Seismological wave theory is largely based on the assumption of linear elasticity. In large parts of the Earth this is a very useful approximation. The stress-strain relation can be linearized over the strain interval of seismic waves. Under certain conditions, however, there are significant deviations from this linearization that carry information about the material which is not accessible to the conventional approach. Unlocking this information opens a new perspectives to investigate natural hazards with seismological means.

Heterogeneous Earth's materials are composed of competent parts that are connected by a much weaker bond system which occupies a tiny volume fraction, only. This holds on all spatial scales ranging from the difference between minerals and contacts to tectonic blocks separated by faults. The rheological difference between these constituents leads to strain accumulations in the weak part - significantly enhancing the nonlinearity of the assemblage in comparison to individual constituents. This form of nonlinearity is called non-classical nonlinearity that is characteristic of all heterogeneous materials like rocks and granular materials.

There are two faces of this non-classical nonlinearity. On one side it affects the propagation of elastic waves due to the nonlinearity of the stress strain relation. This effect is referred to as anomalous fast dynamics which leads for example to amplitude dependent propagation velocities and attenuation. It changes the waveforms of seismic signals. On the other side the nonlinear behavior results from internal damage of the bond system that recovers over time scales far slower than the wave propagation (slow dynamics). This damage softens the material leading to a long term reduction of the seismic velocity and gradual recovery after perturbing events. Presumably the internal damage also weakens the material with profound consequences for natural hazards resulting from material failure.

In the training network SPIN, new seismological measurement methods are combined with the investigation of this exotic material behavior in order to improve observation techniques and develop new approaches for the assessment of natural hazards. At GFZ this project is a collaboration with section 4.2 Geomechanics and Scientific Drilling.