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| Titel |
Local thermal pressurization triggered by flash heating causes dramatic weakening in water-saturated gouges at subseismic slip rates |
| VerfasserIn |
Lu Yao, Shengli Ma, Toshihiko Shimamoto, Tetsuhiro Togo, Jianye Chen, Hiroko Kitajima, Yu Wang, Honglin He |
| Konferenz |
EGU General Assembly 2017
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| Medientyp |
Artikel
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| Sprache |
en
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 19 (2017) |
| Datensatznummer |
250146844
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| Publikation (Nr.) |
 EGU/EGU2017-10897.pdf |
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| Zusammenfassung |
| High-velocity friction studies on water-saturated gouges in recent years have demonstrated
that the wet gouges subjected to high-velocity shear tend to have smaller peak and
steady-state friction, much shorter slip-weakening distance and lower fracture energy, as
compared to the air-dry gouges. Thermal pressurization, compaction-induced pressurization,
and flash heating were previously recognized to be the important weakening mechanisms
in causing these behaviors. However, in spite of theoretical expectation, there is
few evidence to support the occurrence of flash heating in wet gouges, mainly due
to the superimposition of multiple weakening mechanisms especially for thermal
pressurization.
We devised friction experiments to study the role of flash heating in dynamic weakening
of water-saturated gouges. In each experiment, we used a pressure vessel to impose a pore
pressure of 2.0 MPa on the gouge layer sandwiched between porous ceramics blocks, and
applied a long preslide of 1.0 m in displacement before starting the experiment at the
target slip rate. By doing so we could (1) suppress rapid thermal pressurization
in the bulk gouge layer by means of the designed drained condition and elevated
temperature of phase transition of pore water; (2) suppress or even eliminate the
pressurization effects due to compaction especially at the very beginning of the
experiment.
The experiments were performed on a granular gouge (mainly quartz, plagioclase, calcite
and illite) and a clay-rich gouge (illite and chlorite ∼58 wt%), which were both collected
from the Qingchuan fault of the Longmenshan fault system. For the granular gouge, the
steady-state friction coefficients (μss) are 0.39–0.42 at slip rates (V ) of 100 μm/s–10 mm/s;
however, at V ≥40 mm/s, the friction coefficients (μ) decrease suddenly at the onset of the
slip. For instance, μ reduces by 0.29 within displacement of 0.05–0.08m at V =100
mm/s. For the clay-rich gouge, μss increases from 0.24 to 0.34 as V increasing
from 10 μm/s to 100 mm/s. At V =0.4 and 1.0 m/s, the evolutions of friction are
characterized by sharp weakening, quick strengthening and slight weakening as slip
proceeds. It is noteworthy that the sharp initial weakening is always accompanied by a
contemporaneous axial dilatancy of 10–20 μm for both gouges, and the latter friction
evolutions are accompanied by axial shortening for the granular gouge and by further
dilatancy for the clay-rich gouge. Moreover, microstructure observations reveal that
only 40% of the gouge layer was involved in shear deformation for the granular
gouge at V =10–100 mm/s, as compared to distributed shear over the entire clay-rich
gouge layer at all the tested velocities. The observed data, microstructures and
modeling results suggest that flash heating probably triggers thermal pressurization
at asperity-contacts or within extremely localized slip zones, causing the sudden
initial weakening and contemporaneous dilatancy. The difference in the efficiency of
flash heating could explain the different frictional behaviors of the two gouges.
Given the extremely fast weakening caused by flash heating and the resulting local
thermal pressurization, seismic faults could be weakened more rapidly at much
lower slip rates below characteristic weakening velocities previously recognized. |
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