Short communication: Runout of rock avalanches limited by basal friction but controlled by fragmentation

Rock avalanches produce exceptionally long run-outs that correlate with their rock volume. This relationship has been attributed to the size-dependent dynamic lowering of the effective basal friction. However, it has also been observed that run-outs of rock avalanches with similar volumes can span several orders of magnitude, suggesting additional controlling factors. Here, we analyse analogue models of rock avalanches, with the experiments designed to test the role of dynamic fragmentation. We show that for a fixed low basal friction, the run-out of experimental rock avalanches varies over 2 orders of magnitude and is determined by their degree of fragmentation, while the basal friction acts only as an upper limit on run-out. We interpret the run-out's dependence on fragmentation as being controlled by the competition between mobility enhancing spreading and energy-consuming fragmentation limited by basal friction. We formalize this competition into a scaling law based on energy conservation, which shows that the variation in the degree of fragmentation can contribute to the large variation in run-out of rock avalanches seen in nature.

Short communication: Runout of rock avalanches limited by basal friction but controlled by fragmentation

Rock avalanches display exceptionally long runouts, which are found to correlate with their volume and attributed to size dependent dynamic lowering of the effective basal friction. However, even for similar volumes, runouts are seen to span several orders of magnitude suggesting additional controlling factors. Here, we here analyse experiments with analogue models of rock avalanches aimed at testing the role of dynamic fragmentation. We show that for a fixed low basal friction, the runout of experimental rock avalanches varies over two orders of magnitude and is determined by their degree of fragmentation while the basal friction acts only as an upper limiter. We interpret the runout's dependence on fragmentation to be controlled by the competition between mobility enhancing spreading and energy consuming internal friction. We formalize this competition into a scaling law based on energy conservation which shows that variation in the degree of fragmentation can explain the large variation in runout of rock avalanches seen in nature.

The StraboSpot data system for geological research

<p><strong>The StraboSpot digital data system is designed to allow researchers to digitally collect, store, and share both field and laboratory data.  Originally designed for structural geology field data, StraboSpot has been extended to field-based petrology and sedimentology.  Current efforts will integrate micrographs and data related to microscale and experimental rock deformation. The StraboSpot data system uses a graph database, rather than a relational database approach.  This approach increases its flexibility and allows the system to track geologically complex relationships. StraboSpot currently operates on two different platform types: (1) a field-based application that functions with or without internet access; and (2) a web interface (Internet-connected settings only). </strong></p><p><strong> The data system uses two main concepts - spots and tags - to organize data. A spot consists of a specific area at any spatial scale of observation.  Spots are related in a purely spatial manner, and consequently, one spot can enclose multiple other spots that themselves contain spots.  Spatial data can thus be tracked from regional to microscopic scale.  Tags provide conceptual grouping of spots, allowing linkages between spots that are independent of their spatial position.  A simple example of a tag is a geologic unit or formation. Multiple tags can be assigned to any spot, and tags can be assigned throughout a field study. The advantage of tags is their flexibility, in that they can be completely defined by individual scientists. Critically, tags are independent of the spatial scale of the observation. Tags may also be used to accommodate complex and complete descriptions. </strong></p><p><strong>The strength of the StraboSpot platform is its flexibility, and that it can be linked to other existing and future databases in order to integrate with digital efforts across the geological sciences.  The StraboSpot data system – in coordination with other digital data efforts – will allow researchers to conduct types of science that were previously not possible and allows geologists to join big data initiatives.</strong></p>

Thermodynamic consistent formulation for the multiphysics of a brittle ductile lithosphere - semi-brittle semi-ductile deformation and damage rheology

<p>We provide details on a novel formulation derived to describe the multiphysics controlling the deformation of porous rock under lithospheric conditions. The theory is developed consistent with the principles of thermodynamics and enables to capture the behaviour of porous rocks at the transition from frictional brittle behaviour to ductile viscous behaviour. It also accounts for the nonlinear feedback mechanisms derived from energetic consideration for the bi-phasic fluid-rock matrix system.</p><p>The formulation depicts a consistent, implicit visco-elasto-(visco)plastic rheology accounting for both a volumetric and a deviatoric response to applied loads, thereby avoiding the use of, the commonly assumed, plasticity limiter concept. The overstress plastic formulation introduces rate dependent mechanical behavior, an aspect that is consistent with experimental rock mechanics evidence and is also demonstrated to improve numerical stability when addressing problems related to plastic strain accumulation even in the absence of energetic feedbacks.</p><p>The introduction of a damage rheology permits to account for microstructural processes responsible for brittle-like material weakening and rate-dependent dissipative material behavior. The presence of a fluid phase is considered via a dynamic porosity, the evolution of which is demonstrated to primarily control the volumetric mechanical response of the stressed rock during incremental loading.</p><p>The above formulation has been integrated in a massively parallel, open source numerical framework with interfaces to state of the art HPC clusters. The results of a scalability and profile performance analysis on multi-core supercomputer are presented alongside with dedicated applications describing lithospheric rock deformation under different confining conditions as well as the bulk macroscopic material response recorded by laboratory experiments under triaxial conditions.</p>

Introduction to this special section: Rock physics

Rock physics links the physical properties of rocks to geophysical and petrophysical observations and, in the process, serves as a focal point in many exploration and reservoir characterization studies. Today, the field of rock physics and seismic petrophysics embraces new directions with diverse applications in estimating static and dynamic reservoir properties through time-variant mechanical, thermal, chemical, and geologic processes. Integration with new digital and computing technologies is gradually gaining traction. The use of rock physics in seismic imaging, prestack seismic analysis, seismic inversion, and geomechanical model building also contributes to the increase in rock-physics influence. This special section highlights current rock-physics research and practices in several key areas, namely experimental rock physics, rock-physics theory and model studies, and the use of rock physics in reservoir characterizations.