The Earth’s outer shell is not a single, solid piece, but rather a collection of large, moving segments called tectonic plates. Earthquakes occur at the boundaries, known as faults, when built-up stress is suddenly released. A tsunami is a series of long ocean waves caused by a large-scale disturbance that displaces a massive volume of water. Since most large earthquakes occur under the ocean, it is often questioned why powerful undersea quakes on transform faults rarely result in a devastating tsunami.
How Transform Faults Move
Transform faults, also known as strike-slip faults, are boundaries where two tectonic plates slide horizontally past one another. The plates move in a side-by-side motion, neither pulling apart nor pushing together. The San Andreas Fault in California is the most well-known example of a transform boundary on land, but these faults are also numerous on the ocean floor, often connecting segments of mid-ocean ridges. The motion is primarily lateral, meaning the displacement of the seafloor during an earthquake is almost entirely horizontal. This characteristic side-to-side movement establishes why these earthquakes do not generate ocean-spanning waves.
The Essential Condition for Generating a Tsunami
Generating a destructive, deep-ocean tsunami requires the rapid, permanent, and vertical displacement of the seafloor. This process must lift or drop the entire column of water above the rupture zone across an area that can span hundreds of kilometers. When the seafloor moves upward, it pushes the overlying water, creating a bulge that collapses under gravity and forms the initial wave series. This massive vertical shift disturbs the ocean’s equilibrium enough to propagate a powerful wave train across an entire ocean basin. This action is most effectively achieved at subduction zones, where one tectonic plate is forced beneath another and springs upward during a megathrust earthquake.
Why Horizontal Slip Does Not Displace the Water Column
The energy released by an earthquake must translate into a sustained vertical shift of the water column to form a true tsunami. Because the motion along a transform fault is overwhelmingly horizontal, the ocean floor is simply sheared laterally instead of being lifted or dropped. While intense shaking can certainly disturb the water, this motion is more akin to stirring the water than permanently displacing it. The horizontal movement does not create the necessary large-scale, sustained uplift or subsidence of the seabed required to generate a massive wave. Therefore, the side-to-side slip effectively releases tectonic strain without creating the widespread vertical offset that is the hallmark of a tsunamigenic event.
Secondary Ways Transform Earthquakes Can Cause Coastal Hazards
Although transform fault earthquakes do not generate tectonic tsunamis, they still pose a significant, localized threat to coastal communities. The intense ground shaking produced by a large strike-slip event can destabilize steep underwater slopes, triggering massive submarine landslides. If a large mass of sediment or rock slumps rapidly into a bay or off a continental shelf, it displaces a significant volume of water, generating a highly destructive wave often referred to as a local tsunami. These waves are focused and dangerous only near the landslide source, unlike the ocean-wide reach of a tectonic tsunami. Furthermore, in areas where a transform fault is not perfectly straight, the lateral movement can create localized zones of compression or extension (bends) that introduce minor vertical motion, potentially generating a small, localized wave near a coast.