 |
| Titel |
Role of fluids in experimental calcite-bearing faults at seismic deformation conditions. |
| VerfasserIn |
M. Violay, S. Nielsen, D. Cinti, E. Spagnuolo, G. Di Toro, S. Smith |
| Konferenz |
EGU General Assembly 2012
|
| Medientyp |
Artikel
|
| Sprache |
Englisch
|
| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 14 (2012) |
| Datensatznummer |
250065620
|
|
|
|
|
|
| Zusammenfassung |
| Fluids play a fundamental physical (fluid pressure, temperature buffering, etc.) and chemical
(dissolution, hydrolytic weakening, etc.) role in controlling fault strength and earthquake
nucleation, propagation and arrest. However, due to technical challenges, the influence of
water at deformation conditions typical of earthquakes (i.e., slip rates of 1 m/s,
displacements of 0.1-5 m, normal stress of tens of MPa) remains poorly constrained
experimentally.
Here we present results from high velocity friction experiments performed with a rotary
shear apparatus (SHIVA: Slow to HIgh Velocity (friction) Apparatus) on Carrara marble.
SHIVA is equipped with (1) an environmental/vacuum chamber to perform experiments in
the absence of room-humidity, (2) a pressure vessel to perform experiments with fluids (up to
15 MPa confining pressure), including devices to determine fluid composition (Ca2+, Mg2+,
HCO3-, etc).
Experiments were conducted on hollow cylinders (50/30 mm ext/int diameter) of Carrara
(98% calcite) marble at velocities of 1–6.5 m/s, displacements up to a few meters, normal
stresses up to 40 MPa and fluid pressures between 0 (under vacuum) and 15 MPa
(fluid-saturated conditions, with H2O in chemical equilibrium with the marble). Rock and
fluid samples were recovered for post-run analysis to determine deformation mechanisms and
changes in fluid composition.
Under these deformation conditions:
1) the friction coefficient decays rapidly from a peak (= static) μp ~ 0.8 at the
initiation of sliding towards a steady-state μss ~ 0.1. The absolute values of both
peak and steady-state friction are not significantly influenced by the presence of
fluids;
2) the decay from peak to steady-state friction is more abrupt in presence of
fluids;
3) during deceleration of the friction apparatus, the friction coefficient recovers almost
instantaneously to a value, μr, of 0.2-0.6 ( strength recovery) resulting in a small static stress
drop. Strength recovery is smaller in the presence of fluids.
4) the fluid (H2O) after the experiment is enriched in Ca2+, Mg2+ and HCO3-. This
chemical evolution suggests breakdown reactions (decarbonation of calcite) promoted by
frictional heating and controlled by the presence of H2O.
We conclude that the large decrease in friction and abrupt weakening, especially in the
presence of fluids, indicates that calcite-bearing rocks are prone to earthquake nucleation and
seismic rupture propagation (see the L’Aquila 2009 earthquake sequence). The chemical
changes observed in water springs after large earthquakes in carbonatic rocks is similar to
those found in these experiments, suggesting that the weakening mechanisms triggered in the
experiments might occur in nature. |
|
|
|
|
|
|