Jean-Pierre Pozzi

Streaming potential and permeability of saturated sandstones under triaxial stress: Consequences for electrotelluric anomalies prior to earthquakes

The streaming potential, due to fluid circulation in rock, was measured on saturated sediments (Fontainebleau sandstones). The electrokinetic coupling coefficient, which is the ratio of the streaming potential and the excess pore pressure, is proportional to the fluid resistivity. Additionally, for a fluid conductivity of 10−3 S/m, the electrokinetic coupling coefficient varies from 10 to 6642 mV/0.1 MPa for sample permeability in the range of permeabilities from 0.15 × 10−15 to 1220 × 10−15 m2. The different values of the electrokinetic coupling coefficient have been explained by the effect of increasing surface conductivity which becomes nonnegligible compared to fluid conductivity for low permeability. When the sample is deformed under triaxial stress up to failure, the vertical permeability (along the principal stress) drops by about 0.20%/0.1 MPa when failure occurs. The typical variation of the electrokinetic coupling coefficient is a large increase beginning with the onset of the localization of the shear band at about 75% of the yield stress and stopping at the failure. This increase of the electrokinetic coupling coefficient is due to an increase of ζ potential in the shear zone when new surfaces are created and connected. Possible consequences of our results are given concerning the electrical fields which could appear during the preparation of an earthquake. It is shown that in some cases, self-potential anomalies reported in the deformed zone preceding an earthquake occurrence could be due to an increase of the electrokinetic coupling coefficient from 75% of the yield stress to rupture in the vicinity of one of the electrodes. Any variation of fluid resistivity or permeability in the vicinity of one electrode could change the electrokinetic coupling coefficient, inducing a surface electrokinetic potential anomaly. In regard to the interpretation of the electrokinetic effect which occurs at large distance from the epicenter, a larger electrokinetic potential anomaly could be measured between electrodes situated along a vertical fluid flow, for instance, in a shallow borehole. An electrokinetic potential anomaly up to 30 mV, for a fluid conductivity of 0.01 S/m and a rock permeability of 10−12 m2, could be observed with a change of the underground water table level as slight as 50 cm (50 mbar). Moreover, if the permeability between the electrodes is increased by a factor of 8 × 103, the electrokinetic coupling coefficient could be enhanced by a factor up to 650.

Streaming potential in volcanic rocks from Mount Pelée

Streaming potential and electric conductivity have been measured in a laboratory on 11 consolidated samples coming from five deposits of the different evolutionary stages of Mount Pelde volcano. The streaming potential coupling coefficient ranges from-35 to-4905 mV MPa-1 and increases with increasing permeability. This increase is mainly due to the dependency of rock effective conductivity with permeability. The permeability of the samples varies from 0.146x10-12 to 34x10-12 m 2. The zeta potential, at pH-7 and water conductivity of 2.1x10-4 S m-l• is relatively small for the majority of the samples. It ranges from-4 to-19 reV. According to water conductivity analysis on Mount Pel•e, streaming potential coupling coetficients of-25 to-406 mV MPa-1 can be expected for this volcano.

Permeability dependence of streaming potential in rocks for various fluid conductivities

Streaming potentials have been measured on sandstone and limestone samples in a large range of permeabilities. The electrokinetic coupling coefficient increases with permeability and we explain this effect by the related variation of surface conductivity. A model is proposed to study this effect for various fluid conductivities and it is shown that the dependence of the electrokinetic coupling coefficient on permeability is stronger for high fluid resistivity and is weaker for lower fluid resistivity. When fluid resistivity is below 1 Ω.m permeability and streaming potential are no more related.

The 1975–1977 crisis of la Soufriere de Guadeloupe (F.W.I): A still-born magmatic eruption

On July 8, 1976, eruptive activity broke out at la Soufriere de Guadeloupe (F.W.I) after about one year of increasing seismic activity. Seismic activity continued to increase until August 1976, reaching more than 1500 events (a 200-fold increase over the preceding quiet period of a few years) and an energy output of about 1017 ergs in a day. A total of 26 major phreatic eruptions similar to the July 8 outburst took place during an eight-months period. The steam blasts that characterized the eruptions gave rise to particle- and sometimes block-charged plumes that deposited an estimated 106 m3 of solids. The H2O-rich gases emitted during the blasts presumably contained other gases (H2S, SO2, CO2...) that were partly adsorbed on solid particles. All material was erupted at temperatures of the order of 100° to 200°C. The observation of vertical migration of earthquake foci in less than a few hours and over about 6 km depth, and of abnormal variations of the geomagnetic field, indicate a deep energy source for the phreatic eruptions. A small proportion of the gases adsorbed on solid particles had a magmatic origin. However, most of the steam and the tephra seemed to originate from superficial levels of a hydrothermal system. Similar phreatic eruptions have occurred several times in recorded history. In the case of la Soufriere, the origin of the phreatic eruptions is best described by an abnormal energy input (versus steady-state) from a crustal magma chamber. The occurrence of truly magmatic eruptions is presumably inhibited by an extensive hydrothermal system. The abrupt release of more power from the magma chamber could have resulted in an explosive pyroclastic eruption. Substantial improvement of the Guadeloupe volcano observatory has followed the 1975–1977 crisis. Permanent telemetered geophysical networks and regular geochemical observations have provided a five year data base of the volcano behavior in its noneruptive state which can be compared to crisis situations.

New paleomagnetic results from Cretaceous sediments near Lisboa (Portugal) and implications for the rotation of Iberia

Abstract Paleomagnetic study of outer platform sediments of Late Jurassic—Early Cretaceous age from Portugal (Lisboa area) leads to characteristic remanent magnetization directions for the Hauterivian-Barremian ( D = 317°, I = 58°) and the Aptian ( D = 344°, I = 47°). The absolute rotation of Iberia between 125 and 110 Ma is consequently of about 27° in an anticlockwise sense. A later rotation of about 14° follows the first one in the same sense from 110 Ma. The total absolute rotation of Iberia from the Hauterivian age determined by paleomagnetism is thus about 41°. The comparison of declinations measured with paleodeclinations calculated in Lisboa from the Eurasian and African poles of similar age indicates that the main phase of relative rotation of Iberia with respect to Eurasia occurs during the first stage of relative motion (i.e. Hauterivian to Aptian). The tectonic history of the Bay of Biscay is in good agreement with these two stages of rotation. The total relative rotation with respect to Europe is 34°. This value is consistent with results previously established by Van der Voo [2,3,25], but we have established that the main phase of rotation (30°) occurs in the Hauterivian-Aptian interval. The movement of Iberia is clearly linked to the movement of Africa in direction and age, but the amount of rotation of Iberia, during the Early Cretaceous, is much larger and implies that Iberia was not a real part of the African plate.

Un gisement sparnacien exceptionnel à plantes, arthropodes et vertébrés (Éocène basal, MP7): Le Quesnoy (Oise, France)

A new fossil locality is reported from the argiles a lignite du Soisonnais (Early Ypresian, MP7) of the Oise region (France). After the preliminary survey of the flora and the vertebrate and arthropod faunas, we propose a reconstruction of a fluvio-lacustrine palaeoenvironment with a forest, under a warm and wet seasonal climate. This site is outstanding because of the richness, diversity and the state of preservation of the fossils. The present discovery opens a unique window on terrestrial life during the Earliest Eocene.

Laboratory measurements anomalous 0.1–0.5 Hz streaming potential under geochemical changes: Implications for electrotelluric precursors to earthquakes

Streaming potentials resulting from flow of various salt solutions in rock were measured on saturated sediments (Fontainebleau sandstones). The streaming potential ΔV was found to be proportional to the driving pore pressure ΔP. Pulses of amplitude 15–40 mV in the frequency range of 0.1 to 0.5 Hz were observed when the conductivity of the injected water was decreased and the fluid flow rate was relatively low, corresponding to a Darcian velocity of 17 to 30 cm/h. The amplitudes of these pulses are 47% to 133% of the corresponding steady components of the ΔV values. Such geochemically induced effects may possibly be responsible for the frequency signals from 0.1 to 0.5 Hz that were sometimes observed before an earthquake.

Paleomagnetic evidence for the location of Madagascar in Gondwanaland

The combined paleomagnetic results obtained from all formations of the Karroo Supergroup of Madagascar are compared with corresponding data obtained in Africa and South America. This provides unequivocal evidence in favor of the reconstructions of du Toit and Smith and Hallam.

Paleomagnetic study of Sicily: consequences for the deformation of Italian and African margins over the last 100 million years

Abstract Paleomagnetic analysis of Cenozoic sediments from the Iblean platform (12 sites, 100 samples) has provided four new poles. These data are combined with those available from volcanic series to yield a new apparent polar wander path for “stable” Sicily since the Upper Cretaceous. This curve, when compared to a revised curve for Africa (using a selection of available data points), demonstrates a 15° rotation of Sicily with respect to Africa during the Plio-Pleistocene. This rotation accounts for the extensional tectonic regime that prevails in the Pelagian Sea and which is evidenced by “en relais” rifts (Malta, Pantelleria, Linosa), associated with alkaline volcanism of Pliocene and Quaternary age. Paleomagnetic directions from various areas of the Italian Peninsula also indicate anticlockwise rotations with diverse amplitudes. The chronology of these rotations is consistent with a deformation of the western Italian margin during progressive suturation from north to south of the Corsican-Sardinian block since the end of the Oligocene.

Magnetic observations made on la soufriere volcano (guadeloupe) during the 1976–1977 crisis

Abstract An array of fourteen stations for measurement of the total intensity ( B ) of the earths magnetic field was installed on the Soufriere volcano during the 1976 seismovolcanic crisis. Measurements were performed daily from September 1976 to April 1977. Differences between values of the total intensity measured at each station and values of B measured continuously at a reference station (located near the summit of the volcano) have been computed. For each value of the difference, an estimate of an upper bound of the reduction error has been obtained. The effect of lateral conductivity anomalies has been directly measured for four stations and estimated for the others. The main conclusions of these experiments are the following: 1. (1) Significant variations are found in the differences in intensity with characteristic time constants of a few days. These variations are very small for stations located near the summit and within 5 km of the reference station; they reach an amplitude of 15 nT for the more remote stations and often show good correlation from station to station. 2. (2) The amplitude of these variations decreased with the end of the seismovolcanic crisis, at the end of March 1977.