New Delhi:
A strong earthquake with a preliminary magnitude of 8.6 hit waters off westernmost Aceh province in Indonesia today.
The U.S. Geological Survey said that the quake was centered 20 miles (33 kilometers) beneath the ocean floor, around 308 miles (495 kilometers) from the provincial capital of Banda Aceh.
A tsunami warning was issued after the quake was felt across the Asia Pacific region.
Here are the ten most frequently asked questions on tsunami. (Information source: Pacific Tsunami Warning Centre.)
1. What is a tsunami?
A tsunami is a series of large waves of extremely long wavelength and period caused by the displacement of a large volume of a body of water, typically an ocean or a large lake, earthquakes, volcanic eruptions and other underwater explosions. Tsunami waves in deep oceans can travel at high speeds for long periods of time for distances of thousands of kilometres and lose very little energy in the process. The deeper the water, the greater the speed of tsunami waves will be. For example, at the deepest ocean depths the tsunami wave speed will be as much as 800 km/h, about the same as that of a jet aircraft.
2. How are tsunami waves different from tidal waves?
Tsunami waves can be generated by disturbances such as volcanic eruptions or underwater landslides. However, astronomical tides are caused by the gravitational influences of the moon, sun and planets.
3. What does the word tsunami mean?
The word tsunami comes from the Japanese words for harbour ("tsu") and wave ("nami").
4. Which has been the worst tsunami in recent times?
A giant 9.1-magnitude quake off on December 26, 2004, triggered a tsunami in the Indian Ocean that killed 230,000 people in 13 Indian Ocean countries, including Thailand, Sri Lanka and India.
5. Can tsunamis be predicted?
Since earthquakes cannot be predicted, there is no way to know when a tsunami will happen before the earthquake occurs. Once a large potentially tsunamigenic earthquake does occur, we can forecast tsunami arrival times and wave heights through the use of computer modeling.
6. With what speed do tsunami waves travel?
In the open ocean, tsunami waves travel at 600 kmph to 700 kmph. In the deep ocean, waves from a large tsunami may be as little at 60 cm high. As they encounter shallow water they slow down to about 30 kmph and increase in height.
7. How long do tsunamis last when they happen?
Since a tsunami consists of a series of waves, the danger can last for many hours. Large tsunamis are generally recorded for a few days following the event.
8. What is the difference between a tsunami alert and a tsunami warning?
As per international standards, a tsunami alert is issued automatically for any earthquake with magnitude 7.0 or 7.5, provided the epicentre is in an area capable of generating a tsunami, in this case off Aceh.
A tsunami warning is issued if a tsunami has been generated, or if an earthquake powerful enough to generate one occurs nearby.
9. Why are locally generated tsunamis so dangerous?
Locally generated tsunamis can reach nearby shores in under ten minutes, which might not be sufficient for authorities to issue a warning.
10. What does a tsunami look like when it reaches the shore?
A tsunami wave looks and acts like a giant river of water on top of the ocean that floods the shore. Tsunami waves are bigger than tidal waves.