Automatic near real-time determination of co-seismic displacements (Ground Tracking System, GTS): GFZ

Automatic near real-time determination of co-seismic displacements (Ground Tracking System, GTS)

The strength (magnitude) and location (epicenter and depth) of a strong earthquake can be determined exactly and fast with the measurement systems and methods of seismology. Seismology also allows an estimation, but until now, not an accurate determination of possible movements (deformations) of the Earth’s surface in the earthquake’s area. To give the important answer to the question, whether a strong earthquake has generated a tsunami or not, the knowledge of these co-seismic deformations is of very high relevance for tsunami early warning. Help comes from GPS-instrumentation (GPS-stations) and a special analysis system which detects and precisely determines ground motions on the Earth’s surface. GPS-based displacement measurements have their strength close to earthquake epicenters, where displacement values are high. Seismological sensors close to an epicenter may have problems (oversteer effects) while measurements from seismological sensors being farer away are delayed according to seismic signal travel times. In combination with other techniques (determination of the mechanism and area that generates a Tsunami) the GPS-based displacement determinations may increase the reliability of tsunami early warnings significantly (figure 1).

The image shows three simplified diagrams on the determination of displacement vectors on tectonic plates with GNSS-measurements, which allow conclusions on the probability for the generation of a Tsunami. In the first case a tsunami generation may be unlikely or the tsunami may be small, due to a strike slip movement of the tectonic plates with no vertical displacement. A higher probability for a bigger tsunami is given in the other two cases with vertical movements of the plates. In the third case only a part of an involved plate is moved, which corresponds to a smaller Tsunami generation area resulting in a smaller Tsunami. — Figure 1: Simplified relation between earthquakes, tectonic plates movements, GNSS-based displacement determination and tsunami generation: In case a) a tsunami generation may be unlikely or the tsunami may be small (strike slip mechanism, no vertical displacement), while there is a higher probability for a bigger tsunami in cases b) and c) with vertical movements. In case c) only a part of plate 2 is moved (plate 2b) which corresponds to a smaller tsunami generation area as in case b). *(C. Falck, GFZ)*

A system for an automatic determination of ground motions, respectively co-seismic deformations, was developed by GFZ within the project GITEWS. It is supported by an automatic GPS processing system for near real-time processing of continuously measured data from GPS-stations in Indonesia and surrounding areas. This system has been established at the Indonesian warning center as the so called Ground Tracking System. It can deliver first results (3-d displacement vectors) for relevant locations with GPS-equipment being close to the earthquake within less than 3 minutes, with an update interval of 2 minutes in the further process.

The image shows a simulated displacement vectors as they would have been displayed by the GTS on December 26th 2004 just a few minutes after the Sumatra-Andaman Earthquake. Basis for the simulation are subsequently determined movement parameters for GNSS-station locations, displayed as green symbols, caused by the movements of the tectonic plates. In point of fact, at the time of the strong earthquake and the following tsunami there was no GTS and no useable network of GNSS-stations in Indonesia. — Figure 2: Simulated view of displacement vectors as they would have been displayed by the GTS on December 26th 2004 just a few minutes after the Sumatra-Andaman Earthquake. Basis for the simulation are subsequently determined movement parameters for GNSS-station locations (green symbols), caused by the movements of the tectonic plates. In point of fact, at the time of the strong earthquake and the following tsunami there was no GTS and no useable network of GNSS-stations in Indonesia. *(C. Falck, GFZ)*

Precise information about possible movements is also delivered for tide gauge locations, if they have appropriate GPS-equipment onsite. If the location of a tide gauge station has moved during an earthquake, then the corresponding sea level data need to be corrected or have to be discarded for early warning applications.

Reference: Falck, C., Merx, A., Ramatschi, M. (2013): Design and benefit of GFZ's GNSS-based Ground Tracking System (GTS) - Poster, The IAG Scientific Assembly 2013, 150th Anniversary of the IAG (Potsdam 2013).