We present the MIGA experiment, an underground long baseline atom interferometer to study gravity at large scale. The hybrid atom-laser antenna will use several atom interferometers simultaneously interrogated by the resonant mode of an optical cavity. The instrument will be a demonstrator for gravitational wave detection in a frequency band (100 mHz–1 Hz) not explored by classical ground and space-based observatories, and interesting for potential astrophysical sources. In the initial instrument configuration, standard atom interferometry techniques will be adopted, which will bring to a peak strain sensitivity of at 2 Hz. This demonstrator will enable to study the techniques to push further the sensitivity for the future development of gravitational wave detectors based on large scale atom interferometers. The experiment will be realized at the underground facility of the Laboratoire Souterrain à Bas Bruit (LSBB) in Rustrel–France, an exceptional site located away from major anthropogenic disturbances and showing very low background noise. In the following, we present the measurement principle of an in-cavity atom interferometer, derive the method for Gravitational Wave signal extraction from the antenna and determine the expected strain sensitivity. We then detail the functioning of the different systems of the antenna and describe the properties of the installation site.

S U M M A R YThis paper is devoted to the simultaneous determination of the coseismic and postseismic gravitational changes caused by the great 2004 December 26 Sumatra-Andaman earthquake from the time-variable global gravity fields recovered by the Gravity Recovery And Climate Experiment (GRACE) mission. Furthermore, a complete modelling of the elasto-gravitational response of a self-gravitating, spherically layered, elastic earth model is carried out using a normal-modes summation for comparison with the observed coseismic gravitational change. Special attention is paid to the ocean mass redistribution. Special care is paid during the inversion of the data to avoid contamination of tectonic gravity changes by ocean tidal model errors, seasonal and interannual signals originating from continental hydrology and oceanic circulation as well as contamination of the coseismic gravity change by the postseismic relaxation. We use a 4.6-yr-long time-series of global gravity solutions including 26 months of postseismic data, provided by the Groupe de Recherche en Géodésie Spatiale (GRGS). For comparison, the Release-04 solutions of the Center for Space Research (CSR) are also investigated after a spectral windowing or a Gaussian spatial smoothing. Results are shown both in terms of geoid height changes and gravity variations. Coseismic and postseismic gravitational changes estimated from the different gravity solutions are globally similar, although their spatial extent and amplitude depend on the type of filter used in the processing of GRACE fields. The highest signal-to-noise ratio is found with the GRGS solutions. The postseismic signature has a spectral content closer to the GRACE bandwidth than the coseismic signature and is therefore better detected by GRACE. The coseismic signature consists mainly of a strong gravity decrease east of the Sunda trench, in the Andaman Sea. A gravity increase is also detected at a smaller scale, west of the trench. The model for the coseismic gravity changes agrees well with the coseismic signature estimated from GRACE, regarding the overall shape and orientation, location with respect to the trench and order of magnitude. Coseismic gravity changes are followed by a postseismic relaxation that are well fitted by an increasing exponential function with a mean relaxation time of 0.7 yr. The total postseismic gravity change consists of a large-scale positive anomaly centred above the trench and extending over 15 • of latitude along the subduction. After 26 months, the coseismic gravity decrease has been partly compensated by the postseismic relaxation, but a negative anomaly still remains south of Phuket. A dominant gravity increase extends over 15 • of latitude west of the trench, being maximal south of the epicentre area. By investigating analyses of two global hydrology models and one ocean general circulation model, we show that our GRACE estimates of the coseismic and postseismic gravitational changes are almost not biased by interannual variations originating from continental hyd...

International audienceA network of 54 survey GPS sites, 28 continuous GPS stations and three absolute gravity (AG) observation sites have been set up in the Alborz mountain range to quantify the present-day kinematics of the range. Our results allow us to accurately estimate the motion of the South Caspian block (SCB) for the first time, and indicate rotation of the SCB relative to Eurasia, accounting for the left lateral motion in the Alborz range. In light of these new results, it clearly appears that deformation rates vary along the range, the eastern part accommodating mainly left lateral strike slip (2 mm yr(-1) south of the range and 5 mm yr(-1) north of the range) with a very low range normal shortening rate on the Khazar thrust fault (similar to 2 mm yr(-1)), and the western part accommodating range normal shortening (similar to 6 mm yr-1) on the Khazar thrust fault with a left lateral component of similar to 2 mm yr(-1) north of the range and 1 mm yr(-1) south of the range. These present-day kinematics agree with geomorphologic estimated slip rates, but not the long-term deformation, corroborating the idea that the kinematics of the range have changed recently due to the change of SCB motion.;Modelling of the interseismic deformation suggests a deep locking depth on the central-western segment of the Khazar fault (similar to 30 km) in agreement with the Baladeh earthquake rupture and aftershock ranging between 10 and 30 km. Given this unusual deep locking depth and the 34 degrees dip of the thrust, a large part of the Alborz range is located above the seismically coupled part of the fault. Based on our AG measurements this part of the range seems to uplift at a rate of 1-5 mm yr(-1), in agreement with terrace uplift

A global network of superconducting gravimeters (SGs) is compiling significant data for a range of important studies spanning a number of disciplines concerned with the Earth's gravity, tides, environment, and geodetics. Among phenomena being looked at are seismic normal modes, the Slichter triplet, tidal gravity, ocean tidal loading, core nutations, and core modes. Hydrologists and volcanologists also may benefit from SG data. The network was set up by the Global Geodynamics Project (GGP),an international program of observations of temporal variations in the Earth's gravity field. Observations began in 1997 and will continue until 2003. Eighteen SGs currently are in operation in the network (see Figures 1 and 2).

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