Dynamic pulverization by rapid decompression
Abstract
In recent years several studies have identified so-called ';pulverized rocks' on various crustal-scale faults, a type of intensely damaged fault rock which has undergone minimal shear strain, and the occurrence of which has been linked to damage induced by transient stress perturbations during earthquake rupture. Several dynamic mechanisms have been proposed for the generation of pulverized rocks, such as compressive loading by high-frequency stress pulses due to the radiation of seismic waves and/or explosive dilation in tension in rocks containing pressurized pore fluids due to instantaneous reductions in fault-normal stress. Here, we demonstrate an explosive pulverization mechanism by imparting rapid drops in gas confining pressure for gas-saturated (effectively unconfined) rock samples. Using a specially designed pressure vessel allowing near-instantaneous decompression of rock samples via a large-diameter blow-out diaphragm, we show that low-permeability granitic rocks can pulverize by pore fluid-driven volumetric expansion (i.e. hydrofracture) where the confining pressure drops faster than the pore pressure of the rock. Microstructural observations show pervasive pulverization with minor shear in granitic samples, and significantly less damage in limestone and sandstones which have higher initial permeabilities. Permeability measurements on granitic samples following rapid decompression show increases of nearly 4 orders of magnitude in permeability, to values as high as 10-15 m2, and reductions in ultrasonic P-wave velocities of up to 60 %, compared to the starting samples. We propose that for ruptures that generate dynamic reductions in local stress, absolute tension is not necessarily required for pervasive damage or pulverization; rather, providing that the permeability of the fault rocks is low enough, a rapid drop in confining pressure below the pore fluid pressure by at least the tensile strength of the rock may cause pulverization. This may explain field observations suggesting that pulverized rocks surround faults cutting low-permeability crystalline rock. Pulverization by rapid decompression in seismogenic faults is likely controlled by complex interplays between rock permeability and tensile strength, pore fluid pressure and the magnitude and duration of the transient stress reduction.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2013
- Bibcode:
- 2013AGUFMMR41B..04M
- Keywords:
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- 8030 STRUCTURAL GEOLOGY Microstructures;
- 7230 SEISMOLOGY Seismicity and tectonics;
- 8010 STRUCTURAL GEOLOGY Fractures and faults;
- 8020 STRUCTURAL GEOLOGY Mechanics;
- theory;
- and modeling