Introduction

Tsunamis are powerful, fast-moving waves caused by sudden displacement of large water volumes, typically triggered by tectonic processes in subduction zone settings on the ocean floor.

Causes of Tsunamis

Subduction Zones and Tectonic Activity

Tsunamis are primarily caused by sub-marine earthquakes at convergent plate boundaries, specifically subduction zones. These zones occur where an oceanic plate is forced beneath a continental or another oceanic plate due to plate convergence.

When stress builds along the fault line of a subduction zone, it can be suddenly released as an earthquake. If this earthquake occurs beneath the ocean floor and results in vertical displacement of the seabed, it displaces the overlying water column, generating a tsunami.

Schematic cross-section of a subduction zone illustrating how vertical displacement of the ocean floor above a subducting plate generates tsunami waves by displacing the water column. Source

The most common cause of tsunamis is a shallow-focus, high-magnitude earthquake beneath the ocean. These earthquakes typically have a focal depth of less than 70 km and often occur in the Benioff zone, a dipping zone of earthquake activity along the subducting plate.

The sudden slip along a fault causes the seabed to move vertically, lifting or dropping a large volume of water. This sudden movement initiates a series of waves that travel outward from the point of displacement.

Key Characteristics of Tsunami-Generating Earthquakes:

• Occur at convergent boundaries with reverse or thrust faulting.

• Often exceed magnitude 7.0 on the Moment Magnitude Scale (MMS).

• Result in vertical seabed displacement, rather than horizontal, which is more effective at generating tsunamis.

Mechanisms of Tsunami Formation

Displacement of the Sea Floor

The fundamental mechanism involves the vertical displacement of the ocean floor due to fault movement. This displacement pushes the water column above upward or causes it to fall, disturbing the equilibrium of the sea surface.

This disturbance generates waves that radiate outwards in all directions. The energy transfer from the sea floor to the water is almost instantaneous, allowing tsunamis to travel rapidly across the ocean.

Illustration showing multiple tsunami wave fronts propagating through deep water, including direct wave travel and reflections off the surface and seabed. Source

Water Column Displacement

The size and energy of the tsunami depend on several factors:

• Magnitude of the earthquake – stronger earthquakes displace more water.

• Area of sea floor displaced – larger rupture zones create more extensive wave systems.

• Depth of water above the rupture – deeper water allows waves to grow in wavelength and speed.

• Amount of vertical displacement – more vertical movement results in more energetic tsunamis.

Wave Propagation and Behaviour

After the initial disturbance, the tsunami waves:

• Travel at speeds of 500–800 km/h in deep water.

• Have long wavelengths (often exceeding 100 km).

• Are barely noticeable in deep ocean due to low wave height (under 1 metre).

• Grow rapidly in wave height as they approach the shore due to wave shoaling, where the seabed slows the wave, compressing its energy into a smaller volume.

As waves approach the coastline, the seabed becomes shallower, causing:

• Wave speed to decrease.

• Height to increase dramatically, potentially exceeding 30 metres.

• Wavelength to shorten, making the waves more destructive upon landfall.

Additional Causes and Contributing Factors

Secondary Seismic Effects

While the primary trigger is tectonic movement, other associated seismic processes can also contribute:

• Underwater landslides, often triggered by earthquakes, can displace water and generate tsunamis.

• Volcanic eruptions and caldera collapses beneath the sea may also produce water column disturbances.

However, the syllabus focus remains on tectonic-driven tsunami generation, especially due to earthquakes at subduction zones.

Geographical Factors Affecting Tsunami Impact

• Coastal shape: Bays and estuaries can funnel and amplify tsunami waves.

• Offshore topography: Underwater slopes and ridges can intensify or deflect wave energy.

• Distance from epicentre: Tsunami arrival time varies depending on location. Close regions may have only minutes to respond.

• Warning systems: Regions with effective early warning systems and evacuation plans can greatly reduce casualties.

Summary of the Tsunami Generation Process

Step-by-step tsunami formation process:

• Tectonic stress builds at a subduction zone.

• Sudden fault movement (usually thrust) causes vertical sea floor displacement.

• The overlying water column is disturbed, producing a wave train.

• Waves radiate outward at high speeds across the ocean.

• As waves approach land, shoaling increases their height.

• The waves strike the coast, causing flooding and destruction.

Key Terminology Review

• Subduction zone: Where one plate is forced beneath another.

• Benioff zone: Sloping zone of earthquake activity in subduction areas.

• Vertical displacement: Key driver of tsunami generation.

• Water column displacement: The direct mechanism that forms tsunami waves.

• Wave shoaling: The process that increases wave height near shore.

• Sub-marine earthquake: Underwater seismic event that displaces the sea floor.

Tsunamis are therefore not merely giant waves, but complex geophysical phenomena directly tied to plate tectonic activity, especially at convergent boundaries. Understanding these causes and mechanisms is vital to managing and mitigating the hazards they present.