Final answer:
Tsunamis are destructive sea waves caused by underground seismic activities that can travel at high speeds and grow in height as they reach shallow waters, often resulting in significant human and environmental impact, as seen in the 2004 Indian Ocean tsunami and the 2011 Japan tsunami.
Step-by-step explanation:
Tsunamis, commonly referred to as "tidal waves," are powerful and often devastating sea waves triggered by seismic activities such as earthquakes, landslides, or volcanic eruptions underneath the ocean floor. These waves can travel at incredible speeds of up to 700 km/h in deep water, where they may appear as insignificant swells less than 30 cm high. However, as they approach shallower coastal waters, tsunamis slow down, and their energy is compressed upwards, drastically increasing their height up to 30 meters or more, with wavelengths potentially reaching 100 km.
The lethality of tsunamis was starkly demonstrated in the catastrophic 2004 Indian Ocean tsunami, caused by an earthquake with the energy of thousands of atomic bombs. It resulted in waves that ravaged the coastlines of 11 countries, claiming over 150,000 lives initially and culminating in a final death toll of 283,000. Similarly, the 2011 earthquake off Japan's east coast spawned a tsunami that caused additional destruction, particularly to nuclear energy facilities, which heightened the disaster's impact by exposing large areas to radiation risks.