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| Titel |
Frictional behavior and BET surface-area changes of SAFOD gouge at intermediate to seismic slip rates |
| VerfasserIn |
Michiyo Sawai, Toshihiko Shimamoto, Thomas Mitchell, Hiroko Kitajima, Takehiro Hirose |
| 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 |
250075420
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| Zusammenfassung |
| The San Andreas Fault Observatory at Depth (SAFOD) Drilling site is located near the
southern end of the creeping section of the San Andreas fault. Experimental studies on the
frictional properties of fault gouge from SAFOD drill cores may provide valuable
information on the cause of diverse fault motion. We conducted friction experiments on
gouge from the southwest deformation zone (SDZ, Phase III core; Hole G-Run 2-Section 8)
where creep is confirmed by ongoing borehole casing deformation, at intermediate to high
slip rates (10-5 to 1.3 m/s), at a normal stress of about 1 MPa, and under both dry (room
humidity) and wet (25 wt% of H2O added, drained tests) conditions. Experiments were
performed with two rotary-shear friction apparatuses. One gram of gouge was placed
between specimens of Belfast gabbro 25 mm in diameter surrounded by a Teflon sleeve to
confine the gouge. Slip rate was first decreased and then increased in a step-wise
manner to obtain the steady-state friction at intermediate slip rates. The friction
coefficient increases from about 0.13 to 0.37 as the slip rate increases from 0.8
x 10-5 to 9.7 x 10-3 m/s. Our results agree with frictional strength measured at
higher effective normal stress (100 MPa) by the Brown University group in the
same material. Data shows pronounced velocity strengthening at intermediate slip
rates, which is unfavorable for rupture nucleation and may be a reason for having
creep behavior. On the other hand, the steady-state friction markedly decreases at
high velocity, and such weakening may allow earthquake rupture to propagate into
the creeping section, once the intermediate strength barrier is overcome. Gouge
temperature, measured at the edge of the stationary sample during seismic fault motion,
increased to around 175oC under dry conditions, but increased up to 100oC under wet
conditions.
We measured BET surface area of gouge before and after deformation to determine the
energy used for grain crushing. The initial specific surface area (2.6-3.4 m2/g) increases to
14-24 m2/g for dry gouge deformed at intermediate slip rates and to 45-60 m2/g for
wet gouge deformed at subseismic to seismic slip rates. The results suggest that
approximately 2 % and less than 1 % of the frictional work is absorbed in grain crushing for
dry and wet gouges, respectively, if the fracture surface energy of muscovite (0.38
J/m2) is used as the surface energy of phyllosilicate-rich SAFOD gouge. Thus grain
crushing cannot be an important energy sink during seismic fault motion. The surface
area tends to be lower for gouge deformed at high slip rates for both dry and wet
gouges. This results and SEM observations of gouge strongly suggests that welding of
grains takes place at high slip rate due to frictional heating and counteracts the
surface-area increase due to grain crushing. Thus intrafault processes are more
complex than in a simple scenario of “grain crushing and surface-area increase”
assumed in recent studies. Surface area is greater for wet gouge than for dry gouge
suggesting that pore water separating gouge particles suppresses grain welding. |
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