through time. The back azimuth of a incoming P-wave is expected to change accordingly while the earthquake source is moving away from the hypercentre. By polarization analysis we may be able to monitor the temporal change in P-wave back azimuth in order to follow the rupture progress in near real time. Three component P-wave seismograms are scanned to determine the azimuthal variation as a function of time. Back azimuth of the moving rupture front is determined by calculation of co-variance matrix. Seismic stations in local and regional distances (less than ~30°) are suitable for the method, partially because there are no disturbing multiple P-waves (e.g., PP) and partially as the effect of 3d earth heterogeneity on polarization analysis is getting more significant for larger distances. We note that the method may only track part of rupture length for very large earthquakes due to early arrival of S-waves. We note also that the method does not work for bi-lateral earthquake propagation. We tested the new method with theoretical simulation of the Great Andaman earthquake (December 26th, 2004, Mw = 9.3) and with real data from the same earthquake as well as from the Sichuan earthquake (May 12nd, 2008, Mw = 6). Introduction of the new method The main idea bases upon a grid search of the smallest orthogonal distance weighted with
the correspondent linearity (weighted-distance grid). To be more precise, for each grid point and for each time point and additionally for each station the (spherical and orthogonal) distances to a ray that points in direction of the corresponding polarization value is being calculated and weighted with the corresponding linearity. The latter is essential as it helps to qualify a measurement by enhancing very good data sets. This procedure yields for each grid point on the Earth’s surface a weighted sum of the distances to all rays that are calculated from the time dependent P-polarization at the seismological stations. Finally, the smallest value within the summed weighted-distance grid is being determined and can then be linked to the location, where the P-wave was generated. By this means the uni-lateral rupture propagation can be monitored. New method verified with synthetics Working with synthetics is important to verify an idea. For that purpose, we used the algorithm of R. Wang (pers. comm., 2009) which is based on the calculations of Greens functions for a multi-layered half-space (Wang, 1999b). The program enables the modeling of an extended source and thus we modeled the rupture of the Great Andaman earthquake with 30 single sources (orange dots on Fig. 4) according to the rupture geometry obtained by
Fig. 4. Rupture propagation of about 240 s of a moving source (blue crosses) derived from synthetics.
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