Dominique Remy

Interseismic coupling and seismic potential along the Central Andes subduction zone

We use about two decades of geodetic measurements to characterize interseismic strain build up along the Central Andes subduction zone from Lima, Peru, to Antofagasta, Chile. These measurements are modeled assuming a 3-plate model (Nazca, Andean sliver and South America Craton) and spatially varying interseismic coupling (ISC) on the Nazca megathrust interface. We also determine slip models of the 1996 M_w = 7.7 Nazca, the 2001 M_w = 8.4 Arequipa, the 2007 M_w = 8.0 Pisco and the M_w = 7.7 Tocopilla earthquakes. We find that the data require a highly heterogeneous ISC pattern and that, overall, areas with large seismic slip coincide with areas which remain locked in the interseismic period (with high ISC). Offshore Lima where the ISC is high, a M_w∼8.6–8.8 earthquake occurred in 1746. This area ruptured again in a sequence of four M_w∼8.0 earthquakes in 1940, 1966, 1974 and 2007 but these events released only a small fraction of the elastic strain which has built up since 1746 so that enough elastic strain might be available there to generate a M_w > 8.5 earthquake. The region where the Nazca ridge subducts appears to be mostly creeping aseismically in the interseismic period (low ISC) and seems to act as a permanent barrier as no large earthquake ruptured through it in the last 500 years. In southern Peru, ISC is relatively high and the deficit of moment accumulated since the M_w∼8.8 earthquake of 1868 is equivalent to a magnitude M_w∼8.4 earthquake. Two asperities separated by a subtle aseismic creeping patch are revealed there. This aseismic patch may arrest some rupture as happened during the 2001 Arequipa earthquake, but the larger earthquakes of 1604 and 1868 were able to rupture through it. In northern Chile, ISC is very high and the rupture of the 2007 Tocopilla earthquake has released only 4% of the elastic strain that has accumulated since 1877. The deficit of moment which has accumulated there is equivalent to a magnitude M_w∼8.7 earthquake. This study thus provides elements to assess the location, size and magnitude of future large megathurst earthquakes in the Central Andes subduction zone. Caveats of this study are that interseismic strain of the forearc is assumed time invariant and entirely elastic. Also a major source of uncertainty is due to fact that the available data place very little constraints on interseismic coupling at shallow depth near the trench, except offshore Lima where sea bottom geodetic measurements have been collected suggesting strong coupling.

Validation and Comparison of Different Techniques for the Derivation of Digital Elevation Models and Volcanic Monitoring (Vulcano Island, Italy).

High accurate digital elevation models (DEM) acquired periodically over a volcanic area can be used for monitoring crustal deformations. Airborne stereoscopic photography is a powerful tool for the derivation of high resolution DEM, especially when combined with Global Positioning System (GPS). We analyse data acquired on Vulcano Island (Italy) to assess the performance of two photogrammetry methods for DEM generation. The first method is based on automatic digital processing of scanned airborne stereo images from a film camera (Wild RC20). In the second method digital stereo data from the multi-spectral High Resolution Stereo Camera-Airborne (HRSC-A) are used. Accuracy assessment through comparison with kinematic GPS height profiles shows that both DEMs have accuracy on the order of few decimetres. Direct comparison of the two DEMs on the La Fossa volcanic cone provides a standard deviation of the residuals of 78 cm. Residuals greater than two metres between the two DEMs acquired at one year interval are locally evidenced in unstable areas with uneven morphology. The application of photogrammetric DEMs is also discussed within a SAR interferometry study carried out on Vulcano Island to evaluate the potentialities of such techniques for ground deformation monitoring. Although accuracy better than 1 m or 2 m is not required for satellite SAR interferometry, we show how the precise photogrammetric DEMs could still significantly improve SAR interferograms of Vulcano Island.

Persistent uplift of the Lazufre volcanic complex (Central Andes): New insights from PCAIM inversion of InSAR time series and GPS data

We reanalyzed the surface displacements observed at the Lazufre volcanic complex in the Southern Andean Central Volcanic Zone using GPS measurements made between 2006 and 2008 and a large InSAR data set. We performed a detailed spatiotemporal analysis of the displacements using a principal component analysis inversion method (PCAIM). The PCAIM reveals a source with no significant changes in shape and dimension and with a remarkably linear strength increase over the whole period of observation (i.e., 2003–2010). Then we used a three-dimensional mixed boundary element method (MBEM) to invert the first component of surface displacement as obtained from PCAIM. We explored a continuum of geometries from a shallow elliptic crack to a deep massive truncated elliptical cone that could represent a sill or a large magma chamber, respectively. The best models indicate a large flat-topped source with a roof area between 40 and 670 km2 and a depth of between 2 and 14 km below ground surface. Lastly, on the basis of the limited data available for the thermomechanical structure of the crust in the Southern Andean Central Volcanic Zone, we consider some possible scenarios to explain the spatial and temporal pattern of displacements at Lazufre.

Quality and safety of bovine clones and their products

A multidisciplinary research programme was developed to get a scientific expertise for the quality assessment of products obtained from cloned livestock. Thirty-seven bovine Holstein female clones of five different genotypes and their products were analysed in comparison with 38 control animals obtained by conventional artificial insemination and raised under the same conditions at the same experimental farm. Animal evaluation included over 150 criteria and more than 10 000 measurements to check the physiological status and health over a 3-year period. All the parameters studied were in the normal range for age and breed, but some significant differences were detected between clone and control groups in terms of delayed onset of puberty in clones, higher neutrophil counts in haematology or lower biochemical plasma concentrations of gamma glutamyl transferase. Milk and meat analyses were conformable to expected values. We, however, found some differences in fatty acid (FA) composition of milk and muscle suggesting a possible deviation in lipid metabolism as assessed by higher delta-9 desaturase activity indexes in both milk and muscles from clones compared with controls. Repeated muscle biopsies in the semitendinosus muscle of the same animals demonstrated a higher oxidative activity in muscle of young clones (8 months of age) compared with controls, suggesting a delayed muscle maturation in clones. Nutritional evaluation of milk and meat using the rat feeding trials did not show any difference between clone and control products for food intake, growth rate, body composition of the rats, nor for possible allergenicity. Possible reactivation of bovine endogenous retroviruses (BERVs) was analysed and compared between normal and cloned cattle. As expected, these BERV sequences are not transcribed and no RNA was detected in the blood of clones, donor animals or controls; therefore, it may be assumed that the sanitary risk associated with BERV sequences is not higher in cattle derived from somatic nuclear transfer than in cattle born from conventional reproduction. Our results confirm that the quality and safety of products (milk and meat) from adult and clinically healthy cloned cattle is globally similar to normal animals. However, from a strictly biological point of view, the slightly delayed maturation we observed in the muscle of clones together with some marginal differences identified in FA composition of both muscle and milk, point to the need for more refined analysis to totally exclude any risks from the consumption of those products.

Postseismic relocking of the subduction megathrust following the 2007 Pisco, Peru, earthquake

Characterizing the time evolution of slip over different phases of the seismic cycle is crucial to a better understanding of the factors controlling the occurrence of large earthquakes. In this study, we take advantage of interferometric synthetic aperture radar data and 3.5 years of continuous Global Positioning System (GPS) measurements to determine interseismic, coseismic, and postseismic slip distributions in the region of the 2007, Mw 8.0 Pisco, earthquake, Peru, using the same fault geometry and inversion method. Our interseismic model, based on pre-2007 campaign GPS data, suggests that the 2007 Pisco seismic slip occurred in a region strongly coupled before the earthquake while afterslip occurred in low coupled regions. Large afterslip occurred in the peripheral area of coseismic rupture in agreement with the notion that afterslip is mainly induced by coseismic stress changes. The temporal evolution of the region of maximum afterslip, characterized by a relaxation time of about 2.3 years, is located in the region where the Nazca ridge is subducting, consistent with rate-strengthening friction promoting aseismic slip. We estimate a return period for the Pisco earthquake of about 230 years with an estimated aseismic slip that might account for about 50% of the slip budget in this region over the 0–50 km seismogenic depth range. A major result of this study is that the main asperity that ruptured during the 2007 Pisco earthquake relocked soon after this event.

Revised interpretation of recent InSAR signals observed at Llaima volcano (Chile)

We analyzed C band and L band interferometric synthetic aperture radar (InSAR) data acquired from 2003 to 2011 to search for volcanic deformations at Llaima volcano, Southern Andes (38.69°S, 71.73°W). There, specific environmental conditions (steep slopes, snow- or ice-capped summit, dense vegetation cover, and strong tropospheric artifacts) and limited amount of radar data available make it challenging to accurately measure ground surface displacement with InSAR. To overcome these difficulties, we first performed a careful analysis of the water vapor variations using Medium-Resolution Imaging Spectrometer and Moderate Resolution Imaging Spectroradiometer near-infrared water vapor products and then we inverted wrapped interferograms for both topographic correlated phase delays and a simple model source strength. In the light of our results, we conclude that there is no detectable ground displacement related to a deep magmatic source for the 2003–2011 period and that most of the fringes observed in the interferograms were produced by tropospheric delays.

Slow movements observed with in situ and remote-sensing techniques in the central zone of Chile

ABSTRACT Slow movement processes correspond to slow to extremely slow deformation in slopes. Slow movement is often known to be the cause of severe damage to structures and infrastructures. In the Northern Biobío hillside (NBH), many slow movements were not previously identified, which led to building damage. To evaluate their triggering factors, we use two complementary techniques: interferometry of synthetic aperture radar (InSAR) and electrical resistivity tomography (ERT). We observed deformation patterns using SAR data derived from PALSAR-1 during a year that exhibited extreme rainfall (2006). We established that reactivation of slow movements occurs during extreme rainfall periods, which can be observed through deformation patterns. ERT inversion results reveal a formation of silt and clay layers. Our results indicate the existence of slow movement processes close to densely populated areas. An interpretation of InSAR and ERT results showed that during extreme rain events, soil saturation occurs. The silt layer loads the hillslope and the clay layer acts as a lubricant below the silt.

Migrating pattern of deformation prior to the Tohoku-Oki earthquake revealed by GRACE data

Understanding how and when far-field continuous motions lead to giant subduction earthquakes remains a challenge. An important limitation comes from an incomplete description of aseismic mass fluxes at depth along plate boundaries. Here we analyse Earth’s gravity field variations derived from GRACE satellite data in a wide space-time domain surrounding the Mw 9.0 2011 Tohoku-Oki earthquake. We show that this earthquake is the extreme expression of initially silent deformation migrating from depth to the surface across the entire subduction system. Our analysis indeed reveals large-scale gravity and mass changes throughout three tectonic plates and connected slabs, starting a few months before March 2011. Before the Tohoku-Oki earthquake rupture, the gravity variations can be explained by aseismic extension of the Pacific plate slab at mid-upper mantle depth, concomitant with increasing seismicity in the shallower slab. For more than two years after the rupture, the deformation propagated far into the Pacific and Philippine Sea plate interiors, suggesting that subduction accelerated along 2,000 km of the plate boundaries in March 2011. This gravitational image of the earthquake’s long-term dynamics provides unique information on deep and crustal processes over intermediate timescales, which could be used in seismic hazard assessment.Deformation migrated from depth towards the surface in the months leading up to the 2011 Tohoku-Oki earthquake, according to analyses of satellite gravity data.