The simple answer to whether a true tsunami can occur in a lake is no, as the term specifically refers to seismically generated oceanic waves. However, inland bodies of water can experience extremely large, destructive, and fast-moving waves that behave similarly to their oceanic counterparts near the shore. These high-energy events are generated by different mechanisms than deep-ocean earthquakes, leading scientists to use more precise terminology, often calling them impulse waves.
Physical Differences Between Ocean and Lake Waves
True oceanic tsunamis are defined as shallow-water waves regardless of ocean depth, based on their extremely long wavelengths. These wavelengths can span hundreds of kilometers in the open ocean, allowing the wave to interact with the entire water column. The speed of a true tsunami is proportional to the square root of the water depth, enabling it to travel across deep ocean basins at speeds comparable to a jet airliner.
Lakes are significantly smaller and shallower than oceans, meaning they cannot support the vast wavelengths necessary for a true tsunami’s global propagation. The confined nature of a lake fundamentally changes how any wave event behaves and dissipates energy. This small scale means lake impulse waves generally have much shorter wavelengths and different propagation patterns than trans-oceanic tsunamis.
Inland water bodies often experience standing waves called seiches, which are oscillations of the entire water body caused by sustained winds or atmospheric pressure changes. Seiches are characterized by long periods of oscillation, often exceeding three hours. Impulse waves, which are the focus of inland hazard assessment, are distinct because they are generated by a single, rapid displacement of mass.
Triggers Unique to Inland Water Bodies
The most common and powerful trigger for destructive lake waves is the rapid movement of earthen material into the water. These events, broadly categorized as landslides, displace a massive volume of water instantaneously, generating a powerful impulse wave. The energy of the resulting wave is directly proportional to both the volume and the velocity of the material entering the lake.
Landslides can be sub-aerial, meaning the material falls from above the water line, such as a large rockfall from a steep cliff face. Alternatively, they can be sub-aqueous, involving the collapse of unstable sediment beds beneath the lake surface. Sub-aqueous slides are particularly hazardous because they can be triggered by seemingly minor events and are often undetected until the wave forms.
Localized seismic activity can still play a role. Earthquakes that may not be strong enough to cause a widespread tsunami can easily destabilize slopes and saturated sediments surrounding or beneath a lake. This localized ground shaking then acts as the immediate trigger for a destructive landslide, which subsequently generates the impulse wave.
Other Impulse Wave Triggers
Other high-energy events contributing to impulse waves include glacial calving, where large ice masses break off in high-latitude mountain lakes. Human activities, such as the failure of mining tailings or reservoir dams, also represent a catastrophic and rapid displacement of water mass. These non-natural triggers must be included in risk assessments for engineered water bodies.
A classic, albeit extreme, example occurred in Lituya Bay, Alaska, where a massive rockfall generated a wave that reached an incredible run-up height of 524 meters. Although Lituya Bay is a fjord, the physics of the impulse wave generated by the rockfall are identical to what can happen in a steep-sided lake. This event demonstrates the immense destructive potential inherent in rapid mass displacement into a confined water body.
Characteristics of High-Energy Lake Waves
High-energy lake waves can exhibit extreme localized run-up, which is the vertical height the water reaches on the shoreline. Since the wave has less volume to spread across, the energy is concentrated, often resulting in wave heights many times greater than typical wind waves. In modeled scenarios for mountain lakes, maximum wave crest elevations have been calculated to range from 5.9 to 29 meters along the landslide’s travel direction.
Unlike oceanic tsunamis that can travel across an entire ocean basin, lake impulse waves dissipate energy much more rapidly. The waves are constantly interacting with the lakebed and shoreline due to the relatively shallow and confined nature of the water body. This friction and reflection against the boundaries cause the destructive energy to diminish quickly over short distances, a process dependent on the lake’s specific geometry.
The damage caused by these events is typically highly localized, focusing on the immediate vicinity of the triggering event and the shoreline directly opposite. Areas adjacent to the impact zone often experience little more than a strong surge or fluctuation in water level. This specific pattern of destruction emphasizes the importance of hazard mapping focused on steep slopes and unstable shorelines.
After the initial massive wave, the displacement event often creates a series of subsequent, smaller waves, known as a wave train. While these secondary waves are significantly less powerful than the first impulse, they can still pose a danger to boats and infrastructure. The entire high-energy event usually concludes within minutes to hours, unlike oceanic tsunamis which can involve hours of surging activity.