Quantification du couplage au long de la subduction chilienne

La subduction chilienne entre les plaques Nazca et Amérique du Sud est un laboratoire d'exception pour étudier l'accommodation de la convergence sur l'interface de subduction. La mesure par GPS de la déformation de la plaque supérieure pendant la phase intersismique permet de quantifier l'intensité du blocage via le coefficient de couplage cinématique. Il peut être déterminé en utilisant un modèle type "backslip". Ici, nous déterminons sa valeur sur la subduction chilienne (18-38S) et analysons ces variations en relation avec la segmentation sismotectonique. Les vitesses intersismiques collectées depuis 1990 ont été combinées et de nouvelles données GPS ont été acquises sur des réseaux installés ou rénovés depuis 2009. Cet effort instrumental et le travail de taitement des données conduit à un jeu de vitesses intersismiques sur plusieurs centaines de points. Ces données ont été utilisées pour élaborer une carte précise du couplage. La distribution de couplage montre des variations latérales et en profondeur et dessine une segmentation de la marge. Les segments fortement couplés semblent corrélés aux ruptures historiques et les intersegments découplés semblent se comporter comme des barrières. Le séisme de Maule du 27 février 2010 a permis de proposer un lien entre le couplage apparent et le comportement mécanique de l'interface. Il semble possible d'utiliser les cartes de couplage pour estimer l'aléa sismique au Chili. Nous apportons un éclairage nouveau sur la lacune du Grand Nord Chili. Les régions de l'atacama, du Paranal et du Loa correspondent à des zones fortement couplées et sont susceptibles de produire un grand séisme de subduction.The chilean subduction zone between the Nazca and South American plates is an ideal laboratory to understand the processes that take place on such a plate boundary. Measuring the elastic deformation of the upper plate during the interseismic phase using GPS can help assessing the degree of locking between both plates. We calculate the kinematic coupling using a simple backslip model. My aim was to obtain the coupling distribution along the entire chilean trench (18-38S) and to compare it to seismotectonic segmentation of the megathrust. I collected all the published interseismic velocities and combined them into a single data set. New data were collected and processed since 2009 on new or renovated campaign networks. We obtain a new date set on more than a hundred benchmarks. Those data were used to map precisely the coupling on the interface. It strongly varies both along strike and along dip and draws a segmentation of the megathrust. The highly coupled segments correlate well with historical megathrust earthquakes and intersegments, that are low coupled, correlate with zones that behave as barriers. The Maule earthquake that occured on February 27, 2010 ruptured a highly locked segment and stopped in low coupled intersegment areas. It gave us new insights on the relationship between apparent coupling and mechanical behavior of the subduction interface. Coupling maps should help estimating the seismic hazard along the Chilean subduction zone. In particular, we show here that the seismic hazard in the North Chile seismic gap may be lower than expected. Finally, Loa, Paranal and Atacama regions are zones that may rupture alone with a big subduction earthquake.PARIS7-Bibliothèque centrale (751132105) / SudocSudocFranceF

Interseismic Coupling, Megathrust Earthquakes and Seismic Swarms Along the Chilean Subduction Zone (38A degrees-18A degrees S)

International audienceThe recent expansion of dense GPS networks over plate boundaries allows for remarkably precise mapping of interseismic coupling along active faults. The interseismic coupling coefficient is related to the ratio between slipping velocity on the fault during the interseismic period and the long-term plates velocity, but the interpretation of coupling in terms of mechanical behavior of the fault is still unclear. Here, we investigate the link between coupling and seismicity over the Chilean subduction zone that ruptured three times in the last 5 years with major earthquakes (Maule Mw 8.8 in 2010, Iquique Mw 8.1 in 2014 and Illapel Mw 8.4 in 2015). We combine recent GPS data acquired over the margin (38A degrees-18A degrees S) with older data to get the first nearly continuous picture of the interseismic coupling variations on the subduction interface. Here, we show that at least six low coupling zones (LCZ), areas where coupling is low relatively to the neighboring highly coupled segments can be identified. We also find that for the three most recent Mw \textgreater 8 events, co-seismic asperities correlate well with highly coupled segments, while LCZs behaved as barriers and stopped the ruptures. The relation between coupling and background seismicity in the interseismic period before the events is less clear. However, we note that swarm sequences are prone to occur in intermediate coupling areas at the transition between LCZ and neighboring segments, and that the background seismicity tends to concentrate on the downdip part of the seismogenic locked zone. Thus, highly coupled segments usually exhibit low background seismicity. In this overall context, the Metropolitan segment that partly ruptured during the 2015 Illapel earthquake appears as an outlier since both coupling and background seismicity were high before the rupture, raising the issue of the remaining seismic hazard in this very densely populated area

Bridging the gap between North and Central Chile: insight from new GPS data on coupling complexities and the Andean sliver motion

International audienceGPS surveys have been extensively used over the past 20 yr to quantify crustal deformation associated with the Andean subduction zone in Chile. Such measurements revealed the coupling variations associated with the seismic segmentation of the subduction. However, because of data gaps mostly due to access difficulties, the Atacama-Antofagasta regions of North Chile remain poorly known. We present here an upgraded interseismic velocity field aggregating new data acquired between 2012 and 2016 in the region of Taltal (24 degrees S-26 degrees S), over a small-scale network of 20 benchmarks. This denser data set reveals a new complexity regarding the modelling methodology commonly used. We first show that a large-scale rigid Andean sliver, running from central to North Chile, does not allow to explain the velocities measured near the cordillera in the region of Taltal. This region exhibits an additional coherent block motion of almost 8 mm yr(-1) with respect to the inland motion generated by the rotation of the sliver proposed by, for example, Brooks et al. 2003; Metois et al. 2013, 2014, which works well everywhere else. Second, once this local block motion is taken into account, the coupling in the Taltal area is refined, which brings new insights about the subduction segmentation there. The Taltal area shows as a relative low in coupling (although coupling values are still high), potentially cutting a long section of the subduction into two independent highly coupled segments: the Paranal segment-north of Taltal, between 23 degrees S and 25 degrees S-and the Chanaral segment-south of Taltal, between 26 degrees S and 28 degrees S. These segments may rupture individually with magnitude similar to 8 earthquakes or simultaneously which would produce a larger earthquake, especially if a third segment (Atacama-more to the south-between 28 degrees S and 29 degrees S) is also involved

Tectonic-geomorphology of the Litang fault system, SE Tibetan Plateau, and implication for regional seismic hazard

The Litang fault system (LTFS) in the eastern Tibetan Plateau has generated several large (7.5\textgreaterM\textgreater7) historical earthquakes and has exhumed granitic peaks rising \textgreater1700 m above the mean elevation of the plateau, despite being located within a tectonic block surrounded by highly active faults. We study horizontally offset moraine crests from the Cuopu basin and a vertically offset alluvio-glacial fan from the eastern Maoya basin. We determine a left-lateral rate of 0.09 +/- 0.02 mm/yr along a slowly slipping secondary fault at Cuopu, while the main active fault at present is the normal range-front N Cuopu fault, along which we determined a left-lateral rate of 2.3 +/- 0.6 mm/yr since 173 ka. At Maoya fan, matching the vertical 12 +/- 1 m cumulative offset with the 21.7 +/- 4.2 ka fan age yields a vertical (normal) rate of 0.6 +/- 0.1 mm/yr. This rate is very similar to that recently determined at the same location using low-temperature thermochronology (0.59 +/- 0.03 mm/yr since 6.6 +/- 05 Ma). Left-lateral rates along the main faults of the LTFS range between 0.9 and 23 mm/yr at all time scales from a few years to similar to 6 Ma. The facts that the LTFS is highly segmented and that at present, the Cuopu, Maoya and South Jawa segments are mostly normal (while the Litang and Dewu segments are left-lateral/normal), could prevent the occurrence of M\textgreater7.5 destructive earthquakes along the LTFS, as is generally assumed. However, motion on the normal faults appears to be linked with motion on the strike-slip faults, potentially allowing for exceptional larger earthquakes, and implying that the area is not experiencing pure similar to NS extension but rather NW-SE left-lateral transtension. (C) 2016 Elsevier B.V. All rights reserved

The Seismic Sequence of the 16 September 2015 M-w 8.3 Illapel, Chile, Earthquake

On 16 September 2015, the M-w 8.3 Illapel, Chile, earthquake broke a large area of the Coquimbo region of north-central Chile. This area was well surveyed by more than 15 high-rate Global Positioning System (GPS) instruments, installed starting in 2004, and by the new national seismological network deployed in Chile. Previous studies had shown that the Coquimbo region near Illapel was coupled to about 60%. After the M-w 8.8 Maule megathrust earthquake of 27 February 2010, we observed a large-scale postseismic deformation, which resulted in a strain rate increase of about 15% in the region of Illapel. This observation agrees with our modeling of viscous relaxation after the Maule earthquake. The area where upper-plate GPS velocity increased coincides very well with the slip distribution of the Illapel earthquake inverted from GPS measurements of coseismic displacement. The mainshock started with a small-amplitude nucleation phase that lasted 20 s. Backprojection of seismograms recorded in North America confirms the extent of the rupture, determined from local observations, and indicates a strong directivity from deeper to shallower rupture areas. The coseismic displacement shows an elliptical slip distribution of about 200 km x 100 km with a localized zone where the rupture is deeper near 31.3 degrees S. This distribution is consistent with the uplift observed in some GPS sites and inferred from field observations of bleached coralline algae in the Illapel coastal area. Most of aftershocks relocated in this study were interplate events, although some of the events deeper than 50 km occurred inside the Nazca plate and had tension (slab-pull) mechanisms. The majority of the aftershocks were located outside the 5 m contour line of the inferred slip distribution of the mainshock

A Combined Velocity Field of the Mediterranean Region

International audienceWe present a full 3-D velocity field of the Earth's surface in the Euro-Mediterranean area obtained from a combination of three different velocity solutions computed at the Centro Nazionale Terremoti (CNT) of the Istituto Nazionale di Geofisica e Vulcanologia (INGV). All the publicly available GPS data since 1993, have been fully reprocessed by three different software tools and the final velocity field is estimated combining three independent velocity solutions in a least squares sense. The input velocity solutions are treated as stochastic samples of the true velocity field by loosening the reference frame constraints in the associated variance-covariance matrix. The proposed approach allows for a fast and efficient combination of multi velocity solutions, taking into account the full network covariance, if available. The velocity map for the Euro-Mediterranean region will be updated and released regularly on the web portal of the National GPS Network (http://ring.gm.ingv.it) and made available to the scientific community. Here we show and discuss the data analysis and the combination schemes, and the results of the combined velocity field

Rapid response to the Mw 4.9 earthquake of November 11, 2019 in Le Teil, Lower Rhône Valley, France

International audienc