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SUMMARY The M= 8.1, 1 April 2014 Iquique earthquake, which broke part of the northern Chile seismic gap, was preceded by a strong foreshock sequence starting early January 2014. The reported analysis of the continuous records of the nearby GPS stations from the Integrated Plate Observatory Chile, North Chile array lead to contradictory results concerning the existence and location of slow slip events (SSEs) on the interplate contact. Resolving this controversy is an important issue, as although many SSEs are reported in subduction zones, only a few were found to be precursory to large earthquakes. Here we show that the records of a long base tiltmeter installed near Iquique, when corrected for coseismic steps, long-term drift, tidal signals and oceanic and atmospheric loading, show significant residual signals. These can be modelled with a sequence of four SSEs located close to Iquique. Their signature was already reported on some GPS stations, but their source was then characterized with a very low resolution in time and space, leading to contradicting models. With the tilt records, we can rule out the previously proposed models with a single large SSE closer to the main shock. Combining tilt with GPS records greatly improves the resolution of GPS alone, and one could locate their sources 100–180 km south–southeast to the main shock epicentre, with moment magnitudes between 5.8 and 6.2, at the edge of the main aftershock asperities. These moderate SSEs thus did not directly trigger the main shock, but contributed to trigger the main foreshock and the main aftershock. Only the sensitivity and resolution of the tiltmeter, added to the GPS records, allowed us to describe with unprecedented accuracy this precursory process as a cascade of cross-triggered, short-term aseismic slip events and earthquakes on the interplate contact. This three months of precursory activation appears to be the final acceleration burst of a weaker, longer term SSE which started mid-2013, already reported, with a moment release history which we could quantify. From the methodological point of view, our study takes advantage of an interesting complementarity of tilt and GPS measurements, due to their different dependence in distance to the source of strain, which turns out to be very efficient for resolving location and moment of strain sources, even when both instruments are close to each other. It finally demonstrates the efficient removal of sequences of small or even undetected coseismic steps from high resolution tilt record signal in order to retrieve the purely aseismic signal, a presently impossible task for high time resolution GPS records due to low signal to noise.

Air pressure reduction in local gravity measurements is usually done by means of an empirical transfer function (barometric admittance) between pressure and gravity both measured at the same site. In the local zone ( 3.0 cycles/day). The atmospheric reduction from the barometric array is also compared to global loading computations from ECMWF-4Dvar surface pressure data, having a sampling rate of 3 h and a spatial resolution of 0.5°. We also tested a hybrid method based on a spline interpolation of joint ECMWF-4Dvar surface pressure data and local observations collected on the barometric array. The findings are discussed in light of the main features (dimension, geometry and instrumental accuracy) of the barometric array.

The geodetic rates for the gravity variation and vertical uplift in polar regions subject to past and present-day ice-mass changes (PDIMCs) provide important insight into the rheological structure of the Earth. We provide an update of the rates observed at Ny-Alesund, Svalbard. To do so, we extract and remove the significant seasonal content from the observations. The rate of gravity variations, derived from absolute and relative gravity measurements, is −1.39 ± 0.11 μGal yr −1 . The rate of vertical displacements is estimated using GPS and tide gauge measurements. We obtain 7.94 ± 0.21 and 8.29 ± 1.60 mm yr −1 , respectively. We compare the extracted signal with that predicted by GLDAS/Noah and ERA-interim hydrology models. We find that the seasonal gravity variations are well-represented by local hydrology changes contained in the ERA-interim model. The phase of seasonal vertical displacements are due to non-local continental hydrology and non-tidal ocean loading. However, a large part of the amplitude of the seasonal vertical displacements remains unexplained. The geodetic rates are used to investigate the asthenosphere viscosity and lithosphere/asthenosphere thicknesses. We first correct the updated geodetic rates for those induced by PDIMCs in Svalbard, using published results, and the sea level change due to the melting of the major ice reservoirs. We show that the latter are at the level of the geodetic rate uncertainties and are responsible for rates of gravity variations and vertical displacements of −0.29 ± 0.03 μGal yr −1 and 1.11 ± 0.10 mm yr −1 , respectively. To account for the late Pleistocene deglaciation, we use the global ice evolution model ICE-3G. The Little Ice Age (LIA) deglaciation in Svalbard is modelled using a disc load model with a simple linear temporal evolution. The geodetic rates at Ny-Alesund induced by the past deglaciations depend on the viscosity structure of the Earth. We find that viscous relaxation time due to the LIA deglaciation in Svalbard is more than 60 times shorter than that due to the Pleistocene deglaciation. We also find that the response to past and PDIMCs of an Earth model with asthenosphere viscosities ranging between 1.0 and 5.5 × 10 18 Pa s and lithosphere (resp. asthenosphere) thicknesses ranging between 50 and 100 km (resp. 120 and 170 km) can explain the rates derived from geodetic observations.

International audience; We investigate in this paper various approaches to correct gravity changes for the effect of atmospheric pressure changes. Two specific locations are considered: Strasbourg (France) as mid-latitude station, where regular pressure fronts occur and Djougou (Benin) as equatorial station with large thermally driven S1 and S2 waves of planetary extent. We first review the classical approaches based on a constant or frequency-dependent admittance using only local pressure and gravity data. We consider then a model of atmospheric loading and show the barometric admittance in terms of elastic, Newtonian and total load, as a function of the distance from the station. We consider both a 2D pressure model (surface loading) and a 2.5D model, where the density decreases with height (standard atmosphere). Assuming horizontal advection in the atmospheric dynamics, we convert this spatially dependent admittance into a frequency-dependent admittance. Using global pressure data from European Centre for Medium-Range Weather Forecasts (ECMWF) at about 12 km spatial resolution and 3 h sampling, we compute the model-predicted pressure admittance for Djougou and Strasbourg and we simulate the frequency dependence inferred from gravity and pressure observations below 4 cycle per day. A long gravity and pressure data set (1996-2013) from Strasbourg is used to investigate the low frequency part of the pressure admittance while a common 2.5 year data set (August 2010-February 2013) for Strasbourg and Djougou is then analyzed to investigate the high frequency part of the admittance. In both cases, our results are in close agreement with the predictions inferred from an atmospheric 2.5D loading model with a distance-time relationship due to horizontal advection. The frequency dependence of the barometric admittance is explained by the competing contributions of Newtonian attraction and elastic surface deformation according to the distance from the gravimeter. In the far field (low frequencies), the magnitude of the admittance decreases with frequency because of the combined elasticity effect and Newtonian attraction (when the atmosphere is below the horizon) while, on the contrary, in the near field (high frequencies), elasticity becomes negligible and the pressure admittance mainly decreases with increasing frequency because of the decreasing attraction effect of the atmospheric masses inside the cylindrical pressure cell centered on the sensor location of decreasing radius. In the last part, we show that there is variability in time in the pressure admittance for both stations.

International audience; The Gravimetric Observatory J9 located near Strasbourg (France), has a long history of recording solid Earth tides. We present here one of the longest available gravity records (1973-2011) from three different instruments operating at the J9 station: a LaCoste-Romberg spring gravimeter (ET005) from 1973 to 1985, a GWR (TT70-T005) superconducting gravimeter from 1987 to 1996 and a GWR compact superconducting gravimeter (C026) since 1996. We give the results of a tidal analysis performed with ETERNA on these data sets. We show the improvement in terms of noise levels from the oldest type to the newest one and check the time stability of the observed tides for every instrument. This stability is also dependent on the stability of the scale factor of the cryogenic gravimeter which is derived from a direct comparison with repeated absolute gravity measurements. The response of the three gravimeters to the atmospheric pressure changes is also checked. With all these results we found that the improvement from a mechanical spring gravimeter to a modern cryogenic instrument is obvious. And also that the improvement from the T005 SG to the C026 SG is less impressive than from L&R to the T005 SG, but still notable.