We use the scattered seismic waves and ambient seismic noise to investigate structures and processes in the solid Earth. Both types of signals are often considered nuisances in classical seismology but can in fact provide useful and complementary information.

Scattered waves are caused by the heterogeneity of the Earth, which extends in size from the scale of continents down to the scale of mineral grains assembled in a rock specimen. While we can use seismological techniques to image structures above a certain size, structures that are smaller than this resolution limit influence the seismic waves in a complex way that cannot be described with classical means. These scattered waves are integral part of any high frequency seismogram and cause a complex wave train that form a coda of each deterministic wave arrival (direct and reflected/converted at major boundaries). Since scattered waves interrogate the propagation medium at length scales below the resolution limit of classical methods we can investigate the distribution of small scale heterogeneity and attenuation inside the Earth.

The second cause of complexity in any seismic record is the presence of ambient sources of seismic waves. Oceans, atmosphere, industrial activity and small but numerous earthquakes which are not individually observable excite seismic waves in an unpredictable way, generating the ambient seismic field or seismic noise. Using seismic interferometry we can use the continuous presence of the seismic noise to monitor time-dependent minute perturbations of the elastic properties in the propagation medium, which are sensitive to hydrological influences, the tides, shaking and other effects.

With this research we are contributing to the Topic “Living on a Restless Earth – Towards Forecasting Geohazards” of the Helmholtz research programme “Changing Earth – Sustaining our Future”, Subtopics 3.1 “The drivers of geohazards” and 3.2 “Temporal variations and the late stage before catastrophic events”.