A tsunami is a series of ocean waves generated by the large-scale displacement of a substantial volume of water. Unlike typical wind-driven waves that only affect the ocean’s surface layer, a tsunami involves the entire water column from the surface to the seafloor. This displacement usually happens due to a powerful, sudden event beneath or near the ocean. The resulting waves are characterized by extremely long wavelengths and propagate across entire ocean basins with tremendous speed.
Defining Tsunami Magnitude
Scientists cannot use a single, simple number to define the overall size or magnitude of a tsunami, as its impact changes dramatically between the deep ocean and the coast. Size is often determined by measuring the wave’s effects once it reaches land. One of the most common measurements is the runup, which is the maximum vertical height the water reaches above the normal sea level at the shore.
Another way to quantify a tsunami’s effect is by measuring inundation, which tracks the maximum horizontal distance the water travels inland from the shoreline. These coastal measurements provide an understanding of the destructive potential of the event.
However, for early detection, the wave height must be measured far offshore where the tsunami is still a subtle change in the ocean surface. In the deep ocean, the wave’s amplitude is quite small, often only a few inches high, because it has not yet slowed down and grown in height. Measuring this subtle wave height offshore is crucial for early warnings and is distinct from the runup or inundation values calculated after the wave hits the coast.
The Minimum Threshold for Detection
The smallest tsunamis are those that are barely larger than the ocean’s background noise and are classified as “micro-tsunamis.” Modern technology has allowed scientists to detect events that were previously invisible. The Deep-ocean Assessment and Reporting of Tsunami (DART) buoy systems are the primary tool for this detection, using highly sensitive bottom pressure recorders (BPRs) anchored to the seafloor.
These BPRs are capable of measuring changes in water pressure that correspond to a wave displacement as small as 1 millimeter (mm) in 6,000 meters of water. This 1 mm figure represents the smallest theoretical change in sea level that the latest-generation DART sensors can resolve in the deep ocean environment. This level of sensitivity allows for the recording of even the most minute displacement events.
While the instruments can register changes this small, the operational threshold for triggering a rapid warning alert is set higher to avoid false alarms from random oceanic noise. In many regions, such as the North Pacific, DART systems enter a rapid reporting mode only when they detect a wave amplitude exceeding 3 centimeters (cm) in the deep water. This 3 cm displacement is considered the smallest measurable tsunami event that meets the criteria for classification and real-time reporting.
The smallest recorded tsunamis are often those that barely clear this operational threshold, appearing as a slight ripple of a few centimeters in the deep ocean and causing no noticeable change at the coast. These minimal events confirm that a displacement occurred, but they pose no threat.
Sources of Minimal Tsunami Events
Minimal tsunami events are created by smaller, localized disturbances that are not powerful enough to generate destructive trans-oceanic waves. One common source is a minor seismic slip, where a small movement of the seafloor displaces water in a limited area. These events often result from lower magnitude earthquakes that cause sufficient vertical movement to create a measurable pressure change.
Small, localized submarine landslides can also generate minimal tsunamis, particularly in areas with steep underwater topography. Unlike the massive slides that cause large waves, these smaller slides displace a limited volume of water, creating a wave that quickly dissipates as it moves away from the source.
The meteotsunami is a wave generated by rapid changes in atmospheric pressure. These are not caused by geological activity but by fast-moving severe weather events like squalls or thunderstorms. The drop in air pressure over the water surface pulls the ocean level up slightly, and this bulge moves with the weather system. If the speed and direction of the atmospheric disturbance match the natural wave speed in the water, the wave can be amplified. While some meteotsunamis can reach significant heights, many are localized, barely-measurable oscillations.