Annie Souriau

Reevaluation of the Chandler wobble seismic excitation from recent data

Abstract Since 1977, seismic moment tensors of major earthquakes have been systematically determined while high accuracy polar motion data have been made available from space techniques. The seismic excitation of the Chandler wobble during the period 1977–1983 has been obtained using the centroid moment tensor solutions of 1287 strong and moderate earthquakes occurring during that period. Changes in the Earth inertia tensor induced by earthquakes, which are derived from the elastic theory of dislocation, are assumed to be step functions of time. From a comparison between the derived synthetic Chandler wobble and the observed one, it turns out that seismic excitation is far too small (by about two orders of magnitude) to explain the Chandler wobble amplitude variations during 1977–1983, in particular the increase in the wobble amplitude between 1980 and 1983. During the last two decades, seismic moments of major earthquakes remained on the level of 10 27 to 10 28 dyne cm, whereas in the past, events of high magnitudes corresponding to seismic moments on the order of 10 30 to 10 31 dyne cm have been frequently reported. As the later values are those needed to explain the amplitude variations of the Chandler wobble, the problem of the validity of magnitude estimations in the past and frequency of occurrence of great earthquakes is raised, as well as the question of validity of previous positive conclusions on the influence of earthquakes on the Chandler wobble excitation.

Geoid anomalies over Gorringe Ridge, North Atlantic Ocean

Abstract Gorringe Ridge is a strong uplifted block of oceanic crust and upper mantle lying at the eastern end of the Azores-Gibraltar plate boundary. The geoid over this structure derived from Seasat altimeter data exhibits a 9-m height anomaly with a north-south lateral extension smaller than 200 km. An attempt is made to interpret this geoid together with the gravity anomalies and with the seismicity, which has been compiled as a function of depth. It is first shown that the flexure of the oceanic lithosphere due to the ridge loading does not provide a good fit of the geoid anomalies and probably should be discarded, as it assumes a continuous unfractured elastic plate. Models involving local heterogeneities are then tested. The comparison of the observed geoid anomalies with the anomalies due to the uncompensated relief indicates that the topographic high has no shallow compensation. Uncompensated models, previously proposed to explain the gravity anomalies, are tested using the geoid. One model (Purdy and Bonnin, in Bonnin [11]), which involves an uplift of upper mantle material at depth, generates too strong geoid anomalies and must be discarded. Another model, which represents a nascent subduction zone (Le Pichon et al. [25]), fits both the gravity and geoid anomalies, but leads to difficulties in explaining the deep seismicity north of Gorringe Ridge. A model in isostatic equilibrium is also able to fit both gravity and geoid anomalies. This model involves a deep root of density 3.0 g cm −3 , as has been previously proposed for many oceanic ridges and plateaus. This model is compatible with the deep seismicity, but the origin of this low-density material at great depth is up to now an unresolved question. More likely, dynamical models taking into account the forces induced by the convection related to the slow plate convergence in this area will have to be considered.

Global tectonics and the geoid

Abstract A digitised tectonic model, initially built up for regionalization of Rayleigh waves, is applied to the geoid in order to define the mean geoid heights of the following regions: 3 oceanic regions, namely young oceans (0–30 Ma) middle-aged oceans (30–80 Ma) and old oceans (> 80 Ma); trenches and subduction zones; mountains; and shields. The relative importance of the deep sources is damped or enhanced by progressively removing or adding the lower or higher degrees of the geoid. A statistical approach allows us to quantify the success of the correlation between tectonics and these filtered geoids. Significant variations are observed in these correlations for oceanic regions (including subduction zones) with a cut-off between degree-2 and higher degrees. For degrees ⩾ 3, a well-known trend is observed: high values correspond to young oceans (ridges) and low values to old oceans, high values are also obtained for subduction zones. On the contrary, and unexpectedly, for the degree-2 alone a trend reversal is observed: geoid lows are observed over ridges and geoid highs over old oceans; trenches give the same geoid amplitude than old oceans. Clearly this denotes a degree-2 convection pattern connected to plate tectonics. In addition it is shown that the minimum and maximum inertia axes of the surface distribution of young oceans, and independently of old oceans and trenches, coincide with the Earths equatorial inertia axes (74°E and 164°E), i.e., with the equatorial extremes of the degree-2 geoid. Plate tectonics is uncorrelated with the polar anomaly of the degree-2 geoid, namely the flattening which is not accounted for by Earth rotation. A north-south axisymmetric convection with a degree-2 pattern is proposed to explain this extra flattening; this model is supported by the latitude dependence of the depth of oceanic ridges.

Attenuation of Rayleigh waves across the volcanic area of the Massif Central, France☆

Abstract Rayleigh wave attenuation is investigated for periods ranging from 20 to 90 s, along a 450 km-long profile following the Oligocene tensile zone of the French Massif Central. A model is deduced by inversion, assuming that the S-wave intrinsic quality factor Q β is frequency-independent, and yields a mean value Q β = 43 ± 10 for the first 100 km in the upper mantle. This value, far lower than the mean value obtained in Eurasia, is close to those obtained in other recent tensile areas, e.g., the western United States or mid-oceanic ridges. A velocity-depth model for S-waves, deduced in a previous study from surface-wave propagation, has been corrected for the attenuation effect. We find a discrepancy between the corrected S-model and P-wave residuals in the same area, implying that Q β must be frequency-dependent. This can be a clue for partial melting in the upper mantle beneath this region.

A possible local anomaly in the 670-km discontinuity beneath Kerguelen

Abstract The mapping of the 670-km discontinuity in regions of ascending or descending flows is of major interest for a better understanding of the nature of this discontinuity. In this paper, an attempt is made to investigate the properties of deep mantle discontinuities beneath the Kerguelen hot spot. Anomalous P to S converted waves are observed for some earthquakes recorded at the Kerguelen long-period GEOSCOPE station; the characteristics of these phases are specified owing to a three-dimensional polarization analysis of the signals. A tilted discontinuity is able to explain the data; this discontinuity dips toward the northeast with at 20°. The depth at which the conversion takes place is ∼ 880 km, assuming standard velocities in the upper mantle beneath Kerguelen, or 810 km if a hot upper mantle with a Poissons ratio of 0.30–0.31 is present. However, the constraints given by the data are not strong enough to specify unambiguously this depth, a conversion at 600 km (or 550 km assuming a hot upper mantle) may alternatively be proposed. A strong velocity increase at the discontinuity is in each case necessary to explain the amplitude of the converted waves. Geophysical arguments show that this tilted discontinuity must be a local feature; possible other mechanisms, such as wave focusing due to undulations of the discontinuity, may also be proposed.

Present limitations of accurate satellite Doppler positioning for tectonics. An example: Djibouti

One of the possibilities of the Doppler positioning from satellite is to provide geodetic measurements continuous in time without the need for reference stations. If measurements of sufficient accuracy can be achieved they may be used to study local surface displacements in relation to tectonic activity. A Doppler receiver of the MEDOC network is located near Djibouti in the Ghoubhat-Asal rift region which corresponds to the accreting plate boundary between the Arabian and African plates. In November 1978, a seismic and volcanic crisis occured in this area. Surface geodetic measurements and levellings performed in 1973 and 1978–79, just after the crisis, reveal a 60–80 cm sinking of the graben floor and a lateral extension of about 2 meters.Here we analyse Doppler measurements for the period January 1977 to November 1980. Point positions are computed for 7 to 10 day intervals using a precise ephemeris, and a moving window analysis is applied to the data. An apparent 2 meter uplift preceding the November 1978 seismic crisis is detected at Djibouti, whereas no similar phenomenon is observed at the two closest stations, Pretoria and Uccle-Brussels. However, field observations rule out a tectonic origin for this uplift.In Djibouti, the correlation between the apparent vertical station position and the electron density in the ionospheric F-layer reveals that a bias may be induced by the third order term of the ionospheric refractive index not previously taken into account, or more probably by the ray curvature through the ionosphere. This bias is particularly strong for our data set, from a station located close to the magnetic equator, and related to a period of rapid increase in the solar activity.Although our analysis fails to detect any tectonic displacement related to the rift seismic activity, it shows that similar tectonic studies by the Doppler method will be possible once the residual ionospheric errors are removed, for example by use of higher radio frequencies.