A tsunami is a huge wave generated by changes in the seafloor caused by an earthquake. It has properties completely different from ordinary wind waves.
Basic Knowledge
A tsunami is a wave generated when the seafloor suddenly shifts because of an undersea earthquake, underwater landslide, underwater volcanic eruption, or another event, causing the entire body of seawater to be pushed upward. While wind waves move only the water near the surface, a tsunami moves the entire body of seawater from the seafloor to the surface.
Tsunami speed depends on water depth: V ≈ √(gd) (g: gravitational acceleration9.8 m/s², d: water depth)
・Open ocean at a water depth of 4000 m: V = √(9.8×4000) ≈ 198 m/s ≈ approximately 713 km/h(about the speed of a jet)
・Coast at a water depth of 10 m: V = √(9.8×10) ≈ 9.9 m/s ≈ approximately 36 km/h(faster than a person can run)
Tsunamis are fast in the open ocean, but their wave height is only several tens of cm〜1m, making them difficult to notice. Near the coast, they slow down and their wave height increases(shallow-water transformation), becoming a high tsunami.
Tsunamis have wavelengths of several tens to several hundreds of km, and they do not arrive as just one wave; they arrive repeatedly over several hours(a drawdown may arrive first). The first wave is not necessarily the largest.
Tsunami（A wave produced by the transmission of displacement of the entire body of seawater caused by seafloor movement）
Shallow-water transformation（The phenomenon in which a tsunami slows and its wave height increases as the water becomes shallower）
Run-up height（The maximum height reached by a tsunami as it runs up the land）
Tsunami warning・Major tsunami warning（A warning for an expected wave height of 1m or more; a major tsunami warning for more than 3m）
Drawdown（The phenomenon in which seawater recedes offshore before a tsunami arrives. A dangerous warning sign）
In Depth
During the 2011 Tohoku earthquake and tsunami, a maximum run-up height of approximately 40.1 m(Ayari Bay, Ofunato City, Iwate Prefecture) was recorded. The wave height increased so much because the topography of the ria coast concentrated the tsunami's energy.
For early tsunami warnings, data from pressure gauges (seafloor tsunami meters) installed on the seafloor are used in real time. In Japan, the Dense Oceanfloor Network for Earthquakes and Tsunamis (DONET) has been installed from offshore of the Kii Peninsula to offshore of Shikoku and is used for early detection of Nankai Trough earthquakes.
Historically, the tsunami from the 1960 Chile earthquake (M9.5) crossed the Pacific Ocean and reached Japan approximately 22 hours later, causing 142 deaths. This is a typical example of a distant tsunami.
- Tsunamis=the entire body of seawater moves (wind waves affect only the surface)
- They are fast in the open ocean (approximately 700 km/h), slow in shallow water, and increase in height
- V ≈ √(gd): speed is proportional to the square root of water depth
- The first wave is not necessarily the largest
- A drawdown is also a dangerous warning sign
- If you feel an earthquake, evacuate immediately to higher ground
Points to Note
① A drawdown may arrive first, but a wave of rising water may also arrive first. Do not feel safe just because a drawdown has arrived. ② Even if the wave height is low in the open ocean, it becomes large near the coast (shallow-water transformation), so do not underestimate it as “small.” ③ A distant tsunami may arrive even if you do not feel an earthquake, so evacuate to higher ground when a tsunami warning is issued.
Practice
- Give the approximate speed of a tsunami in the open ocean at a water depth of 4000 m (km/h).
- What is the phenomenon in which a tsunami's wave height increases as it approaches the coast called?
- Explain why one must not decide that “everything is safe because a drawdown has arrived” when a tsunami occurs.