Ocean based Tsunami Warning Systems
Since reducing the frequency of tsunami occurrence is not possible we have to ensure people in endangered coastlines are prepared and know what to do in case a tsunami warning has been issued by the mandated national authorities. An essential part of a tsunami warning system (TWS) is the awareness and preparedness of the coastal population and thus reducing the vulnerability of a coastal area. For an effective TWS three conditions must be met:
- the warning system has to be swift, reliable and operational;
- procedures to protect the population are the subject of a pre-established evacuation plan;
- population and stakeholders are aware of the risk and countermeasures are part of the local coastal zone management
In order to be operational, the TWS must be capable of detecting and localizing tsunamigenic subsea earthquakes within a few minutes and based on a prescribed decision matrix assessing the tsunami risk, predicting its propagation and its time of arrival for different coastlines and finally issuing the tsunami watch message.
Real time data from seismic instrumentation networks allow regional tsunami watch centres (RTWC) to locate the epicentre of an earthquake and to measure its magnitude. As from a certain magnitude threshold, which varies with the source zone, subsea earthquakes have a potential to generate a tsunami. So just based on these seismic data, the regional centre will issue a tsunami watch message to national authorities in the endangered region. Depending on the vicinity of the tsunami source zone to a certain coastline the national tsunami warning focal points might issue immediately a tsunami warning or wait for the first tide gauge readings if the tsunami generation can be confirmed or not. The real-time sea-level measurements also allow these centres to refine the information concerning the tsunami amplitude. Therefore, the quick detection of a tsunami requires not only a sufficient number of measuring devices and networks, but also networks with advanced means of communication for the real-time transmission of data. Besides the monitoring capability a fully fledged RTWC can also swiftly process and analyze these data around-the-clock, 24 hours a day, seven days a week.
Devices used for detecting a tsunami
Seismometers pick up tremors and movements like earthquakes in earths crust. There are hundreds of real-time transmitting seismometers around the globe while the RTWCs focus on networks in their watch area. See the EMSC or the GEOFON programme websites for further details. Strong sub-sea earthquakes can trigger tsunamis, but this first assessment based on seismic data needs to be confirmed by sea-level measurements.
A tide gauge is a device which measures the sea level at a specific location. It is generally situated in a harbour and sometimes combined with a GPS station. These instruments are also used to measure tides for shipping or long term sea-level rise for climate predictions. The international network is coordinated through IOC GLOSS.
A tsunameter is usually an open-ocean buoy system with a pressure sensor at sea-bottom capable of detecting long waves of very low amplitude (a few centimetres). The real-time data are transmitted via satellites. But there are also cabled systems which can be used for near-shore locations. However, these systems are very expensive, because they have to be frequently serviced. Therefore tsunameters are mainly used for locations far off any coastline or island.
The Intergovernmental Oceanographic Commission - Tsunami Programme
The Intergovernmental Oceanographic Commission (IOC) of UNESCO has been mandated by its Member States to coordinate the establishment of the four regional tsunami warning systems around the globe (see: www.ioc-tsunami.org).