ALBERT

Description: Synthetic portlandite single crystals were used to measure thermal diffusivity and elastic constants. The full tensor of elastic constants cijkl is derived by Brillouin spectroscopy at ambient conditions. The resultant aggregate bulk and shear moduli are KS, VRH=32.2(3) GPa and GVRH=21.2(2) GPa, respectively. The thermal diffusivity D was measured from −100∘C to 700∘C parallel [001] and perpendicular [100] to the crystallographic c-axis using laser flash method. The dehydration of the crystals influences the thermal diffusivity determination depending on sample size, orientation and heating rate. Thermal diffusivity and the derived thermal conductivity show a pronounced anisotropy with a maximum perpendicular to the c-axis, i.e. in the plane of the [CaO6] octahedral layers. In the same direction the highest sound velocities (vP and vmean) and longest mean free path length of phonons are determined. The thermal diffusivity as well as the derived thermal conductivity show a distinct temperature dependence.

Description: Being a part of ongoing continental collision between the Arabian and Eurasian plates, the Caucasus region is a remarkable site of moderate to strong seismicity, where devastating earthquakes caused significant losses of lives and livelihood. In this article, we survey geology and geodynamics of the Caucasus and its surroundings; magmatism and heat flow; active tectonics and tectonic stresses caused by the collision and shortening; gravity and density models; and overview recent geodetic studies related to regional movements. The tectonic development of the Caucasus region in the Mesozoic-Cenozoic times as well as the underlying dynamics controlling its development are complicated processes. It is clear that the collision is responsible for a topographic uplift / inversion and for the formation of the fold-and-thrust belts of the Greater and Lesser Caucasus. Tectonic deformations in the region is influenced by the wedge-shaped rigid Arabian block indenting into the relatively mobile region and producing near N-S compressional stress and seismicity in the Caucasus. Regional seismicity is analysed with an attention to sub-crustal seismicity under the northern foothills of the Greater Caucasus, which origin is unclear – whether the seismicity associated with a descending oceanic crust or thinned continental crust. Recent seismic tomography studies are in favour of the detachment of a lithospheric root beneath the Lesser and Greater Caucasus. The knowledge of geodynamics, seismicity, and stress regime in the Caucasus region assists in an assessment of seismic hazard and risk. We look finally at existing gaps in the current knowledge and identify the problems, which may improve our understanding of the regional evolution, active tectonics, geodynamics, shallow and deeper seismicity, and surface manifestations of the lithosphere dynamics. Among the gaps are those related to uncertainties in regional geodynamic and tectonic evolution (e.g., continental collision and associated shortening and exhumation, lithosphere structure, deformation and strain-stress partitioning) and to the lack of comprehensive datasets (e.g., regional seismic catalogues, seismic, gravity and geodetic surveys).

Description: Lava dome growth is a surface manifestation of the subsurface magma dynamics. We study the dome growth at Volcán de Colima during 2007-2009 using quantitative modelling of history matching. This modelling is based on minimization of misfits between the morphological shapes of modeled and observed domes using model parameters such as the characteristic crystal content growth time and the eruption temperature. We consider two models: (a) a dome growth model where the lava viscosity depends on the volume fraction of crystals, and (b) a thermomechanical model of the dome growth, where the lava viscosity depends on both the volume fraction of crystals and the temperature. In model (a), the evolution of the modelled lava dome for several months agrees well with the observed lava dome. Meanwhile, to match the dome growth for a few years, a highly thin carapace is introduced in the model preventing the dome to significantly advance laterally. The model carapace mimics a colder and more viscous part of the uppermost lava dome. In model (b), we analyse the dome growth depending on the heat transfer at the dome’s interface with the air, conductive heat transfer at the crater’s floor, and the latent heat of crystallization. The carapace develops self-consistently in response to a cooling at the dome’s interface. For dome-building episodes of about few months, dome growth can be well described by a model with crystallization due to degassing. For longer dome-building episodes, cooling plays a significant role in the dome building.

Description: The prognosis of the state of stress in the subsurface can be improved by using numerical models in addition to data from stress measurements, as such models allow for consideration of variability stemming from structural complexities and an inhomogeneous distribution of rock properties. These models are generally set up in two consecutive steps. In the first step one an initial stress is established that accounts for gravity and a reasonable ratio of horizontal to vertical stress. This represents a reference stress state in the absence of tectonic stress and ensures equilibrium with gravity: i.e. no strain is produced once gravity acts. In the second step tectonic stress is included via displacement boundary conditions which induce horizontal differential stress to come up with the final stress state. Both initial stress and tectonic stress are chosen in such a way that calibration data are reproduced by the model at the locations and depths where the data were measured. We present generic models to investigate whether the choice of initial and tectonic stress affects the final state of stress in areas of the model domain where no stress data are available. We find that there is in general an ambiguity as different combinations of initial stress and tectonic stress yield the same final state of stress at the points where data are available. However, in those areas of the model domain where calibration data do not exist, these different choices of initial stress and tectonic stress produce differing results. These deviations are largest in the vicinity of lithological interfaces. We find that this ambiguity is reduced if more stress data exist, particularly in not just one lithology and at one depth. On the other hand, if more data are available, it becomes increasingly more difficult to find a combination of initial and tectonic stress to match them all. In view of the uncertainties of the data, such deviations between modelled stress and data may be expectable to some degree. However, such deviations may indicate that inelastic rock properties do play a role in some lithologies.

Description: Fault reactivation potential is a crucial aspect for many underground utilizations, such as the construction and long-term safety of a nuclear waste repository, as seismic events can endanger these operations. An estimation of the fault reactivation potential requires information about the stress field, but stress data are only available pointwise and are not evenly distributed throughout Germany. Geomechanical–numerical modeling can be used to derive a spatially continuous description of all six independent components of the stress tensor as shown by the model of Germany by Ahlers et al. (2022). Information about the geometry of faults extending several kilometers in depth is provided for most areas in Germany by the geological models of the federal states and geological models created in the framework of projects such as GeoMol (Assessing subsurface potentials of the Alpine Foreland Basins for sustainable planning and use of natural resources) or GeORG (Geopotenziale des tieferen Untergrundes im Oberrheingraben). We use the 3D fault geometries provided by such models and map the stress data from the Germany model by Ahlers et al. (2022) onto these faults. Then, assuming hydrostatic pore pressure, we calculate the so-called slip tendency (TS), the ratio between resolved shear stress and the effective normal stress on the fault plane as a measure of fault reactivation potential. A fault is considered critical when its TS value exceeds its coefficient of friction. In general, TS ranges between 0 and 0.7 for the analyzed faults. The highest overall TS values are observed along the NNE–SSW-striking Upper Rhine Graben, where TS routinely reaches and exceeds values of 0.7. In the North German Basin, the Ore Mountains and Saxony only very few TS values exceed 0.7. The area with the lowest overall TS is the Molasse Basin, where the TS of the mostly WSW–ENE-striking faults only rarely exceeds values of 0.4. In general, N–S- to NNE–SSW- and NW–SE-striking faults show the highest TS values, whereas WSW–ENE-striking faults show the overall lowest values. With increasing depth, TS decreases. Pore pressure and overpressure have the potential to significantly influence the resulting TS.