What are the types of tsunami alerts? What are the intensity scales?

New Delhi: A magnitude 8.8 struck Russia’s Far East early Wednesday, triggering a tsunami in the northern Pacific. Warnings have been sounded in Japan, Alaska, Hawaii, New Zealand and several other islands in the Pacific.

The Japan Meteorological Agency reported that a tsunami measuring 40 centimeters (1.3 feet) was recorded in Tokachi, on the southern coast of Hokkaido, Japan’s northernmost main island.

The Oregon Department of Emergency Management said that small tsunami waves, between 1 and 2 feet (30 to 60 centimeters) high, were expected to reach the coast starting around 11:40 pm local time.

Philippine officials have warned coastal provinces facing the Pacific about potential tsunami waves of under 1 meter (3 feet).

This is considered the strongest earthquake in the world since the 9.0-magnitude quake that hit Japan in March 2011, which caused a deadly tsunami and nuclear disaster. Only a few earthquakes in history have been stronger. It was also the most powerful quake in Russia’s Kamchatka Peninsula since 1952, according to the Geophysical Survey of the Russian Academy of Sciences.

When a disturbance occurs in the ocean, Tsunami Warning Centres carry out a quick analysis it and issue alerts based on its potential to trigger tsunamis and the forecasted height of the waves.

Once the level of threat is assessed, these alerts and warnings may be upgraded, downgraded, or cancelled based on the changing situation. Therefore, it is very important to stay alert until authorities officially declare that the tsunami threat is over.

Tsunami information statement: First, when a disturbance in the ocean is detected, a tsunami information statement is issued. This is issued to let emergency officials and the public know that an earthquake has occurred or that a tsunami watch, advisory, or warning has been issued for another part of the ocean.

Tsunami watch: It is issued well in advance to alert emergency officials and the public about an event that could later impact the area. The warning is essentially based on earthquake data, even if there’s no confirmation yet of a destructive tsunami.

This can later be upgraded to a warning or advisory, or simply cancelled. This will depend on new information and analysis. Officials and the public should be on their toes and fully prepared to take action if required.

Tsunami advisory: This is issued when a tsunami is imminent, expected, or already happening and has the potential to cause strong currents or dangerous waves near the shore. The threat may persist for several hours after initial arrival, but significant flooding is not expected. This advisory is usually updated to be extended, to adjust affected areas, to be upgraded to a warning, or to be cancelled.

Tsunami warning: This is issued at a time when a tsunami with the potential to cause widespread inundation is imminent, expected, or happening. Dangerous coastal inundation and strong currents are possible and could last for several hours.

For earliest possible warning, this is issued mostly based only on seismic information. Afterwards, it can be updated, changed geographically, downgraded, or cancelled.

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According to UNESCO Tsunami Programme, "this scale incorporates 12 divisions – consistent with the 12-grade seismic intensity scales – arranged according to the effects on humans, nature, and objects, including vessels of variable size, and on buildings and other engineered constructions."

As per the UNESCO Tsunami Programme, the scales based on the impact include: 1. Not Felt (not felt even under the most favourable circumstances), 2. Scarcely Felt (Felt by few people on board in small vessels but not observed in the coast); 3. Weak (Felt by most people on board in small vessels. Observed by few people in the coast); 4. Largely Observed (Felt by everyone on board in small vessels and by few people on board in large vessels. Observed by most people in the coast); 5. Strong (Felt by everyone on board in large vessels and observed by everyone in the coast); 6. Slightly Damaging (Most small vessels move violently onshore, or crash strongly into each other, flooding in a few wooden structures; 7. Damaging (Many wooden structures damaged, few are demolished or washed away); 8. Heavily Damaging (All people escape to higher ground, a few are washed away, extensive flooding; 9. Destructive (many people are washed away); 10. Very Destructive (Complete damage in many masonry buildings, artificial embankments collapse, port water breaks damaged; 11. Devastating (Many masonry buildings are destroyed; 12. Completely Devastating (Practically all masonry buildings demolished. Most reinforced concrete buildings suffer at least heavy damage).