Underwater earthquakes occur when stress builds up in the ocean floor and rocks suddenly break or slip along a fault, releasing energy in the form of seismic waves. Most are caused by the movement of tectonic plates, the enormous pieces of Earth’s outer shell that constantly shift over the planet’s surface. Because much of the world’s active plate boundaries lie beneath the oceans, the seafloor is one of the most important regions for earthquake activity.
These earthquakes can happen thousands of feet below the ocean’s surface, far from any coastline. Some are too small to be felt on land, while others can trigger tsunamis, damage submarine infrastructure, and reshape the seafloor. Understanding why they happen requires looking at the forces that move Earth’s crust, the structure of the ocean floor, and the ways energy travels through rock and water.
How tectonic plates cause underwater earthquakes
Earth’s outermost solid layer consists of the crust and the rigid uppermost part of the mantle. Together, these form the lithosphere, which is divided into tectonic plates. These plates move slowly over the hotter, more deformable material beneath them, generally at rates of millimeters to centimeters per year.
Although the movement is gradual, the boundaries between plates can experience enormous forces. Rocks along these boundaries may become locked together by friction even as the plates continue moving. Stress accumulates in the surrounding rock until the force becomes sufficient to overcome that friction. The rocks then suddenly slip, releasing stored elastic energy as seismic waves.
This sudden release is an earthquake. The point inside Earth where the rupture begins is called the focus, or hypocenter. The point directly above it on the seafloor or Earth’s surface is the epicenter.
Underwater earthquakes follow the same basic physical principles as earthquakes on land. The main difference is their setting: they occur beneath seawater, often along plate boundaries, where the geology and depth of the ocean can influence their consequences.
The three main types of plate boundaries beneath the ocean
The arrangement and movement of tectonic plates determine where many underwater earthquakes occur. Three types of plate boundaries are especially important.
Convergent boundaries
At convergent boundaries, tectonic plates move toward one another. When one plate is forced beneath another and descends into the mantle, the process is called subduction.
Subduction zones are among the most powerful earthquake-producing regions on Earth. The descending plate and the overriding plate can remain locked across large areas of their contact surface. As the plates continue to move, stress builds until sections of the boundary rupture.
These earthquakes can be exceptionally large because the rupture may extend across a vast area. Many occur beneath the ocean near deep-sea trenches, where one plate bends downward into the mantle.
If a large rupture causes substantial vertical displacement of the seafloor, it can push or pull the overlying water. This movement may generate a tsunami, a series of waves that can travel across entire ocean basins.
Not every subduction-zone earthquake produces a tsunami. The outcome depends on factors such as the earthquake’s size, depth, location, rupture direction, and the amount and pattern of seafloor displacement.
Divergent boundaries
At divergent boundaries, tectonic plates move apart. These boundaries are common along mid-ocean ridges, extensive underwater mountain systems that run through the world’s oceans.
As plates separate, hot mantle material rises beneath the gap. Some of this material melts, producing magma that cools and forms new oceanic crust. This process, known as seafloor spreading, gradually creates new sections of the ocean floor.
Earthquakes occur as the crust stretches, breaks along faults, and adjusts to the movement of magma. These events are generally smaller than the largest earthquakes at subduction zones, although their intensity varies with local conditions.
Divergent boundaries also contain volcanic systems. Magma moving through cracks can cause earthquakes, while earthquakes can alter the pathways through which magma travels. The two processes are closely related, but they are not identical.
Transform boundaries
At transform boundaries, tectonic plates slide horizontally past one another. Friction can lock sections of the boundary until accumulated stress causes sudden sideways movement.
These boundaries commonly connect segments of mid-ocean ridges, although transform faults also occur in other tectonic settings. They can produce shallow earthquakes that release substantial energy, even when there is little vertical movement of the seafloor.
Because transform earthquakes often involve mainly horizontal displacement, they are generally less likely to generate major tsunamis than large earthquakes that cause significant vertical seafloor movement. However, tsunami potential depends on the full geometry of the rupture and any associated vertical displacement or underwater slope failure.
Why underwater earthquakes happen even far from plate boundaries
Most significant underwater earthquakes are associated with plate boundaries, but earthquakes can also occur within tectonic plates. These are called intraplate earthquakes.
The interior of a plate may appear relatively stable, yet it can contain old faults, fractures, and zones of weakened rock. Stresses transmitted from distant plate boundaries, changes in the distribution of forces within the plate, or adjustments to earlier geological structures can reactivate these weaknesses.
Some underwater earthquakes also occur along fracture zones, which are long geological features associated with the movement of oceanic plates. Although many fracture zones preserve evidence of ancient tectonic activity, some sections can remain mechanically active.
Earthquakes can also occur in volcanic regions where magma moves through the crust. As magma pushes into surrounding rock, it changes local stresses and may open or reactivate fractures. The resulting seismic activity can accompany volcanic unrest beneath the seafloor.
These different mechanisms explain why underwater earthquakes are not confined to a narrow set of trenches or ridges. The ocean floor contains a complex network of faults, volcanic structures, and older geological features that can respond to changing stresses.
How stress builds up and suddenly releases
The slow movement of tectonic plates does not usually produce a smooth, continuous slide along every fault. Instead, friction and the physical properties of rocks can cause sections of a fault to alternate between periods of relative stability and sudden movement.
When a fault is locked, surrounding rocks gradually deform as tectonic forces continue to act on them. This deformation stores elastic energy, much like a bent spring. Eventually, the stress may exceed the fault’s resistance to slipping, and the fault ruptures.
The rupture can spread along the fault, sometimes traveling several miles or more. Different sections may slip by different amounts, depending on the fault’s geometry, the stresses acting on it, and the strength and frictional properties of the surrounding rocks.
The released energy travels through Earth as seismic waves. Some waves move through the interior of the planet, while others travel along its surface or through the oceanic crust. Instruments called seismometers detect these waves and help scientists determine where an earthquake occurred, how deep it was, and how large it was.
Faults do not all behave in the same way. Some remain locked for long periods before producing large earthquakes. Others experience frequent small slips, and some move slowly in episodes known as slow-slip events. The timing of a future earthquake on any particular fault remains difficult to predict.
The role of underwater volcanoes and magma
Volcanic activity is another important source of seismic activity beneath the ocean. Many submarine volcanoes occur along mid-ocean ridges and above subduction zones, where geological processes supply magma to the crust.
As magma rises, it can force open fractures or push against surrounding rock. These movements produce small earthquakes known as volcanic earthquakes. Changes in pressure within magma chambers, the movement of volcanic fluids, and the collapse of unstable material can also generate seismic signals.
However, an earthquake near an underwater volcano does not automatically mean that an eruption is imminent. Tectonic faults and volcanic systems can produce similar seismic signals, and scientists must interpret earthquake patterns alongside other observations, such as changes in seafloor elevation, volcanic gas emissions, and local heat flow.
Some underwater volcanic eruptions occur with little warning detectable by available instruments. Others are preceded by increasing seismic activity as magma moves through the crust. The relationship varies from one volcanic system to another.
Earthquakes associated with submarine volcanoes can affect the seafloor locally, and an eruption or collapse may disturb the surrounding water. Nevertheless, most underwater earthquakes are tectonic rather than directly volcanic.
How the ocean influences an earthquake’s effects
Water does not prevent an earthquake from occurring. The main forces responsible for tectonic earthquakes operate within solid rock, far below the ocean’s surface.
The ocean does, however, influence how an underwater earthquake affects its surroundings. Seismic energy can travel through the seafloor and water, while the pressure of the overlying water affects the conditions under which sediment and rock deform. The depth of the earthquake, the shape of the seafloor, and the thickness and properties of the sediments all influence the resulting signals and hazards.
An earthquake may shake the ocean floor without producing noticeable movement at the surface of the water. This is because the passage of seismic waves through the Earth does not necessarily displace a large volume of water vertically.
Tsunamis require a different process: the displacement of water over a sufficiently large area. Large earthquakes can produce that displacement when they lift or lower the seafloor, particularly along shallow subduction zones. Underwater landslides can also displace water and generate tsunamis, sometimes with little contribution from an earthquake’s direct movement of the seafloor.
Water depth affects how a tsunami travels, but the waves behave differently from ordinary wind-generated waves. In deep water, a tsunami can move rapidly while its height remains relatively small. As it enters shallower coastal waters, its speed decreases and its height may increase, creating dangerous flooding and strong currents.
Why some underwater earthquakes are more dangerous than others
The magnitude of an earthquake describes its overall size, based on the energy released during the rupture. But magnitude alone does not determine the consequences.
An earthquake’s depth matters. Shallow earthquakes can produce stronger shaking at the nearby seafloor and may be more effective at displacing the seabed. Deep earthquakes can be powerful, but their effects on the seafloor and the ocean above them differ from those of shallow events.
The location and direction of movement also matter. A large earthquake on a fault that produces little vertical seafloor displacement may generate a smaller tsunami than a somewhat different rupture that lifts or lowers a broad area of the seabed.
The stability of the ocean floor is another factor. Earthquake shaking can destabilize steep underwater slopes or loose sediments, triggering submarine landslides. These failures can damage cables and pipelines, bury habitats, and sometimes generate additional waves.
Distance from populated coastlines affects human risk as well. A major earthquake in a remote part of the ocean may cause little direct damage to people, while an earthquake closer to a densely populated coast can leave much less time for warnings and evacuation.
Underwater earthquakes can also damage submarine communication cables. These fiber-optic cables carry much of the world’s international digital traffic, and disruptions can affect communications between regions. Damage may result from direct ground movement, seafloor deformation, or landslides triggered by shaking.
How scientists detect earthquakes beneath the ocean
Detecting underwater earthquakes is more challenging than monitoring many earthquakes on land because ocean-bottom instruments are expensive to deploy, difficult to maintain, and spread across a vast area.
Scientists rely on networks of seismometers on land and, where available, instruments installed on the ocean floor. Seismic waves from a submarine earthquake can travel long distances through Earth, allowing land-based stations to detect events occurring far offshore.
By comparing the arrival times of seismic waves at multiple stations, researchers can estimate an earthquake’s location and depth. The pattern and strength of the recorded waves help them calculate its magnitude and study the type of fault movement involved.
Ocean-bottom seismometers provide more direct measurements in areas that are poorly covered by land-based networks. Some underwater observatories also monitor changes in pressure, temperature, and seafloor movement, offering additional information about tectonic and volcanic processes.
Scientists use these measurements to map active faults, study how stress accumulates, and understand how earthquakes rupture. They also combine seismic observations with satellite measurements of ground deformation, geological surveys, and records of past earthquakes and tsunamis.
For tsunami monitoring, seafloor pressure sensors can detect changes associated with passing tsunami waves. Buoys and coastal tide gauges provide additional observations that help determine whether a potentially dangerous wave has formed and how it is progressing.
Although monitoring systems have improved the detection of underwater earthquakes and the ability to issue tsunami warnings, they cannot reliably predict the exact time, location, and magnitude of a future earthquake.
What underwater earthquakes reveal about Earth’s interior
Underwater earthquakes are not merely hazards. They provide valuable evidence about how Earth works.
The locations of earthquakes reveal where tectonic plates meet and how they move. Their depths help scientists trace the paths of descending plates at subduction zones. The seismic waves they produce also travel through Earth’s interior, changing speed and direction as they encounter different materials.
By studying these waves, researchers can investigate the structure of the crust and mantle, including variations in rock composition, temperature, and physical properties. Earthquake patterns also help identify faults that may be difficult to observe directly beneath thick sediments or deep water.
Seafloor spreading and the distribution of submarine earthquakes provide evidence that Earth’s surface is continually renewed and rearranged. New crust forms at mid-ocean ridges, older oceanic crust is recycled into the mantle at subduction zones, and transform faults accommodate sideways plate movement.
These processes operate over geological timescales, but earthquakes reveal that change does not always happen gradually. Long periods of accumulated stress can be interrupted by sudden ruptures that release energy in seconds or minutes.
Underwater earthquakes are therefore a direct expression of Earth’s dynamic interior. They occur because the planet’s outer shell is divided into moving plates, because rocks respond to stress, and because faults sometimes release stored energy abruptly. Their consequences depend on how that energy is released, how the seafloor moves, and how the surrounding ocean and coastal environment respond.