Learning lessons from disastersWhile humans, through the emission of greenhouse gases, bear some measure of responsibility for sea-level rise, ocean warming and acidification, the coasts are also exposed to a number of natural threats as well. These include earthquakes, landslides, tsunamis and volcanic eruptions, as well as natural climate phenomena, particularly the Pacific climate anomaly known as El Niño. Although humans have no direct influence on the occurrence of such events, a variety of technological solutions have been developed to protect coastal communities as far as possible and to minimize damage to property. Many lessons have been learned from past disasters, as evidenced by modern disaster preparedness schemes for tsunamis.
- 3.19 > It was one of the most devastating natural disasters in the history of Europe. When the earth shook on 1 November 1755 in Lisbon, tens of thousands of people died beneath the rubble of buildings, in the fire storm, and in the floods of a tsunami.
- Tsunamis are especially large waves that can travel for thousands of kilometres across the sea. As they approach a coast they are slowed down by the shallow water, which causes them to rise up many metres in height. Up to 70 per cent of all tsunamis are triggered by earthquakes, mostly in the sea. Other causes include volcanic eruptions or landslides, in which large amounts of sand, rocks or sediments surge downslope like an avalanche. The more material that is set in motion or the faster it moves, the more energy the resulting tsunami possesses.
Catastrophes out of the blueFor a long time people in coastal areas were simply at the mercy of tsunamis because they came with no warning at all. A tsunami on 1 November 1755 caught the people of Lisbon, Portugal’s capital city, completely unprepared. On that day, about 200 kilometres to the west of the Strait of Gibraltar, a powerful submarine earthquake occurred that was so violent it destroyed most of the city. To add to the devastation, it triggered an enormous tsunami that flooded large areas of the city around 40 minutes after the earthquake. According to various estimates, between 30,000 and 100,000 people lost their lives due to the earthquake and tsunami in the Portuguese capital alone. Other cities and villages on the Portuguese and the Moroccan coasts were also devastated. Even on the other side of the Atlantic, in the Caribbean Islands, the tsunami caused damage to harbour structures and boats.
Especially high-risk regionsRegions in the Pacific are especially threatened by tsunamis because of the tectonic plate boundaries that run parallel to the coasts and are often characterized by heavy seismic and volcanic activity. This is why the term “Ring of Fire” is used to describe these regions. In the western Pacific they include the coasts of the Philippines, Indonesia, Japan and Russia, while in the east large segments of the coasts of North and South America make up the ring. Many sites within the Ring of Fire have repeatedly experienced strong earthquakes throughout history that have also triggered large tsunamis.
Japan also lies on the Ring of Fire. Because multiple plate boundaries meet here, the country has frequently been shaken by strong earthquakes. Likewise, numerous giant waves have occurred in this area throughout the past, and the phenomenon received its name there long ago. The term “tsunami” is of Japanese origin and derives from the words “tsu” (harbour) and “nami” (wave). The term tells us that the waves are especially destructive to harbour cities.
- 3.20 > The Ring of Fire circling the Pacific. The coastlines run parallel to plate boundaries, where many earthquakes originate. These may often be followed by tsunamis.
Development of the Japanese tsunami warning systemIt was a long time before people began to understand how to interpret the initial warning signs. On 15 June 1896, a tsunami with a wave height of 38 metres hit the northeast coast of Japan. Around 20,000 people died. It was unusual that the earthquake preceding it was only weakly felt on the Japanese coast, but still produced such a powerful tsunami. This led to a debate in Japan over the origin of this enormous wave. Some specialists attributed the tsunami to submarine landslides. Although the causes remained unclear, the ensuing discussion led to an increased awareness of tsunamis in Japan.
Among the citizens of Japan, the perception generally spread that earthquakes were an important warning sign for possible tsunamis. The general rule was accepted that “When the ground shakes it’s time to evacuate”. In 1933 a tsunami hit the northeast coast of Japan, again following closely after an earthquake. This time the population was better prepared and many saved themselves by escaping to higher ground. Still, around 3000 people lost their lives.
In 1941 Japan became the first country in the world to implement a tsunami warning system – at the meteorological station at Sendai, a large city on the east coast. A seismometer was permanently installed there that could be used to estimate the strength and approximate distance of earthquakes. From then on tsunami warnings were announced on the radio, and police stations were informed in the affected regions. As a rule, it took 20 minutes from assessment of the earthquake data until a tsunami warning was given.
- 3.23 > A tsunami hit the coast of Hawaii on 1 April 1946. The triggering event was an earthquake that occurred 4000 kilometres away near the Aleutian Islands. It took 4.5 hours for the tsunami to travel from the site of its origin to Hawaii, where it took the lives of 159 people.
- In the following years more seismometers were installed in various other regions and finally, in 1952,
the Japan Meteorological Agency (JMA) implemented a nation-wide tsunami warning system. By 1999, increasingly technologically advanced seismometers had been installed that constantly improved the speed and quality of determinations of the intensity and location of earthquakes. Tsunami warnings could ultimately be announced within 3 minutes after an earthquake. But in spite of the use of mathematical simulation models, it was not possible from the earthquake data alone to reliably determine the size of the tsunami to be expected. After the tragedy of
11 March 2011, the tsunami warning system in Japan was finally significantly improved. On this day, off the coast of the northeast Japanese region of To–hoku, a strong submarine earthquake occurred. Around 16,000 people died in the quake and the great wave it produced.
As a consequence, sensors were installed on the sea floor off the Japanese coast that could recognize a tsunami wave passing overhead based on discernable changes in pressure. Thanks to the deployment of these additional sensors, the ability to determine the path of a tsunami and estimate the size of the wave expected to hit land has been greatly improved.
Development of the tsunami warning system in the USANot only in Japan but also in the USA, efforts to develop a warning system began relatively early. In the Aleutian Islands, which extend from the coast of Alaska far out into the Pacific, a strong earthquake occurred in 1946 and triggered a large tsunami. The wave was so enormous that it completely destroyed a steel-reinforced concrete lighthouse that stood atop a 12-metre-high cliff on the Aleutian Island of Unimak.
4.5 hours later the tsunami reached the Hawaiian Island group 4000 kilometres away. It hit the inhabitants with no advance warning, because the earthquake was not felt here at all. The waves were up to 16 metres high and in some locations the water penetrated a thousand metres inland. 159 people died. The tsunami was also felt on the northwest coast of the USA. The waves here were only about 2 metres high, but there was still some damage to boats in a number of harbours.
After this experience, US officials established a tsunami warning centre in 1949 near Honolulu, Hawaii. Similar to the system in Japan, their method was based on identifying earthquakes and calculating the travel time of a potential tsunami. If a partner country reported an earthquake, the centre would calculate the travel time of a possible tsunami wave to its arrival on the coast of the USA.
Earthquake intensity The intensity of an earthquake is determined on the basis of a moment magnitude scale. The scale ends at a value of 10.6. This maximum intensity is reached when the Earth’s crust breaks completely apart in the area of the earthquake. Theoretically it is not possible for an earthquake to release more energy than this. The structure of the scale is logarithmic. This means that the strength of the earthquake increases exponentially with the scale value. One point on the scale is equivalent to an increase in energy of about 30 times. For the purpose of visualization, the seismic energy of an earthquake can be compared to the explosive power of TNT. The energy of an earthquake with a magnitude of 5 is equal to about 475 tonnes of TNT; that of a magnitude 6 earthquake is equal to about 15,000 tonnes of TNT.
International cooperation emergesFor a long time Japan only generated warnings for its own coast, while the system in the United States rapidly developed into an international warning centre for the entire Pacific realm. The impetus for this international cooperation was an earthquake that struck near the large city of Valdiva in Chile on 22 May 1960. The Earth’s crust ruptured on land in Chile from north to south over a length of 1000 kilometres. This included the jarring movement by 20 metres toward the west of a 200-kilometre-wide block situated between the continental margin and the Andes. This triggered an enormous tsunami wave that caused immense damage nearby, particularly on the coast of Chile, and then spread westward across the entire Pacific.
Hawaii experienced 10-metre waves, while the more distant east coast of Japan was subjected to 5-metre waves. Because other countries in the Pacific and island nations in particular were impacted, UNESCO (United Nations Educational, Scientific and Cultural Organization), beginning in 1960, strongly pushed for the implementation of a Pacific-wide warning system. The Intergovernmental Oceanographic Commission (IOC), established by UNESCO after the earthquake in Chile, was responsible for the international cooperation. The IOC member states decided to integrate the system with the existing warning centre in Hawaii. It began operations in 1965 as the Pacific Tsunami Warning Center (PTWC). The PTWC still coordinates tsunami warning and prediction for the entire Pacific realm on behalf of the USA’s National Oceanic and Atmospheric Administration (NOAA).
As in Japan, the accuracy of tsunami prediction was limited in the beginning. The warning system essentially consisted of member states informing each other by telephone whenever an earthquake was recorded. Aided by the seismographic information and travel-time maps, calculations could then be made to determine whether and when a possible tsunami could hit land. This information was augmented by water-level measurements in various coastal areas. But predictions still remained uncertain.
75 per cent of all tsunami warnings were false alarms, which often led to expensive evacuation measures. In 1986 an alarm led to the evacuation of Waikiki, a district of Honolulu. Many public buildings in this part of the city had to be vacated. Although waves arrived at the beach at the expected time, they were only slightly larger than the normal surf. Officials in Hawaii estimate that the interruption of normal business as a result of the false alarm caused losses equivalent to 41 million US dollars. This was followed by considerable criticism of the efforts of the PTWC tsunami warning centre.
- 3.25 > Since the 1980s a tsunami warning system of buoys has been installed around the Pacific that receive signals from pressure sensors on the sea floor. These sensors detect tsunami waves.
- 3.24 > The earthquake off the east coast of Japan on 11 March 2011 lasted around 5 minutes. It triggered a tsunami that devastated broad areas of northeast Japan and also breached the seawall of the Fukushima I nuclear power plant.
- In 1987, therefore, NOAA decided to install a completely new warning system that delivers tsunami data in real time. This consisted of pressure sensors on the sea floor that send data to buoys at the surface through an acoustic signal. The buoys then transmit the data via a satellite connection to the PTWC. The advantage of this is that the system directly measures the strength of the tsunami wave, providing a reliable indication of its magnitude and behaviour at landfall. It therefore complements very well the classic seismographic earthquake measurements.
This buoy system is known in the USA as DART (Deep-ocean Assessment and Reporting of Tsunamis). It is operated by the National Data Buoy Center (NDBC) of NOAA and continues to be upgraded even today. Australia, Chile, Colombia, Ecuador, India, Russia and Thailand are now also using these kinds of buoys. Japan developed its own buoy system, but is gradually giving this up in favour of pressure sensors connected by cables. In the Pacific and adjacent marine regions today, more than 50 buoys have been installed that can be used by the PTWC. >