Abstract

Friction is a well-known, yet little understood phenomenon in mechanical, as well as geological, systems. It not only dissipates energy, but in metals and alloys also alters surface and subsurface properties by modifying the internal structure of the material, aka its microstructure. Under tribological loading, phenomena such as dislocation self organization, grain refinement and oxidation can occur. The complete rotation of the entire subsurface parts of the materials was observed as an underlying mechanism that takes place prior to grain refinement.
In this talk, we will look at all of these phenomena, as they take place in metals and alloys. It will be shown how modern materials science tools in combination with model experiments, can illuminate basic deformation mechanisms in metallic materials subjected to a frictional shear. We will then briefly describe, how these general phenomena can also be found in ceramic materials, despite their different binding states.

Abstract

Abstract1:

The UVT project was designed to develop, construct, and validate a novel urban mobile seismic vibrator through geophysical field experiments. The engineering development of the vibrator has been led by Herrenknecht GmbH, while field validation is currently being conducted by the Geophysical Institute at the Karlsruhe Institute of Technology (KIT). A principal objective of the project is to validate the ground force emitted by the vibrator using multiple methodologies, thereby allowing for a comprehensive evaluation of the system’s performance. These methodologies necessitate homogeneous subsurface conditions to decouple the effects of vibrator performance from those of ground heterogeneity.

This work presents the first geophysical characterization of the subsurface at the Herrenknecht UVT test site. The objective was to provide an initial model for future full-wave inversion processes and for subsequent validation of the ground force signal generated by the urban vibration truck. To achieve this, a one-dimensional shear-wave velocity (1D VS) profile was created using the multichannel analysis of surface waves (MASW), along with an estimate of the Rayleigh wave quality factor (Q) using the spectral ratio method.

The MASW methodology was implemented through a structured three-step approach: seismic data acquisition, dispersion analysis, and inversion analysis. The resulting 1D VS models revealed significant near-surface heterogeneity—up to 2 m in one of the measured profiles, attributed to variable fill material. In contrast, the deeper layers exhibited consistent stratigraphy and velocity structures. The attenuation properties were estimated using the spectral ratio method, which had first been validated through analytical and synthetic viscoelastic models prior to application to the field data. The resulting Q values provided a global measure of site attenuation and demonstrated robustness under well-controlled conditions.

Abstract2:

The vp/vs ratio provides insights into the medium properties, as fluid content, porosity and stress distribution. A commonly used method to estimate the vp/vs ratio in situ within spatial closely distributed earthquakes is the approach proposed by Lin and Shearer (2007). There, pairs of differential P- and S-arrival times measured at the same station are used to estimate the vp/vs ratio within the earthquake cluster. The vp/vs ratio is the slope of a line fitted to differential P- and S- times after subtraction of the mean value over all stations. This statistical approach is highly dependent on the geometry of the earthquake locations and the station distribution. Only in a medium with a homogeneous vp/vs ratio, the estimated vp/vs ratio has no bias, as shown by Palo et al. (2016), which limits the usage of “Lin and Shearer”.

In this master thesis, a new method is developed to overcome these limitation, as it is directly inverted for the vp/vs ratio after a precise location, e.g. with the double-difference earthquake location of Waldhauser and Ellsworth (2000). The new approach is based on the ansatz of relative location presented by Fréchet (1985). In an iterative process the double difference relocation of the earthquake hypocentres is combined with the new method, that uses P and S differential time pairs, to improve locations and velocity estimates. The new method is tested in a 2D and 3D subduction zone setting and applied to the dataset of the 2022 Mw 5.8 Esmeraldas earthquake. The observation of pair azimuthal anisotropy within the seismogenic interface in the subduction zone offshore Ecuador is modelled using the crack-induced anisotropy relations of Schubnel and Guéguen (2002).