New scientific processes and innovative technologies distinguish this system from the previous tsunami warning systems. Due to the specific geological situation in Indonesia, the previously used, established tsunami warning systems are not optimal for Indonesia. The earthquakes in the Indian Ocean at Indonesia originate along the Sunda Trench, a subduction zone which extends in an arch from the northwest tip of Sumatra to Flores in eastern Indonesia. If a tsunami originates here, in an extreme case, the waves reach the coast within 20 minutes, so that only very little time remains for an early warning. Therefore, the concept of the entire system was based on this prevailing condition.
Technical concept of GITEWS
Due to the local geology, the advance warning time is extremely short. Therefore, an alarm must be triggered within five minutes after a strong earthquake. That is why a new approach was developed, which is primarily based on model-based coupling of seismological data with GPS measurements and level measurements.
More than 300 sensors are distributed across all of Indonesia and supply their data to the warning centre in real time. From this, a newly developed, automated Decision Support System (cf. below Decision Support System DSS) compiles a picture of the situation from this, on the basis of which the decision is made whether to issue an alarm.
Therefore, the following steps are taken in the process:
- Quake location and strength: Ascertainment with seismological data; all earthquakes (worldwide) are recorded. All earthquakes M≥2 are evaluated in the national warning centre. Earthquake information is basically provided. Tsunami warning alerts are only issued if a tsunami is expected (earthquake magnitude >7).
- Fracture mechanism: only strong sub-oceanic earthquakes with a distinct vertical component can cause tsunamis. An initial assessment as to whether the ocean floor has moved vertically can be determined using land fixed points (cf. below GPS Shield).
- Ascertainment of a tsunami: On the coasts and on the offshore islands of Indonesia, tide gauges were installed with GPS components, which monitor the sea level. The data are integrated into the warning process (cf. below GPS Level).
- Decision-making: within less than five minutes, the decision can be made as to whether a warning needs to be issued and - if so - to which coastal sections (cf. below DSS).
GITEWS forms the core structure of the InaTEWS Indonesian Tsunami Early Warning System. With the setup of GITEWS, due to the specific conditions of Indonesia, with its extremely short advance warning times, the experiences of the previously existing tsunami early warning systems for the Pacific in the USA and Japan could only be exploited to a limited extent. As a result of this challenge, the newly developed components and procedures and their interaction in GITEWS/InaTEWS make the system one of the most state-of-the-art tsunami early warning systems worldwide.
The basic requirement for the early warning system is fast and reliable ascertainment of the site and magnitude of an earthquake. SeisComp3 was developed by the GEOFON working group of the GFZ and can reliably determine earthquake strength and location within around four minutes, even with strong earthquakes. This makes SeisComp3 unique worldwide. GFZ provided this system to the community free of charge, so that all countries bordering the Indian Ocean have implemented this system quasi as standard.
Strong quakes cause a considerable horizontal and vertical displacement on the Earth's surface, which can be several metres long, both horizontally and vertically and can be measured with GPS. Subject to an accordingly dense measurement network, this "GPS Shield", together with the seismological data, is able to characterise the earthquake fracture within 5 minutes, so that the strength and expansion of a tsunami can be calculated. This new procedure has been made ready for use in GITEWS and is now used as a standard method for tsunami identification in the near field.
GPS tide gauges monitor the sea level. The changes in water level caused by a tsunami are recorded and integrated into the warning process. The GITEWS gauges record the changes using three types of sensors: Pressure, radar and floaters and are additionally equipped with GPS receivers to determine a possible vertical displacement of the surface. In the meantime, reliable level data are not only available in Indonesia, but also in other countries bordering the Indian Ocean. The data are also available in public databases of the IOC. Webcams are also installed for observation at individual exposed sections of coastline.
The Decision Support System is one of the key elements of the warning centre in Jakarta. The results of the sensor data networks merge here, are compared to pre-calculated modelling and thereby create a picture of the situation and propose a warning alert, if necessary, which must then be released by the scientists on duty. If necessary, pre-calculated risk maps can also be displayed for decision-making. The DSS was developed by the German Aerospace Centre (DLR) within the context of the GITEWS.
The situation assessment and generation of warning alerts is based on modelling results. From a small amount of data, which is available within the first approx. 5 minutes after the occurrence of an earthquake (earthquake location, magnitude, information of the GPS Shield, if applicable), an extensive situation status can only be generated using modelling. This takes place, on the one hand, with pre-calculated, high-resolution scenarios in a database and on the other hand (and only in the last few years) through a less high-resolution, but online calculating computer process. In addition to the tsunami calculation (running time to the coast, wave height at the coast), the high-resolution scenarios also contain calculations of the subsequent floods, which is a crucial input factor for all risk assessments and e.g. evacuation measures. This is supplemented with constant updates using the online tool. Therefore, both options (pre-calculated scenarios and online tool) will always be used. The dispatch of the warning alerts by BMKG takes place through various and technically autonomous communication channels and is defined by "standard operating procedures".
Tsunami buoys (also referred to as tsunameters) are not autonomous WARNING systems. In all tsunami warning systems worldwide, they are MEASUREMENT instruments for the verification of a tsunami. The most important information, namely, the fast earthquake location and magnitude, without which either a simulation or a warning can be generated, can NOT be supplied by buoy systems.
Buoy systems for the direct measurement of a tsunami were initially part of the research concept. The further development of the GPS Shield made it possible to discontinue pursuing the buoy concept. Therefore, buoys have no longer by part of the operational warning system since 2010, so that the high maintenance cost of buoy installations near coastlines can also be omitted.
The system is based on 300 different land-based sensor systems. The data from these sensors are transferred in real-time to the control room in the warning centre and are aggregated there in the state-of-the-art Decision Support System (DSS) and implemented into a situation status. The warning takes place on the basis of very fast, precise earthquake recording and evaluation, which forms the heart of the warning system. The fast determination of earthquake parameters (location, depth, magnitude) through 160 seismometers on land is the first and most important basis for the tsunami preview through modelling and the generation of a warning alert, which is based on this. The first situation status is then substantiated further through additional data from GPS stations and tide gauges along the coast of Indonesia. The verification of a tsunami takes place with tide gauges, which are also equipped with GPS sensors.
Right from the start, GITEWS was planned with an end-to-end approach. This is comprised of setting up instrument networks for measuring the natural disaster (tsunami, earthquake), the decision-making support on the basis of a modelling system for generating situation assessments, a country-wide risk assessment with the creation of hazard, vulnerability and risk maps and the capacity development with authorities, local decision-makers and administrations, as well as affected local companies and the hotel industry. This work on the various fields of activity was performed in parallel right from the start, whereas constant coordination took place between the fields of activity and the national and international partners involved.
The installation phase of GITEWS was characterised by the development of necessary system components, on the one hand, and by the development of appropriate strategies, information materials, standards and approaches, on the other hand. The BMKG (Badan Meteorologi, Klimatologi dan Geofisika), operator of the early warning system, reached the operational status step-by-step and over various phases of setting up the system. During the GITEWS phase (2005 to 2011), a series of German institutions were involved, under the auspices of the German Centre for Geosciences GFZ, whose task was the technical setup of the system. The contributions of other donor countries were integrated. Capacity development measures in the downstream area (Disaster Reduction Strategy) were implemented in pilot regions, in cooperation with the local administrations and the population. The approaches, processes and products, e.g. the TsunamiKiT are transferable to other parts of Indonesia and form the bases for country-wide implementation.