Distribution géographique de la sismicité mondiale

Example of Fault plane solution: Atlantic ridge Fault-plane solutions for earthquakes along the Mid- Atlantic Ridge, showing the prevalence of extensional tectonics with normal faulting in the axial zone of the spreading center (based on data from Huang et al., 1986). Romanche fault

Example of Fault plane solution: Romanche fault Fault-plane solutions for earthquakes on the St. Paul, Romanche and Chain transform faults in the Central Atlantic ocean (after Engeln et al., 1986). Most focal mechanisms show right lateral (dextral) motions on these faults, corresponding to the relative motion between the African and American plates.

Photo Modèle à partir des données du satellite GRACE

Ellipsoid

Gravité La gravité et la composante radiale de L'accélération centrifuge étant très faible on admet que la direction du champ de la pesanteur est confondue avec la direction l'accélération gravitationnelle.

Ellipsoid-Geoid: definition

Measurement of the Geoid : spatial geodesy A satellite orbiting around the Earth will be sensitive to gravity: Its motion is such that the rotation force exactly equilibrates the gravity forces. If the gravity is stronger (i.e. the gravity potential higher), then the satellite will have to orbit a little bit farther away from the earth (to increase the rotation force, and remain in equilibrium) Conclusion : an orbiting satellite will follow exactly the Gravity potential! measuring the satellite orbit will give us the gravity potential (i.e. the Geoid)

Explication : Géoïde généré par une masse Géoïde total (masse + creux de topo) Géoïde du à la masse La surface s enfonce La gravité diminue le géoïde descend

Geoid: examples m m Earth s geoide from GRACE (Gravity Recovery And Climate Experiment) satellite (NASA-GFZ)

Geoid: examples Non-hydrostatic Geoid GEM-T1 m m

Altitude: definition Altitude (height) is not a purely geometrical concept (i.e. distance from one point to the other) it is defined with respect to the gravity potential. 2 points A and B at ground level One might think their altitude is d A and d B But it is not!!!! The altitude is the distance to the geoid (i.e. the sea level) : h A and h B If the Geoid is not flat (i.e. at the same distance from the center of the Earth at A and B), then the altitude changes

Altitude-Geoid

Satellite altimetry The result is a high resolution map of the Geoid on 70% of the earth surface

Satellite altimetry A zoom of the oceanic Geoid shows that we see in detail short wavelength gravity anomalies These come from density anomalies at the surface of the sea bottom. They are ridges, sea mounts, transform faults, etc

Satellite altimetry An anomaly of the sea surface can also be related to water anomaly The precision of current altimeter allow to map swells of no more than 10 cm. Doing this, we can trace oceanic currents like the Gulf Stream

Satellite altimetry: Sea level variation Blue = decrease of sea level Red = sea level rise

Das Oberflächenrelief

Géodésie Pierre Bouguer 1698-1758 Georg Everest 1790-1866

Isostasy: models

Mesure de gravité 1) Correction à l'air libre. Le gravimètre embarqué dans un ballon (station B) mesure une gravité réduite parce que la distance au centre de la Terre est augmenté de l'altitude h.2) Correction de Bouguer. Sur le plateau (station C), la masse de roches sous-jacente, d'épaisseur h, augmente la gravité. La correction est positive si la station est au-dessus du niveau de référence (niveau de la mer),négative dans le cas contraire.3) Correction de terrain. Au pied d'une montagne (station D), la masse des matériaux de la montagne provoque une attraction H, diminuant la valeur de la gravité. La vallée V provoque aussi une attraction vers le bas.

Isostasy: examples Hypothetical Bouguer anomalies over continental and oceanic areas. The regional Bouguer anomaly varies roughly inversely with crustal thickness and topographic elevation (after Robinson and Çoruh, 1988)

Isostasy: examples

Isostasy: examples Oceanic ridge

Isostasy: examples