“Tempat Evakuasi Sementara” saves lives during the tsunami

Not even the German Foreign Office provides travellers to Indonesia with truly concrete recommendations in the event of a tsunami though the country has historically experienced a (small) tsunami every two to three years. Even experienced travellers to Asia often do not know that the tsunami warning alarms sound different in each country. As a traveller, how do I find out about a tsunami? What happens in case of an emergency? Whether in Japan, Indonesia or Hawaii – obtaining appropriate and concrete information isn’t entirely straightforward for foreigners.

Indonesia is a real tsunami hotspot. About two-thirds of all tsunami-related global deaths in the last two hundred years have occurred in Indonesian territory. Even excluding the two largest tsunami events in 2004 (160,000, Aceh) and in 1883 (36,000 fatalities, Krakatau), Indonesia accounts for approximately as many deaths from tsunamis as does Japan. This numbers even more than such deaths on the entire continent of South America combined. Analysing historical data shows that the risk of death during a tsunami is greatest near the coasts. In Indonesia, one should however be aware that tsunamis usually inundate areas no more than five hundred metres inland. The Aceh flood in 2004, which reached up to four kilometres inland, was the worst-case scenario. It was triggered by a massive earthquake, the scale of which is only expected every five to seven hundred years. The adjacent map shows the Indonesian coastal sections most at risk of tsunamis.

Indonesia is a real tsunami hotspot. About two-thirds of all tsunami-related global deaths in the last two hundred years have occurred in Indonesian territory. Even excluding the two largest tsunami events in 2004 (160,000, Aceh) and in 1883 (36,000 fatalities, Krakatau), Indonesia accounts for approximately as many deaths from tsunamis as does Japan. This numbers even more than such deaths on the entire continent of South America combined. Analysing historical data shows that the risk of death during a tsunami is greatest near the coasts. In Indonesia, one should however be aware that tsunamis usually inundate areas no more than five hundred metres inland. The Aceh flood in 2004, which reached up to four kilometres inland, was the worst-case scenario. It was triggered by a massive earthquake, the scale of which is only expected every five to seven hundred years. The adjacent map shows the Indonesian coastal sections most at risk of tsunamis.  

The Sunda Trench, where the strong subsea quakes occur, stretches in an arc from the north-western tip of Sumatra to the island of Flores in eastern Indonesia, running approximately five thousand kilometres in length. If a tsunami is triggered here, the waves would reach the coast within twenty minutes in an extreme-case scenario, and very little time remains for early warning and evacuation. The response time on many Indonesian islands is even shorter. Hardly five minutes often remain between quakes and tsunamis in these areas.

An evacuation plan serves as perhaps the most vital assistance for tourists in Indonesia when planning a trip. It maps out areas of potential flooding for individual cities down to the street level when tsunami waves hit. Perhaps that bungalow directly on the Indonesian waterfront isn't always the best choice. Travellers should also familiarise themselves with the early warning system. This system differs from country to country. Every traveller to Indonesia should have a small packed suitcase prepared for an emergency: the operators of the Indonesian early warning system recommend that travellers have at hand their identification documents in waterproof packaging, drinking water, a torch (flashlight) and a radio.

A constant three-minute siren will be heard in the event of a tsunami in Indonesia. This will be repeated in an infinite loop as long as the warning is still active. An official message, also in English, should follow within five minutes at the latest. The BMKG*, the operator of the early warning system in Indonesia, will have quickly simulated the spread of the tsunami waves and will issue a warning accordingly. There are three different warning levels: “Major Warning (Awas)” means a tsunami wave measuring more than three metres high is likely to follow; the second stage, “Warning (Siaga)”, estimates wave heights between 0.5 and 3 metres; The third warning stage, “Advisory (Waspada)”, assumes a tsunami wave measuring half a metre at most. The first two warning stages require direct evacuation.

Three zones are then distinguished cartographically in the evacuation plan: extremely dangerous (red) and potentially dangerous (yellow). In addition, zones that aren't affected are also depicted. These areas are marked in grey. Individuals in the yellow zones do not need to leave, but should seek higher floors in buildings. Safe locations would lie too far inland to reach in time. Densely populated areas in particular cannot be evacuated in the time available.  

Anyone who must quickly access up-to-date information in the event of a catastrophe, cannot rely on the cellular network. In the particular event of a tsunami, the networks are quickly overloaded and tend to break down. A simple ultra-high frequency radio is the most vital source for obtaining reliable information in an emergency. But who still travels with a radio in their luggage? In Indonesia, however, it can be extremely useful. In villages, far from larger cities, drums, “Kentongans” (Indonesian slot or wood drums), mosque loudspeakers and other traditional means of communication are utilised for alerting the general public. Drummers on Bali have even developed distinct rhythms for tsunami warnings. It is therefore worthwhile to listen closely, as sirens are regularly tested in Indonesia every month. On the 26^th of each month, sirens sound at exactly 10:00 AM for one minute, the same sirens that will be heard in the event of an emergency. This, at the same time, commemorates the victims of the 2004 catastrophe. The siren is quieter during the exercise alarms, and the warning sound is interrupted three times with the message: “This is a tsunami early warning test. This is just a test”.

If you feel a strong earthquake in the immediate vicinity of the Indonesian coast, then it's best not to wait for an official warning. People should immediately seek higher ground or a safe place away from the coast. For tourists this means that they should watch what the locals do. How do they react when the ground is shaking?

Strong earthquakes last at least twenty seconds. Some of the areas on the outlying islands of the Indonesian coast may seem to be underdeveloped, but the history of tsunamis is systematically recounted in these traditional societies. Despite their greatest proximity to the epicentre in 2004, there were few fatalities on some of the small islands, as almost everyone immediately reached safety. In the large Indonesian cities, however, collective memory is short. Many people sought information in 2004 via digital media. Valuable time was therefore lost without people acting and seeking suitable refuge.

Not every quake that triggers a tsunami is directly noticeable. On the other hand, in approximately only ten to twenty per cent of cases do dangerous tsunamis follow an earthquake. An undersea quake must register at least a seven on the Richter scale in order to trigger a tsunami. The intensity is then so high that it vertically lifts the water masses.

A tsunami is preceded either by a rapid decline or rise in water level. “Rapid” means the water rises or sinks within five to ten minutes. When the water quickly recedes, fish cannot always keep pace and they end up floundering in the tidal zone. This would never happen when the sea recedes during the daily ebb and flow. The first high tsunami wave will surely follow in less than twenty minutes after the water quickly recedes.  

In addition, the sound of a tsunami can hardly be missed. The breaking wave is extremely loud, like a thundering cannon. What is absolutely crucial to know is that the first tsunami wave is usually not the largest and, more importantly, never the only one. Subsequent waves, sometimes the fifth or sixth, can be many times stronger. Because these subsequent waves often occur many hours after the first, people should never leave their safe refuge after the first wave(s) retreat.  Because a wave can only be expected every ten to sixty minutes in Indonesia, sometimes even two hours can pass until the next wave arrives. A great deal of patience can be a lifesaver. Sometimes more than ten waves can follow one after another at lengthy intervals.

This means that it is essential to remain in a safe place because the retreating tsunami waves are often devastating and lead to the most fatalities. When the waves recede, vast amounts of material are pulled back with the current into the sea.

It is particularly unsafe along rivers in the coastal flatlands. Waves can spread unhindered here and can reach quite far inland. River bridges should therefore be avoided at all costs. Also, cars often become traps during tsunamis. They are believed to be the fastest means of transport for leaving the danger zone. However, driving a car should be avoided at all costs, particularly in larger cities, because the streets become quickly clogged. In addition, panicked evacuation rarely runs smoothly. Everyone should make their way to higher ground as quickly as possible—for example, to mountains or taller hills, or flee to the interior.

All buildings that serve as suitable refuge in Indonesia are designated in writing with the words: “Tempat Evakuasi Sementara”. These buildings would even withstand a tsunami and would have enough floors to provide sufficient height. There are currently many parking structures in Indonesia that have been specially built as shelters. Lightweight structures made of wood, clay or concrete blocks, however, offer no protection. Incidents in Japan have shown that even tsunami waves measuring two to three metres high destroy these less robust buildings. Travellers to Indonesia should therefore inquire at their hotel whether their accommodations are located in a flood region and if the structure is stable enough to withstand a tsunami. One should also ask if the upper floors can be used as shelter. In addition, travellers should inquire about nearby hills that can be used for refuge.

Those aboard a ship should immediately head farther out to sea. The wave in the deep sea can reach a velocity of eight hundred kilometres an hour, but it is often only a few centimetres to decimetres high and measures a wavelength of one hundred kilometres or more. If, however, the water becomes shallower near the coast, the wave speed decreases in favour of the wave height. Tsunami waves can therefore measure several tens of metres high when they hit the coast. The Aceh tsunami waves reached up to twenty metres high, and during the last tsunami in 2010, which hit the Mentawai Islands, they reached a height of more than twelve metres. The shallower the water, the shorter the interval between waves and the larger the amplitudes.

The interactive science poster, “Submarine Landslide”, is currently on a touring exhibition again. Those interested can compare sediment thickness, population density, continental and plate boundaries and bathymetric data (profile/terrain maps of the seabed) with information on historical tsunamis, earthquakes, landslides and volcanic activity. This comparison results in statistical consolidation for certain potentially hazardous regions. A potential submarine landslide simulation off of Spitsbergen (Svalbard, Norway) with a subsequent tsunami shows how a threat could develop for the entire North Atlantic region. The interactive science poster has been on view since May aboard the exhibition ship „MS Wissenschaft – Zukunftsprojekt Erde“ zu sehen.

* BMKG: Badan Meteorologi, Klimatologi dan Geofisika – Indonesian Institute for Meteorology, Climatology and Geophysics.

Text: Jana Kandarr (ESKP), Dr. Jörn Lauterjung (GFZ)

The information leaflet "Tsunami" (in German), published by the German Research Centre for Geosciences, offers travellers to Indonesia detailed information on what to do in the event of a tsunami. It provides information to individuals who will be staying abroad for a longer period of time in coastal regions in danger of tsunamis. The informational leaflet also explains the causes and characteristics of tsunamis and depicts the most important regions vulnerable to such events throughout the world. A condensed summary (in German) is also available.