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| Titel |
Localization-controlled transition to extreme weakening in granular fault gouge |
| VerfasserIn |
Steven Smith, Giulio Di Toro, Stefan Nielsen |
| Konferenz |
EGU General Assembly 2013
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 15 (2013) |
| Datensatznummer |
250079919
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| Zusammenfassung |
| Observations of both ancient and active faults suggest that much slip during earthquakes
occurs within thin (millimeter-scale) layers of granular fault gouge. It is therefore of primary
importance to understand the rheological behavior of gouges at the conditions expected
during earthquakes (slip velocities ~0.1 – 1 m s-1, normal stresses >50 MPa, total
displacements typically 0.05 m of slip) that accounts in some
cases for >50% of the experimental fracture energy. This strengthening phase is
followed by weakening that initiates at a slip velocity (0.5 – 2 m s-1) up to an order of
magnitude higher than in solid rocks (0.05 - 0.2 m s-1). Microstructural observations of
experiments stopped during the transition from strengthening to weakening show that at
peak strength the gouges are crosscut by a continuous c. 100-μm wide shear band
that parallels the gouge layer boundaries. Immediately following the transition to
weakening, the shear band internally develops discontinuous, elongate patches (up to
100 μm long and 20 μm wide) of recrystallized gouge that we interpret as local
“hot spots”. By the end of weakening, continued localization within the shear band
forms a through-going, 2-3-μm wide slip zone that remains stable with increasing
displacement. This discrete slip zone has a shiny surface appearance and is flanked by
dynamically recrystallized gouge layers up to 300 μm thick with well-developed
shape and crystallographic preferred orientations. Our current interpretation of
the experiments involves the following; localization in the gouge layers occurs
during the initial strengthening phase (accompanied by layer dilation). This causes
progressively increasing inter-particle slip velocities within the zones of localization.
After a critical amount of localization, the average inter-particle slip velocity is
sufficiently high to induce local heating (and recrystallization) of particle contacts and a
degradation in contact shear strengths. We believe that this corresponds to the onset
of weakening in the gouge experiments. More generally, our results suggest that
localization during the early stages of seismic slip may be one of the most important
mechanical processes controlling the dynamic strength evolution of gouge-bearing faults. |
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