Reconstruction of seismic faulting by high-velocity friction

experiments: An example of the 1995 Kobe earthquake

Kazuo Mizoguchi,1 Takehiro Hirose,2 Toshihiko Shimamoto,2 and Eiichi Fukuyama,1

Received 25 August 2006; revised 31 October 2006; accepted 15 November 2006; published 12 January 2007.

[1] High-velocity friction experiments on a fault gougecollected from the Nojima fault activated during the 1995Kobe earthquake showed that the friction coefficientdecreased from 0.63 to 0.18 over a slip weakeningdistance, Dc, at high slip rates of 1 m/s. The dramaticdrop in friction coefficient of more than 0.3 is consistentwith that for the Kobe earthquake estimated fromseismological observations. Experimentally determinedDc becomes 5 m at a higher normal stress of 1.85 MPa,close to the order of magnitude of seismologicallydetermined Dc of 0.5 to 1 m. The difference in Dc is notsignificant because the fracture energy consumed duringfrictional slip is the same order of 106 N/m for both cases.Here we show that frictional behavior of a fault during anearthquake can be predicted by conducting high-velocityfriction experiments. Citation: Mizoguchi, K., T. Hirose,T. Shimamoto, and E. Fukuyama (2007), Reconstruction of

seismic faulting by high-velocity friction experiments: An

example of the 1995 Kobe earthquake, Geophys. Res. Lett., 34,

L01308, doi:10.1029/2006GL027931.

1. Introduction

[2] Recently, a great interest has been paid to frictionexperiments on rocks at high slip rates, comparable to thoseof 1 m/s during seismic fault motion. They show dramaticslip weakening of frictional strength associated with fric-tional melting [Tsutsumi and Shimamoto, 1997; Hirose andShimamoto, 2005] and formation of a silica gel layer on thefault [Goldsby and Tullis, 2002; Di Toro et al., 2004].However these studies were performed on bare rock withoutgouge materials. The fault gouge, an accumulated wearproduct of slip between fault surfaces, is present in majorfaults, such as the San Andreas fault [Wu et al., 1975]. Toaccurately extrapolate the frictional behaviour of a naturalfault during an earthquake, we perform high-velocity fric-tion experiments using its fault gouge.[3] The dynamic rupture process during large earth-

quakes has been analysed using slip weakening models[Ida, 1972]. In the model, the friction coefficient of the faultdecreases from an initial value mi to a residual value mr overthe slip weakening distance Dc. Thus these frictionalparameters related to the slip dependent behavior are crucial

for strong ground motion associated with the earthquakefaulting. The 1995 Kobe earthquake (M7.2), southwestJapan, has been seismologically well-examined. The rela-tion between stress and slip at each location on the fault isdetermined based on the inversion of the seismic wave-forms and Dc is estimated at 0.5 1 m [Ide and Takeo,1997]. The Nojima fault, southwest Japan, is one of thefaults that moved during the Kobe earthquake [Nakata andYomogida, 1995]. In-situ stress measurements at boreholesdrilled along the fault after the earthquake and the defor-mation rate analysis (DRA) of the core samples indicated mrof less than 0.3 [Ikeda et al., 2001; Tadokoro et al., 2001;Tsukahara et al., 2001; Yamamoto and Yabe, 2001]. mi wasestimated to be about 0.6 from the difference of stressorientation before and after the earthquake [Yamashita etal., 2004]. The value of mi is approximately consistent withthe general results of friction coefficient of rocks measuredin laboratory experiments [Byerlee, 1978]. However thevalue of Dc is more than several orders of magnitudes, andthe friction coefficient decrease during slip of more than0.3 is one order of magnitude, larger and than those ofprevious laboratory results at low slip rates [Marone, 1998].From the high-velocity fiction experiments on the Nojimafault gouge, we present data on mi, mr, and Dc and discusstheir relationship with those determined from geophysicalobservations.

2. Experimental Procedure

[4] All experiments were conducted on fault gouge sam-ples collected from an outcrop of the Nojima fault [Monzawaand Otsuki, 2003; Otsuki et al., 2003], using a rotary-shear,high-speed frictional testing apparatus [Shimamoto andTsutsumi, 1994]. The gouge sample of 1 g in weight wasput between a pair of granite cylinders with rough endsurfaces ground by #80 SiC powders (Figure 1a). The gougelayer, about 1 mm thick, was sheared under applied normalstress between the cylinders. The gouge layer was notsaturated with aqueous water, but contained moisture inair. A Teflon sleeve covered the fault to confine the gougein the fault during shearing. As the slip rate varies within thesample as a function of distance from the center of rotationaxis, we used an equivalent slip velocity Veq, defined suchthat t Veq S gives the rate of total frictional work on faultarea S, assuming that shear stress t is constant over the faultsurface [Shimamoto and Tsutsumi, 1994].

3. Results

[5] The frictional behaviour of the fault gouge sheared ata normal stress of 0.62 MPa was strongly dependent on Veq(Figure 1b). The coefficient of friction at Veq of 1.03 m/s

GEOPHYSICAL RESEARCH LETTERS, VOL. 34, L01308, doi:10.1029/2006GL027931, 2007

1Earthquake Research Department, National Research Institute forEarth Science and Disaster Prevention, Tsukuba, Japan.

2Department of Geology and Mineralogy, Division of Earth andPlanetary Sciences, Graduate School of Science, Kyoto University, Kyoto,Japan.

Copyright 2007 by the American Geophysical Union.0094-8276/07/2006GL027931

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(HVR178) decreased remarkably with displacement, whilethat at Veq of 0.006 m/s (HVR 239) did not change withdisplacement, being at a high constant level of 0.75. High-velocity sliding dramatically decreased the frictionalstrength of the fault gouge. The slip weakening curve atthe high slip rate can be fitted by the following exponentialdecay equation

m mr mi mr expln 0:05 d

dc

1

where mr is residual coefficient of friction, mi is initialcoefficient of friction, d is displacement after an initialfriction point. Because the exponential decay friction curvecontinues to decrease at a displacement to infinite, wedefine dc as a displacement at which (mi mr) reduces to5% of (mi mr), and assume that dc represents Dc.Mechanical parameters determined by an exponential curvefit with the above equation (1) for HVR178 run are: initialfrictional coefficient mi = 0.62, residual frictional coefficientmr = 0.08, slip weakening distance Dc = 22.5 m.[6] We conducted a series of experiments at Veq of

1.03 m/s, varying normal stress from 0.34 to 1.87 MPa.The exponential curve fitting on the results shows depen-dency of normal stress on shear stress at the initial andresidual state, and slip-weakening distance (Figure 2). Thereis a linear relationship between normal and shear stressesat each state, which have the similar intercept value of0.1 MPa at a normal stress of 0 MPa. If the high velocityfrictional property of the Nojima fault gouge obeys thetraditional friction law (Coulomb-Mohr equation), thisintercept is attributed to the frictional resistance of theTeflon sleeve. The data shown in Figure 1b were correctedfor this Teflon friction. From the slopes of the linearrelationships, the average mi and mr in the normal stressrange of 0.34 to 1.87 MPa were determined to be 0.63 and

0.18, respectively (Figure 2a). Contrary to this, Dcdecreased from 40 to 5 m with increasing normal stressup to 1.87 MPa (Figure 2b).

4. Discussion and Conclusions

[7] The friction coefficient of the Nojima fault wasestimated to decrease from 0.6 to less than 0.3 duringthe Kobe earthquake [Ikeda et al., 2001; Tadokoro et al.,2001; Tsukahara et al., 2001; Yamamoto and Yabe, 2001;Yamashita et al., 2004]. The high-velocity friction experi-ments on the Nojima fault gouge show that the frictionalstrength decreased from 0.63 to 0.18. The strength dropobserved for this earthquake is found by the high-velocityfrictional properties of the gouge. Dc of 5 to 40 m obtainedin our high-velocity friction experiments is larger than thatof 0.5 1 m which was seismically determined for the1995 earthquake [Ide and Takeo, 1997]. Considering thenormal stress dependence observed in the experiments,however, the experimental results approach the same orderof magnitude as seismological Dc (Figure 2b). The analysisof the seismic waveform inversion also showed shear stressdrop of 2 4 MPa [Ide and Takeo, 1997]. Assuming asimple slip weakening curve of linear stress decrease asslip increases to Dc [Andrews, 1976], the fracture energydescribed as an area of 1/2 Dc (stress drop) are

Figure 1. High-velocity friction experiment on faultgouge. (a) Schematic diagram of sample assembly duringthe experiments. (b) Representative mechanical behaviors atfast (black) and slow (grey) slip rates. Dashed grey lineshows a curve fitted by exponential decay equation (1).

Figure 2. Relevance of high-velocity frictional behavior ofthe fault to normal stress. (a) Normal stress versus shearstress for the initial and residual state. Frictional coefficientsfor peak and steady state determined by a least square fit withthe Coulomb-Mohr linear equation (dash lines) were 0.6258(0.0612) and 0.1701 (0.0197), respectively. (b) Slipweakening distance (Dc) plotted as a function of normalstress. Dc at lower normal stresses ranges from 20 to 40 m,while Dc at higher normal stress decrease to around 5 m.

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approximately calculated to be about 0.5 1 106 N/m.The calculation based on our results of high velocity slipweakening behaviour results in 1 3 106 N/m(Figure 3), which is the same order of fracture energy asthe seismological estimation. Frictional strength of a faultmight reach steady state when the fault consumes fractureenergy of 106 N/m in cases of both nature [Mikumo andFukuyama, 2006] and laboratory experiments. The longerDc in laboratory experiments can be explained by the lowerstress drop than that in nature. Here, it should be noted thatthis consideration can be applied only to high-velocityexperiments conducted at seismic slip rates. For the slowerrate experiments in which the fault did not show slipweakening behaviour, it is difficult to estimate the valueof fracture energy.[8] Frictional melting and thermal pressurization are

widely accepted as a weakening mechanism associated withearthquake faulting [e.g., Sibson, 1973; Lachenbruch,1980]. They might not cause the decrease in frictionalstrength of the fault gouge observed in these high-velocityfriction experiments because the experiments were con-ducted with dry condition and temperature rise in the faultwas estimated to be around 380C, below the melting point[Mizoguchi and Shimamoto, 2004]. It should be consideredthat new weakening mechanisms, such as silica gel lubri-cation [Goldsby and Tullis, 2002; Di Toro et al., 2004] andmoisture-drained weakening [Mizoguchi et al., 2006], havebeen reported in recent high-velocity friction experiments. Itshould be our future subject to clarify the weakeningmechanism of our experiments.[9] In this paper, we demonstrated that high-velocity

friction experiments on the Nojima fault gouge yieldssimilar frictional behaviour to that of the same fault duringthe Kobe earthquake. Here we suggest that we can predictseismic fault motion during an earthquake from conductinghigh-velocity friction experiments before an earthquake,although the weakening mechanism of our experiments isnot clear in present.

[10] Acknowledgments. We thank Christopher A. J. Wibberley forhis constructive comments that helped to improve this manuscript. We aregrateful to A. Tsutsumi, H. Noda and D. Sone for scientific discussions.

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E. Fukuyama and K. Mizoguchi, Earthquake Research Department,

National Research Institute for Earth Science and Disaster Prevention,Tsukuba 305-0006, Japan. (mizo@bosai.go.jp)T. Hirose and T. Shimamoto, Department of Geology and Mineralogy,

Division of Earth and Planetary Sciences, Graduate School of Science,Kyoto University, Kyoto 606-8502, Japan.

Figure 3. Fracture energy estimated from the high-velocity friction curves at each normal stress. Meshed areadefined by mi, mr and Dc in the exponential fitting curve(upper right) corresponds to the calculated fracture energy.

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