Annals 47, 1, 2004, 01/07def 21 ANNALS OF GEOPHYSICS, VOL. 47, N. 1, February 2004 Key words Pressure Stimulated Currents (PSC)- Piezo Stimulated Current – rocks – marble-electric precursors 1. Introduction Transient electric phenomena in the litho- sphere have been observed for a long time (Varotsos and Alexopoulos, 1984a,b; Fujinawa and Takahashi, 1990; Nomicos and Vallianatos, 1997; Hayakawa, 1999). During the last decade, interest in transient electric signals has been growing and observation networks have been extended in many countries of the world (see Park et al., 1993; Hayakawa, 1999; Kopytenko et al., 2001; Hayakawa and Molchanov, 2002). Many models have been suggested to explain such transient electric phenomena accompanied by fracture. Piezoelectric effects constitute one of several factors for modeling (e.g., Finkelstein et al., 1973). However the proposed mechanism cannot explain why non-piezoelectric minerals or rocks generate electric phenomena. Moreover, electrokinetic effects (Mizutani et al., 1976) are also limited to the cases of wa- ter-saturated rocks or water flowing through rocks. Since electric phenomena are also ob- served during fracturing of dried rocks it is ev- ident that both piezoelectric and electrokinetic effects may not be the main factors for model- ing such phenomena. Many researchers accept that the transient electric phenomena are related to crack genera- tion and propagation in the Earth’s crust (e.g., Molchanov and Hayakawa, 1995, 1998; Val- lianatos and Tzanis, 1998, 1999a,b; Tzanis and Vallianatos, 2002). In order to understand the mechanisms that produce these electric signals, many fracture tests in the laboratory have been conducted us- ing various kinds of minerals and rocks under Pressure Stimulated Currents (PSC) in marble samples Cimon Anastasiadis (1), Dimos Triantis (1), Ilias Stavrakas (1) and Filippos Vallianatos (2) (1) Department of Electronics, Technological Educational Institution of Athens, Greece (2) Department of Natural Resources Engineering, Technological Educational Institute of Crete, Greece Abstract The electrical behaviour of marble samples from Penteli Mountain was studied while they were subjected to uni- axial stress. The application of consecutive impulsive variations of uniaxial stress to thirty connatural samples produced Pressure Stimulated Currents (PSC). The linear relationship between the recorded PSC and the applied variation rate was investigated. The main results are the following: as far as the samples were under pressure cor- responding to their elastic region, the maximum PSC value obeyed a linear law with respect to pressure varia- tion. In the plastic region deviations were observed which were due to variations of Young’s modulus. Further- more, a special burst form of PSC recordings during failure is presented. The latter is emitted when irregular lon- gitudinal splitting is observed during failure. Mailing address: Dr. Cimon Anastasiadis, Depart- ment of Electronics, Technological Educational Institu- tion of Athens, Athens-Egaleo 12210, Greece; e-mail: ci- mon@teiath.gr 22 Cimon Anastasiadis, Dimos Triantis, Ilias Stavrakas and Filippos Vallianatos both dry and saturated conditions (e.g., Nitsan, 1977; Ogawa et al., 1985; Brady and Rowell, 1986; Cress et al., 1987; Yamada et al., 1989; Enomoto and Hashimoto, 1990; Hadjicontis and Mavromatou, 1994,1995; O’Keefe and Thiel, 1995; Freund, 2000; Takeuchi and Naga- hama, 2001; Stavrakas et al., 2003). Since transient electric phenomena are promising candidates as earthquake precursors, a series of laboratory experiments of uniaxial compression of marble samples were carried out to understand the underlying physical mecha- nisms of electric signal generation. In the first set of experiments, marble samples were sub- jected to a time-varying uniaxial compressional stress at both variable and constant stress rates, not exceeding the elasticity limit (Stavrakas et al., 2003; Vallianatos et al., 2004). The applied stress henceforth in the experiments is uniaxial compressional stress. The technique used to measure the current emitted from rock samples while applying stress at various rates will hence- forth be referred to as pressure stimulated cur- rent technique. The experimental results support the validity of the Moving Charged Dislocation (MCD) model (see Vallianatos and Tzanis 1998, 1999a,b; Tzanis and Vallianatos, 2002). In the present paper, we show experimental results obtained in the case of applying stress that produced deformations in the plastic range up to fracture. The results suggest that the pro- portionality factor γ between the emitted cur- rent (I) and the stress rate (dS/dt) changes as we pass to the plastic region, in consistency with the MCD model. Furthermore, an experimental attempt to un- derstand an electrical activity (i.e a series of short pulses) observed approaching failure is given. 2. Sample and experimental description Marble belongs to the class of metamorphic rocks. Its structural inhomogeneities are due to either natural or man-made causes such as the application of mechanical stress or chemical processing. In the described experiment, thirty Dio- nysos marbles (see table I) collected from Mt. Penteli, Attica were used. The Dionysos mar- ble, which has been typically used since ancient times for the construction of artifacts and mon- uments, is mainly composed of calcite (98%) and other minerals, depending on the variety of marble, such as muscovite, sericite and chlorite (Kleftakis et al., 2000). Its content in quartz is very low, about 0.2%. Its density is 2.7 g/cm3 and its porosity is approximately 0.4%. Calcite crystals are polygonic, mainly equisized, some- times exhibiting twinning and their texture may be characterized as quasi-homoblastic. The rock is white with a few thin parallel ash-green coloured veins containing silver-shaded areas due to the existence of chlorite and muscovite. Matrix rocks were intentionally selected to be quasi single-grained. The experiment was con- ducted in a Faraday shield to prevent electric noise. The noise-protected system comprised a uniaxial hydraulic load machine (Enerpac- RC106) that applied compressional stress to the sample, which was placed on a stainless steel base. The marble sample was placed between two thin teflon plates in the direction of stress to provide electrical insulation. The values of the externally applied stress were recorded us- ing a manometer. A pair of electrodes was at- tached to the marble sample using conductive paste. The electrodes were attached in a direc- Table I. Information table containing the samples used during the described experiments. Sample code Dimensions Experimental technique applied Marble Dionysos (mm) MD001 to MD010 50 × 50 × 50 PSC technique applied on sequential stress variations MD011to MD030 50 × 50 × 45 PSC measurements closely to the failure with constant stress rate 23 Pressure Stimulated Currents (PSC) in marble samples tion perpendicular to the axis of the applied stress (see fig. 1). For electrical measurements, a sensitive programmable electrometer Keithley 617 was used, (current range from 0.1 fA to 20 mA). 3. Experimental results and discussion In a set of previously conducted PSC exper- iments on marble samples (Stavrakas et al., 2003; Vallianatos et al., 2004), the samples were subjected to uniaxial stress in the elastic range of the material. In the present set of experiments, multiple incremental stress variations were applied to the sample to pass progressively from the elas- tic into the plastic range. Figure 2a-c shows the measured time series of the applied stress S (fig. 2a), the stress rate dS/dt (fig. 2b) and the current emission (fig. 2c) which is of the order of pA. The recording de- scribed corresponds to a stress range that the material behaves elastically. Our experimental data obey a scaling law relating the emitted cur- rent (I) and the stress rate dS/dt, (see Hadjicon- tis and Mavromatou, 1994; Vallianatos and Tza- nis, 1998, 1999a; Stavrakas et al., 2003). Fig. 1. Experimental setup. Fig. 2a-c. Time records of (a) stepwise applied stress to Penteli marble sample (MD007), b) the cor- responding stress rate (ds/dt) and (c) the emitted PSC. Recordings of the currents emitted due to successive abrupt changes of the applied stress both in regions where the material behaves elas- tically and in regions where the material is in the plastic range are shown in fig. 3a-c. Figure 3a depicts the sequence of steps of incremental stress variations. We note that the secondary ax- is was graded in values of normalized stress S/Smax where Smax is the maximum applied stress on the material close to failure. Figure 3b shows the stress rate dS/dt with respect to time. Figure 3c is the emitted current (I) with respect to time. We proceed now to the study of the re- lation between the emitted current (I) and the stress rate dS/dt, when the applied stress S takes values in both the elastic and plastic ranges. In a b c 24 Cimon Anastasiadis, Dimos Triantis, Ilias Stavrakas and Filippos Vallianatos Fig. 3a-c. Time recordings of (a) successive abruptly applied stresses onto the sample (MD007), b) the corre- sponding stress rates and (c) PSC. Fig. 4. Scaling factor γ with respect to the normal- ized stress. Fig. 5. Normalized experimental stress-strain dia- gram from a Penteli marble sample. b c a 25 Pressure Stimulated Currents (PSC) in marble samples previous papers, based on the MCD model, Val- lianatos and Tzanis (1998, 1999a) propose a scaling between the emitted current and the stress rate dS/dt, when the material is uniaxial- ly compressed I dt dS = c (3.1) where γ is a scaling factor which has a recipro- cal dependence to the Young’s modulus Y (i.e. γ ∼∼ ∼∼ 1/Y) of the material. Since we study the behaviour of marble samples in both the elastic and plastic ranges we may estimate the dependence of the scaling factor γ on stress. Figure 4 demonstrates the de- pendence of factor γ on the normalized stress (S/Smax). The scaling factor γ was calculated us- ing the experimental data according to the rela- tionship dt dS I max max =c b l where Imax is the maximum value of the emitted current during the application of uniaxial stress (S) and (dS/dt)max is the corresponding maxi- mum stress rate. In the calculation of the quan- tity S/Smax, the stress S corresponds to its aver- Fig. 6a-c. Time records of two PSC peaks taken from a marble sample (MD014) at fracture: a) curves, b) de- pict stress and (c) stress rate respectively. a b c 26 Cimon Anastasiadis, Dimos Triantis, Ilias Stavrakas and Filippos Vallianatos Fig. 7. Sample photo after fracture to show the fracture planes (sample: MD027). Fig 8a,b. Fracture modes of geomaterials: a) planes parallel to the direction of stress, b) planes di- agonal to the direction of stress. age value during each stress step. This is prac- tically equal to the instantaneous stress on the sample at the time when the maximum value of the stress rate (dS/dt)max is exerted. It becomes clear in the diagram of fig. 4 that when the ap- plied stress is less than 0.5 Smax, the value of the factor γ remains practically constant. Notice- able is the fact that as far as S/Smax < 0.5 the ma- terial behaves elastically and Young’s modulus remains constant. This becomes evident in the normalized stress-strain diagram (fig. 5). The diagram was constructed using data of marble samples from Mt. Penteli, Attica (Kleftakis et al., 2000). When the ratio S/Smax > 0.5 the ma- terial exits the elastic range and gradually en- ters the plastic range thus Young’s modulus Y is continuously decreasing. According to the MCD model, the scaling factor γ is proportional to 1/Y. This is consistent with the experimental result indicating that as the values of normalized stress S/Smax increase, the factor γ increases too. The latter becomes evident if the calculated values of γ that corre- spond to the plastic range for 0.6 < S/Smax < 0.7 are considered (fig. 4). From the microphysical point of view we note that applying stresses in the plastic range, structural changes are intro- duced into the samples depending on the stress state. According to Hallbauer et al. (1973), when the sample is stressed uniaxially with stress beyond 0.55 Smax up to 0.65 Smax then mi- crocracks appear. These cracks are the most dominant factor of all heterogeneities that gov- ern the failure nucleation process in rock sam- ples (Lei et al., 2000) and are the sources of macrocracks that will appear when stress ex- ceeds 0.85 Smax and increases up to failure. We proceed now to study PSC near the fail- ure range. Figure 6a-c shows PSC emission when the applied stress was greater than 0.95 Smax. The continuously increasing stress on the sample and the corresponding stress rate dS/dt are depicted in fig. 6a and 6b respectively. The maximum recorded stress Smax on the sample is recorded at t = 334 s, accompanied by a short abrupt decrease of the stress value. Simultane- ously, the first current peak is recorded (fig. 6c). At the time interval between 340 s and 350 s stress was kept approximately constant and was followed by sample fracture accompanied by a second more intense current peak. A photograph of the status of the specimen after the experiment is shown in fig. 7. The two main fracture planes (which were created while stress was instanta- neously decreased) can be seen to lie along the direction of stress (point A in fig. 6a). Such frac- tures could be the result of a large number of mi- crocracks that had already been generated when the sample had suffered a stress between 0.55 Smax and 0.85 Smax (Jaeger and Cook, 1979). Systematic laboratory study of the PSC emitted by marble samples due to stress slight- ly before fracture suggests that when irregular longitudinal splitting is observed during the failure process (fig. 8a), then for each fracture plane corresponding to a macrocrack a PSC peak is observed. Thus, the number of PSC peaks appearing just before dynamic failure is a b 27 Pressure Stimulated Currents (PSC) in marble samples Fig. 9a-c. Multiple PSC peaks of a marble sample (MD016) in the time interval of the appearance of mi- crocracks (parallel to the direction of the applied stress) and fracture time: a) stress, b) stress rate and (c) PSC. fig. 7) gave multiple PSC peaks (see fig. 9a-c). On the other hand, when the failure of the sample forms a shear fracture, then a «single» PSC peak is observed (see fig. 10). The latter experimental results could possibly be related to the two types of electric earthquake precur- sors (i.e. single signals and electrical activi- ties) reported by Varotsos and Lazaridou (1991). 4. Concluding remarks In this paper, Pressure Stimulated Currents (PSC) were studied on a typical geomaterial (Penteli marble). We first established a correlation between the emitted PSC and the applied stress rate. When the material was stressed within its elas- tic range, a linear relation between PSC and stress rate (dS/dt) was observed. Deviation from linearity exists when the applied stress on the geomaterial is driven to the plastic range. This is due to the dependence of the scaling factor be- tween PSC and stress rate on Young’s modulus. We have shown that slightly before fracture, PSC emissions were detected associated with the fracture mode of the geomaterial. When the failure of the sample forms a shear fracture, then a «single» PSC peak is detected. When ir- regular longitudinal splitting is observed during the failure process then a PSC sequence is recorded which may suggest that each fracture plane corresponding to a macrocrack activates an electrical process. REFERENCES BRADY, B.T. and G.A. ROWELL (1986): Laboratory investi- gation of the electrodynamics of rock fracture, Nature, 321, 488-492. CRESS, G.O., B.T. BRADY and G.A. ROWELL (1987): Sources of electromagnetic radiation from fracture of rock samples in laboratory, Geophys. Res. Lett., 14, 331-334. ENOMOTO, J. and H. HASHIMOTO (1990): Emission of charged particles from indentation fracture of rocks, Nature, 346, 641-643. FINKELSTEIN, D., R.D. HILL and J.R. POWELL (1973): The piezoelectric theory of earthquake lightning, J. Geo- phys. Res., 78, 992-993. FREUND, F. (2000): Time-resolved study of charge genera- Fig. 10. PSC peak at diagonal fracture of a marble sample (MD018). Curve (a) corresponds to stress changes on a normalized axis; curve (b) corresponds to PSC. associated with the number of macrocracks created in directions nearly parallel to the di- rection of the applied stress. The experimental stressing of marble samples with multiple fail- ure planes (i.e. similar to the one depicted in a b c 28 Cimon Anastasiadis, Dimos Triantis, Ilias Stavrakas and Filippos Vallianatos tion and propagation in igneous rocks, J. Geophys. Res., 105 (B5), 11001-11019. FUJINAWA, Y. and K. TAKAHASHI (1990): Emission of elec- tromagnetic radiation preceding the Ito seismic swarm of 1989, Nature, 347, 376-378. HADJICONTIS, V. and C. MAVROMATOU (1994): Transient electric signals prior to rock failure under uniaxial compression, Geophys. Res. Lett., 21, 1687-1690. HADJICONTIS, V. and C. MAVROMATOU (1995): Electric sig- nals recorded during uniaxial compression of rock samples: Their possible correlation with preseismic electric signals, Acta Geophys. Polon., 43, 49-61. HALLBAUER, D.K. H. WAGNER and N.G.W. COOK (1973): Some observations concerning the microscopic and mechanical behaviour of quartzite specimens in stiff, triaxial compression tests, Int. J. Rock Mech. Min Sci. Geomech. Abstr., 10, 713-726. HAYAKAWA, M. (Editor) (1999): Atmospheric and Ionospher- ic Electromagnetic Phenomena Associated with Earth- quakes (Terra Scientific Publishing Co., Tokyo), pp. 996. HAYAKAWA, M. and O. MOLCHANOV (Editors) (2002): Seis- mo Electromagnetics: Lithosphere-Atmosphere-Ionos- phere Coupling (Terra Scientific Publishing Co., Tokyo), pp. 477. JAEGER, J.C. and N.G.W. COOK (1979): Fundamentals of Rock Mechanics (Chapman and Hall, London), pp. 593. KLEFTAKIS, S., Z. AGIOUTANTIS and C. STIAKAKIS (2000): Numerical simulation of the uniaxial compression test including the specimen-platen interaction, in Computa- tional Methods for Shell and Spatial Structures (IASS- IACM). KOPYTENKO, Y., V. ISMAGILOV, M. HAYAKAWA, N. SMIRNOVA, V. TROYAN and T. PETERSON (2001): Investigation of the ULF electromagnetic phenomena related to earth- quakes: contemporary achievements and perspectives, Ann. Geofis., 44 (2), 325-334. LEI, X.L., K. KUSUNOSE, O. NISHIZAWA, A. CHO and T. SATOH (2000): On the spatiotemporal distribution of acoustic emissions in two granitic rocks under triaxial compression: the role of pre-existing cracks, Geophys. Res. Lett., 27, 1997-2000. MIZUTANI, H., T. ISHIDO, T. YOKOKURA and S. OHNISHI (1976): Electrokinetic phenomena associated with earthquakes, Geophys. Res. Lett., 3, 365-368. MOLCHANOV, O.A. and M. HAYAKAWA (1995): Generation of ULF electromagnetic emissions by microfracturing, Geophys. Res. Lett., 22, 3091-3094. MOLCHANOV, O.V. and M. HAYAKAWA (1998): On the gener- ation mechanism of ULF seismogenic electromagnetic emissions, Physic. Earth Planet. Int., 105, 201-210. NITSAN, U. (1977): Electromagnetic emission accompany- ing fracture of quartz-bearing rocks, Geophys. Res. Lett., 4, 333-337. NOMICOS, K. and F. VALLIANATOS (1997): Transient electric variations associated with large intermediate-depth earthquakes in South Aegean, Tectonophysics, 269, 171-177. OGAWA, T., K. OIKE and T. MIURA (1985): Electromagnetic radiation from rocks, J. Geophys. Res., 90, 6245-6249. O’KEEFE, S.G. and D.V. THIEL (1995): A mechanism for the production of electromagnetic radiation during fracture of brittle materials, Physic. Earth Plane. Inter., 89, 127-135. PARK, S.K., M.J.S. JOHNSTON, T.R. MADDEN, F.D. MORGAN and H.F. MORRISON (1993): Electromagnetic precur- sors to earthquakes in the ULF band: a review of ob- servations and mechanisms, Rev. Geophys., 31, 117- 132. STAVRAKAS, I., C. ANASTASIADIS, D. TRIANTIS and F. VAL- LIANATOS (2003), Piezo stimulated currents in marble samples: Precursory and concurrent-with-failure sig- nals, Nat. Hazards Earth Syst. Sci., 3, 243-247. TAKEUCHI, A. and H. NAGAHAMA (2001): Voltage changes induced by stick-slip of granites, Geophys. Res. Lett., 28, 3365-3367. TZANIS, A. and F. VALLIANATOS (2002): A physical model of electrical earthquake precursors due to crack propaga- tion and the motion of charged edge dislocations, in Seismo Electromagnetics: Lithosphere-Atmosphere- Ionosphere Coupling, edited by M. HAYAKAWA and O.A. MOLCHANOV (Terra Scientific Publishing Co., Tokyo), 117-130. VALLIANATOS, F. and A. TZANIS (1998): Electric current generation associated with the deformation rate of a solid: preseismic and coseismic signals, Phys. Chem. Earth, 23, 933-938. VALLIANATOS, F. and A. TZANIS (1999a): A model for the generation of precursory electric and magnetic fields associated with the deformation rate of the earthquake focus, in Atmospheric and Ionospheric Electromagnet- ic Phenomena Associated with Earthquakes, edited by M. HAYAKAWA (Terra Scientific Publishing Co., Tokyo), 287-305. VALLIANATOS, F. and A. TZANIS (1999b): On possible scal- ing laws between Electric Earthquake Precursors (EEP) and earthquake magnitude, Geophys. Res. Lett., 26 (13), 2013-2016. VALLIANATOS, F., D. TRIANTIS, A. TZANIS, C. ANASTASIADIS and I. STAVRAKAS (2004): Electric earthquake precur- sors: from laboratory results to field observations, Phys. Chem. Earth, 29, 339-351. VAROTSOS, P. and K. ALEXOPOULOS (1984a): Physical prop- erties of the variations of the electric field of the Earth preceding earthquakes, 1, Tectonophysics, 110, 73-98. VAROTSOS, P. and K. ALEXOPOULOS (1984b): Physical prop- erties of the variations of the electric field of the Earth preceding earthquakes, 2. Detection of epicentre and magnitude, Tectonophysics, 110, 99-125. VAROTSOS, P. and M. LAZARIDOU (1991): Latest aspects of earthquake prediction in Greece based on seismic elec- tric signals, Tectonophysics, 188, 321-347. YAMADA, I., K. MASUDA and H. MIZUTANI (1989): Electro- magnetic and acoustic emission associated with rock fracture, Phys. Earth Planet. Inter., 57, 157-168.