Experimental Investigation of Flash Weakening in Limestones
Abstract
Frictional properties of limestone and dolostone are crucial to understand earthquake mechanics where seismic ruptures nucleate and propagate in carbonate rocks (e.g., Mw 6.3 L’Aquila 2009 earthquake). We performed 27 rock friction experiments in a compression-torsion apparatus on ring-shaped (50/60 and 70/80 mm int/ext diameter) 100% CaCO3-samples at sub-seismic to seismic slip rates (0.05 to 350 mm/s), small displacements (50-60 mm) and under normal stresses of 3-8 MPa. The experiments involved four steps: 1) loading step to apply the normal stress, 2) “low-speed” step to verify the low-speed behavior, 3) “high-speed” step to determine the velocity dependence and, 4) final deceleration step to stop the experiment. The friction coefficient gradually increased during the low-speed step (0.05 mm/s and 7 mm displacement) from 0 to 0.7-0.8, a typical value for friction in limestone. During the high-speed step, slip rate was abruptly (< 0.04 s) increased in about 5 mm of slip to 250 mm/s (for 30 mm displacement, samples 50/60 mm in diameter) and to 350 mm/s (for 40 mm displacement, samples 70/80 mm in diameter). All experiments show a dramatic decrease, up to 60%, in friction for slip rates > 100 mm/s. During the final deceleration step (< 7 mm of slip in < 0.04 s), the friction coefficient recovered its initial value (0.7). A possible weakening mechanism is flash heating-induced thermal decomposition of calcite (CaCO3 -> CaO + CO2) at asperity contacts. In the experiments, weakening was contemporaneous with a peripheral temperature increase of 60-170°C measured with an infrared camera. This temperature range yields a lower limit to the temperature achieved in the slipping zone and at the asperity contacts. Energy Dispersive Spectroscopy equipped with Field Emission Scanning Electron Microscope (FE-SEM), X-Ray powder diffraction and Raman Spectroscopy analyses did not detect decarbonation products (CaO, Ca(OH)2) in the slipping zone. Instead, FE-SEM observation reveals the presence of CaCO3 nano-particles (< 40 nm in size) decorating the slipping zone after the experiments. We conclude that flash heating and nano-powder lubrication may operate together to decrease fault dynamic friction in limestone rocks in experiments and likely in nature.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2010
- Bibcode:
- 2010AGUFM.T31D..04D
- Keywords:
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- 7209 SEISMOLOGY / Earthquake dynamics;
- 8004 STRUCTURAL GEOLOGY / Dynamics and mechanics of faulting;
- 8012 STRUCTURAL GEOLOGY / High strain deformation zones;
- 8034 STRUCTURAL GEOLOGY / Rheology and friction of fault zones