Time-dependent deformation: bridging the strain rate gap in brittle rocks.

Lead Research Organisation: University of Edinburgh
Department Name: Sch of Geosciences

Abstract

Earthquake rupture and volcanic eruptions are the most spectacular manifestations of dynamic failure of critically-stressed crust. However, these are actually rather rare, discrete events in both space and time. Most of the crust spends most of its time in a highly-stressed but sub-critical state. Furthermore, water is ubiquitous in the crust. It is well-known that water-rock chemical reactions can lead to time-dependent deformation enabling rocks to fail over extended periods of time at stresses far below their short-term failure strength; a phenomenon known as 'sub-critical crack growth'. Quantifying sub-critical crack growth is crucial to unravelling the complexities of the evolution and dynamics of the brittle crust. The presence of cracks allows the crust to store and transport fluids, and even modest changes in crack size, density or linkage can produce major changes in fluid transport properties. Time-dependent rock deformation therefore has both a scientific and a socio-economic impact since it controls the duration and detectability of any precursory phase of important geohazards such as earthquake rupture and volcanic eruptions. Such deformation mechanisms cause compaction and cracking, both of which affect porosity and permeability. The results may therefore also be of relevance in the effective recovery of hydrocarbon and geothermal energy resources, and the integrity of long-term storage facilities for hazardous waste. Our current lack of understanding in this area has recently been highlighted by UNESCO, and 'Understanding Slow Deformation before Dynamic Failure' is one of the two priority areas for study within the Natural Hazards theme of its International Year of Planet Earth. We are therefore seeking funds for an integrated laboratory experiment, deep-sea observatory and quantitative analysis study of time-dependent brittle rock deformation, involving the Rock & Ice Physics Laboratory (RIPL) in the Department of Earth Sciences at University College London (UCL), the School of Geo-Science at Edinburgh University (EdU) and the Italian National Institute of Geophysics and Volcanology (INGV). The aim of the project is to discriminate between competing models of time-dependent rock deformation. That is currently not possible given the range of strain rates achievable in conventional laboratory experiments. Our main objective is therefore to use the deep-sea environment to bridge the strain rate gap between laboratory and crustal strain rates. The project will build on previous collaborative work between the groups, and take advantage of recent developments in experimental methodology (RIPL) and theoretical analysis (EdU), and preliminary results achieved during a deep-sea observatory pilot study funded by NERC, where infrastructural support was provided by INGV. The pilot study was originally proposed as a feasibility study for the current proposal, and achieved positive results. We now have a timely opportunity to build on this earlier collaboration and take advantage of access to a unique, new, multi-million Euro deep-sea observatory facility established by INGV on the bed of the Ionian Sea off the east coast of Sicily.

Publications


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Description We examined the problem of rocks deforming spontaneously at constant stress by chemically-assisted growth of a population of micro-cracks - a process known as 'brittle creep'. Our role was to analyse results from laboratory creep tests. We demonstrated the results fitted a previously published model for creep, and that its parameters depended on rock type and deformation rate.
Exploitation Route Brittle creep is, to varying extents, an important component of the total deformation of rocks and the Earth at low temperatures. Accordingly the results could be used in quantifying natural hazards such as earthquakes, volcanic eruptions and slope instability, induced hazards from subsurface engineering projects, or the non-destructive testing of engineering structures made of composite materials.
Sectors Construction,Energy,Environment