Decision Support system (DSS) is the set of rules to be followed for issue of Tsunami bulletins. These rules are appropriately coded in the form of software that automatically generates bulletins by accessing the real-time data from the observing network as well as the model scenario database. The Standard Operating Procedure (SOP) is the set of routine activities to be followed by the staff at the tsunami warning centre for Observation, Evaluation, Confirmation, and Dissemination of Bulletins.
The early warning centre continuously monitors the Seismic activity in the two Tsunamigenic source regions of Indian Ocean through the network of national and international seismic stations. This network enables us to detect any tsunamigenic earthquakes within a time period of 10 minutes of occurrence. Water-level data is also monitored continuously from the network of national and international tide gauges and BPRs. This network enables confirmation or cancellation of a tsunami. All the datasets are continuously monitored in the Early Warning Centre using a custom-built software application that generates alarms/alerts in the warning centre whenever a pre-set threshold is crossed. Tsunami bulletins are then generated based on pre-set decision support rules and disseminated to the concerned authorities for action, following a Standard Operating Procedure.
The criteria for generation of different types of advisory bulletin messages (Warning/Alert/Watch) for a particular region of the coast are based on the earthquake parameters, available warning time (i.e. time taken by the tsunami wave to reach the particular coast) and expected run-up from pre-run model scenarios.
The Indian Warning Criteria is based on the premise that coastal areas falling within 60 minutes travel time from a tsunamigenic earthquake source need to be warned based solely on earthquake information, since enough time is not available for confirmation of water levels from BPRs and Tide Gauges. Those coastal areas falling outside the 60 minutes travel time from a tsunamigenic earthquake source are put under a watch status and upgraded to a warning only upon confirmation of water-level data. For example if a tsunamigenic earthquake happens in the coast of Northern Indonesia, parts of the A&N Islands falling within 60 minutes travel time of a tsunami wave are put under warning status. Other areas are put under Watch Status and upgraded to a warning only if the BPRs or tide gauges reveal significant change in water level. This implies that the possibility of false alarms is higher for areas close to the earthquake source; however for other regions since the warnings are issued only after confirmation of water-level data, the issue of false alarms doesnt arise. To reduce the rate of false alarms even in the near source regions, alerts are generated by analysing the pre-run Model scenarios, so that warnings are issued only to those coastal locations that are at risk.
Category of tsunami advisory bulletins, time-line for generation, content of the alert and dissemination contact information is detailed below:
Earthquake Information Bulletin (T+20 Min) contains information about Origin time, latitude and longitude of the epicenter, name of geographical area, Magnitude, and Depth of an Earthquake. This message also contains preliminary evaluation of tsunami potential based on the magnitude. (For e.g. Earthquake occurring on land or Earthquake with < M6.5 or Earthquake occurring > 100 KM depth or Earthquake occurring in very shallow water column, etc. No Tsunami is expected; For larger magnitude earthquakes in the ocean, a qualitative statement on the tsunamigenic potential may be given). No Immediate Action is required. Provided to MHA.
Tsunami Warning (T+30 Min) contains information about the Earthquake and a tsunami evaluation message indicating that Tsunami is expected. (eg. For Earthquakes with > M6.5 occurring in the Ocean within a depth of < 100 KM, a Tsunami Warning will be issued for those areas falling within 60 Minutes Travel Time from the Earthquake Source and if Expected Run up is > 2 Meters). This is the Highest Level wherein immediate actions are required to move Public to higher grounds. Message also contains information on the Travel Times and Tsunami Grade (based on run-up estimates) at various Coastal Locations from Pre-run Model Outputs. Provided to MHA and Public
Tsunami Alert (T+30 Min) contains information about the Earthquake and a tsunami evaluation message indicating that Tsunami is expected. (eg. For Earthquakes with > M6.5 occurring in the Ocean within a depth of < 100 KM, a Tsunami Alert will be issued for those areas falling within 60 Minutes Travel Time from the Earthquake Source and if Expected Run up is between 0.5 to 2 Meters as well as for those areas falling above 60 Minutes Travel Time from the Earthquake Source and if Expected Run up is >2 Meters). This is the Second Highest Level wherein Immediate Public Evacuation is Not Required. Public should avoid beaches since b current are expected. Local officials should be prepared for evacuation if it is upgraded to warning status. Message also contains information on the Travel Times and Tsunami Grade (based on run-up estimates) at various Coastal Locations from Pre-run Model Outputs. Provided to MHA and Public
Tsunami Watch (T+30 Min) contains information about the Earthquake and a tsunami evaluation message indicating that Tsunami is expected. (eg. For Earthquakes with > M6.5 occurring in the Ocean within a depth of < 100 KM, a Tsunami Watch will be issued for those areas falling within 60 Minutes Travel Time from the Earthquake Source and if Expected Run up is < 0.5 Meters and for those areas falling above 60 Minutes Travel Time from the Earthquake Source and if Expected Run up is 0.5 to 2 Meters). This is the Third Highest Level wherein Immediate Public Evacuation is Not Required, Local officials should be prepared for evacuation if it is upgraded to warning status. Message also contains information on the Travel Times and Tsunami Grade (based on run-up estimates) at various Coastal Locations from Pre-run Model Outputs. Provided to MHA
Tsunami Information Bulletins contains information on Tsunami Confirmation (wherever a Tsunami Warning already Exists) or Upgradation (Watch to Alert to Warning or bringing new areas under Watch) or Cancellation (withdraw Watch or Alert or Warning) or Observed Water Level Heights or New estimates of Travel Time and Tsunami Grade (generated by inverting observed water-level data) and any other additional information that becomes available during the course of the Event.
A Tsunami cancellation will be issued if the Tsunami Warning was issued on the basis of erroneous data or if the warning center determines from subsequent information that only an insignificant wave has been generated. In addition, Tsunami Warning may be canceled on a selective basis when a significant wave that has been generated clearly poses no threat to one or more of the areas the warning center warns, either because of intervening continents or islands which screen them or because the orientation of the generating area causes the tsunami to be directed away from these areas. To maintain credibility the warning center will use the terminology non-destructive tsunami in the cancellation message whenever applicable.
A Tsunami All Clear Bulletin indicates that the Tsunami Threat is passed and No more dangerous waves are expected.
An explanation of the Standard Operating Procedure followed at the Tsunami Warning Centre is listed below:
I. Evaluation of Tsunami based on Earthquake Parameters
Time Line: To to T0 + 30 min
Occurrence of a Tsunamigenic earthquake (T0)
Detection of Earthquake (Determination of Hypocenter and Magnitude)
The seismic network established by IMD as part of Tsunami Warning System enables rapid detection of earthquakes occurring in both the known tsunamigenic zones of Indian Ocean, viz., Andaman-Sumatra-Java subduction zone and the Makran subduction zone. A reasonable estimate of hypocenter and magnitude is available within 15 minutes (maximum T0 + 15 Min) from the occurrence of the earthquake (T0).
Qualitative Evaluation of Tsunami
After detecting an earthquake of M > 6.5, in any of the two Tsunamigenic zones, tsunami warning center sends a message (Earthquake Information Bulletin) of such an occurrence to MHA and Secretary, Ministry of Earth Sciences. In case this earthquake is located in Sumatra-Andaman Tsunamigenic zone, Andaman & Nicobar administration is also informed (T0 + 20 Min). The following rules are used for evaluating the Tsunamigenic potential of an earthquake.
The Tsunami Information bulletin with qualitative evaluation of Tsunamigenic potential as above, will be disseminated to Ministry of Earth Sciences (MoES), Ministry of Home Affairs (MHA) with the time line T0 + 20 Min
Quantitative Evaluation of Tsunami:
The tsunami affect is evaluated quantitatively using the numerical model outputs. The closest scenario is selected for quantitative tsunami forecast from scenario database using above earthquake parameters. The costal regions to be warned are identified based on tsunami wave travel time and expected water level height as follows:
The Tsunami Warning, Alert and Watch bulletins with quantitative evaluation are sent to Ministry of Earth Sciences (MoES), Ministry of Home Affairs (MHA) with the time line T0 + 30 Min.
II Detection and Re-Evaluation of Tsunami based on observed water levels
Timeline: T0 + 30 Min PLUS
Detection of Tsunami through Bottom Pressure Recorder (BPR) gauge:
Each Bottom Pressure Recorder (BPR) gauge is designed to detect and report tsunamis on its own, without instructions from land. The tsunami detection algorithm in the gauge's software works by first estimating the amplitudes of the pressure fluctuations within the tsunami frequency band and then testing these amplitudes against a threshold value. The amplitudes are computed by subtracting predicted pressures from the observations, in which the predictions closely match the tides and lower frequency fluctuations. The predictions are updated every 15 seconds, which is the sampling interval of the BPR gauges.
Background oceanic noise determines the minimum detection threshold. Based on past observations, a reasonable threshold for the Deep Ocean is 3 cm (or 30 mm). If the amplitudes exceed the threshold, the gauge goes into a rapid reporting mode to provide detailed information about the tsunami. It remains in this mode for at least three hours
The Bottom Pressure Recorders near to the earthquake source are also detect / pick up the signals due to the passage of seismic wave in addition to the tsunami wave. To distinguish between the signals due to seismic wave and tsunami wave detected by BPR the tsunami wave arrival time can be checked by overlying the numerical modelling result over the observation de-tide data.
Detection and Monitoring progress of Tsunami through Tide gauge:
For each Tide Station at coastal region the predicted tide value is calculated by the computation of Harmonic constituents and is used to compute the residual from the tide observations. The residual is very useful to detect the amplitudes of tsunami waves ranging from 0.5 M to 2 M and above. To identify tsunami arrival in real time Tide water level data observations, the numerical modelling results are overlaid on the tide residuals to check the tsunami wave arrival at that Tide Gauge location.
Dissemination of Confirmed Tsunami Information Bulletin:
The triggering of tsunami is confirmed with real time observations if they are showing more than the threshold values. The Confirmed Tsunami Information bulletin is generated with observed water levels. The identified coastal areas, which are under watch status, will be upgraded to either warning or alert status. If the observations are not showing more than the threshold values then Tsunami Cancellation bulletin will be sent to MoES, MHA and Authorized people with the water level observations.
Confirmed Tsunami Information bulletin will be sent to MoES, MHA and Local Authorities after T0 + 30 Minutes, as and when the real-time water-level observations are available (not later than T0 + 60 Minutes).
Re-Evaluation of Tsunami
The numerical modeling results are corrected / adjusted on-the-fly by a real-time water level observation data. The model requirement in this case is the solution must provide the best fit to the observations.
Real time water level Data Assimilation and Inversion is done to get the best fit model scenario automatically, where initial conditions are determined from an approximated solution in terms of source magnitude and location. An effective implementation of the inversion is achieved by using a discrete set of Greens functions Unit sources to form a model source. The algorithm chooses the best fit to a given BPR data among a limited number of unit solution combinations by direct sorting, using a choice of misfit functions.
Then the closest scenario is selected or selected scenario will be corrected for quantitative tsunami forecast from scenario database using above earthquake parameters. If real time water level observations show the significant water level changes above the thresholds, accordingly the coastal regions that fall above 1Hr travel times, which are under Alert status will be upgraded to Warning and those under Watch status will be upgraded to alert status.
Parallely, the real time numerical simulation will be initialized using the updated fault parameters to get the inundation levels at vulnerable coastal regions.
As per standard operating procedure, warning center sends the Tsunami Warning Bulletin to MOES, MHA and Public; The Tsunami Alert Bulletin to MOES, MHA and Public; Tsunami Watch Bulletin to MOES and MHA. If real time doesnt show any significant water level changes all previously issued bulletins are canceled by issuing cancellation bulletin.
In cases where a tsunami warning/alert/watch is issued, the warning center continues to monitor the tsunami activity through tide gauge water level observations and other complementary observations. If observations indicate that the tsunami activity has decreased and no more damaging tsunami waves are expected, then warning center updates the status and Issues the Tsunami ALL CLEAR bulletin to respective coastal regions.