Tsunami early warning system - how it works
by Kapila Dahanayake, Senior Professor of Geology, University of Peradeniya
Tsunamis or earthquakes can be neither prevented nor predicted. An early warning system that can give sufficient hours of advance notice to coastal communities is a primary way of effective mitigation of this disaster.
Water level gauges are an essential element of a tsunami warning system. They can be used to confirm the generation of tsunami waves following an earthquake. To be effective for warning purposes, water level gauges should be located near the tsunami source region to get the most rapid confirmation of the event. Forecasting tsunamis requires adequate understanding of the phenomenon, good and quick collection of earthquake and sea level data and accurate and fast assessment and interpretation of data.
A tsunami warning centre such as that in operation around the Pacific ocean could have saved thousands of people who died in the Indian Ocean earthquake. Both Sri Lanka and India did not have such a facility mainly because large tsunamis were in the past extremely rare in the Indian Ocean.
In the Pacific Ocean a few tsunamis are reported in any given decade. Between 1975 and 1998 there have been at least eighteen (18) in the Pacific and the adjoining seas resulting in significant human casualties and damage to property. In countries like Japan or USA, people living in coastal areas of the Pacific Ocean where more than 90% of all tsunamis recorded have occurred in the past, are warned and taught to move away from coastal regions after a tsunamigenic earthquake. The Pacific Tsunami Warning Center (PTWC) operated by the US in Hawaii and supported by 26 member countries, including Australia, Canada, Chile, China, Japan, USA etc., has many seismic stations, water level reporting stations and dissemination points scattered in the Pacific Basin. Functioning of the system begins with the detection by any participating seismic laboratory of an earthquake of sufficient size to trigger alarms, set at the threshold value of 6.5 on the Richter Scale. PTWC collects seismic data, locates the earthquake and computes its magnitude. When reports from water level stations show that a tsunami has been generated posing a threat, a warning is transmitted to the relevant dissemination centres within shortest possible time.
Japan is located in the circum Pacific tectonic zone where seismic activity is extremely high. Japan being an island nation surrounded by sea is one of the most tsunami-prone countries in the world.
Since 22,000 people died in the Sanriku Tsunami of 1896 and another 3000 in 1933, Japanese Meteorological Agency (JMA) has been in the forefront of providing tsunami warnings for more than 100 years.
Over the years, JMA has been able to reduce the target time of tsunami warning to just 3 minutes since 1994 by using state-of-the-art technology. JMA has improved its seismometer networks (numbering more than 180) and seismic data processing by getting rapid estimation of magnitude of earthquake and location and observing sea level changes using tidal data from more than 100 stations. Even the most modern computer will require much time to study tsunami generation and propagation. JMA has calculated tsunami generation and propagation for 100,000 different simulated cases and obtained estimated tsunami heights and arrival times which are stored as a database.
Minimal information required for tsunami warning constitutes time of origin, location, magnitude and depth of earthquake (hypocentre) and tidal data giving sea level rise. Principal components are the following:
* Network of seismic stations to observe seismic waves * Tidal stations to monitor sea level changes * Dedicated telephone lines to transmit observed data in real time * System to determine immediately the hypocentre and magnitude of the earthquake * Evaluation of possibility of generation of tsunami using sea level changes obtained from tidal stations via satellite * Communication network to disseminate warnings to mass media , local authorities, police and eventually to residents at risk.
Once the tsunami warning is rapidly communicated to people at risk they should be able to respond positively without panicking.
For smooth evacuation in potentially danger areas, prior to the event it is essential to (i) conduct awareness programmes about tsunamis (ii) prepare hazard maps giving previous heights of tsunamis (iii) identify evacuation routes and shelters and (iv) conduct evacuation drill practices periodically for schoolchildren and villagers at risk.
Continuous effort is essential for education and training since people tend to forget the dangers as probability of tsunami occurrence in the Indian Ocean countries particularly is low if old records are any indicator.
In the aftermath of the massive and disastrous tsunamigenic earthquake of 26/12 we have to learn from the long and successful post-tsunami experiences of the Pacific Rim countries and take appropriate steps to establish a tsunami warning facility in Sri Lanka / SAARC region and be prepared for any possible future threat from the Indian Ocean. It should be noted that the geological conditions of the occurrence of earthquakes in the Indian Ocean region are different from those in the Pacific Ocean. It is necessary to recognize the type of tsunami -local or distant or both - that could attack the countries in the region. If there is no risk from local tsunamis, it would be sufficient to establish a system to monitor a tsunami warning from a regional Tsunami Warning Center 24 hours a day and each country could determine its responsibility and initiative and decide if a warning should be issued or not.
Although research studies indicate that the probability of an immediate repetition of a tsunami is low from the same location of the earthquake, there is always the possibility of an earthquake from a different location.
Past records and years of earthquake research have shown that it is not yet possible to predict with precision the exact date and time of earthquakes even with modern equipment available in countries like USA and Japan. Present thinking is that an earthquake can happen anywhere anytime. Same is true for tsunamis. But with a tsunami warning system in place, most people will have time to evacuate to high ground or safe locations (e.g 3rd floor or above of nearby strong buildings- ideally reinforced concrete buildings) unless the earthquake occurred very close to the coast.
Since the minimum period required to give warnings about tsunamis currently is three minutes, it may be too late for residents of such coastal areas by the time they receive the warning.
Therefore they should run immediately to high ground if they feel a strong earthquake had occurred as judged by strong earth shaking and damage to houses. Even in countries like ours, until the tsunami warning system is established - which may take a considerable period of time- people living in coastal areas should be on high alert if they feel a strong earthquake and run to high ground or to 3rd floors or above (considering the height of the previous tsunami) of strongly built houses of the vicinity.
Awareness and education programmes are sometimes more effective than sophisticated and expensive warning systems.Such programmes also can save hundreds and thousands of human lives. In this regard it is essential that measures are taken to include Disaster Management and Mitigation as a subject in the school curriculum so that awareness programmes will continue without much effort.
(The writer was recently in Tokyo attending a Workshop on Tsunami Early Warning Systems and conducted field visits to tsunami preparedness facilities in Numazu City, Shizuura District).