800 km/h
Explanation
Tsunamis are large ocean waves caused by the sudden displacement of a large volume of water, typically due to undersea earthquakes or volcanic eruptions. The physical characteristics of a tsunami, such as speed and wave height, are determined primarily by the depth of the ocean water.
Analysis:
- Speed in Deep Ocean: In the open ocean, where the water is deep, tsunami waves travel at extremely high speeds. The speed is calculated using the formula $v = \sqrt{g \cdot d}$, where $g$ is gravity and $d$ is depth. Consequently, in deep water, speeds can reach approximately 800 km/h, which is comparable to the speed of a jet aircraft.
- The 2004 Event: The 2004 Indian Ocean Tsunami, triggered by a massive earthquake off the coast of Sumatra, traveled across the deep ocean at speeds ranging between 700 km/h and 800 km/h.
- Shoaling Effect: While the speed is high in the open ocean, the wave amplitude (height) is very low (often less than 1 meter). As the tsunami approaches the coast and water depth decreases, friction slows the wave down (to roughly 30–50 km/h), causing the wave height to increase dramatically.
Key Takeaway:
Tsunami waves travel at speeds of approximately 800 km/h in the deep open ocean with low wave height, but slow down significantly while increasing in height as they enter shallow coastal waters.