About Tsunami

The Indian National Centre for Ocean Information Services (INCOIS)provides round-the-clock monitoring and warning services for the coastal population on tsunamis, storm surges, and high waves through the in-house Indian Tsunami Early Warning Centre (ITEWC). ITEWC has also been designated as the Regional Tsunami Service Provider (RTSP) to provide tsunami warnings to countries on the Indian Ocean Rim by the Intergovernmental Oceanographic Commission (IOC) of UNESCO.

When an earthquake occurs in a Tsunami genic source and if the magnitude is more than 6.5 and the depth is less than 100 km, INCOIS automatically starts issuing a real-time tsunami warning. The information on the occurrence of a tsunami genic earthquake is also transmitted to the State Government. The levels of disaster can be confirmed based on the real time wave buoy and Bottom Pressure Recorder (BPR) data from the EWS. An earthquake with magnitude of 6.5 to 7.0 has lesser potential to generate Tsunami and earthquakes with magnitude  more than 7.1 can generate large tsunami.

Nature of Advisories issued by INCOIS

Real-time tsunami warnings are issued by INCOIS by first announcing the area of warning, alert and watch based on travel time together with an estimate of the height of the tsunami calculated on pre-run numerical models of tsunami propagations. Based on the hazard – ‘warning’, ‘alert’ and ‘watch’ are issued.

a) Area under Warning:

Area that is within 60 minutes from the tsunami generic sources and wave height expected is more than 2 meters.

b) Area under Alert:

Area within 60 minutes travel time of the tsunami and wave height is less than 2m and Area more than 60 minutes travel time of tsunami and the expected wave height is more than 2 m.

 c) Area under Watch:

Areas that are outside the 60 minutes travel time of tsunami and are kept under watch when the wave height is expected to be less than 2 m.

The status of hazard is modified according to the travel time. The expected wave height is also modified by the pressure and wave height data from the BPR and Wave buoy on real-time basis. Based on the data from BPR and the tsunami wave buoy, if the passage of tsunami is not detected or even if it passes and  the wave was insignificant and may not cause destruction even on a minor scale, the warning is cancelled and “A Tsunami All Clear Bulletin” is issued. The All Clear bulletin is also issued when all the waves, picked up by the tsunami wave buoy, reach the coast with or without destruction. The status of hazard will be modified according to the travel time. The expected wave height is also modified by the pressure and wave height data from the BPR and Wave buoy on real-time basis.

Any Tsunami originating in the Banda ache, Sumatra (Andaman – Sunda sub plate) will take more than 60 minutes to reach Tamil Nadu. Hence, the alert is first issued and there is time for action. However, for regions like Andaman and Nicobar the time interval is very limited.

INCOIS and issue of bulletins

Over a period of four to five hours as many as six bulletins may be issued by INCOIS.

The First Bulletin (Type – I) is issued before the lapse of 20 minutes of the earthquake when the seismic network detects an earthquake occurring in the Andaman Sumatra Subduction Zone (ASSZ). The First bulletin informs that an earthquake has occurred and the preliminary estimates of the location of the epicentre, magnitude, depth of focus and time are informed.

The Second Bulletin (Type – II) is issued not later than 30 minutes of the earthquake. If the earthquake is in the subduction zone and has a magnitude of more than 6.3 and the depth of focus is less than 100 km, the bulletin is issued. The bulletin identifies the areas under warning and threat based on pre-run numerical models from which the expected wave height is estimated.

The Third Bulletin (Type – II Supplementary) is issued with updated earthquake parameters and revised information on tsunami wave height and accordingly the status of the threat is updated.

The Fourth Bulletin (Type III) is issued when the tsunami passes the EWS instrumental setup i.e. BPR. Based on the water level and pressure data, the origin of the tsunami is confirmed in the Fourth Bulletin. The sea level data is crucial for confirming whether a tsunami has originated or not. The IV Bulletin is very important for regions like Tamil Nadu which are far from the tsunami genic source. If the threat is upgraded to warning status evacuation will have to be started immediately.

The Fifth Bulletin (Type – III Supplementary) is issued when the tsunami reaches the coast with hourly updates and also whenever real-time water level information are available. The run-up is recorded when the tsunami reaches the near source regions and the magnitude of the tsunami is confirmed. If the run-up is insignificant in the near source regions, the warning is downgraded. When the earthquake is strike slip, without vertical displacement, it will not generate a significant tsunami. The threat is PASSED for individual zones.

The Final – “ALL CLEAR” bulletin is  issued when water level from multiple gauges confirm that a tsunami has not been generated or if the tsunami has formed 120 minutes after the first wave or significant tsunami pass the tide gauges giving provision for subsequent waves.

Notification Phase

The Early Warning System setup by IMD and INCIOS under the Ministry of Earth Sciences, GoI, disseminates the information on the origin of the Cyclone and Tsunami as early as possible. In the case of a Cyclone, the formation of the cyclone may be made known even over 48 hrs before it strikes the coast but for Tsunami the time given is only 90 to 150 minutes. The notification of the hazard should be made taking into consideration the time interval left to prepare.

In addition, an early warning system for earthquake/tsunami in Indian Ocean is operational at Madras Atomic Power Station, Kalpakkam.  This PC-based Earthquake Notification System (ENS) is installed in Control room of MAPS.

Pre – Disaster:

  • An earthquake with serious magnitude anywhere in and around the Bay of Bengal is good reason for the entire coastal region/districts to be on alert for the possibility of a tsunami.
  • All coastal habitations should be warned if a tsunami is to strike. This will require a full-fledged warning system which can be triggered from a central location in each district. The DEOC will need to ensure that this is working during Non – Disaster period.
  • The steps to be followed to mitigate the disaster due to tsunami are similar to those that are followed for that of Cyclones – hence the instructions to mitigate cyclonic effect disasters are relevant and should be followed.
  • Natural Bio-shields will help break the impact of the tsunami.

Since tsunami is a disaster that has very little warning period, it demands the fastest response.