How do tsunami buoys work

Early warning of tsunami waves

The 2004 tsunami disaster in the Indian Ocean shocked people in the region. Since then, an early warning system has been set up there under the direction of the GFZ German Research Center for Geosciences in Potsdam. The focus is on Indonesia. Seismometers, buoys, ocean sensors, coastal gauges and satellites provide crucial data that is converted into targeted warning notices within a few minutes. It is particularly important that the authorities and the local people learn what to do when the alarm sounds. All components of the early warning system must interlock so that the disaster does not repeat itself one day.

Earthquake zone on the Sunda Arch

The tsunami of December 26, 2004 in the Indian Ocean was devastating. More than 200,000 people were killed. The tsunami was triggered by the third strongest earthquake in the past hundred years with a magnitude of 9.3 on the Richter scale, which is open to the top, off the coast of northern Sumatra. The disaster struck the affected regions completely unprepared. There were no alarm plans or evacuation plans. Several organizations published the first reports on the location and strength of the quake on the Internet after just twelve minutes. But after less than 20 minutes, the first tsunami wave reached Banda Aceh beach in northern Sumatra and caused devastating devastation there. Even an hour and a half after the earthquake, when the first wave reached Thailand, and two hours after the earthquake, when Sri Lanka was hit, no reliable reports had yet been passed on to the local authorities - or the reports could not be implemented in action.

Immediately after the disaster, a consortium of German research institutions led by the GFZ German Research Center for Geosciences in Potsdam presented the federal government with a concept for setting up a tsunami early warning system in the Indian Ocean. It has been implemented with a focus on Indonesia since March 2005 (German Indonesian Tsunami Warning System, GITEWS for short).

The geological situation of Indonesia posed a major challenge: In the Indian Ocean, the Indo-Australian plate sinks below the Eurasian plate at the Sunda Arc at a rate of 6 to 7 cm per year. The fact that the earthquake zone runs largely parallel to and closely spaced from the country's coast over a length of several thousand kilometers is particularly tricky. That is why the warning times for a tsunami are between 20 and 40 minutes. This constellation explains the structure of the early warning system that was ultimately chosen.

Earthquake monitoring

The components of the tsunami warning system

The backbone of the early warning system is the earthquake observation system. The earthquakes are measured with a network of seismometers. At the same time, the displacements of the earth's surface are monitored with the help of stations that use the GPS satellite navigation system. The location of the seismometers and the structure of the network were chosen so that an earthquake - regardless of where it occurs - is registered within two minutes at at least three stations in the network. In this way, an earthquake can be located very quickly.

There are now more than 120 seismic stations in Indonesia. In addition, the earthquake early warning center in Jakarta went into operation in mid-2007, to which around 300 international earthquake stations in the Indian Ocean region are connected via satellite communication. With this system, earthquakes can be precisely located within three to four minutes to an accuracy of 20 to 30 kilometers. The strength of the quake is also calculated at the same time. The warning of the earthquake monitoring system is only the first step - it triggers the further chain of detection and action.

Detection and size prediction using oceanographic methods

Not every earthquake causes a tsunami. If you took all earthquakes into account, there would often be false alarms. To avoid this, the wave generated by the earthquake must be measured oceanographically. This is done with ocean floor pressure levels and specially equipped GPS buoys. They were installed in 10 strategically important locations along the west and south coast of Indonesia.

Ocean floor unit with pressure sensor

The oceanographic measuring systems each consist of an ocean floor unit and a buoy on the sea surface. To register the tsunami waves, use is made of the fact that the waves cause pressure changes on the ocean floor. The deep ocean acts like a filter that filters out pressure changes caused by surface waves and normal swell. The ocean floor unit is equipped with a pressure sensor, a process computer, a data storage unit and a communication modem for the acoustic transmission of the data to the buoy.

The buoy anchored above the ocean floor unit transmits the data to a data center via communications satellite. It is therefore equipped with a receiver modem for the acoustic signals from the ocean floor unit, an on-board computer, a data storage unit and satellite communication. Batteries and solar cells supply the buoy with energy.

Warning buoy with solar cells

The buoy itself can also record sea state data using GPS measurements of its vertical position. The registered signal is a superposition of the normal sea state and the tsunami wave. Because normal swell has significantly shorter wavelengths than tsunami waves, the two effects can be separated by mathematical filtering. The buoy enables a method to register a tsunami wave that is independent of the pressure measurement, which increases the safety of the overall system.

The oceanographic measurements are supplemented by observations of coastal levels, which in the case of Indonesia are installed on the islands off Sumatra and Java and on other islands in the Indian Ocean.

Simulation of the tsunami waves

Model calculations can be used to determine when a tsunami hits the coast and how high the waves pile up. In addition, the simulations provide detailed information about the damage potential of the tsunami and local differences in the effects. If the simulations for the entire Indian Ocean basin were only started after the earthquake had already happened, they would not be ready in time for the warning. That is why numerous simulations are carried out in advance. A large variety of possible epicentres along the Sunda Arc as well as possible earthquake strengths and earthquake crack lengths are taken into account. The simulations are collected in a database.

As soon as the monitoring system for the earthquakes and the oceanographic measurements indicate a tsunami, the most suitable simulation with the measured parameters is selected from the database. The parameters used for this are the location of the earthquake, the magnitude of the earthquake, the deformation of the earth's crust and the wave height in the deep ocean. The simulations, in turn, contain information on arrival times, wave heights on the beach and floodplain areas

A hazard map for the coastal sections in question is then created from the most suitable simulation. Together with other data - for example on settlement - this results in an initial picture of the situation for the authorities and the population. The entire process runs in a so-called decision support system, which forms the core of the early warning center, automated and within a few seconds. With the help of the scenarios and new incoming data, the assessment of the situation is continuously refined and stabilized.

Data centers and the early warning center

The data from the instruments available via satellite communication - seismometers, GPS stations, buoy systems and coastal gauges - converge in local and national data centers. There the data is analyzed and evaluated in a decision support system. Using special procedures, the employees examine the data for possible signs of a strong earthquake or the occurrence of an anomaly in the sea level.

The actual warning messages are then sent from the Indonesian national warning center in Jakarta. You use several channels for this. The main line of communication is the direct phone line to local police stations. These initiate specific measures - for example evacuations. In addition, other institutions in Indonesia are informed of the tsunami warning via the Internet and fax. In addition, SMS messages are sent and the radio and television stations are informed.

Create resources

Information event on evacuations

The creation of resources, known as “capacity building”, is a mainstay of the project. Scientists and technical staff are trained for technical operation, maintenance of the instruments and the further development of the system. A legal framework was also created in which all activities relating to early warning, disaster control and preventive measures such as the control of building standards or the creation of zoning plans were embedded. That didn't exist before.

But the most important aspect of early warning concerns the actual target group, i.e. the population. So that effective measures can be taken in view of the extremely short early warning times, awareness of a latent hazard and preventive protective measures must be awakened and strengthened among the population. In the event of an alarm, the people affected on site should react correctly. For this purpose evacuation exercises and regular information events are carried out. In addition, the schoolchildren already take special lessons. A working group in the early warning project deals specifically with these questions and with the implementation of preparatory measures such as the inclusion of risk and vulnerability maps in urban and landscape planning.