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Electromagnetic signals associated with stick–slip of quartz-free rocks
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Tectonophysics 450 (2008) 79 – 84 www.elsevier.com/locate/tecto
Electromagnetic signals associated with stick–slip of quartz-free rocks Akito Tsutsumi a,⁎, Nobumasa Shirai b a
Division of Earth and Planetary Sciences, Graduate School of Science, Kyoto University, Kyoto 606-8502, Japan b National Institute of Advanced Industrial Science and Technology, AIST, Ibaraki 305-8567, Japan Received 25 June 2007; received in revised form 11 December 2007; accepted 8 January 2008 Available online 15 January 2008
Abstract We recorded clear transients in the electric and magnetic fields upon sudden slip in stick–slip experiments on dry, cylindrically shaped, quartzfree rock specimens of basalt and peridotite with a 30° saw-cut (representing a fault) at confining pressures of up to 120 MPa. The amplitudes of the measured electric field signals were always higher at the electrode pair oriented parallel to the strike of the fault than at the pair oriented perpendicular. This anisotropy suggests a preferred electric polarization normal to the slip surface. The transients in the electric and magnetic fields were observed only when the fault slip occurred by stick–slip mode, not by a stable mode of the sliding, and the amplitudes of the electric field signals increased with increasing stress drop. It is suggested that the generation process of the electromagnetic signals is closely related to the characteristic behavior of the fault at the time of the initiation of slip during stick–slip events, probably with respect to the intensity of the signals. We propose that one or both of the following two processes characteristic of the fault at the time of the initiation of slip during stick–slip events are essential for the generation of detectable electromagnetic signals: rapid slip along the simulated fault and separation of the rock masses across the fault. © 2008 Elsevier B.V. All rights reserved. Keywords: Stick-slip; Electromagnetic signal; Rock friction
1. Introduction Various electromagnetic anomalies have been considered as candidates for a precursor signal of earthquake occurrence, and several physical models have been proposed for the mechanism that generates the observed anomalous signals (Nitsan, 1977; Warwick et al., 1982; Ogawa et al., 1985; Yamada et al., 1989; Freund, 2000, Rabinovitch et al., 2000; Freund, 2002; Frid et al., 2003); however, a theoretical basis for the generation mechanism of these electromagnetic phenomena has yet to be fully established. It is well known that rock samples produce electromagnetic signals upon failure, and many different experiments have been performed to evaluate the mechanisms that produce the observed signals. The piezoelectric effect of quartz has been considered as a favorable candidate for the electric signals recorded during deformation experiments on quartz-bearing granitic rocks ⁎ Corresponding author. Fax: +81 75 753 4189. E-mail address: [email protected] (A. Tsutsumi). 0040-1951/$ - see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.tecto.2008.01.001
(Nitsan, 1977; Warwick et al., 1982); however, the occurrence of electromagnetic signals during the deformation of quartz-free rocks (Ogawa et al., 1985; Yamada et al., 1989; Freund, 2000, Rabinovitch et al., 2000; Freund, 2002; Frid et al., 2003) suggests the existence of an alternative mechanism. Several models have been proposed to explain the universal features of the generation of electromagnetic signals from rocks, including electrification upon separation of the walls of microcracks (Ogawa et al., 1985; Yamada et al., 1989), the emission of electrons and charged particles from fracture surfaces (Enomoto and Hashimoto, 1990; Kawaguchi, 1998; Tsutsumi et al., 2003), atomic oscillations upon crack surfaces (Frid et al., 1999, 2003), and the bombardment of atmospheric gases due to the turbulent flow of net charged particles (Brady and Rowell, 1986; Cress et al., 1987). These models have been proposed based mainly on fracture experiments of rock samples, such as uniaxial and triaxial compression tests. Aside from these fracture experiments of rocks, the amount of experimental work on rock friction remains relatively limited. Previous studies have reported data from friction experiments (Lockner et al., 1986;
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Yoshida et al., 1994, 1997; Takeuchi and Nagahama, 2001; Rabinovitch et al., 2005); however, the samples used in most of these experiments were granitic rocks that contained quartz. In the present study, we performed a series of friction experiments on quartz-free rocks under confined conditions within a triaxial pressure vessel and measured changes in the electric and magnetic fields associated with fault sliding. We recorded clear transients in the electric and magnetic fields upon sudden slip motion during stick–slip. This paper described the characteristics of the recorded electric and magnetic signals. 2. Experimental procedure We analyzed samples of basalt (Toyooka, Japan) and peridotite (Hidaka, Japan). Thin section analysis under an optical microscope confirmed that the analyzed rocks contain no quartz. Cylindrical test specimens were prepared (50 mm in length and 20 mm in diameter) with a through-going saw-cut inclined at 30° to the long axis of the cylinder (Fig. 1). The pre-cut surfaces were ground by hand lapping with a #1000 silicon carbide abrasive. We chose this fine-grained abrasive so as to produce relatively smooth surfaces. Previous studies of rock friction have shown that stick–slip instability is highly dependent on machine stiffness, as well as normal stress and surface roughness (Dieterich, 1979). Unstable slip is favored by conditions of low stiffness, high normal stress, and smooth fault surfaces. Fig. 1 shows a schematic diagram of our experimental setup. Frictional experiments were performed using an oil-medium triaxial testing apparatus at confining pressures ranging from 30 to 120 MPa. The pressure vessel was made of stainless steel. Specimens were encased in four layers of a 0.5-mm-thick polyolefin heat-shrink tube to isolate the specimen from the Fig. 2. Record of the electric signals measured at the copper plate electrodes and the coil for a stick–slip experiment on basalt at 80 MPa confining pressure. (a) Data for the entire recording period. (b) Enlargement of that part of the record in (a) that contains the stick–slip event (vertical dashed lines in (a)).
Fig. 1. Schematic diagram of the configuration of the test system. The rock specimen with a saw-cut surface, copper plate electrodes, and coil were all set within a triaxial pressure vessel. Electrode pairs A and B were set normal and parallel to the strike of the fault plane, respectively. An AE sensor was attached to the moving piston at a distance of 160 mm from the center of the fault.
confining pressure medium (mineral oil). To reduce electrical noise, the ram used in applying axial differential stress was operated manually with a hand-pump and a volume-screwequipped pumping system. The displacement rate of the ram was controlled to be almost constant at about 0.003 mm/s by adjusting the speed using the handle of the volume-screw. In this system, axial load and axial displacement are both measured outside the pressure vessel. The load cell, which is mounted on the outer end of the moving piston, measures both the force applied to the specimen through the piston and the frictional resistance of o-rings around the moving piston. The distance between the pre-cut plane of the specimen and the load cell was 376 mm. Displacement was measured using a straingauge-type transducer mounted between the moving piston and the fixed base of the system. For the electric field detector, two sets of paired parallel copper plate electrodes (30 × 30 mm2) were set around the specimen, with one pair arranged normal to the strike of the fault (electrode pair A in Fig. 1) and the other parallel (electrode pair B). For the magnetic field detector, a solenoid coil (206 turns with a diameter of 42 mm) was set
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Fig. 3. Record of the electric signals measured at the copper plate electrodes and the coil for a stick–slip experiment on peridotite at 80 MPa confining pressure.
around the specimen just below the electrode pairs (Fig. 1). One of the electrodes of each electrode pair and one end of the winded coil wire were connected to ground; the pressure vessel was not grounded in this study. We measured the potential differences between the parallelfaced electrodes of each pair and between the two ends of the coil using a wide-band preamplifier (BX-31A; NF Corporation) with an input impedance of 107 ohm and a frequency response from DC to 70 MHz. To detect the acoustic emission signals, an acoustic emission (AE) sensor (AE-905U-T; NF Corporation) was attached to the piston at a distance of 160 mm from the center of the fault (Fig. 1). Output signals from the electrode pairs, coil, AE sensor, and axial load cell were recorded using a 4-channel digital storage oscilloscope (DS-9244 AM; Iwatsu Electric Co., LTD). The oscilloscope was equipped with just four signal input channels; consequently, in most cases the two electrode pairs, the coil, and the load cell were attached to the input connecters for recording. In some recordings, the AE sensor was attached to one of the input connecters. Depending on the mode of sliding, either a signal from the coil or the load cell was selected as a trigger for recording. During stick–slip sliding, signal from the axial load cell was used as a trigger for recording because the rapid force drop of the axial load associated with stick–slip provided a reliable trigger. During stable sliding, signal from the axial load cell could not be used as a trigger because it remained largely constant during sliding; in this case, output from the coil was used as a trigger to ensure that we recorded any electromagnetic signals emitted during stable sliding.
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followed by a period of no slip during which the stress was recharged. Stable sliding of the specimens was not accompanied by a detectable signal from the magnetic field sensor; in contrast, each of the stick–slip events was accompanied by clear signals in the electric and magnetic fields for all of the tested samples. Representative sets of recorded signals are shown in Figs. 2 and 3 for experiments on basalt and peridotite, respectively, both at a confining pressure of 80 MPa. In these cases, the stick–slip event was successfully triggered at the initial oscillation in the axial load (stress) record. The records show a clear delay by 0.3 ms in the timing of the onset of the first oscillation in the axial load relative to the electric and magnetic field signals. It should be noted here that the recorded axial load is not a direct measure of the stress near the fault because, as noted above, the force applied on the piston was measured outside the pressure vessel. The highpressure confining medium was sealed with o-rings around the moving piston; these o-rings would have affected the motion of the piston, thereby causing a delay in the response of the piston to sudden slip events on the fault surface. Thus, the delayed reaction of the load cell is probably due to viscous coupling between the moving piston and the o-ring seals (Shi et al., 1986).
3. Experimental results In all experiments, the simulated fault slid initially in stable mode for several millimeters of displacement. The slip mode then changed to periodic stick–slp, characterized by sudden slip
Fig. 4. Effect of stress on the intensity of the electric signal recorded at the time of stick–slip. (a) Records of the change in the electric field measured at electrode pair B during stick–slip events for basalt at between 30 and 100 MPa of confining pressure. (b) Relationship between the intensity of the electric field signal and the magnitude of the stress drop for basalt and peridotite samples.
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The electric field measured by the electrodes shows a rapid change toward the first transient-a sharp peak with negative polarity in these cases — followed by a broader peak of opposite polarity and a gradual return to the background level (Figs. 2 and 3). We observed both negative and positive polarities of the initial stepwise transient of the signal in individual runs. The maximum amplitudes of the signals (peakto-peak height) are higher for the signal measured at electrode pair B, oriented parallel to the strike of the fault, than those for the signal measured at electrode pair A, oriented normal to the strike of the fault (Figs. 2 and 3). Our preliminary results show that the maximum amplitude of the electric field increases with normal stress, whereas the periods between the first and second peaks and the relaxation time taken to return to background level are largely constant, showing no change with changing normal stress (Fig. 4). The signals recorded at the coil are different from those recorded at the electrodes, showing an initial high-frequency oscillation of about 500 kHz for about 0.1 ms, followed by an attenuating lower-frequency oscillation of about 10 kHz (Fig. 2b). There was no tuning between the oscillation of the axial load record and that of the low-frequency oscillation of the coil.
Fig. 5. Representative record showing the relationship between the timing of the AE signal and that of electromagnetic field signals for a stick–slip event within basalt at 30 MPa confining pressure. (a) Data for the entire recording period. (b) Enlargement of that part of the record in (a) that contains the stick–slip event.
The initiation of slip during stick–slip was always associated with an AE. Fig. 5 shows the temporal relationship between the timing of the AE signal and that of changes in the electric and magnetic fields for an experiment on basalt at a confining pressure of 30 MPa. In this record, the electric and magnetic field signals precede the AE signal by about 0.025 ms, reflecting the travel time of the acoustic wave. If we take into account the travel time of the AE signal from the stick–slip fault to the AE sensor (Fig. 1), the initiation of the electric and magnetic field signals can be considered to be coincident with the AE signal. 4. Discussion The electrification of crack surfaces (e.g., contact or separation electrification) has been considered as one of the important source processes in the generation of electromagnetic signals that occur during the fracture of rocks (e.g., Ogawa et al., 1985; Yamada et al., 1989). It was recently suggested in a friction experiment that the surfaces of real contacts (asperities) upon a fault are electrified due to fault shearing (Takeuchi and Nagahama, 2002, 2006), as with the electrification that accompanies crack-wall separation during fracture. The generation of charges upon asperity surfaces results in the formation of an electric dipole along a fault in a direction perpendicular to the fault surface, potentially leading to the directivity of electromagnetic signals (Takeuchi and Nagahama, 2006). In the present study, we observed a clear difference in the amplitude of change in the electric field recorded by electrode pairs A and B: the signal intensity was higher for electrode pair B, oriented parallel to the strike of the fault (Figs. 2 and 3). The asymmetric nature of the signal could be explained by the above model of asperity electrification on a fault plane during the sliding of rocks. Polarization of the electromagnetic field from faults has been considered in recent field studies (Lichtenberger, 2005, 2006). In our experiments, electric signal intensity shows a rough increase with normal stress (Fig. 4 a); more precisely, it seems that the electric signal intensity increases with increasing stress drop (Fig. 4b). The peridotite data shown in Fig. 4b demonstrate that the magnitude of the electric field change shows a stronger correlation with the shear stress drop than with the applied normal stress. In contrast, for basalt the magnitude of the electric field change shows a strong correlation with both the shear stress drop and normal stress. For example, the magnitude of the electric signal recorded for the peridotite experiment performed at a normal stress of 162.3 MPa is similar to the signal recorded for the experiment at a normal stress of 124.1 MPa, and smaller than that recorded for the experiment at 145.5 MPa. Similar results have been reported for a stick–slip experiment on granite (Takeuchi and Nagahama, 2001). To explain the observed correlations between the electromagnetic signal and the stress conditions in the present stick– slip experiments, it is necessary for the source process to include a mechanism whereby the signal intensity increases with increasing stress. Takeuchi and Nagahama (2001, 2002) proposed a model of surface electrification associated with hole-and electron-trapping centers, and proposed that charges on
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the asperities increase with increasing area of the asperities. Based on the well known increase in asperity area with normal stress (Dieterich and Kilgore, 1994) and the linear correlation between normal stress and the amount of stress drop during stick–slip (Sholz and Engelder, 1976), Takeuchi and Nagahama (2001) argued that the increasing intensity of the electromagnetic signal with stress drop is due to an increase in the area of asperities. The model proposed by Takeuchi and Nagahama (2001) is able to explain our results if we consider only the relationship between the electric signal intensity and the amount of stress drop; however, our results show that electric signals are only recorded when fault slip occurs by stick–slip, not by stable sliding. In Takeuchi and Nagahama's (2001) model, it is assumed that electric signals are generated due to the shearing of fresh asperities, thereby implying that an electric signal is generated whenever asperities are sheared in friction experiments; this is inconsistent with our finding that electromagnetic signals are only recorded during stick–slip. It is therefore suggested that the generation process of the electromagnetic signals is closely related to a process associated with the onset of stick–slip, probably with respect to the intensity of the signals. We propose that the rapid slip velocity upon a fault during stick–slip may be an important factor in terms of generating electric signals during the shearing of asperities. It is well known that every slip event during stick–slip takes place over the same duration of time, over which time the mass and stiffness of the loading system are fixed (Johnson and Scholz, 1976). This constant sliding time results in a linear relation between total slip and the average slip velocity. Because the stress drop during stick–slip shows a linear relationship with the total displacement via stiffness (Ohnaka, 1973), the average slip velocity increases linearly with increasing stress drop (Johnson et al., 1973; Johnson and Scholz, 1976; Shimamoto et al., 1980). The exact mechanism that leads to an increase in electromagnetic intensity is unknown, but it may involve a process in which the source of the electric signals increases with the amount of total slip or with the slip velocity along the fault. The durations of the electric field transients (i.e., the periods between the first and second peaks and the relaxation time taken to return to background level) are largely constant, showing no change with changing normal stress or the amount of shear stress drop (Fig. 4a). This result appears to support the above proposal that the generation process of the electromagnetic signals may be related to the rapid rate of sliding along the fault plane. Slip velocities as high as several tens of centimeters/ second are expected for slip during stick–slip (Johnson et al., 1973), much higher than the velocities ofb1 mm/s expected for stable sliding. Stick–slip has been observed in many different materials, including rock, metal, plastic, and foam rubber (Brune et al., 1993). The most extreme known case of stick–slip motion is that associated with earthquakes. Our results suggest that the intensity of coseismic electromagnetic signals shows a linear correlation with the magnitude of stress drop and/or the maximum slip rate during the event. Many previous studies have proposed that materials vibrate in a direction normal to the sliding surfaces during stick–slip
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(Brune et al., 1993; Bodin et al., 1998; Bouissou et al., 1998). In stick–slip experiments on a synthetic rock material, Bouissou et al. (1998) showed that the fault surfaces were at least partially separated during slip. The authors found that the maximum relative normal displacement of the fault surfaces increased with increasing normal stress. Their interpretation of this phenomenon is that a large amount of relative normal displacement is required to separate the fault surfaces at high normal stress because of the interlocking of asperities. If instantaneous normal vibrations or separation of the fault surfaces had occurred in our experiments, and if the normal separation displacement had increased with increasing stress drop, the observed relationship between the electromagnetic signal intensity and the amount of stress drop could be attributed to the expected increase in the amount of instantaneous separation of the charged fault surfaces. As with the crack-opening model (capacitor model) proposed by O'Keefe and Thiel (1995), an increase in the amount of instantaneous separation of a fault should result in a reduction in the equivalent capacitance between the surfaces across the fault leading to an increase in voltage across the fault. The low-frequency component recorded by the coil during the later stages of stick–slip events is not evident in the signals detected by the electrode (Fig. 3), possibly indicating that the lowfrequency component was induced by interaction between the vibrating coil and the geomagnetic field at the time of slip. The exponential nature of the recovery of the electrode signal to the zero level may reflect the macroscopic neutralization process of charges across the fault. The high-frequency component that appeared in the initial stages of stick–slip events is also clearly observed for the coil, but not for the electrodes (Fig. 3). This result might suggest that the high-frequency component of the coil signal was due to vibration of the system, as for the low-frequency component; however, we also wonder if the high-frequency component of the signal, at about 500 kHz, could have been generated due to elastic vibration of the experimental system at the time of the stick–slip event. A further possible interpretation is that the high-frequency signal (and possibly also the lowfrequency component) was recorded only for the coil because of the contrasting sensitivities of the coil and the electrode used in this study. As noted above, electric and magnetic sensors were both set at a distance of several centimeters from the fault, much less than one wavelength of the targeted electromagnetic waves. The recorded data could therefore represent the near-field type in an electromagnetic sense, making it difficult to define the impedance of each source of the signals. However, the clear differences in the observed waveforms of the electric and magnetic signals indicate that two different source processes operated in the generation of the signals detected during stick–slip events: a process with a high-impedance source such as bulk polarization of the fault surfaces, and a magnetic-field-dominated mechanism with a low-impedance source such as electric charge neutralization processes that operated locally within or across the fault surface. It is possible that the initial high-frequency component observed in the coil record was due to the neutralization process that operated locally in the vicinity of the asperities during the initial part of the stick-slip event, when the slip velocity was expected to be close to its peak value.
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5. Conclusions The main results of this study are summarized as follows. (1) Electric and magnetic signals were recorded only at the time of the initiation of individual slip events during stick–slip, associated with AE emission, but not during the remainder of the slip. The recorded electric and magnetic field signals were produced by a mechanism related to the initiation of abrupt slip and associated stress drop during stick–slip events. (2) The amplitudes of the electric potential signals increased with increasing normal stress or increasing stress drop during stick–slip events. It is likely that the generation process of the signals involves a mechanism that acts to increase the signal intensity with increasing stress. It is proposed that one or both of the following two processes characteristic of the fault at the time of the initiation of slip during stick–slip events are necessary for the generation of detectable electromagnetic signals: rapid slip of the simulated fault and separation across the fault at the time of the stick–slip event. (3) The amplitudes of the measured electric potential signals were always higher at the electrode pair (pair B in Fig. 1) oriented parallel to the strike of the fault than at the pair (A in Fig. 1) oriented normal to the fault. This anisotropy in the electric field signals is ascribed to electric polarization normal to the fault surface. Acknowledgements This study was supported in part by a grant awarded by the National Institute of Advanced Industrial Science and Technology (AIST). A.T. was supported in part by KAKENHI (13640459). We thank F. Freund for his constructive comments and assistance in editing the English of an earlier version of this paper. Constructive comments from two reviewers led to substantial improvements in the manuscript. References Bodin, P., Brown, S., Matheson, D., 1998. Laboratory observation of faultnormal vibrations during stick slip. J. Geophys. Res. 103, 29931–29944. Bouissou, S., Petit, J.P., Barquins, M., 1998. Experimental evidence of contact loss during stick–slip: possible implications for seismic behaviour. Tectonophysics 295, 341–350. Brady, B.T., Rowell, G.A., 1986. Laboratory investigation of the electrodynamics of rock fracture. Nature 231, 488–492. Brune, J.N., Brown, S., Johnson, P.A., 1993. Rupture mechanism and interface separation in foam rubber models of earthquakes: a possible solution to the heat flow paradox and the paradox of large overthrusts. Tectonophysics 218, 59–67. Cress, G.O., Brady, B.T., Rowell, G.A., 1987. Sources of electromagnetic radiation from fracture of rock samples in the laboratory. Geophys. Res. Lett. 14, 331–334. Dieterich, J.H., 1979. Modeling of rock friction. 1. Experimental results and constitutive equations. J. Geophys. Res. 84, 2161–2168. Dieterich, J.H., Kilgore, 1994. Direct observation of frictional contacts: new insights for state-dependent properties. Pure Appl. Geophys. 143, 283–302.
Enomoto, Y., Hashimoto, H., 1990. Emission of charged particles from indentation fracture of rocks. Nature 346, 641–643. Freund, F., 2000. Time-resolved study of charge generation and propagation in igneous rocks. J. Geophys. Res. 105, 11001–11019. Freund, F., 2002. Charge generation and propagation in igneous rocks. J. Geodyn. 33, 543–570. Frid, V., Rabinovitch, A., Bahat, D., 1999. Electromagnetic radiation associated with induced triaxial fracture in granite. Phil. Mag. Lett. 79, 79–86. Frid, V., Rabinovitch, A., Bahat, D., 2003. Fracture induced electromagnetic radiation. J. Phys. D: Appl. Phys. 36, 1620–1628. Johnson, T., Wu, F.T., Scholz, C.H., 1973. Source parameters for stick–slip and for earthquakes. Science 179, 278–279. Johnson, T., Scholz, C.H., 1976. Dynamic properties of stick–slip friction of rocks. J. Geophys. Res. 81, 881–888. Kawaguchi, Y., 1998. Charged particle emission and luminescence upon bending fracture of granite. Japan. J. Appl. Phys. 37, 3495–3499. Lichtenberger, M., 2006. Underground Measurements of Electromagnetic Radiation Related to Stress-induced Fractures in the Odenwald Mountains (Germany). Pure Appl. Geophys. 163, 1661–1677. Lichtenberger, M., 2005. Regional stress field as determined from electromagnetic radiation in a tunnel. J. Struct. Geol. 27, 2150–2158. Lockner, D.A., Byerlee, J.D., Kuksenko, V.S., Ponomarev, A.V., 1986. Stick slip, charge separation and decay. Pure Appl. Geophys. 124, 601–608. Nitsan, U., 1977. Electromagnetic emission accompanying fracture of quartzbearing rocks. Geophys. Res. Lett. 4, 333–336. Ogawa, T., Oike, K., Miura, T., 1985. Electromagnetic radiations from rocks. J. Geophys. Res. 90, 6245–6249. Ohnaka, M., 1973. Experimental studies of stick–slip and their application to the earthquake source mechanism. J. Phys. Earth 21, 285–303. O'Keefe, S.G., Thiel, D.V., 1995. A mechanism for the production of electromagnetic radiation during fracture of brittle materials. Phys. Earth Planet. Inter. 89, 127–135. Rabinovitch, A., Frid, V., Bahat, D., Goldbaum, J., 2000. Fracture area calculation from electromagnetic radiation and its use in chalk failure analysis. Int. J. Rock Mech. Min. Sci. 37, 1149–1154. Rabinovitch, A., Shay, A., Liraz, R., Frid, V., Bahat, D., 2005. Electromagnetic radiation emitted during friction process. Int. J. Fract. 131, L21–L27. Shi, X.J., Guo, Z.Q., Wang, C.Y., Hasegawa, T., 1986. Direct measurement of stick–slip and shear fracture energy of granite at elevated pressures. Geophys. Res. Lett. 13, 1027–1030. Shimamoto, T., Handin, J., Logan, J.M., 1980. Specimen-apparatus interaction during stick–slip in a triaxial compression machine: a decoupled twodegree-of-freedom model. Tectonophysics 67, 175–205. Sholz, C.H., Engelder, J.T., 1976. The role of asperity indentation and ploughing in rock friction — I. Int. J. Rock. Mech. Min. Sci. & Geomech. Abstr. 13, 149–154. Takeuchi, A., Nagahama, H., 2001. Voltage changes induced by stick–slip of granites. Geophys. Res. Lett. 28, 3365–3368. Takeuchi, A., Nagahama, H., 2002. Interpretation of charging on fracture or frictional slip surface of rocks. Phys. Earth Planet. Inter. 130, 285–291. Takeuchi, A., Nagahama, H., 2006. Electric dipoles perpendicular to a stick–slip plane. Phys. Earth Planet. Inter. 155, 208–218. Tsutsumi, A., Tanaka, S., Shirai, N., Enomoto, Y., 2003. Electric signals accompanying fracture of granite. Japan J. Appl. Phys. 42, 5208–5212. Warwick, J.W., Stoker, C., Meyer, T.R., 1982. Radio emission associated with rock fracture: possible application to the great Chilean earthquake of May 22, 1960. J. Geophys. Res. 87, 2851–2859. Yamada, I., Masuda, K., Mizutani, H., 1989. Electromagnetic and acoustic emission associated with rock fracture. Phys. Earth Planet. Inter. 57, 157–168. Yoshida, S., Manjgaladze, P., Zilpimani, D., Ohnaka, M., Nakatani, M., 1994. Electromagnetic emissions associated with frictional sliding of rock. In: Hayakawa, M., Fujinawa, Y. (Eds.), Electromagnetic phenomena related to earthquake prediction. Terra Scientific Publishing Company, Tokyo, pp. 307–322. Yoshida, S., Uyeshima, M., Nakatani, M., 1997. Electric potential changes associated with slip failure of granite: Preseismic and coseismic signals. J. Geophys. Res. 102, 14883–14897.
Tectonophysics 450 (2008) 79 – 84 www.elsevier.com/locate/tecto
Electromagnetic signals associated with stick–slip of quartz-free rocks Akito Tsutsumi a,⁎, Nobumasa Shirai b a
Division of Earth and Planetary Sciences, Graduate School of Science, Kyoto University, Kyoto 606-8502, Japan b National Institute of Advanced Industrial Science and Technology, AIST, Ibaraki 305-8567, Japan Received 25 June 2007; received in revised form 11 December 2007; accepted 8 January 2008 Available online 15 January 2008
Abstract We recorded clear transients in the electric and magnetic fields upon sudden slip in stick–slip experiments on dry, cylindrically shaped, quartzfree rock specimens of basalt and peridotite with a 30° saw-cut (representing a fault) at confining pressures of up to 120 MPa. The amplitudes of the measured electric field signals were always higher at the electrode pair oriented parallel to the strike of the fault than at the pair oriented perpendicular. This anisotropy suggests a preferred electric polarization normal to the slip surface. The transients in the electric and magnetic fields were observed only when the fault slip occurred by stick–slip mode, not by a stable mode of the sliding, and the amplitudes of the electric field signals increased with increasing stress drop. It is suggested that the generation process of the electromagnetic signals is closely related to the characteristic behavior of the fault at the time of the initiation of slip during stick–slip events, probably with respect to the intensity of the signals. We propose that one or both of the following two processes characteristic of the fault at the time of the initiation of slip during stick–slip events are essential for the generation of detectable electromagnetic signals: rapid slip along the simulated fault and separation of the rock masses across the fault. © 2008 Elsevier B.V. All rights reserved. Keywords: Stick-slip; Electromagnetic signal; Rock friction
1. Introduction Various electromagnetic anomalies have been considered as candidates for a precursor signal of earthquake occurrence, and several physical models have been proposed for the mechanism that generates the observed anomalous signals (Nitsan, 1977; Warwick et al., 1982; Ogawa et al., 1985; Yamada et al., 1989; Freund, 2000, Rabinovitch et al., 2000; Freund, 2002; Frid et al., 2003); however, a theoretical basis for the generation mechanism of these electromagnetic phenomena has yet to be fully established. It is well known that rock samples produce electromagnetic signals upon failure, and many different experiments have been performed to evaluate the mechanisms that produce the observed signals. The piezoelectric effect of quartz has been considered as a favorable candidate for the electric signals recorded during deformation experiments on quartz-bearing granitic rocks ⁎ Corresponding author. Fax: +81 75 753 4189. E-mail address: [email protected] (A. Tsutsumi). 0040-1951/$ - see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.tecto.2008.01.001
(Nitsan, 1977; Warwick et al., 1982); however, the occurrence of electromagnetic signals during the deformation of quartz-free rocks (Ogawa et al., 1985; Yamada et al., 1989; Freund, 2000, Rabinovitch et al., 2000; Freund, 2002; Frid et al., 2003) suggests the existence of an alternative mechanism. Several models have been proposed to explain the universal features of the generation of electromagnetic signals from rocks, including electrification upon separation of the walls of microcracks (Ogawa et al., 1985; Yamada et al., 1989), the emission of electrons and charged particles from fracture surfaces (Enomoto and Hashimoto, 1990; Kawaguchi, 1998; Tsutsumi et al., 2003), atomic oscillations upon crack surfaces (Frid et al., 1999, 2003), and the bombardment of atmospheric gases due to the turbulent flow of net charged particles (Brady and Rowell, 1986; Cress et al., 1987). These models have been proposed based mainly on fracture experiments of rock samples, such as uniaxial and triaxial compression tests. Aside from these fracture experiments of rocks, the amount of experimental work on rock friction remains relatively limited. Previous studies have reported data from friction experiments (Lockner et al., 1986;
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Yoshida et al., 1994, 1997; Takeuchi and Nagahama, 2001; Rabinovitch et al., 2005); however, the samples used in most of these experiments were granitic rocks that contained quartz. In the present study, we performed a series of friction experiments on quartz-free rocks under confined conditions within a triaxial pressure vessel and measured changes in the electric and magnetic fields associated with fault sliding. We recorded clear transients in the electric and magnetic fields upon sudden slip motion during stick–slip. This paper described the characteristics of the recorded electric and magnetic signals. 2. Experimental procedure We analyzed samples of basalt (Toyooka, Japan) and peridotite (Hidaka, Japan). Thin section analysis under an optical microscope confirmed that the analyzed rocks contain no quartz. Cylindrical test specimens were prepared (50 mm in length and 20 mm in diameter) with a through-going saw-cut inclined at 30° to the long axis of the cylinder (Fig. 1). The pre-cut surfaces were ground by hand lapping with a #1000 silicon carbide abrasive. We chose this fine-grained abrasive so as to produce relatively smooth surfaces. Previous studies of rock friction have shown that stick–slip instability is highly dependent on machine stiffness, as well as normal stress and surface roughness (Dieterich, 1979). Unstable slip is favored by conditions of low stiffness, high normal stress, and smooth fault surfaces. Fig. 1 shows a schematic diagram of our experimental setup. Frictional experiments were performed using an oil-medium triaxial testing apparatus at confining pressures ranging from 30 to 120 MPa. The pressure vessel was made of stainless steel. Specimens were encased in four layers of a 0.5-mm-thick polyolefin heat-shrink tube to isolate the specimen from the Fig. 2. Record of the electric signals measured at the copper plate electrodes and the coil for a stick–slip experiment on basalt at 80 MPa confining pressure. (a) Data for the entire recording period. (b) Enlargement of that part of the record in (a) that contains the stick–slip event (vertical dashed lines in (a)).
Fig. 1. Schematic diagram of the configuration of the test system. The rock specimen with a saw-cut surface, copper plate electrodes, and coil were all set within a triaxial pressure vessel. Electrode pairs A and B were set normal and parallel to the strike of the fault plane, respectively. An AE sensor was attached to the moving piston at a distance of 160 mm from the center of the fault.
confining pressure medium (mineral oil). To reduce electrical noise, the ram used in applying axial differential stress was operated manually with a hand-pump and a volume-screwequipped pumping system. The displacement rate of the ram was controlled to be almost constant at about 0.003 mm/s by adjusting the speed using the handle of the volume-screw. In this system, axial load and axial displacement are both measured outside the pressure vessel. The load cell, which is mounted on the outer end of the moving piston, measures both the force applied to the specimen through the piston and the frictional resistance of o-rings around the moving piston. The distance between the pre-cut plane of the specimen and the load cell was 376 mm. Displacement was measured using a straingauge-type transducer mounted between the moving piston and the fixed base of the system. For the electric field detector, two sets of paired parallel copper plate electrodes (30 × 30 mm2) were set around the specimen, with one pair arranged normal to the strike of the fault (electrode pair A in Fig. 1) and the other parallel (electrode pair B). For the magnetic field detector, a solenoid coil (206 turns with a diameter of 42 mm) was set
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Fig. 3. Record of the electric signals measured at the copper plate electrodes and the coil for a stick–slip experiment on peridotite at 80 MPa confining pressure.
around the specimen just below the electrode pairs (Fig. 1). One of the electrodes of each electrode pair and one end of the winded coil wire were connected to ground; the pressure vessel was not grounded in this study. We measured the potential differences between the parallelfaced electrodes of each pair and between the two ends of the coil using a wide-band preamplifier (BX-31A; NF Corporation) with an input impedance of 107 ohm and a frequency response from DC to 70 MHz. To detect the acoustic emission signals, an acoustic emission (AE) sensor (AE-905U-T; NF Corporation) was attached to the piston at a distance of 160 mm from the center of the fault (Fig. 1). Output signals from the electrode pairs, coil, AE sensor, and axial load cell were recorded using a 4-channel digital storage oscilloscope (DS-9244 AM; Iwatsu Electric Co., LTD). The oscilloscope was equipped with just four signal input channels; consequently, in most cases the two electrode pairs, the coil, and the load cell were attached to the input connecters for recording. In some recordings, the AE sensor was attached to one of the input connecters. Depending on the mode of sliding, either a signal from the coil or the load cell was selected as a trigger for recording. During stick–slip sliding, signal from the axial load cell was used as a trigger for recording because the rapid force drop of the axial load associated with stick–slip provided a reliable trigger. During stable sliding, signal from the axial load cell could not be used as a trigger because it remained largely constant during sliding; in this case, output from the coil was used as a trigger to ensure that we recorded any electromagnetic signals emitted during stable sliding.
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followed by a period of no slip during which the stress was recharged. Stable sliding of the specimens was not accompanied by a detectable signal from the magnetic field sensor; in contrast, each of the stick–slip events was accompanied by clear signals in the electric and magnetic fields for all of the tested samples. Representative sets of recorded signals are shown in Figs. 2 and 3 for experiments on basalt and peridotite, respectively, both at a confining pressure of 80 MPa. In these cases, the stick–slip event was successfully triggered at the initial oscillation in the axial load (stress) record. The records show a clear delay by 0.3 ms in the timing of the onset of the first oscillation in the axial load relative to the electric and magnetic field signals. It should be noted here that the recorded axial load is not a direct measure of the stress near the fault because, as noted above, the force applied on the piston was measured outside the pressure vessel. The highpressure confining medium was sealed with o-rings around the moving piston; these o-rings would have affected the motion of the piston, thereby causing a delay in the response of the piston to sudden slip events on the fault surface. Thus, the delayed reaction of the load cell is probably due to viscous coupling between the moving piston and the o-ring seals (Shi et al., 1986).
3. Experimental results In all experiments, the simulated fault slid initially in stable mode for several millimeters of displacement. The slip mode then changed to periodic stick–slp, characterized by sudden slip
Fig. 4. Effect of stress on the intensity of the electric signal recorded at the time of stick–slip. (a) Records of the change in the electric field measured at electrode pair B during stick–slip events for basalt at between 30 and 100 MPa of confining pressure. (b) Relationship between the intensity of the electric field signal and the magnitude of the stress drop for basalt and peridotite samples.
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The electric field measured by the electrodes shows a rapid change toward the first transient-a sharp peak with negative polarity in these cases — followed by a broader peak of opposite polarity and a gradual return to the background level (Figs. 2 and 3). We observed both negative and positive polarities of the initial stepwise transient of the signal in individual runs. The maximum amplitudes of the signals (peakto-peak height) are higher for the signal measured at electrode pair B, oriented parallel to the strike of the fault, than those for the signal measured at electrode pair A, oriented normal to the strike of the fault (Figs. 2 and 3). Our preliminary results show that the maximum amplitude of the electric field increases with normal stress, whereas the periods between the first and second peaks and the relaxation time taken to return to background level are largely constant, showing no change with changing normal stress (Fig. 4). The signals recorded at the coil are different from those recorded at the electrodes, showing an initial high-frequency oscillation of about 500 kHz for about 0.1 ms, followed by an attenuating lower-frequency oscillation of about 10 kHz (Fig. 2b). There was no tuning between the oscillation of the axial load record and that of the low-frequency oscillation of the coil.
Fig. 5. Representative record showing the relationship between the timing of the AE signal and that of electromagnetic field signals for a stick–slip event within basalt at 30 MPa confining pressure. (a) Data for the entire recording period. (b) Enlargement of that part of the record in (a) that contains the stick–slip event.
The initiation of slip during stick–slip was always associated with an AE. Fig. 5 shows the temporal relationship between the timing of the AE signal and that of changes in the electric and magnetic fields for an experiment on basalt at a confining pressure of 30 MPa. In this record, the electric and magnetic field signals precede the AE signal by about 0.025 ms, reflecting the travel time of the acoustic wave. If we take into account the travel time of the AE signal from the stick–slip fault to the AE sensor (Fig. 1), the initiation of the electric and magnetic field signals can be considered to be coincident with the AE signal. 4. Discussion The electrification of crack surfaces (e.g., contact or separation electrification) has been considered as one of the important source processes in the generation of electromagnetic signals that occur during the fracture of rocks (e.g., Ogawa et al., 1985; Yamada et al., 1989). It was recently suggested in a friction experiment that the surfaces of real contacts (asperities) upon a fault are electrified due to fault shearing (Takeuchi and Nagahama, 2002, 2006), as with the electrification that accompanies crack-wall separation during fracture. The generation of charges upon asperity surfaces results in the formation of an electric dipole along a fault in a direction perpendicular to the fault surface, potentially leading to the directivity of electromagnetic signals (Takeuchi and Nagahama, 2006). In the present study, we observed a clear difference in the amplitude of change in the electric field recorded by electrode pairs A and B: the signal intensity was higher for electrode pair B, oriented parallel to the strike of the fault (Figs. 2 and 3). The asymmetric nature of the signal could be explained by the above model of asperity electrification on a fault plane during the sliding of rocks. Polarization of the electromagnetic field from faults has been considered in recent field studies (Lichtenberger, 2005, 2006). In our experiments, electric signal intensity shows a rough increase with normal stress (Fig. 4 a); more precisely, it seems that the electric signal intensity increases with increasing stress drop (Fig. 4b). The peridotite data shown in Fig. 4b demonstrate that the magnitude of the electric field change shows a stronger correlation with the shear stress drop than with the applied normal stress. In contrast, for basalt the magnitude of the electric field change shows a strong correlation with both the shear stress drop and normal stress. For example, the magnitude of the electric signal recorded for the peridotite experiment performed at a normal stress of 162.3 MPa is similar to the signal recorded for the experiment at a normal stress of 124.1 MPa, and smaller than that recorded for the experiment at 145.5 MPa. Similar results have been reported for a stick–slip experiment on granite (Takeuchi and Nagahama, 2001). To explain the observed correlations between the electromagnetic signal and the stress conditions in the present stick– slip experiments, it is necessary for the source process to include a mechanism whereby the signal intensity increases with increasing stress. Takeuchi and Nagahama (2001, 2002) proposed a model of surface electrification associated with hole-and electron-trapping centers, and proposed that charges on
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the asperities increase with increasing area of the asperities. Based on the well known increase in asperity area with normal stress (Dieterich and Kilgore, 1994) and the linear correlation between normal stress and the amount of stress drop during stick–slip (Sholz and Engelder, 1976), Takeuchi and Nagahama (2001) argued that the increasing intensity of the electromagnetic signal with stress drop is due to an increase in the area of asperities. The model proposed by Takeuchi and Nagahama (2001) is able to explain our results if we consider only the relationship between the electric signal intensity and the amount of stress drop; however, our results show that electric signals are only recorded when fault slip occurs by stick–slip, not by stable sliding. In Takeuchi and Nagahama's (2001) model, it is assumed that electric signals are generated due to the shearing of fresh asperities, thereby implying that an electric signal is generated whenever asperities are sheared in friction experiments; this is inconsistent with our finding that electromagnetic signals are only recorded during stick–slip. It is therefore suggested that the generation process of the electromagnetic signals is closely related to a process associated with the onset of stick–slip, probably with respect to the intensity of the signals. We propose that the rapid slip velocity upon a fault during stick–slip may be an important factor in terms of generating electric signals during the shearing of asperities. It is well known that every slip event during stick–slip takes place over the same duration of time, over which time the mass and stiffness of the loading system are fixed (Johnson and Scholz, 1976). This constant sliding time results in a linear relation between total slip and the average slip velocity. Because the stress drop during stick–slip shows a linear relationship with the total displacement via stiffness (Ohnaka, 1973), the average slip velocity increases linearly with increasing stress drop (Johnson et al., 1973; Johnson and Scholz, 1976; Shimamoto et al., 1980). The exact mechanism that leads to an increase in electromagnetic intensity is unknown, but it may involve a process in which the source of the electric signals increases with the amount of total slip or with the slip velocity along the fault. The durations of the electric field transients (i.e., the periods between the first and second peaks and the relaxation time taken to return to background level) are largely constant, showing no change with changing normal stress or the amount of shear stress drop (Fig. 4a). This result appears to support the above proposal that the generation process of the electromagnetic signals may be related to the rapid rate of sliding along the fault plane. Slip velocities as high as several tens of centimeters/ second are expected for slip during stick–slip (Johnson et al., 1973), much higher than the velocities ofb1 mm/s expected for stable sliding. Stick–slip has been observed in many different materials, including rock, metal, plastic, and foam rubber (Brune et al., 1993). The most extreme known case of stick–slip motion is that associated with earthquakes. Our results suggest that the intensity of coseismic electromagnetic signals shows a linear correlation with the magnitude of stress drop and/or the maximum slip rate during the event. Many previous studies have proposed that materials vibrate in a direction normal to the sliding surfaces during stick–slip
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(Brune et al., 1993; Bodin et al., 1998; Bouissou et al., 1998). In stick–slip experiments on a synthetic rock material, Bouissou et al. (1998) showed that the fault surfaces were at least partially separated during slip. The authors found that the maximum relative normal displacement of the fault surfaces increased with increasing normal stress. Their interpretation of this phenomenon is that a large amount of relative normal displacement is required to separate the fault surfaces at high normal stress because of the interlocking of asperities. If instantaneous normal vibrations or separation of the fault surfaces had occurred in our experiments, and if the normal separation displacement had increased with increasing stress drop, the observed relationship between the electromagnetic signal intensity and the amount of stress drop could be attributed to the expected increase in the amount of instantaneous separation of the charged fault surfaces. As with the crack-opening model (capacitor model) proposed by O'Keefe and Thiel (1995), an increase in the amount of instantaneous separation of a fault should result in a reduction in the equivalent capacitance between the surfaces across the fault leading to an increase in voltage across the fault. The low-frequency component recorded by the coil during the later stages of stick–slip events is not evident in the signals detected by the electrode (Fig. 3), possibly indicating that the lowfrequency component was induced by interaction between the vibrating coil and the geomagnetic field at the time of slip. The exponential nature of the recovery of the electrode signal to the zero level may reflect the macroscopic neutralization process of charges across the fault. The high-frequency component that appeared in the initial stages of stick–slip events is also clearly observed for the coil, but not for the electrodes (Fig. 3). This result might suggest that the high-frequency component of the coil signal was due to vibration of the system, as for the low-frequency component; however, we also wonder if the high-frequency component of the signal, at about 500 kHz, could have been generated due to elastic vibration of the experimental system at the time of the stick–slip event. A further possible interpretation is that the high-frequency signal (and possibly also the lowfrequency component) was recorded only for the coil because of the contrasting sensitivities of the coil and the electrode used in this study. As noted above, electric and magnetic sensors were both set at a distance of several centimeters from the fault, much less than one wavelength of the targeted electromagnetic waves. The recorded data could therefore represent the near-field type in an electromagnetic sense, making it difficult to define the impedance of each source of the signals. However, the clear differences in the observed waveforms of the electric and magnetic signals indicate that two different source processes operated in the generation of the signals detected during stick–slip events: a process with a high-impedance source such as bulk polarization of the fault surfaces, and a magnetic-field-dominated mechanism with a low-impedance source such as electric charge neutralization processes that operated locally within or across the fault surface. It is possible that the initial high-frequency component observed in the coil record was due to the neutralization process that operated locally in the vicinity of the asperities during the initial part of the stick-slip event, when the slip velocity was expected to be close to its peak value.
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5. Conclusions The main results of this study are summarized as follows. (1) Electric and magnetic signals were recorded only at the time of the initiation of individual slip events during stick–slip, associated with AE emission, but not during the remainder of the slip. The recorded electric and magnetic field signals were produced by a mechanism related to the initiation of abrupt slip and associated stress drop during stick–slip events. (2) The amplitudes of the electric potential signals increased with increasing normal stress or increasing stress drop during stick–slip events. It is likely that the generation process of the signals involves a mechanism that acts to increase the signal intensity with increasing stress. It is proposed that one or both of the following two processes characteristic of the fault at the time of the initiation of slip during stick–slip events are necessary for the generation of detectable electromagnetic signals: rapid slip of the simulated fault and separation across the fault at the time of the stick–slip event. (3) The amplitudes of the measured electric potential signals were always higher at the electrode pair (pair B in Fig. 1) oriented parallel to the strike of the fault than at the pair (A in Fig. 1) oriented normal to the fault. This anisotropy in the electric field signals is ascribed to electric polarization normal to the fault surface. Acknowledgements This study was supported in part by a grant awarded by the National Institute of Advanced Industrial Science and Technology (AIST). A.T. was supported in part by KAKENHI (13640459). We thank F. Freund for his constructive comments and assistance in editing the English of an earlier version of this paper. Constructive comments from two reviewers led to substantial improvements in the manuscript. References Bodin, P., Brown, S., Matheson, D., 1998. Laboratory observation of faultnormal vibrations during stick slip. J. Geophys. Res. 103, 29931–29944. Bouissou, S., Petit, J.P., Barquins, M., 1998. Experimental evidence of contact loss during stick–slip: possible implications for seismic behaviour. Tectonophysics 295, 341–350. Brady, B.T., Rowell, G.A., 1986. Laboratory investigation of the electrodynamics of rock fracture. Nature 231, 488–492. Brune, J.N., Brown, S., Johnson, P.A., 1993. Rupture mechanism and interface separation in foam rubber models of earthquakes: a possible solution to the heat flow paradox and the paradox of large overthrusts. Tectonophysics 218, 59–67. Cress, G.O., Brady, B.T., Rowell, G.A., 1987. Sources of electromagnetic radiation from fracture of rock samples in the laboratory. Geophys. Res. Lett. 14, 331–334. Dieterich, J.H., 1979. Modeling of rock friction. 1. Experimental results and constitutive equations. J. Geophys. Res. 84, 2161–2168. Dieterich, J.H., Kilgore, 1994. Direct observation of frictional contacts: new insights for state-dependent properties. Pure Appl. Geophys. 143, 283–302.
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