Waves and Tsunamis

Sculpting seawater into crested shapes, waves move energy from one area to another. Waves located on the ocean’s surface are commonly caused by wind transferring its energy to the water, and big waves, or swells, can travel over long distances.

• When waves crash onshore they can make a significant impact to the landscape by shifting entire islands of sand and carving out rocky coastlines.
• Storm waves can even move boulders the size of cars above the high tide line, leaving a massive boulder hundreds of feet inland.
• Until recently, scientists attributed the placement of these rogue boulders to past tsunami damage, however, a 2018 study upended this notion by carefully recording the movement of boulders along a swath of rocky coastline in Ireland over a time period in which no tsunamis occurred.
• In addition to over 1,000 mid-sized boulders, many reaching over 100 tons in weight, scientists recorded the movement of a 620-ton boulder (the same weight as 90 full-sized African elephants), showing that storm waves moved it over 8 feet (2.5 meters) in just one winter.

Anatomy of A Wave

• A wave forms in a series of crests and troughs.
• The crests are the peak heights of the wave and the troughs are the lowest valleys.
• A wave is described by its wavelength (or the distance between two sequential crests or two sequential troughs), the wave period (or the time it takes a wave to travel the wavelength), and the wave frequency (the number of wave crests that pass by a fixed location in a given amount of time).
• When a wave travels, it is passing through the water, but the water barely travels, rather it moves in a circular motion.

Wave Formation

Surface Waves

• Waves on the ocean surface are usually formed by wind.
• When wind blows, it transfers the energy through friction.
• The faster the wind, the longer it blows, or the farther it can blow uninterrupted, the bigger the waves. Therefore, a wave’s size depends on wind speed, wind duration, and the area over which the wind is blowing (the fetch).
• This variability leads to waves of all shapes and sizes.
• The smallest categories of waves are ripples, growing less than one foot (.3 m) high.
• The largest waves occur where there are big expanses of open water that wind can affect.
• Places famous for big waves include Waimea Bay in Hawaii, Jaws in Maui, Mavericks in California, Mullaghmore Head in Ireland, and Teahupoo in Tahiti.
• These large wave sites attract surfers, although occasionally, waves get just too big to surf.
• Some of the biggest waves are generated by storms like hurricanes.
• Giant waves don’t just occur near land.
• ‘Rogue waves,‘ which can form during storms, are especially big—there are reports of 112 foot (34 m) and 70 foot (21 m) rogue waves—and can be extremely unpredictable.
• To sailors, they look like walls of water.
• No one knows for sure what causes a rogue wave to appear, but some scientists think that they tend to form when different ocean swells reinforce one another.
• Many of the largest rogue wavesrecorded have been in the North Sea in the North Atlantic Ocean.
• One was recorded by a buoy in 2013 and measured 62.3 feet (19 m) and another nicknamed the Draupner wavewas a massive wall of water 84 feet (25.6 m) high that crossed a natural gas platform on New Year’s Eve, 1995.

Tsunami Waves

• A classic tsunami wave occurs when the tectonic plates beneath the ocean slip during an earthquake.
• The physical shift of the plates force water up and above the average sea level by a few meters.
• This then gets transferred into horizontal energy across the ocean’s surface.
• From a single tectonic plate slip, waves radiate outwards in all directions moving away from the earthquake.
• A tsunami is a set of waves created by a disturbance, likely an earthquake, which reaches the surface of the sea. 
• When a tsunami reaches shore, it begins to slow dramatically from contact with the bottom of the seafloor.
• As the leading part of the wave begins to slow, the remaining wave piles up behind it, causing the height of the wave to increase.
• Though tsunami waves are only a few feet to several meters high as they travel over the deep ocean, it is their speed and long wavelength that cause the change to dramatic heights when they are forced to slow at the shore.

Formation of Tsunamis

• Powerful undersea earthquakes are responsible for most tsunamis. Seismologists say only earthquakes measuring greater than 7.0 on the Richter scale can produce a major tsunami.
• Most earthquakes that generate tsunamis occur in areas called subduction zones, where pieces of the Earth’s crust press against each other. The friction between two slow-moving plates of the Earth’s crust creates vast amounts of seismic energy which is released in the form of an earthquake.
• When a strong undersea earthquake strikes a relatively short distance below the sea floor, it abruptly pushes up one of the immense plates of the Earth’s crust. That suddenly displaces an enormous amount of ocean water which becomes a tsunami, spreading outward in every direction from the epicenter of an earthquake – like ripples on a pond, only on a much larger scale.
• Tsunamis generated in the open ocean appear to be only small waves, but they can grow rapidly in size as they reach shallow water before crashing into seacoast settlements.
• Tsunami waves become dangerous only when they get close to the coast: the height of a Tsunami wave grows larger as the water becomes more and more shallow in a wave shoaling process
• An increase in wave amplitude results in “shoaling” when waves, including tsunamis, run from deep to shallow water. This is significant in coastal regions. This phenomena occurs because of the force from the seabed as it becomes shallower. This slows down the wave: the shallower the water, the slower the wave.
• Even when tsunamis have only a small amplitude (less than a meter) they can shoal up to many meters high as they hit shallow water. When a tsunami hits shallower coastal waters, the trough or base of the wave contacts the beach floor. As a consequence, the leading edge of the tsunami slows dramatically due to the shallower water, but the trailing part of the wave is still moving rapidly in the deeper water.
• The wave is compressed and its velocity slows below 80 kilometers per hour. Its wavelength diminishes to less than 20 kilometers and its amplitude is magnified many times. This piling up of tsunami energy results in growth of the wave height.
• The form of the adjacent geography to deep water (open bays and coastline), can shape the tsunami into a step-like wave with a steep braking face. The wave height as it crashes upon a shore depends almost entirely upon the submarine topography offshore. Steeper shorelines produce higher tsunami waves.
• Because of the factors of low amplitude in deep water and large wavelength, tsunamis are often not noticed in mid-ocean. As the tsunami hits shallower water, the velocity slows, wavelength decreases and the waves height (amplitude) increases.

Areas that are prone to Tsunami:-

• Damage is usually worst in areas closest to the undersea quake, often because the fast-moving waves will hit land so quickly.
• The areas most prone to tsunamis are coastal areas that are near earthquake-prone regions or near areas of undersea volcanoes and face the open ocean.
• Thus, the coast of Japan, south-east Asia and coastal parts of south-east India and the Andaman and Nicobar Islands are prone to tsunamis, also Ring of fire in the pacific.

Tsunami early warning system

Tsunami waves are capable of destroying seaside communities with wave heights that sometimes surpass around 66ft (20 m). Tsunamis have caused over 420,000 deaths since 1850—over 230,000 people were killed by the giant earthquake off Indonesia in 2004, and the damage caused to the Fukushima nuclear reactor in Japan by a tsunami in 2011 continues to wreak havoc. Although tsunamis cannot be predicted in advance when an earthquake occurs, tsunami warnings are broadcast and any waves can be tracked by a global network of buoys – this early warning system is essential because tsunamis can travel at over 400 miles per hour (644 km/hr). The highest tsunami wave reached about 1,720 ft (524 m), a product of a massive earthquake and rockslide. When the wave hit shore, it was said to destroy everything.

• It is made up of two equally important components:-
  • A network of sensors to detect tsunamis
  • A communications infrastructure to issue timely alarms to permit evacuation of the coastal areas.
  • It ismade up of a network of seismic-monitoring stations and sea-level gauges. These detect earthquakes and abnormal changes in sea level and help scientists decide whether a tsunami has been triggered by an earthquake.
• When operating, seismic alerts are used to instigate the watches and warnings; then, data from observed sea level height (either shore-based tide gauges or DARTbuoys) are used to verify the existence of a tsunami.
• Other systems have been proposed to augment the warning procedures; for example, it has been suggested that the duration and frequency content of t-waveenergy (which is earthquake energy trapped in the ocean SOFAR channel) is indicative of an earthquake’s tsunami potential.
• At present, early warning systems rely on measuring the movement of dart buoys which record changes in the sea level or assessing the bottom pressure of the propagating tsunami. The problem is that these systems require the tsunami to physically reach the measurement location.
• With the speed at which tsunami waves travel through open water, no system can protect against a very sudden tsunami, where the coast in question is too close to the epicenter

MeteoTsunamis

• There are also other, usually less destructive tsunami waves caused by weather systems called meteotsunamis.
• These tsunami waves have similar characteristics to the classical earthquake driven tsunamis described above, however they are typically much smaller and focused along smaller regions of the oceans or even Great Lakes.
• Meteotsunamis are often caused by fast moving storm systems and have been measured in several cases at over 6 feet (2 meters) high.
• A 2019 study found that smaller meteotsunami waves strike the east coast of the U.S. more than twenty times a year.