Before the 2004 tsunami, the Indian Ocean did not have a system to alert residents of coastal areas when a tsunami was imminent. Therefore, the catastrophe of 2004, which left several people dead, triggered the Indian Government to establish Indian Tsunami Early Warning System at the Indian National Centre for Ocean Information Services (INCOIS). The system was developed keeping in view two major objectives: real-time acquisition of data from multiple sensors through various communication channels and dissemination of tsunami advisories to the stakeholders at the earliest possible time.
Tsunamigenic zones in the Indian Ocean
Design of early warning system
The underlying principle of the Early Warning System (EWS) is to monitor earthquakes in realtime. The seismic system is a core component for near-source regions where the EWS is expected Early warning system to mitigate tsunami fury With the help of early warning system, India can issue tsunami alerts within minutes of a major earthquake in Indian Ocean region to issue a warning in just few minutes after a quake. The system monitors large magnitude earthquakes which are ‘tsunamigenic’.
In addition to seismic observation system, sea-level observation system is also a critical. Due to the complexity and uncertainty in forecasting whether an undersea earthquake has the potential to generate a tsunami, the observation of sea level changes is important. The use of actual sea level observations helps in reducing the risk of issuing false tsunami warnings. The EWS’s sea-level system includes a network of tide gauge stations along the coastline of the country and a network of bottom pressure recorders. In addition, EWS also includes a number of coastal sea level stations that can report sea level variations in real time.
In order to integrate these heterogeneous datasets from multiple sensors, a state-of-art computational facility is installed at INCOIS.
To assess the real time scenario and make tsunami forecast, simulation of the event is important. The EWS hypothesises that a certain magnitude of earthquake can cause tsunami is generated using numerical simulation. For this, the simulation system takes earthquake parameters as input and uses numerical models for generating high accuracy wave propagation information in open oceans. This is then extrapolated to the coastline in estimating the maximum wave height and arrival time of destructive waves. Currently, EWS uses the TUNAMI-N2 model to simulate tsunami wave propagation to estimate travel time and run-up height for a particular earthquake.
Since the model cannot be run at the time of an event, due to large computing time as well as due to non-availability of required fault parameters in real-time, a database of pre-run scenarios is essential. The result of the simulation is a huge database of approximately 8 terabytes, consisting of spatial maps depicting the water level in the Indian Ocean region. The application software developed based on geospatial technologies has an interface to store, retrieve, analyse and display the spatial maps from the database. The spatial layers currently being handled by this application include fault lines, fault segments for different earthquake magnitudes, travel time maps, directivity maps, simulation results for about 1800 coastal forecast points, graphs of model and observed tsunami wave profiles at each coastal forecast point.
As the event progresses, the hypothesis is either confirmed or cancelled based on the real time sea-level observations. In case of confirmation of tsunami, the EWS upgrades/downgrades the threat status at all the administrative regions along the Indian Coastline.
Directivity and travel-time maps for the 11 April 2012 quake revealing the threat levels due to the tsunami
Standard operating procedure
The Decision Support Systems (DSS) can be thought of as a codified version of standard operating procedures (SOP) during an event. An EWS functions with a set of operating standards and code of conduct, known as standard operating procedures (SOP). The Indian SOP suits both, near-source as well as far-source earthquake events. For example, if an earthquake strikes the Sunda subduction zone, coastal areas in Andaman & Nicobar Islands with tsunami travel times of < 60 min are categorised as near-source and for those areas where water level exceeds > 2 m, warning is issued. However, the areas in mainland are categorised as far-source and will be under alert/watch based on estimated water levels until confirmation from sea level observations. These decisions are coded into the application software that enables generation and dissemination of location specific advisories in both map and text formats. These guide the scientist-operators at the tsunami warning center on when to initiate the bulletin, how to analyse the situation, whom to issue the notification, threat assessment and cancellation.
Role of geospatial technologies
The advent of GIS has made it possible to connect the simulation results of threat assessment (wave heights and arrival times) at open-ocean to each individual administrative blocks in the country. In its most visible form, geospatial technologies help in disseminating the threat spatially. The geographical user interfaces developed using GIS help the decision makers to handle the tasks easily by visualising the actual scenario.
Performance of the EWS
Since its inception in October 2007, the centre has monitored 364 quakes of M > 6.5, out of which 64 were in the Indian Ocean region. Only on five occasions, warning/ alert/watch were issued. The best example for performance of warning centre is great EQ of Mw 8.6 on April 11, 2012 that occurred off west coast of northern Sumatra wherein only three areas were under warning. Also, a comparison of earthquake parameters estimated by warning centre has been made with other international agencies, like the US Geological Society.
The EWS has been scaled up to meet the tsunami early warning requirements of other countries in the Indian Ocean. INCOIS has been identified as one of the Regional Tsunami Watch Providers in the Indian Ocean. Plans are afoot to achieve tsunami warning capability for earthquake events happening outside the Indian Ocean region. As the world moves towards a multi-hazard approach, EWS has been designed to handle other disaster early warning systems.