P04

High-Velocity Friction Experiments for Samples of Various Size:
Scale Effect in the Critical Slip Rate for Dynamic Weakening○Hiroyuki NODA・Kiyokazu OOHASHI

surface, where the dynamic weakening locally takes

Introduction
There is a large difference in length scale between

place. Because a larger sample hosts heterogeneity of alaboratory and natural large events. Then it is crucial tosmaller wavenumber, it should weaken at a lower slip

study the scale effect in friction and evaluate

rate. In this case, Vw is expected to be inversely

applicability of lab-experiments. It is known that

proportional to the sample size.

friction decreases dramatically at coseismic slip rate

Here, we present our preliminary results of high-due to frictional heating and various associatedvelocity friction experiments for samples of various

physico-chemical

size conducted in order to confirm the scale effect andeffects(e.g.,TsutsumiandShimamoto, 1997 (TS97); Di Toro et al., 2011). High-the hypothesis of the thermoelastic instability.

velocity friction experiments for centimeter-scale
samples showed the dynamic weakening at slip rates VMethodologyof the order of 0.1 m/s. On the other hand, recent

We used gabbro of sub-millimeter grain size,experiments by Yamashita et al. (2015) (Y+15)“Zimbabwe black” which is comparable to experiments

demonstrated that a large sample with 1.5 m-long andby TS97 and by Y+15. Because thermal properties of

0.1 m-wide frictional surface showed the dynamic

the apparatus matters (Yao et al., 2016), we used theweakening at V of the order of 0.01 m/s, arguingsame apparatus that TS97 used, which is now at

existence of the scale effect on the weakening slip rate

Yamaguchi University. Width of the frictional surface

Vw.

W is 4.5 mm in TS97, 0.1 m and 11.5 mm in Y+15. So

Noda (2023) pointed out that it can be explained byfar, we conducted experiments with W of 3 mm andconcentration of frictional power due to thermoelastic

13.5 mm (Figure 1). The experimental conditions areinstability1972).

similar to TS97; the normal stress was kept at 1.5 MPa

Heterogeneous normal stress causes heterogeneousand successive positive velocity steps of a factor ofthermal expansion of wall rocks, which may positively

about 1.5 were applied every about 50 m of slip.

(e.g.,DowandBurton,feeds back to enhance the heterogeneity. If there is notenough time for thermal conduction to smears out theperturbation field, or at high enough slip rate V>Vcr, thethermoelastic effect causes unlimited growth of theheterogeneity. Vcr is the critical slip rate derived fromthe linearized theory for an infinite medium, being
proportional to the wavenumber of the heterogeneity. If
the thermoelastic instability occurs, frictional power
density should concentrate to hot spots on the sliding

Fig1. Samples of different widths of frictionalsurface used in this study.

Results and discussion(e.g., Lachenbruch, 1980; Segall and Rice, 1995) affect

The sample with W=3 mm weakened at V=0.4 m/s,

the fault behavior significantly. Investigation of thewhile that with W=13.5 mm did at V=0.25 m/s. When

dominant mechanism during coseismic slip rate onplotted together with TS97 and Y+15 (Figure 2 left), Vw

natural faults is an important future study,

sizes ranges for about 2 orders of magnitude withinversely correlation with W. Normalization of the slip

Summaryrate by Vcr, which is inversely proportional to W,

Frictional experiments for samples with different

reduces this variation to about 1 order of magnitudefault size using the same apparatus show that a large(Figure 2 right). It is likely that the dynamic weakening

sample weakens at a low slip rate. The observed

occurs at about V=10Vcr. This factor of 10 may be fromcorrelation is consistent with the consequence of

nonlinear effects such as partial opening of thethermoelastic instability and resulting local activation

frictional surface and reduction in the frictionof dynamic weakening. The dynamic weakening occurs

coefficient due to temperature rise (e.g., Noda 2008), orat about V=10Vcr.

from the end effect of the experimental configuration.
Evaluation of this point deserves further study. In any

References

event, the comparison in this study indicates theDi Toro, G., et al. (2011), Nature.

existence of the scale effect, reconfirming the discovery

Dow, Th. A., & Burton, R. A. (1972). Wear.

by Y+15 and inferring significance of thermoelastic

Lachenbruch, A. (1980). JGR.

instability although more data may be needed.

Segall, P., & Rice, J. R. (1995). JGR.

It should be mentioned that the direct applicability ofTsutsumi, A. & Shimamoto, T. (1997). GRL.

the thermoelastic instability remains to be investigated.

Noda, H. (2008). JGR.

The natural fault has wear material (fault gouge)

Noda, H. (2023). EPS.

between wall rocks, and the approximation as a simple

Yamashita, F., et al. (2015). Nature.

rock-on-rock configuration may be inappropriate. In

Yao, L. (2016), GRL.

addition, not only thermal, but also hydraulic effects

Fig 2. Normalized friction coefficients by low-velocity values for experiments in this study (x), TS97 (onlyerrorbars), and Y+15 (other symbols) plotted against V (left) and V/Vcr (right). The dynamic weakening occurs
at around V=10Vcr.