J. C. Rossignol

The Azores triple junction evolution since 10 Ma from an aeromagnetic survey of the Mid-Atlantic Ridge

Abstract In the past two decades several models have been presented to describe the evolution and the present structure of the Azores Triple Junction. These models were mainly based on morphological analysis of sea bottom topography, sparse magnetic profiling, sidescan sonar surveying over the plateaus and global plate kinematic considerations for the North Atlantic. In this paper we follow a different approach: from a detailed aeromagnetic survey covering both sides of the Mid-Atlantic Ridge between 37°N and 40°30′N the magnetic anomalies up to anomaly 5 are accurately identified, allowing careful modelling of the kinematics of this region for the past 10 Ma and thus establishing a coherent framework for the design of geophysical models for the Azores Triple Junction. The analysis of magnetic anomalies and the use of Fourier domain inversion techniques show that the ridge is made up of six segments, each one varying in length from 50 to 60 km. The more continuous section of the ridge can be defined from the first four northern segments, although the North Azores Fracture Zone right-offsets the ridge at 39°30′N, 29°40′W. The fifth and sixth segments are, respectively, right-offset by the Acor Fracture Zone (at 38°23′N, 30°15′W) and by the Princess Alice Fracture Zone (at 38°00′N, 30°50′W). Anomaly identifications using a two-dimensional model and plate tectonics reconstruction techniques allowed the calculation of rotation pole parameters. The results thus obtained reveal that, at least between anomalies 5 and 3 ( ∼ 10–3.85 Ma), the Azores displayed an independent motion relative to the neighbouring plates and after anomaly 2A (2.45 Ma) the Azores moved attached to the Eurasian plate. The triple junction (Azores-North America-Africa, or Eurasia-North America-Africa) moved northward from ∼ 38°00′N, 30°50′W to ∼ 38°20′N, 30°15′W (between 4 and 3A) and probably to 38°50′N, 30°00′W at anomaly 2A time. A detailed reconstruction model is presented.

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.

Tectonic framework of the Azores Triple Junction

The lack of accurate and detailed magnetic information has, in the past, limited the development of well constrained models for the plate tectonic evolution of the Azores Triple Junction. An aeromagnetic survey, made possible by the existing airport facilities, has long been desired as it can provide high quality magnetic data, whose homogeneity and coherency is far better than those provided by classic marine surveys. The results presented in this paper concern only a part of the Aeromagnetic Survey conducted by the Portuguese Instituto Nacional de Meteorologia e Geofisica and already allow an improved definition of the basic tectonic boundaries at the central part of the Azores plateau.

Identification and analysis of electromagnetic signals in Greece: The case of the Kozani Earthquake VAN Prediction

An electric station was installed in July 1993, 4.5 km away from the VAN station of Ioannina, and recorded in the following two years a number of anomalous signals, including those of April 18 and 19, 1995, interpreted by the VAN group as Seismic Electric Signals precursors to the May 13 Kozani earthquake (West-Macedonia). A magnetic station was also installed and clearly recorded the magnetic components of the same events. The amplitude, shape, characteristic pattern and duration, magnetic characteristics and polarisation of the anomalous signals suggest that they are generated by artificial (industrial) sources.

An aeromagnetic survey of the southwest of the Western Mediterranean: Description and tectonic implications

Abstract We present an aeromagnetic survey performed in the summer of 1973 in the southwestern Mediterranean, over the Alboran Sea and part of the Algero-Provencal basin; data acquisition and reduction are described and maps of both the total field intensity and anomalies are given. The anomaly map is described in detail and its implications for the tectonic evolution of the two deep basins included in the surveyed area are discussed. We propose that both basins opened simultaneously along a NNW-SSE direction, and that the Alboran basin was later a site of compression, which led to the formation of the Alboran ridge.

Characteristics of electromagnetic noise in the Ioannina Region (Greece); A possible origin for so called “seismic electric signal” (SES)

The study of electromagnetic noise in the Ioannina region (NW Greece) shows the existence of long transient electric signals (duration>10 s) without measurable correlated magnetic signals, as is the case for Seismic Electric Signal (SES) observed in the VAN network of short-term earthquake prediction. Our signals have an artificial origin and are emitted by different kinds of transmitters implanted in the Ioannina region. This origin can be clearly ascertained from simultaneous observation, at stations in direct sight of the transmitters, of four electromagnetic components (two electric and two magnetic) in a broad band of frequencies (10−3–10³ Hz). Relying on these observations, it is proposed that the April 18 and 19, 1995 electric signals, interpreted by the VAN group as SES precursors to the May 13 Kozani earthquake, are due to some transmitter located far to the North of the IOA station of the VAN network. The transmitters of this area would be of digital type, as is increasingly common in radio-telecommunication networks. We believe that careful studies of electromagnetic noise in a broad range of frequencies should precede lengthy discussions of the statistical significance of recorded signals.