. (2006): PSGRN/PSCMP -a new code for calculating co-and post-seismic deformation, geoid and gravity changes based on the viscoelasticgravitational dislocation theory.

[1] We discuss the impact of uncertainties in computed coseismic stress perturbations on the seismicity rate changes forecasted through a rate-and state-dependent frictional model. We aim to understand how the variability of Coulomb stress changes affects the correlation between predicted and observed changes in the rate of earthquake production. We use the aftershock activity following the 1992 M7.3 Landers (California) earthquake as a case study. To accomplish these tasks, we first analyze the variability of stress changes resulting from the use of different published slip distributions. We find that the standard deviation of the uncertainty is of the same size as the absolute stress change and that their ratio, the coefficient of variation (CV), is approximately constant in space. This uncertainty has a strong impact on the forecasted aftershock activity if a rate-and-state frictional model is considered. We use the early aftershocks to invert for friction parameters and the coefficient of variation by means of the maximum likelihood method. We show that, when the uncertainties are properly taken into account, the inversion yields stable results, which fit the spatiotemporal aftershock sequence. The analysis of the 1992 Landers sequence demonstrates that accounting for realistic uncertainties in stress changes strongly improves the correlation between modeled and observed seismicity rate changes. For this sequence, we measure a friction parameter As n % 0.017 MPa and a coefficient of stress variation CV = 0.95.

S U M M A R YIntensive global positioning system (GPS) monitoring after the 1999İzmit earthquake provides an opportunity to understand the postseismic behaviour of a strike-slip fault and the rheology below the brittle upper crust. Two data sets are available: displacements measured during the first 300 days after theİzmit earthquake and velocity measurements between 2003 and 2005. Using an inversion method and a forward modelling, respectively, we investigate two mechanisms: (1) afterslip on and below the coseismic rupture plane and (2) viscoelastic stress relaxation in the lower crust and upper mantle described by a Maxwell or a standard linear solid (SLS) rheology. The inversion results show that the first several months following thė Izmit earthquake were dominated by afterslip at depths shallower than 30 km and the slip amount decayed with time; after that, apparent afterslip has a very different spatial distribution and is located much deeper. For viscoelastic relaxation, a model with an elastic upper crust and a Maxwell viscoelastic lower crust overlying a Maxwell mantle (E-M-M) fits the data measured in the first 300 days better in the far field than in the near field. However, the observed far-field, 300-day displacement and the long-term (2003)(2004)(2005) displacement, which might be dominated by viscoelastic relaxation, cannot be described by a Maxwell rheological model with constant viscosity: the effective viscosity increases over time. Therefore, we have built a refined rheological model: an elastic upper crust and an SLS lower crust overlying a Maxwell viscoelastic mantle (E-SLS-M). Our best solution yields a viscosity for the lower crust of ∼2 × 10 18 Pa s, a relaxation strength of 2/3 and a viscosity for the Maxwell mantle of 7 × 10 19 Pa s. Finally, we explain the data using a composite model, consisting of the preferred E-SLS-M model and the afterslip model obtained from the residual displacement after correcting for viscoelastic relaxation. For the early time period, the residual displacements can be mainly explained by a shallow afterslip whose magnitude decays with time and whose spatial distribution is stable, whereas the residual displacements for the later time period require negligible afterslip. It indicates that the postseismic deformation in the later time period induced by a deep source can be almost entirely explained by the E-SLS-M model. The composite model can generally explain the data in the entire spatial and temporal space.

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Knowledge about the acting stresses is of crucial importance for understanding the tectonics of a region. Data about the stress ®eld in north-eastern Germany used to be very rare. In general, it was assumed that the orientation of the larger horizontal principal stress (S H ) is similar to that found for western Germany and central West-Europe, i.e. NW±SE. To check this, several borehole logs of the late 1980s were analysed for information on the principal horizontal stress orientations: they include Four-Arm-Dipmeter and borehole televiewer data from 15 boreholes. The depth range of our stress results reaches from 1500 to 6700 m. They were compared to a few other data, especially from hydraulic fracturing, and to recent ®ndings on the stresses in the Northwest German basin. In contrast to expectation, S H derived from breakout orientations below the salt layers displayed N to NE orientation. The latter was found at 10 locations spread over the NE-German basin from Berlin to the Baltic sea, from the Polish border to the former border between East and West Germany. Moreover, this stress rotation in the subsaline formations seems to be the continuation of a trend found in the NW German basin.