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

The strength (magnitude) and location (epicentre and depth) of a strong earthquake can be determined accurately and quickly 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 of whether a strong earthquake has generated a tsunami or not, 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 epicentres, where displacement values are high. Seismological sensors close to an epicentre may have problems (over steer effects) while measurements from seismological sensors that are further away, are delayed according to seismic signal travel times. In combination with other techniques (determination of the mechanism and area that generates a Tsunami), measurement values that are determined using GPS technology may increase the reliability of tsunami early warnings significantly (Figure 1).

Figure 1: Simplified relationship between earthquakes, tectonic plate movements, GNSS-based displacement determination and tsunami generation. In case a) the generation of a tsunami is less likely, a generated tsunami would tend to be small (tectonic plates only move laterally to one another, without vertical displacement). In cases b) and c), there is a higher probability for the generation of a larger tsunami, as the tectonic plates have also moved vertically towards one another. In case c), only a part of plate 2 is moved, which corresponds to a smaller tsunami generation area.

A system for the automatic determination of ground motions, respectively co-seismic deformations, was developed by GFZ within the context of the GITEWS project. It is based on an automatic GPS processing system for near real-time processing of continuously measured data from GPS stations in Indonesia and surrounding areas. This system, introduced at the Indonesian warning centre as the so called Ground Tracking System, can deliver first results (3D 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.

Figure 2: Simulated view of displacement vectors (red arrows and bars), as they would have been displayed by the GTS on 26 December 2004, just a few minutes after the Sumatra-Andaman Earthquake. Subsequently determined movement parameters for GNSS station locations (green symbols), caused by the movements of the tectonic plates, form the basis for the simulation. In fact, at the time of the strong earthquake and the subsequent tsunami, there was no GTS in Indonesia and no regional networks of GNSS stations that were usable for early warning functions.

Precise information about possible movements is also delivered for tide gauge locations, provided that they are fitted with appropriate GPS equipment. If the location of a tide gauge has moved, e.g. during an earthquake, the corresponding sea level data needs to be corrected or 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).
Link to poster: PDF, 1156KB