As current geodetic techniques approach the ability to monitor ground movements with millimeter accuracy over the broadband range of ~1 second to ~10 years, it becomes apparent that GGOS should be exploited for geohazard prediction and early warning systems. Natural phenomena that can be addressed by geodesy include earthquakes, tsunamis, landslides, volcanic eruptions, global climate change and coastal inundation associated with sea level rise and land subsidence. Here we consider the characteristics of GGOS that could meet the requirements of geohazard prediction and early warning, and discuss the implications for inter-operability and consistency between the various geodetic techniques to realize the full potential of GGOS.
Broadly speaking, successful prediction and early warning require two very different sets of requirements. On the one hand, prediction systems are characterized by high accuracy measurements, detailed modeling and understanding, and long term stability to provide a standard frame of reference as a basis for prediction. On the other hand, early warning systems are characterized by real-time sensitivity and automatic response to events, and robustness against false alarms. Since geohazards are often associated with long-term cumulative processes leading to precipitously damaging events, there is an obvious advantage if the two systems being used for prediction and early warning are developed within a self-consistent framework, as could be provided by GGOS. This way, the early warning system design can be better informed by the understanding gained from the prediction system. Prediction also helps to target the warning systems more efficiently.
As a concrete illustration, a specific example is drawn from recent research and development on the potential application of GGOS to trans-oceanic tsunami warning systems. Trans-oceanic tsunamis (that travel far across the ocean) are particularly difficult to detect, and can cause unexpected death and destruction on distant shores, such as in Africa several hours after the 2004 Sumatra earthquake. Such tsunamis are generated by the abrupt motion of the ocean floor, resulting from a very great earthquake (typically Mw > 8.5), that in turn results from the stress accumulation at the locked interface of a megathrust fault located at the boundary between two major tectonic plates that rotate independently around the Earth's center. GGOS is specifically well-adapted to monitor every link in this long physical chain of consequences, which is crucial for accurate prediction and effective early warning. Contributions of GGOS toward prediction start logically with the determination of the Earth's center and frame of reference, then plate rotation, then the accumulation of plate boundary strain, then determination of locked plate interfaces, which can be used to predict the potential locations of very great earthquakes. This then sets the stage as to where to focus early warning efforts, and educate affected populations. Contributions of GGOS toward early warning would logically start with the near real-time detection of ground displacements caused by earthquakes, then the rapid determination of earthquake magnitude and fault slip parameters, leading to the rapid prediction of ocean bottom displacement, which are input to tsunami models for rapid prediction of tsunami waves, and finally the direct confirmation of tsunami waves using space-borne and in-situ measurements. This is not to overstate the role of GGOS with respect to non-geodetic techniques, but rather to emphasize the logical completeness and internal consistency of geodetic-based systems, and to make the case that GGOS has a unique and important role to play.