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| Titel |
Coseismic thermal-mechanical-chemical processes on the Wenchuan Earthquake fault: constraints from experiments and numerical modeling |
| VerfasserIn |
J. Chen, X. Yang, C. J. Spiers |
| Konferenz |
EGU General Assembly 2012
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 14 (2012) |
| Datensatznummer |
250063205
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| Zusammenfassung |
| We have determined the frictional, transport properties and mineralogical composition of the
fault rocks, collected from two exposures along the Longmenshan (Wenchuan Earthquake)
Fault Zone, and used the data obtained to numerically model coseismic slip. Friction test at
1.3 m/s were carried out on principal slip surface gouges with 1 mm thickness under normal
stress of ~ 1 MPa. Experiments of dry samples revealed a peak friction coefficient of around
0.6 followed by dynamic weakening to steady state value of ~ 0.17. Results for wet samples
were characterized by rapid attainment of steady state slip with a very low friction
coefficient (0.07 and 0.13), and little or no dynamic weakening. Fluid transport
properties were measured on transect fault rock samples, in particular on principal slip
surface gouges and nearby rocks, at effective pressure (Pe) cycled in the range of 7 -
165 MPa. The oscillating flow method was applied using water as pore fluid at a
mean pressure of 14 MPa, measuring permeability and porosity change at each
step change of confining hence effective pressure. Permeability, specific storage
and porosity values obtained for the gouge samples at Pe = 165 MPa fell in the
range of 7.3 Ã10-22to 2.6Ã10-20m2, 1.1Ã10-10 to 4.0Ã10-10 Pa-1 and 9.4
% to 5.9 %, respectively. The surrounding rocks show values up to 4 orders of
magnitude, 10 times and 4 times higher than these. The two gouge samples on the
principal slip surface consisted of quartz (29%, 66%), dolomite (27%, 7%) as the
major minerals, illite (17%, 20.6%) and smectite or I/S mixlayers (8%, 13.4%) as
clays.
On basis of these experimental data, numerical modeling of coseismic slip were
conducted, considering the real slip history, chemical reactions (i.e. dehydration of
smectite, decarbonation of dolomite) and the state dependent thermodynamic properties
of the pore fluid, allowing for the formation of SC water and CO2. The results
indicate that (1) thermal pressurization enhanced by chemical reactions played
a significant role during the Wenchuan Earthquake; (2) over pressure may have
developed in impermeable zone at depth, limiting the temperature increase less than
600 oC; (3) the two stage slip mode observed at locations such as Beichuan could
have been associated with a major decrease (60%) in fault strength after the first
stage of slip. We explored a wide range of parameters in order to determine the
contribution from different factors and their sensitivity. Over pressuring is possible
under conditions of low permeability, rapid reaction kinetics, high abundance of
hydrated minerals and/or high slip velocity, as reported in previous studies (i.e.
Brantut et al., 2010; Rice, 2006). Especially, the state dependent thermodynamic
properties of the fluid played a crucial role. Our results showed that local PT condition
within the slip zone can vary over a large range during frictional sliding, and that the
pore fluid may even undergo multiple phase changes, from liquid to vapor and or
supercritical states with increasing temperature, changing the thermodynamic properties of
the mixed fluid phase by up to three orders of magnitude. Such effects may have
determining role in EQ dynamics, much more important than previously realized. |
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