2.2 Tsunamis: generation, propagation, and coastal impacts

Tsunami Generation Mechanisms

Tsunamis are powerful ocean waves triggered by sudden underwater disturbances that displace large volumes of water. Unlike wind-driven waves, tsunamis involve the entire water column from the seafloor to the surface, which is why they carry so much energy across vast distances. Understanding how they form, travel, and strike coastlines is essential for hazard planning and early warning systems.

Earthquakes as the Primary Cause

Earthquakes are by far the most common tsunami trigger, but not every earthquake generates one. The key requirements are:

• Magnitude greater than 7.0 and a shallow focal depth (less than 70 km below the seafloor)
• Vertical motion along a thrust or normal fault, causing sudden uplift or subsidence of the seafloor

That vertical displacement is what matters. A strike-slip earthquake (where plates slide horizontally past each other) moves far less water and rarely produces a significant tsunami.

When the seafloor suddenly shifts upward or downward, the overlying water column is displaced, and a series of waves radiates outward from the source area.

Two major examples illustrate the scale of earthquake-generated tsunamis:

• The 2004 Indian Ocean tsunami was generated by a magnitude 9.1 earthquake along the Sunda Trench off Sumatra, Indonesia. It killed over 230,000 people across 14 countries.
• The 2011 Tōhoku earthquake (magnitude 9.0) off Japan's northeastern coast produced a tsunami that reached wave heights exceeding 10 meters in many locations.

Other Tsunami Triggers: Landslides and Volcanic Eruptions

Submarine or coastal landslides generate tsunamis when large volumes of rock and sediment suddenly slide downslope and displace water. These tsunamis tend to be more localized than earthquake-generated ones, but they can produce extreme wave heights near the source.

• The 1958 Lituya Bay tsunami in Alaska was caused by a massive rockslide (triggered by an earthquake) that fell into a narrow inlet, generating a wave that reached a staggering 524 meters up the opposite slope. This remains the tallest tsunami wave ever recorded.

Volcanic eruptions can trigger tsunamis through several mechanisms: underwater explosions, caldera collapses, pyroclastic flows entering the sea, or flank collapses of volcanic islands.

• The 1883 eruption of Krakatoa in Indonesia generated a destructive tsunami that killed over 36,000 people along the coasts of Java and Sumatra.
• The 1792 collapse of Mount Mayuyama in Japan sent debris into Ariake Bay, triggering a tsunami that killed an estimated 15,000 people in the nearby city of Shimabara.

Tsunami Wave Characteristics

Wave Properties in the Deep Ocean

Out in the open ocean, tsunamis are almost undetectable. They have long wavelengths (up to hundreds of kilometers), low amplitudes (typically less than 1 meter), and travel at speeds up to 800 km/h, roughly the speed of a commercial jet.

Tsunamis behave as shallow-water waves, meaning their wavelength is much greater than the ocean depth. This classification determines how they move. Their speed depends on water depth according to:

$c = \sqrt{gd}$

where $c$ is wave speed, $g$ is gravitational acceleration (9.8 m/s²), and $d$ is water depth.

This equation tells you something important: as a tsunami enters shallower water near the coast, it slows down. But the energy doesn't disappear. Instead, the wave compresses and its amplitude increases dramatically, producing the tall, destructive waves that strike shore.

Wave Train and Dispersion

A tsunami is not a single wave. It arrives as a wave train, a series of waves that can span hours. A critical point for safety: the first wave is often not the largest.

• During the 2004 Indian Ocean tsunami, the third wave was the most destructive in several locations.

Dispersion is the process by which waves of different wavelengths travel at slightly different speeds. Over long distances, this causes the wave train to spread out, so individual waves arrive at the coast separated by minutes or even tens of minutes. This complicates evacuation because people may assume the danger has passed after the first wave recedes, only to be caught by a larger subsequent wave.

Tsunami Height and Power

Influence of Bathymetry and Coastal Configuration

Bathymetry (underwater topography) plays a major role in determining how large a tsunami is when it reaches shore.

• Shallow continental shelves and narrow bays or inlets can amplify tsunami heights significantly.
• Deeper waters and gentle offshore slopes tend to reduce wave heights.
• Underwater ridges or canyons can focus or scatter tsunami energy, creating uneven wave heights along a single stretch of coast.

The shape and orientation of the coastline matter just as much:

• Headlands may experience higher wave heights because wave refraction bends energy toward protruding points of land.
• Funnel-shaped bays concentrate wave energy into a narrowing space, amplifying heights further. The V-shaped bay at Rikuzentakata, Japan, experienced wave heights up to 13 meters during the 2011 Tōhoku tsunami for exactly this reason.

Nearshore Processes and Tidal Influence

As a tsunami approaches the coast, several hydrodynamic processes transform the wave:

• Shoaling causes the wave to grow taller as the water gets shallower.
• Refraction bends the wave so it tends to converge on headlands and diverge in bays.
• Diffraction allows wave energy to spread around obstacles like breakwaters or islands.

Together, these processes determine the final height, direction, and energy distribution of the wave at any given point along the coast.

Tidal stage at the time of arrival also matters. A tsunami arriving at high tide will reach higher elevations and has a greater chance of overtopping coastal defenses. The 2011 Tōhoku tsunami coincided with high tide in some areas, which worsened the flooding.

Role of Coastal Vegetation

Coastal vegetation can reduce tsunami impacts, though it's not a substitute for evacuation or engineered defenses. Dense vegetation acts as a friction barrier that slows the flow and reduces inundation depth.

• Mangrove forests in the Andaman and Nicobar Islands helped reduce damage from the 2004 Indian Ocean tsunami in some areas.
• Dense coastal forests near Sendai, Japan, reduced the extent of inland flooding during the 2011 Tōhoku tsunami.

The effectiveness depends on vegetation density, width of the vegetated zone, and the intensity of the tsunami. A very large tsunami will overwhelm even thick mangrove stands, but vegetation consistently reduces damage from moderate events.

Coastal Impacts of Tsunamis

Inundation and Erosion

Inundation (flooding of low-lying coastal areas) is the most immediately destructive impact. How far inland the water reaches depends on wave height, coastal elevation, slope, and whether natural or artificial barriers are present.

• The 2011 Tōhoku tsunami caused inundation up to 10 km inland in some flat, low-lying areas of Japan.

Erosion from tsunami waves can dramatically reshape coastlines in a matter of minutes. The powerful currents strip away sediment, undermine foundations of coastal structures, and destroy natural features like beaches, dunes, and coral reefs.

• The 2004 Indian Ocean tsunami caused extensive erosion along the coasts of Indonesia, Thailand, and Sri Lanka, damaging beaches, coastal forests, and reef systems that had taken decades or centuries to develop.

Infrastructure Damage and Secondary Hazards

Tsunami damage extends well beyond the initial flooding:

• Ports and harbors: Strong tsunami currents can destroy breakwaters, jetties, and seawalls. The 2011 Tōhoku tsunami destroyed or damaged over 300 ports and harbors in Japan, disrupting supply chains and causing major economic losses.
• Buildings and transportation: Waves and debris impact can flatten structures not designed for tsunami loads. The 2004 Indian Ocean tsunami destroyed or damaged over 570,000 houses in Indonesia alone.
• Tsunami deposits: Sand, silt, and debris carried inland by the waves get left behind when the water recedes, burying structures and agricultural land. Deposits from the 2011 Tōhoku tsunami were found up to 4.5 km inland.

Secondary hazards often compound the destruction:

• Fires from ruptured gas lines or damaged electrical systems
• Hazardous material spills from industrial facilities
• Contamination of freshwater supplies by saltwater and debris
• The most dramatic secondary disaster from the 2011 Tōhoku tsunami was the Fukushima Daiichi nuclear accident, which resulted from tsunami flooding that knocked out the plant's cooling systems. This led to the release of radioactive materials and the evacuation of over 100,000 people.