How Tsunamis Work

Tsunami Generation

Approximately 80 percent of tsunamis result from submarine earthquakes along subduction zones, where one tectonic plate thrusts beneath another. When a megathrust earthquake ruptures the plate interface, the overriding plate snaps upward by several meters over a fault area hundreds of kilometers long. This instantaneous vertical displacement of the seafloor pushes the entire overlying water column upward, creating the initial tsunami wave. The 2011 Tohoku earthquake displaced the seafloor by up to 50 meters horizontally and 10 meters vertically, generating a tsunami that reached heights of 40 meters at the coast.

Volcanic tsunamis can be generated by caldera collapse, flank failure, or pyroclastic flows entering the sea. The 1883 Krakatoa eruption generated tsunamis over 30 meters high that killed approximately 36,000 people in coastal Java and Sumatra. The 2022 Hunga Tonga-Hunga Ha'apai eruption demonstrated that volcanic atmospheric pressure waves can generate tsunamis globally, with measurable waves reaching coastlines across the Pacific, Atlantic, and Mediterranean within hours of the eruption.

Submarine landslides displace water volume proportional to their size and speed. The Storegga Slide off Norway approximately 8,200 years ago involved 3,000 cubic kilometers of sediment and generated tsunamis exceeding 20 meters along the Norwegian coast and 3 to 6 meters in Scotland. Smaller submarine landslides associated with earthquakes can amplify locally generated tsunamis beyond what the earthquake alone would produce, as occurred during the 1998 Papua New Guinea event.

Propagation Across Ocean Basins

In the open ocean, tsunami speed equals the square root of gravitational acceleration multiplied by water depth. At average ocean depth (4,000 meters), this yields speeds of approximately 200 meters per second or 720 kilometers per hour, comparable to a commercial jet aircraft. Despite these enormous speeds, open-ocean tsunami height rarely exceeds 1 meter because the energy is distributed across the entire water column, and the wavelength stretches 100 to 500 kilometers. Ships at sea typically cannot detect a passing tsunami.

As tsunamis approach shore and water depth decreases, conservation of energy forces the wave to slow down, decrease in wavelength, and increase dramatically in height. A tsunami 50 centimeters high in 4,000 meters of water can amplify to 10 meters or more in shallow coastal waters. This shoaling process also causes the leading edge of the wave to steepen, potentially forming a turbulent wall of water (bore) rather than a smooth wave crest.

Coastal topography dramatically modifies tsunami impact. Narrow bays and inlets can focus wave energy through funneling, amplifying heights by factors of 2 to 5. Headlands can deflect energy, creating sheltered areas behind them. Offshore islands and reefs can partially block or diffract incoming waves. The same tsunami can produce heights varying from 2 to 20 meters along a single coastline depending on local bathymetry and coastal geometry.

Warning Systems and Preparedness

The Pacific Tsunami Warning Center (established 1949) and other regional warning centers use seismic networks to detect earthquakes that might generate tsunamis. Within minutes of a large submarine earthquake, automated algorithms estimate magnitude, location, and fault orientation to assess tsunami potential. Deep-ocean pressure sensors (DART buoys) deployed across the Pacific, Indian, and Atlantic oceans detect passing tsunamis in real time, confirming or canceling warnings based on actual wave measurements rather than seismic estimates alone.

Warning time depends on distance from the tsunami source. Distant tsunamis provide hours of warning (the 2011 Japan tsunami took 10 hours to cross the Pacific to California). Near-field tsunamis generated by local earthquakes may arrive at nearby coasts within 10 to 30 minutes, too fast for institutional warning systems. In these situations, the earthquake itself serves as the warning: any earthquake strong enough to make standing difficult near the coast should trigger immediate evacuation to high ground without waiting for official warnings.

Vertical evacuation structures (reinforced buildings designated as tsunami shelters), mapped evacuation routes, and regular community drills save lives when warning times are short. Japan's extensive investment in seawalls, tsunami gates, and evacuation infrastructure following centuries of tsunami experience reduced fatalities during the 2011 event compared to what would have occurred without these preparations, though the tsunami exceeded design standards in many locations, demonstrating that even prepared communities face residual risk from extreme events.