GEOLOGY, December 2007 1131

INTRODUCTIONA common problem encountered in fault-zone

studies is the diffi culty in fi nding geological records of seismic fault slip and locating seis-mic slip zones. Cowan (1999) suggested that seismic fault slip of short duration will not leave geological records except for the small per-centage of faults containing pseudo tachylyte. Natural pseudotachylyte has been studied in detail (e.g., Sibson, 1975; Magloughlin and Spray, 1992); frictional melting processes have been reproduced by conducting high-velocity friction experiments (Spray, 1987; Tsutsumi and Shimamoto, 1997; Hirose and Shimamoto, 2005; Di Toro et al., 2006); and it is becoming possible now to predict mechanical properties of faults during frictional melting by modeling (e.g., Fialko and Khazan, 2005). Thus compre-hensive studies on pseudotachylytes have con-tributed greatly to our understanding of seismic fault slip. For the majority of faults without

pseudotachylytes, however, it is still diffi cult to clearly discriminate between fault zones that underwent seismic slip and those subjected only to slow aseismic slip. Thermal pressurization may occur during seismic fault slip for fl uid-rich fault zones (e.g., Sibson, 1973; Wibberley and Shimamoto, 2005; Noda and Shimamoto, 2005; Rice, 2006), but identifi cation of this process in natural fault zones is not easy.

A promising approach to this problem is to conduct high-velocity friction experiments on simulated fault gouge to get information on physico-chemical processes in fault zones during seismic fault slip and then to observe and analyze natural fault zones with renewed insight. Mizoguchi et al. (2007) started such high-velocity experiments on the Nojima fault gouge and successfully reproduced complex structures observed in the Nojima fault zone. This paper reports results from similar high-velocity friction experiments on siderite-bearing gouges at seismic slip rates and demonstrates that frictional heating decomposes siderite into magnetite and CO

2 gas. Siderite is a common

subsidiary mineral in fault zones and its decom-position product (magnetite) is stable, so that

such decomposition should leave a geological record of seismic slip in fault zones. In con-trast, thermal decomposition of calcite in the high-velocity friction experiments on Carrara marble by Han et al. (2007) produced a highly unstable mineral of lime (CaO). We correlate our experimental results with those from recent observations on slip zones in the Chelungpu fault, Taiwan, that caused the 1999 Chi-Chi earthquake (Li et al., 2005; Hirono et al., 2006a, 2006b). We also argue that thermal decompo-sition may play a signifi cant role in dynamic weakening of faults during seismic slip.

EXPERIMENTAL PROCEDURESWe used a rotary-shear, high-velocity fric-

tion apparatus that is capable of producing seismic slip rates (up to a few m/s) (see Hirose and Shima moto, 2005). Experiments were con-ducted at room temperature and room humid-ity conditions, at normal stresses (σn) of 0.6 or 1.3 MPa, and at slip rates of 1.3 or 2.0 m/s. The slip rate is reported in terms of equivalent slip rate (V ) defi ned such that τVA gives total rate of frictional work on a fault with area A, assum-ing no velocity dependence of shear stress τ

Geology, December 2007; v. 35; no. 12; p. 1131–1134; doi: 10.1130/G24106A.1; 3 fi gures.© 2007 The Geological Society of America. For permission to copy, contact Copyright Permissions, GSA, or editing@geosociety.org.

*E-mail: rhhan@korea.ac.kr.†Current address: Department of Earth and

Planetary Systems Science, Hiroshima University, Higashi-Hiroshima 739-8526, Japan.

Seismic slip record in carbonate-bearing fault zones: An insight from high-velocity friction experiments on siderite gouge

Raehee Han*Department of Earth and Environmental Sciences, Korea University, Seoul 136-701, Republic of Korea

Toshihiko Shimamoto†

Division of Earth and Planetary Sciences, Graduate School of Science, Kyoto University, Kyoto 606-8502, JapanJun-ichi Ando

Department of Earth and Planetary Systems Science, Hiroshima University, Higashi-Hiroshima 739-8526, JapanJin-Han Ree

Department of Earth and Environmental Sciences, Korea University, Seoul 136-701, Republic of Korea

ABSTRACTPseudotachylyte formed by frictional melting has been the only unequivocal evidence of

past seismogenic fault slip. We report from high-velocity friction experiments on siderite-bearing gouge that mineral decomposition due to frictional heating also can leave evidence of paleoseismic events along shallow crustal faults other than pseudotachylyte. Experiments were conducted room dry on simulated gouge composed of siderite or mixture of siderite, calcite, and quartz, initially at room temperature, under normal stresses of 0.6–1.3 MPa and at seismic slip rates of 1.3–2.0 m/s. In all cases, gouge exhibited dramatic slip weakening and siderite was decomposed into nanocrystalline magnetite and CO2 gas, as confi rmed by CO2 measurement, X-ray diffraction analyses, and transmission electron microscopy. The weakening was caused by the low frictional strength of ultrafi ne decomposition products at seismic slip rates. Magnetite formation during shearing changed gouge color to black and increased magnetic susceptibility by a few orders of magnitude. Those changes can be recognized in natural fault zones, and black gouge in the Chelungpu fault zone in Taiwan is perhaps such an example. Thus our results suggest that thermal decomposition in shallow crustal faults can be an important co-seismic process not only for dynamic fault weakening, but also for leaving seismic slip records.

Keywords: seismic fault slip, thermal decomposition, black gouge, siderite, magnetite, dynamic fault weakening.

1132 GEOLOGY, December 2007

(Hirose and Shima moto, 2005). We conducted three experiments on simulated gouge (1 g in weight and ~0.77 mm in thickness) consisting of (1) pure siderite (run number HVR605), (2) pre-decomposed siderite gouge (or ultrafi ne magnetite gouge; HVR732), and (3) gouge consisting of 1:1:1 mixture of siderite, quartz, and calcite (HVR763). Siderite (FeCO

3) gouge

was prepared by crushing a siderite block from Connecticut, USA. We used commercial quartz sands (Nacalai Tesque, Japan) for quartz gouge and crushed Carrara marble for calcite gouge. Pre-decomposed siderite gouge, which can not emit CO

2, was used to test if CO

2 gas pressuri-

zation affects dynamic weakening (see Han et al. [2007] for a similar experiment on cal-cite). Decomposed siderite gouge for HVR732 was prepared by leaving siderite powder in an electronic furnace for 1 h at 600 °C, higher than the siderite decomposition tem-perature of 400–580 °C (e.g., Deer et al., 1992; Isambert et al., 2003). The complete decom-position was confi rmed by weight measure-ment and X-ray diffraction (XRD) analysis (detection of only magnetite).

We used two types of specimen confi gurations: (1) a pair of solid cylinders of Indian gabbro with an outer diameter of 24.8 mm, and (2) a pair of hollow cylinders of Australian sandstone, 34 mm and 15.6 mm in outer and inner diameters, respec-tively (see diagrams in Figs. 1C, 1E). The latter assembly was used in an attempt to minimize variation in slip rate in the radial direction. Fol-lowing the technique developed by Mizo guchi et al. (2007), we put a Tefl on sleeve of ~9 mm width and 5 mm wall thickness on the outside of all specimens and a Tefl on bar of 15.6 mm in diameter into the inner hole of the second assem-bly in order to prevent gouge leakage.

During one experiment (HVR605), the onset of CO

2 emission was measured with two solid

electrolyte-type CO2 sensors (TGS4161, Figaro

Co. Ltd., Osaka, Japan) with an accuracy of ~20% (see Han et al., 2007, for more details of the sensors). Quick-responding sensor 1 with a response time of 0.9–1.0 s was set ~30 mm away from the simulated fault for detecting the onset of CO

2 emission. Slow-responding

sensor 2, set at a corner of sealed specimen chamber, was used for monitoring the amount of emitted CO

2.

EXPERIMENTAL RESULTSA representative result from gouge experi-

ment (HVR 605; σn = 0.6 MPa, V = 1.3 m/s) exhibits dramatic slip weakening of gouge from peak frictional coeffi cient μp of ~1.55 to steady-state friction μss of ~0.23 over the time interval of ~30 s or over slip weakening distance Dc of ~40 m (Fig. 1A). The extraordinary high μp is due to the effect of the Tefl on sleeve, which had a very tight fi t on the specimen assembly. We found later, from the Tefl on friction tests under no nor-

mal load, that the contribution of very tight Tefl on sleeve to the peak friction can be as high as 0.6 in terms of friction coeffi cient μ, but the Tefl on fric-tion decreases very rapidly with slip and becomes as low as 0.01 in μ during the steady state. Thus μp in Figure 1A is not reliable, but μss is close to the real value as the gouge friction.

First CO2 gas emission was detected with sen-

sor 1 ~1 s after the onset of slip (see orange verti-cal bar close to the ordinate of Fig. 1A). Consid-ering the response time of 0.9–1.0 s of sensor 1,

the CO2 emission started immediately (~0.1 s)

after the onset of slip and continued throughout the slip weakening. On the other hand, the output of sensor 2 was set with a detection threshold of 400 ppm, roughly corresponding to CO

2 concen-

tration in air (see red curve in Fig. 1A). The maximum output approaches ~2630 ppm, indi-cating that the total CO

2 emission is ~0.002 mol,

since the volume of the specimen chamber is 16.7 L (or 0.0167 m3). Thus the amount of decom-posed siderite estimated from the amount of CO

2

emitted is ~0.20 g (20% of the total gouge). This is equivalent to a gouge thickness of 0.15 mm being decomposed, assuming a decomposition reaction in the O

2-present condition 3 FeCO

3 +

1/2 O2 = Fe

3O

4 + 3 CO

2 (Koziol, 2004).

The result from pre-decomposed gouge (HVR732), which cannot emit CO

2 during

slip, exhibits a similar weakening behavior to that of siderite gouge, which emits CO

2 during

slip, although the pre-decomposed gouge shows more erratic evolution of friction (cf. Figs. 1A, 1B). This strongly suggests that the effect of CO

2

gas pressurization is not important, as is the case during calcite decomposition (Han et al., 2007).

The third experiment was conducted on gouge composed of mixed siderite, calcite, and quartz with the same weight proportion using a hollow specimen assembly at σn = 1.3 MPa and V = 2.0 m/s (HVR763; Figs. 1D, 1E). The black curve in Figure 1D shows the fi rst run, followed by the second run shown in red, started 10 min after the fi rst run. Despite the complex specimen assembly in HVR763, the initial Tefl on friction was not very large, on the order of 0.1–0.15 in terms of frictional coeffi cient, because of the ini-tial smoothing treatment of Tefl on sleeve under no axial load. Thus the peak friction μp of 0.92 for the fi rst run is fairly close to Byerlee (1978) friction (0.85). This peak stress is followed by marked slip weakening to μss of 0.2–0.3 over a displacement of ~15 m (black curve in Fig. 1D). Overall slip weakening after this is quite similar to that for the fi rst run (cf. black and red curves in Fig. 1D). Because these runs were made at a higher normal stress and at faster slip rate than the other two experiments (in Figs. 1A and 1B), and thus at a higher heat production rate, Dc in Figure 1D is somewhat shorter than that for the earlier two runs.

Change in color of all gouge samples was conspicuous. Siderite gouge before HVR605 run was light brownish powder, but it changed completely to very black powder after the run (Figs. 2A, 2B). A very shiny slickenside surface or slip zone is developed in the gouge, although we show only a disaggregated sample in Figure 2B. The black gouge after the run consists of ultrafi ne magnetite grains (~20 nm), as revealed under transmission electron microscope (TEM) (Fig. 2C). A selected-area electron diffraction (SAED) pattern shows that those grains have random lattice orientations (Fig. 2D).

D

B

HVR732σn = 0.6 MPa V = 1.3 m/s

Time (s)0 5 10 15 20 25 30 35

Fric

tion

coef

ficie

nt

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

Time (s)0 5 10 15 20 25

0.0

0.2

0.4

0.6

0.8

1.0

2nd slide 1st slide

Fric

tion

coef

ficie

nt

HVR763 σn = 1.3 MPa V = 2.0 m/s

A

SR

Gouge

Teflon sleeve

Normal stress

C

E

Teflon sleeve

Teflon bar

Gouge

R S

Normal stress

34 mm15.6 mm

24.8 mm

Pre-heated siderite (magnetite) gouge

Mixture of siderite, quartz and calcite

Time (s)0 10 20 30 40 50 60 70

Fric

tion

coef

ficie

nt

0.0

0.2

0.4

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sor

1 ou

tput

(m

V)

Sen

sor

2 ou

tput

(pp

m)HVR605

σn = 0.6 MPa V = 1.3 m/s

Onset of CO2 emission

0

200

400

600

800

0

500

1000

1500

2000

2500

3000

3500

Siderite gouge

80

Sensor 1

Sensor 2

Figure 1. A: Friction coeffi cient and outputs from two CO2 sensors (sensor 1 without fi lter and sensor 2 with fi lter) plotted against time in HVR605 performed at σn = 0.6 MPa and V = 1.3 m/s (see text). B: Friction coeffi cient plotted against time in HVR732, conducted at same conditions as that in HVR605. Before the run, siderite gouge was completely decomposed by static heating at 592 °C for 1 h in an electronic furnace. C: Schematic profi le of experimental specimen used for runs on siderite gouge (A) and decomposed siderite gouge (B). D: Friction coeffi cient against time in HVR763 at σn = 1.3 MPa and V = 2.0 m/s. The run was conducted with the new specimen confi guration in which the gouge layer was confi ned by an inner Tefl on bar and an outer Tefl on sleeve and two rock cylinders (E). Approximately 10 min after the cessation of the fi rst slide, the second slide was conducted at the same sliding condi-tion. R and S are rotary and stationary rock cylinders, respectively (C and E).

GEOLOGY, December 2007 1133

Pre-decomposed siderite gouge changed its color to very dark brown before the experi-ment (Figs. 2E, 2F). The same gouge after the experiment is composed of magnetite powder of 20–30 nm grain size (Fig. 2G), which is not much different from that of HVR605 (Fig. 2C). Also, a random lattice orientation of intact gouge sheared after pre-decomposition is con-fi rmed by a SAED pattern in Figure 2H.

The color change and development of a shiny slickenside surface are also recognized for mixed siderite-calcite-quartz gouge (HVR763; Figs. 3A–3C). A gouge piece in Figure 3C shows a sequence of changes from the original gouge at the bottom, to black powder, and to a

shiny slickenside surface. Magnetite formation and a remarkable decrease in the intensity of siderite diffraction peaks were recognized by the XRD analysis of the black gouge (Fig. 3D). Lime (CaO) and periclase (MgO), identifi ed by TEM observation (Figs. 3E, 3F) and XRD data in Figure 3D, indicate that at least part of the cal-cite decomposed. We also measured magnetic susceptibility of the undeformed gouge mixture and the deformed gouge in HVR763 run. The magnetic susceptibility of the deformed gouge (4.8 × 10−4 m3/kg) is much higher than that of undeformed gouge (9.4 × 10−7 m3/kg) due to the formation of magnetite.

DISCUSSIONSMeasurement of CO

2 emission (Fig. 1) and

XRD and TEM analyses of deformed gouge (Figs. 2 and 3) all clearly indicate that ther-mal decomposition of carbonate gouges due to frictional heating took place during our experi-ments. Siderite decomposition indicates that temperature transiently exceeded 400–580 °C (e.g., Deer et al., 1992; Isambert et al., 2003). However, the amount of CO

2 emission indicates

that ~20% of siderite gouge was decomposed in HVR605, so that temperature rise was not enough to decompose the whole gouge. This interpretation also holds for HVR763 since the original gouge was preserved adjacent to the black gouge (Fig. 3C). The partial decomposi-tion of siderite-bearing gouge is consistent with our preliminary thermal modeling using a fi nite element method that estimated the lower tem-perature than that of siderite decomposition.

We fi rst detected CO2 emission at an elapsed

time of ~0.1 s immediately after the onset of slip (Fig. 1A). Flash heating at sliding asperities (e.g., Rice, 2006) is likely to have caused at least this initial decomposition. Flash temperature at sliding asperity contacts can be estimated by (Archard, 1959; O’Hara, 2005):

ΔTp

C

aV

Cm

p p

= 0 3452

./

,μπρ κ ρ

(1)

where μ is friction coeffi cient, ρ is density, Cp is heat capacity at a constant pressure, pm is compressive yield strength, a is asperity contact radius, V is slip rate, and κ is thermal conduc-tivity. We used the following values for siderite gouge: ρ = 2699 kg/m3 (measured with the gouge used), Cp = 712 J/kg K (Robie et al., 1984), pm = 0.67 GPa (from Table 1 in Spray, 1992) for siderite (Mohs hardness = 4; Deer et al., 1992), and κ = 3 W/m K (Horai, 1971). Using those values and assuming that the friction at 0.1 s is equal to Byerlee friction (0.85), that a is 10 μm (taken from a range of Dc measured in gouge layers; Paterson and Wong, 2005), and that V is equal to external slip rate (1 m/s at 0.1 s), the cal-culated temperature rise (ΔT ) at an elapsed time of 0.1 s is ~570 °C, enough for siderite decompo-

sition. Although this is very rough estimate since V at sliding asperities and a are not well known, the initial siderite decomposition is highly likely to be due to fl ash heating. As shear is localized along thin zones with continued slip, intense shear heating is expected in the thin zones. The decomposed fraction of siderite, as much as 20% in our experiment, may be explained by the shear heating in the thin zones accompanied by thermal conduction, in addition to fl ash heating. We are now conducting detailed textural analy-ses and thermal calculations to delineate heating mechanisms in the gouge zone.

A very important implication of our results for fault-zone studies is that mineral decom-position can occur during a short-lived seismic fault slip and that the decomposition may leave

Figure 2. A: Color change of gouge before HVR605. B: Color change of gouge after HVR605. C: Transmission electron micro-scope (TEM) photomicrograph (scale bar = 20 nm) and selected-area electron diffraction pattern (D) indicate that randomly oriented ultrafi ne magnetite grains were formed by thermal decomposition of siderite, which was responsible for the color change (A). E: Color of gouge before static heating in an electronic furnace. F: Color of gouge after static heating in an electronic furnace. The heated gouge was used for the run HVR732. G, H: TEM observation on the gouge recov-ered after HVR732 confi rmed that the gouge consisted of randomly oriented ultrafi ne magnetite grains. Scale bar in G = 20 nm.

Black gouge below sliding surface

55 6535 4515

m m m m

c

q

s

scq cq s ssq q cc q s q c qc

252θ (degree)

Sheared

Original

q: Quartzc: Calcites: Sideritem: Magnetite

D

B HVR763

Sheared

A

Original

COriginal gouge

Shiny slickenside (sliding surface) on black gouge

20 nm

20 nm

MgO

CaO

F

E

Figure 3. A, B: Color change of gouge from pale brown to black before and after HVR763. C: Parts of the sheared gouge layer showed shiny sliding surfaces (slickensides) and black gouge layer underlain by original gouge. D: Reduced diffraction peak inten-sities of siderite and calcite after HVR763 and new magnetite peaks were recognized by X-ray diffraction on the sheared gouge. E, F: Transmission electron microscope observation confi rmed that some ultrafi ne MgO and CaO grains were formed by ther-mal decomposition of calcite.

1134 GEOLOGY, December 2007

geological records of seismic fault slip. There are many potential candidates for thermal decomposition, including clay minerals in fault gouge. Calcite decomposition (Han et al., 2007) will not leave such a record since its decompo-sition product (CaO) is very unstable and easily reacts with moisture, forming hydrated lime [Ca(OH)

2], from which calcite can be formed

again under CO2-present conditions. However,

the decomposition product of siderite (magne-tite) is much more stable than CaO and can be a potential indicator of seismic slip, although it may undergo reactions in geological time scale. Siderite decomposition or magnetite for-mation during shearing changes the color to black (Figs. 2 and 3) and increases its magnetic susceptibility by a few orders of magnitude, as recognized in HVR763.

Exactly the same changes have been reported for drill cores of the Chelungpu fault zone that caused 1999 Taiwan Chi-Chi earthquake. Li et al. (2005) reported disappearance of siderite and a small increase in magnetite content in black gouge developed in siderite-bearing gray fault gouge, although they suggested vaporiza-tion of siderite. Carbonate decomposition is also suggested by anomalously high magnetic susceptibility of the black layers (Hirono et al., 2006a) and by the decrease in the inorganic and total carbon contents (Hirono et al., 2006b) in other Chelungpu fault cores. Mineral decom-position by frictional heating may be a wide-spread phenomenon in natural fault zones, and this opens up a new perspective in searching for geological records of seismic slip.

Dramatic weakening behavior of siderite gouge (Fig. 1) is quite similar to that of a fault in Carrara marble (Han et al., 2007). Absence of amorphous material from XRD analysis and TEM observation excludes the possibility of gel formation (e.g., Goldsby and Tullis, 2002) or frictional melting (e.g., Hirose and Shima-moto, 2005) as causes of weakening. Simi-lar behavior of siderite and pre-decomposed siderite gouges (cf. Figs. 1A and 1B) implies that the weakening is not due to the effect of CO

2 gas pressurization, as is also the case for a

fault in Carrara marble. We still don’t know the exact weakening mechanism(s), but fl ash heat-ing of ultrafi ne magnetite grains (decomposi-tion product) is a possible mechanism based on the same reasons discussed in Han et al. (2007). Understanding high-velocity friction between nanometer-scale particles will provide a clue to understanding the exact mechanism of the dynamic weakening of faults.

ACKNOWLEDGMENTSHan’s stay at Kyoto University in 2005 was sup-

ported by the BK 21 Project of the Ministry of Edu-cation, Republic of Korea. This work was supported by the Korea Research Foundation grant C00435

(100699), the Grant-in-Aid for Scientifi c Research, Japan Society for Promotion of Science (16340129 and 18340159), and the Center of Excellence Program for the 21st Century of Kyoto University, “Active Geosphere Investigation.” We thank T. Hirose and K. Mizoguchi for technical help and comments, and W. Kim for measuring magnetic susceptibility. We also appreciate constructive reviews by H. Stünitz and S.F. Cox. Transmission electron microscope work was done at the Natural Science Center for Basic Research and Development (N-BARD), Hiroshima University.

REFERENCES CITEDArchard, J.F., 1959, The temperature of rub-

bing surfaces: Wear, v. 2, p. 438–455, doi: 10.1016/0043–1648(59)90159–0.

Byerlee, J.D., 1978, Friction of rocks: Pure and Applied Geophysics, v. 116, p. 615–626, doi: 10.1007/BF00876528.

Cowan, D.S., 1999, Do faults preserve a record of seismic slip? A fi eld geologist’s opinion: Jour-nal of Structural Geology, v. 21, p. 995–1001, doi: 10.1016/S0191–8141(99)00046–2.

Deer, W.A., Howie, R.A., and Zussman, J., 1992, An introduction to the rock forming minerals (sec-ond edition): Harlow, Longman Scientifi c and Technical, 720 p.

Di Toro, G., Hirose, T., Nielsen, S., Pennacchioni, G., and Shimamoto, T., 2006, Natural and experimental evidence of melt lubrication of faults during earthquakes: Science, v. 311, p. 647–649, doi: 10.1126/science.1121012.

Fialko, Y., and Khazan, Y., 2005, Fusion by the earth-quake fault friction: Stick or slip?: Journal of Geophysical Research, v. 110, p. B12407, doi: 10.1029/2005JB003869.

Goldsby, D.L., and Tullis, T.E., 2002, Low frictional strength of quartz rocks at subseismic slip rates: Geophysical Research Letters, v. 29, p. 1844, doi: 10.1029/2002GL015240.

Han, R., Shimamoto, T., Hirose, T., Ree, J.-H., and Ando, J., 2007, Ultra-low friction of carbon-ate faults caused by thermal decomposition: Science, v. 316, p. 878–881, doi: 10.1126/ -science.1139763.

Hirono, T., Lin, W., Yeh, E.-C., Soh, W., Hashimoto, Y., Sone, H., Matsubayashi, O., Aoike, K., Ito, H., Kinoshita, M., Murayama, M., Song, S.-R., Ma, K.-F., Hung, J.-H., Wang, C.-Y., and Tsai, Y.-B., 2006a, High magnetic susceptibility of fault gouge within Taiwan Chelungpu fault: Nondestructive continuous measurements of physical and chemical properties in fault rocks recovered from Hole B, TCDP: Geophysi-cal Research Letters, v. 33, p. L15303, doi: 10.1029/2006GL026133.

Hirono, T., Ikehara, M., Otsuki, K., Mishima, T., Sakaguchi, M., Soh, W., Omori, M., Lin, W., Yeh, E.-C., Tanikawa, W., and Wang, C.-Y., 2006b, Evidence of frictional melting from disk-shaped black material, discovered within the Taiwan Chelungpu fault system: Geophysi-cal Research Letters, v. 33, p. L19311, doi: 10.1029/2006GL027329.

Hirose, T., and Shimamoto, T., 2005, Growth of molten zone as a mechanism of slip weakening of simu-lated faults in gabbro during frictional melt-ing: Journal of Geophysical Research, v. 110, p. B05202, doi: 10.1029/2004JB003207.

Horai, K., 1971, Thermal conductivity of rock-forming minerals: Journal of Geophysical Research, v. 76, p. 1278–1308.

Isambert, A., Valet, J.-P., Gloter, A., and Guyot, F., 2003, Stable Mn-magnetite derived from Mn-siderite by heating in air: Journal of

Geophysical Research, v. 108, p. 2283, doi: 10.1029/2002JB002099.

Koziol, A.M., 2004, Experimental determination of siderite stability and application to Martian Meteorite ALH84001: American Mineralogist, v. 89, p. 294–300.

Li, W.-H., Wu, S.-Y., Hung, J.-H., and Tsai, Y.B., 2005, Nanometric characteristics of fault gouge in the slip zone from Feng Yuan bore-hole of the 1999 Chi-Chi earthquake in Taiwan: Asia Oceania Geosciences Society, 2nd Annual Meeting, Singapore, Abstracts, 58-SE-A0266.

Magloughlin, J.F., and Spray, J.G., eds., 1992, Fric-tional melting process and products in geologi-cal materials: Tectonophysics, v. 204, p. 197–204, doi: 10.1016/0040–1951(92)90307-R.

Mizoguchi, K., Hirose, T., Shimamoto, T., and Fukuyama, E., 2007, Reconstruction of seismic faulting by high-velocity friction experiments: An example of the 1995 Kobe earthquake: Geo-physical Research Letters, v. 34, p. L01308, doi: 10.1029/2006GL027931.

Noda, H., and Shimamoto, T., 2005, Thermal pres-surization and slip-weakening distance of a fault: An example of the Hanaore fault, southwest Japan: Seismological Society of America Bulletin, v. 95, p. 1224–1233, doi: 10.1785/0120040089.

O’Hara, K., 2005, Evaluation of asperity-scale tem-perature effects during seismic slip: Journal of Structural Geology, v. 27, p. 1892–1898, doi: 10.1016/j.jsg.2005.04.013.

Paterson, M.S., and Wong, T.-f., 2005, Experimen-tal rock deformation—The brittle fi eld (second edition): Berlin, Heidelberg, Springer-Verlag, 348 p.

Rice, J.R., 2006, Heating and weakening of faults during earthquake slip: Journal of Geo-physical Research, v. 111, p. B05311, doi: 10.1029/2005JB004006.

Robie, R.A., Haselton, H.T., Jr., and Hemingway, B.S., 1984, Heat capacities and entropies of rhodochrosite (MnCO3) and siderite (FeCO3) between 5 and 600 K: American Mineralogist, v. 69, p. 349–357.

Sibson, R.H., 1973, Interactions between tempera-ture and pore fl uid pressure during earthquake faulting and a mechanism for partial or total stress relief: Nature, v. 243, p. 66–68.

Sibson, R.H., 1975, Generation of pseudotachylyte by ancient seismic faulting: Royal Astronomical Society Geophysical Journal, v. 43, p. 775–794.

Spray, J.G., 1987, Artifi cial generation of pseudo-tachylite using friction welding apparatus: Simulation of melting on a fault plane: Jour-nal of Structural Geology, v. 9, p. 49–60, doi: 10.1016/0191–8141(87)90043–5.

Spray, J.G., 1992, A physical basis for the fric-tional melting of some rock forming miner-als: Tectonophysics, v. 204, p. 205–221, doi: 10.1016/0040–1951(92)90308-S.

Tsutsumi, A., and Shimamoto, T., 1997, High-velocity frictional properties of gabbro: Geo-physical Research Letters, v. 24, p. 699–702, doi: 10.1029/97GL00503.

Wibberley, C.A.J., and Shimamoto, T., 2005, Earth-quake slip weakening and asperities explained by thermal pressurization: Nature, v. 436, p. 689–692, doi: 10.1038/nature03901.

Manuscript received 21 May 2007Revised manuscript received 28 July 2007Manuscript accepted 2 August 2007

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