Steffen Abe

Grain fracture in 3D numerical simulations of granular shear

We present a new method to implement realistic grain fracture in 3D numerical simulations of granular shear. We use a particle based model that includes breakable bonds between individual particles allowing the simulation of fracture of large aggregate grains during shear. Grain fracture simulations produce a comminuted granular material that is texturally comparable to natural and laboratory produced fault gouge. Our model is initially characterized by monodisperse large aggregate grains and gradually evolves toward a fractal distribution of grain sizes with accumulated strain. Comminution rate and survival of large grains is sensitive to applied normal stress. The fractal dimension of the resultant grain size distributions (2.3 ± 0.3 and 2.9 ± 0.5) agree well with observations of natural gouges and theoretical results that predict a fractal dimension of 2.58. Copyright 2005 by the American Geophysical Union.

Extension of the Lattice Solid Model to Incorporate Temperature Related Effects

Abstract—The elastic and frictional properties of solids are temperature-dependent. Thus, heat has without doubt a major influence on the dynamics of earthquakes, particularly considering the high temperatures generated during large slip events. In order to provide a foundation for the study of these heat related effects, the Lattice Solid Model for the study of earthquake dynamics is extended to incorporate the generation and transfer of heat. The thermal and elastic properties of 2- and 3-D lattice solids in the macroscopic limit are derived. To verify the numerical implementation of heat transfer, a simulation has been performed in a simple case and the results compared to a known analytical solution for the same problem. Thermal expansion and a simple approximation of a temperature-dependent pore fluid pressure are implemented in the 2-D Lattice Solid Model. Simulations confirm that these effects influence the dynamics of the slip of a fault with fault gouge. Whereas thermal expansion only minor influences the dynamics of fault rupture, the influence of the increase in the pore fluid pressure generated by slip heating is more significant. The simulations show that the temperatures generated during slip events accord with those expected for real earthquakes as inferred from geologic evidence.

Scaling evalutation of the lattice solid model on the SGI Altix 3700 [evalutation read evaluation]

The lattice solid model is a particle based method which has been successfully employed for simulating the fracturing of rocks, the dynamics of faults, earthquakes and gouge processes. However, results from initial simulations demonstrate that models consisting of only thousands of particles are inadequate to accurately reproduce the micro-physics of seismic phenomenon. Instead, models with millions or tens of millions of particles are required to produce realistic simulations. Parallel computing architectures, such as the SGI Altix 3700, provide the opportunity to solve much larger computational problems than traditional single processor systems. In order to take advantage of high performance systems, a message passing interface version of the lattice solid model has been implemented. Benchmarks, presented in this paper, demonstrate an 80% parallel efficiency for the parallel lattice solid model on 128 processors of the SGI Altix 3700. These results, for a two-dimensional wave propagation problem, indicate the potential for the lattice solid model to simulate more computationally challenging three-dimensional geophysical processes.

Efficient implementation of complex particle shapes in the lattice solid model

The lattice solid model is a particle based simulation model for the study of earthquake micro-physics and rock mechanics. It consists of particles interacting by various types of mechanisms such as elastic-brittle forces and friction. Results of laboratory experiments have shows that the grain shape has a major influence on the frictional properties of fault gouge. In order to enable realistic simulations it is thus improtant to include the capabillty to model non-spherical particles into the simulation software. To achieve this goal a new class of particles wich variable shapes have been impelman ted in the lattic solid model. The shepe of the particles is described by an arbitrary number of piecewise spherical patches. This leads to a good balance between the computational cost of the contact detection and calculation of interactions between particles and the range of particle shapes available.

ACcESS: Australia's contribution to the iSERVO Institute's development

Solid earth systems simulation is now becoming feasible from the microscopic to the global scale. The ACES international cooperation has shown development of simulation capabilities for solid earth phenomena that are beyond the ability of a single group or country. Each country has different strengths, computational approaches, and laboratory and field observational systems. This range of numerical models is required to model the entire earth system, and calibration is required to ensure that results obtained using these models match with the different laboratory and field observations available in each country. For this reason, international groups have agreed to work toward establishing iSERVO to build the ACES cooperation. iSERVO aims to collaboratively develop a computational infrastructure - accessible through Web portals - that combines models developed across the international community and to conduct collaborative research to solve problems of global significance such as earthquake forecasting, green energy development, and environmental management. The new Australian Computational Earth Systems Simulator research facility provides a virtual laboratory for studying the solid Earth and its complex system behavior. The facilitys capabilities complement those developed by overseas groups, thereby creating the infrastructure for an international computational solid earth research virtual observatory.

Modeling Vertical Seismic Profiles Using a Lattice Solid Model

We use a Lattice Solid Model to model seismic response in Vertical Seismic Profiles configurations, driven by the common problem of high energy dissipation during wave propagation in soils and shallow unconsolidated sediments. We are presenting our very first configuration simulations using this model which has initially been used for modeling earthquake mechanisms and related fault zones dynamics, fracture processes. The most interesting and intriguing potential of this model is that viscosity and internal friction can be incorporated in the wave propagation resulting in realistic approaches of energy loss problems. Similar types of modeling are used in geomechanical and geotechnical applications; their introduction to geophysics could lead to a better integration of disciplines.