthquake magnitude and tsunami size is not a simple one-to-one correspondence. Tsunami generation is a complex process dependent on the earthquake’s size, its depth, and the specific type of fault movement that occurs.
The Minimum Magnitude Required
For an earthquake to generate a destructive and far-reaching tsunami, a considerable amount of energy must be released, typically requiring a minimum magnitude. Most earthquakes capable of causing significant tsunamis register a Moment Magnitude Scale (MMS) reading of 7.5 or higher. Seismologists use the MMS because it directly relates to the physical size of the fault rupture and the total energy released, making it a reliable indicator of tsunami potential.
While magnitude 7.5 is the general threshold for a destructive event, global warning centers often use a slightly lower number for initial alerts. In the Pacific Ocean, which hosts the majority of tsunamis, a magnitude 7.1 event or greater is often the benchmark for issuing an alert. Earthquakes below magnitude 7.0 are unlikely to produce a damaging tsunami.
A magnitude of 8.0 or greater is required to generate a “dangerous distant tsunami,” capable of traveling across an entire ocean basin and causing widespread destruction. For example, the 2011 Tohoku earthquake and the 2004 Sumatra-Andaman earthquake were both magnitude 9.1 events that created devastating, trans-oceanic tsunamis. Since the logarithmic Moment Magnitude Scale means an increase of one whole number represents about 32 times more energy release, the jump from a magnitude 7 to a magnitude 8 earthquake is highly significant.
How Underwater Earthquakes Displace Water
The physical mechanism that converts an earthquake’s energy into a tsunami relies on the sudden vertical movement of the seafloor. This displacement pushes the entire water column above the rupture zone. The most efficient tsunami-generating earthquakes occur at subduction zones, where one tectonic plate is forced beneath another.
In a subduction zone, the overriding plate becomes temporarily locked to the subducting plate, causing massive strain to build up. When the stress overcomes the friction, the overriding plate suddenly snaps upward through a process known as thrust faulting. This rapid upward motion of the seabed acts like a giant paddle, lifting the water above it.
Conversely, earthquakes resulting from strike-slip faulting, where two plates grind past each other horizontally, rarely generate tsunamis. This is because the sideways movement causes minimal vertical shift of the ocean floor. The seafloor’s sudden uplift or subsidence must occur over a large area to effectively push the water and initiate powerful, long-wavelength tsunami waves.
The earthquake must also be relatively shallow, occurring less than 100 kilometers (62 miles) below the Earth’s surface, to transfer energy efficiently to the ocean floor. If the hypocenter is too deep, the seismic energy dissipates before it can cause the necessary large-scale vertical rupture of the seabed. The resulting wave train radiates outward as the displaced water attempts to regain equilibrium.
Geological Conditions That Influence Tsunami Size
Earthquake magnitude is necessary, but not the sole factor, in determining a tsunami’s size and destructive power. Other geological and oceanographic conditions can either amplify or diminish the hazard. Shallower earthquakes, especially those less than 50 kilometers deep, are significantly more effective at generating large tsunamis because the fault rupture is closer to the seafloor.
The water depth above the rupture zone is another variable. Earthquakes that occur under deep ocean water are more likely to create powerful tsunamis that can travel vast distances without losing energy. The deep ocean allows the wave to maintain high speed and a very long wavelength, which is characteristic of a destructive tsunami.
An exception to the magnitude rule involves secondary effects like submarine landslides. Even a magnitude 7.0 earthquake can destabilize the continental slope and trigger a massive underwater landslide. This landslide rapidly displaces a large volume of water, creating a highly destructive local tsunami that can strike the coast within minutes.
The overall size of the fault area that ruptures, not just the single point of the hypocenter, also influences the wave’s power. A larger rupture area means more water is displaced across a wider region, translating into a more sustained and energetic tsunami wave train. Assessing tsunami risk requires analyzing all these variables alongside the earthquake’s magnitude.