Electrical conductivity of carbonbearing granulite at raised temperatures and pressures

Glover, Paul; Vine, F. J.

doi:10.1038/360723a0

Cited by 86 publications

(47 citation statements)

References 12 publications

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“…Other common conductive phases in the crust are graphite and some metal sulphides and oxides (including the magnetite present in serpentinite) (e.g., JONES, 1992). Some unsaturated graphite granulites show an increase in conductivity by up to a factor of three as confining pressure is increased from 20 to 200 MPa (GLOVER and VINE, 1992), due to increasing connectivity of the graphite phase. However, the pressures required to achieve this small conductivity increase are considerably greater than the permissible deviatoric stress in the San Andreas Fault zone (LACHENBRUCH and MCGARR, 1990); the effect is reversed if the graphitic granulites are saturated (GLOVER and VINE, 1992); and we know of no evidence for enhanced graphite (or metal sulphide or oxide) concentrations in the San Andreas Fault zone.…”

Section: Discussionmentioning

confidence: 99%

Modeling Low-frequency Magnetic-field Precursors to the Loma Prieta Earthquake with a Precursory Increase in Fault-zone Conductivity

Merzer

Klemperer²

1997

Pure and Applied Geophysics

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The 1989 M s = 7.1 Loma Prieta earthquake was preceded for 12 days by what have been claimed as precursory ultra-low-frequency (ULF) magnetic noise anomalies ten times background, and by a very high peak up to 100 times background just 3 hours before the earthquake. We propose that these anomalous fields could have been due to the formation of a long thin highly-conductive region along the earthquake fault, which magnified the external electromagnetic waves incident on the earth's surface. We use a simplified quantitative model, assuming a highly-conductive elliptic cylinder embedded in a layered resistivity structure, which we base on independent magnetotelluric measurements. The magneticfield anomaly observed 3 hours before the main shock can be modeled by assuming an elliptic conductor extending from the surface to the hypocenter with a conductivity of 5 S · m −1 . Our computed anomaly matches the observed anomaly to within a deviation of 35% over an observed frequency range of over 2 orders of magnitude, over which the measured anomaly varies from only about twice background (at 5 Hz) to about 100 times background (at 0.01 Hz). In addition, other anomalies recorded up to 12 days before the earthquake, can be modeled in detail by varying only the size of the elliptic conductor.We show that such an increase in conductivity could be caused by a precursory reorganization of the geometry of fluid-filled porosity in the fault-zone, which we call a dilatant-conductive effect. The extreme observed magnetic anomalies can be modeled using the high fault-zone porosity (c. 10%) and fluid conductivity (equivalent to 2 M NaCl) implied by other workers' magneto-telluric measurements, but without requiring the large-scale precursory fluid flow characteristic of other published models for the magnetic-field precursors.

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Section: Discussionmentioning

confidence: 99%

Modeling Low-frequency Magnetic-field Precursors to the Loma Prieta Earthquake with a Precursory Increase in Fault-zone Conductivity

Merzer

Klemperer²

1997

Pure and Applied Geophysics

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“…Upon loading, the conductivity rises sharply, increasing by a factor of 3-4 to about 2.5×10 −6 Sm and then continues to increase slowly. Stress-induced increase in the conductivity is commonly explained by better point-to-point contacts between grains under load (Glover and Vine, 1992). However, the next experiment will show that we are dealing with a more complex and much more interesting phenomenon.…”

Section: Methodsmentioning

confidence: 99%

Pre-earthquake signals – Part I: Deviatoric stresses turn rocks into a source of electric currents

Freund

2007

Nat. Hazards Earth Syst. Sci.

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Abstract. Earthquakes are feared because they often strike so suddenly. Yet, there are innumerable reports of preearthquake signals. Widespread disagreement exists in the geoscience community how these signals can be generated in the Earth's crust and whether they are early warning signs, related to the build-up of tectonic stresses before major seismic events. Progress in understanding and eventually using these signals has been slow because the underlying physical process or processes are basically not understood. This has changed with the discovery that, when igneous or highgrade metamorphic rocks are subjected to deviatoric stress, dormant electronic charge carriers are activated: electrons and defect electrons. The activation increases the number density of mobile charge carriers in the rocks and, hence, their electric conductivity. The defect electrons are associated with the oxygen anion sublattice and are known as positive holes or pholes for short. The boundary between stressed and unstressed rock acts a potential barrier that lets pholes pass but blocks electrons. Therefore, like electrons and ions in an electrochemical battery, the stress-activated electrons and pholes in the "rock battery" have to flow out in different directions. When the circuit is closed, the battery currents can flow. The discovery of such stress-activated currents in crustal rocks has far-reaching implications for understanding pre-earthquake signals.

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“…The sensitivity test indicates that the detection of the conductors is robust and that they can be reliably considered as features of the final model. Conductive features, in general, can be explained at crustal depths with various subsurface mechanisms, such as the presence of fluids in interconnected pore spaces (Hyndman and Shearer 1989), high permeability due to fault gouge, interconnected graphite, sulfides, and oxides (Glover and Vine 1992), and magmatic underplating (Wannamaker et al 2008). Graphite can be immediately ruled out, however, as there are no reports of its presence in the study area.…”

Section: Resultsmentioning

confidence: 99%

Geoelectric structure of northern Cambay rift basin from magnetotelluric data

Danda

Rao

Kumar

2017

Earth Planets Space

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Broadband and long-period magnetotelluric data were acquired over the northern part of the Cambay rift zone along an east-west profile ~ 200 km in length. The decomposed TE-and TM-mode data were inverted using a 2-D nonlinear conjugate gradient algorithm to obtain the lithospheric structure of the region. A highly conductive (~ 1000 S) layer was identified within the Cambay rift zone and interpreted as thick Quaternary and Tertiary sediments. The crustal conductors found in the profile were due to fluid emplacement in the western part, and the presence of fluids and/or interconnected sulfides caused by metamorphic phases in the eastern part. The demarcation of the Cambay rift zone is clearly delineated with a steeply dipping fault on the western margin, whereas the eastern margin of the rift zone gently dips along the NE-SW axis, representing a half-graben structure. A highly resistive body identified outside the rift zone is interpreted as an igneous granitic intrusive complex. Moderately conductive (30-100 Ω-m) zones indicate underplating and the presence of partial melt due to plume-lithosphere interactions.

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Electrical conductivity of carbonbearing granulite at raised temperatures and pressures

Cited by 86 publications

References 12 publications

Modeling Low-frequency Magnetic-field Precursors to the Loma Prieta Earthquake with a Precursory Increase in Fault-zone Conductivity

Modeling Low-frequency Magnetic-field Precursors to the Loma Prieta Earthquake with a Precursory Increase in Fault-zone Conductivity

Pre-earthquake signals – Part I: Deviatoric stresses turn rocks into a source of electric currents

Geoelectric structure of northern Cambay rift basin from magnetotelluric data

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