Stress monitoring by observing changes in shear-wave splitting time from local small earthquakes. The routine observations are carried out by UEDIN.DGG (see Subproject 3). The results are checked and discussed with IMOR.DG (see Subproject 1).
Based on automatic fault plane solutions, horizontal compressions of individual fault planes of earthquakes down to magnitude 0 are observed in real-time. It shows that the strike of the horizontal compression axis is often very stable in Iceland. Exceptions of this have been observed in the preparatory stage of large earthquakes. These variations can be observed directly in real-time on the IMOR.DG website for some areas. A tool is available on the website to monitor other areas and time periods. This work is done both on completely automatic evaluated data as well as on manually checked data (Stefánsson et al. 2000d).
This algorithm defines time and place of a possibly approaching earthquake, on basis of seismicity rate and on sizes of fault planes of small earthquakes. The method has been developed under Subproject 2 and is described there. The algorithm has been checked at IMOR.DG and is routinely used there. See further in Subproject 2.
Spectral amplitude grouping (SAG) is a monitoring algorithm that can be a good indicator of long-term evolution of stress. Two previous algorithms are based on fault plane solutions of earthquakes. The fault plane solutions are based on spectral amplitudes of P- and S-waves, and signs of the P-waves. SAG is based on grouping the earthquakes by using spectral amplitudes of many different phases at some seismic stations. The method is applied to monitor the of state of stress in a region as described in Subproject 2.
Inversion for rock stress tensor based on microearthquake locations and fault plane solutions is a very significant tool in real-time investigations. Algorithms for this are available and routinely applied at the IMOR.DG, and described under Subproject 2.
An algorithm for relative location based on multievent location technique is available for investigation and testing at IMOR.DG. By this technique which provides relative locations for groups of microearthquakes with an error within 10 m, it is possible even to discriminate between the fault plane and the auxiliary plane (Rögnvaldsson et al. 1998b).