Spatiotemporal evolution of surface creep in the Parkfield region of the San Andreas Fault (1993-2004) from synthetic aperture radar.

International audienceThe Parkfield section of the San Andreas Fault (SAF) is defined as a transitional portion of the fault between slip-release behavior types in the creeping section of the SAF to the northwest and the apparently locked section to the southeast. The Parkfield section is characterized by complex frictional fault behavior because it represents a transition zone from aseismic creep to stick-slip regime. At least six historic earthquakes of Mw ~6 have occurred in this area in 1881, 1901, 1922, 1934, 1966, and 2004. It was observed in the 2004 Mw 6.0 Parkfield earthquake that ~70% of the total (coseismic and postseismic) moment release occurred aseismically. To understand the SAF behavior in this area, it is of particular interest to measure and analyze, not only the spatial evolution of the surface displacement in this area, but also its evolution over time. Using radar data acquired by the European Space Agency's European Remote Sensing (ERS1-2) satellites, we constructed descending interferograms and retrieved time series of surface displacements along the central SAF for the decade preceding the 2004 Parkfield earthquake. We focus on characterizing the space and time evolution of surface creep in the Parkfield and Cholame sections. The spatial pattern of the interseismic displacement rate indicates that tectonic strain was not uniformly distributed along the strike of the fault between 1993 and 2004. Our data indicate not only a decrease in the creep rate from the Parkfield section to south of Highway-46 from 1.4 ±0.3 cm/y to 0.6 ±0.3 cm/y, but also a small but significant creep-rate increase in the Cholame section to 0.2 ±0.1 cm/y. The evidence for episodic creep in the Cholame section of the SAF south-east of Parkfield is in contrast with previously published interpretations of GPS and trilateration data. The Cholame section of the SAF merits close monitoring because it was likely the nucleation site of the 1857 Fort Tejón earthquake and because it has shown recent evidence of deep slow slip as revealed by deep tremors

Estimation des effets atmosphériques en interférométrie différentielle : Validation des modèles à partir d'une base de données multitemporelle d'interférogrammes

- L'aspect différentiel des données interférométriques est à l'origine du caractère multitemporel des images manipulées. Dans cet article, nous proposons une méthode de correction des effets atmosphériques en interférométrie différentielle fondée sur cette propriété. L'approche développée commence par sélectionner les pixels les plus cohérents et les plus stables dans le temps en s'appuyant sur la corrélation des effets atmosphériques avec la topographie. L'exploitation de ces pixels nous permet de retrouver un modèle atmosphérique pour chaque interférogramme. Ensuite, les relations existant entre les différents couples interférométriques sont utilisées pour valider les modèles d'atmosphère obtenus. La diversité des données interférométriques et leur répartition dans le temps fixent la robustesse de cette méthode

IMPROVEMENT OF THE TROPOSPHERIC CORRECTION BY ADAPTED PHASE FILTERING

Tropospheric inhomogeneities can form a major error source in DinSAR (Differential SAR Interferometry) measurements used in slow deformation monitoring. Previous studies introduced techniques to correct these artefacts. In [1] they propose to evaluate and correct tropospheric effects directly from raw differential interferograms by estimating the phase/altitude correlation. Since the wrapped phase noise in these interferograms influences the correction of tropospheric artefacts its removal is mandatory. In this paper, we aim to show that adapted wrapped phase filtering greatly improves the retrieval of tropospheric effects. The filtered interferograms are then used to model these artefacts. Filtered and unfiltered results are compared to quantify the improvement

INTERFÉROMÉTRIE RADAR APPLIQUÉE AUX VOLCANS : CAS DE L’ETNA ET DES CHAMPS PHLÉGRÉENS (ITALIE)

During the last few years, the radar images collected by the European satellites ERS1 and ERS2, the Japanese satellite JERS and the Canadian satellite RADARSAT have been used with success to create interferograms. This technique has been applied for geophysical applications like co-seismic deformation mapping, volcano deformation monitoring, landslides monitoring, mining subsidence detection, glaciers monitoring. Here we report the research carried out by our group on Etna volcano (Italy) and in the area of Naples (Italy) where are located several potentially active volcanoes (Vesuvius, Ischia) and where a subsidence of the caldera of Campi Flegrei is still on going in response to the 1982-1984 seismic crisis. Etna is the volcano that has been studied first using ERS SAR interferometry. Using this method, a large scale deflation of the volcano associated with the large 1991-1993 eruption was detected in data covering the second half of the eruption. Further studies showed that the local deformation fields located in Valle del Bove (East of the volcano) where associated with the compaction of the 1986-1987 and 1989 lava fields and also partly with a subsidence of the surrounding terrain in response to the load of the new deposited material. Other local deformation fields have been identified, corresponding to the 1983, 1981 and 1971 lava fields. However, due to its strong topography, interferograms of Etna are affected by tropospheric effects. Those effects must be eliminated in order to correctly interpret the fringes pattern. The problem of the troposphere has been first investigated from its theoretical point of view and using existing local meteorological data as well as radio-soundings data. Recently, thanks to the large amount of available interferograms, another approach has been investigated, consisting in the research of a correlation fringe/elevation in the interferograms themselves. This approach, operated either in automatic mode (automatic fringe unwrapping) or in manual mode proved to be efficient for most of the coherent interferograms. After removal of the tropospheric correction, the evolution of the deformation of the volcano at large scale between 1992 and 1998 has been inferred. The subsidence occurring during the second half of the 1992 eruption as well as the uplift preceding the 1995 unrest of the Southeast crater are visible, but their amplitude is less than previously estimated. The depth of the modelled source of subsidence/uplift related to the large scale deformation is of the order of 6 to 8 km, not well constrained by the data. The study of the correlation fringe/elevation was possible only after a detailed analysis of the spatial and temporal properties of coherence of the Etna area. Indeed, the technique of fringe unwrapping for fringe/elevation correlation analysis is possible only if the poorly coherent pixels are eliminated. A map of the most coherent pixels of the volcano was produced. The recent lava fields as well as the towns and villages surrounding the volcano are the most coherent areas. The quality of the interferograms is also enhanced when high accuracy DEM (Digital Elevation Model) are used. Using kinematic GPS data collected along more than 100 km of road around the volcano, we assessed the accuracy of several DEMs of Etna. The most accurate DEM was produced by digitising 1/25.000e maps of Etna. This DEM does not take into account the topographic changes due to the recent eruptions. Merging other more recent DEMs corresponding to those areas, we produced an updated relatively high accuracy DEM (±3 m) of Etna. In the Naples area, we analysed interferograms in the period 1993-1996 and show that the Campi Flegrei caldera is still subsiding at a rate of about 30 mm/year

A ten years analysis of deformation in the Corinthian Gulf via GPS and SAR Interferometry

The Corinthian Gulf in Greece, is the most active of a series of extending grabens which accomodate the deformation in the highly seismic Aegean region. The geodetic network established in the region has about 200 points: 50 1st order points and ~150 2nd order points. The network covers an area of about 100 x 80 km2, which correspond to an average density of 1 point every 5 km2. This dense network allows to study the main active faults in the region. Eleven field surveys were organized in 1990, 1991, 1992, 1993, 1994, June 1995, October 1995, 1997, and 2001. Two earthquakes occurred in the vicinity during the ten years period: the 1992, 18 November Ms=5.9 Galaxidi earthquake and the 1995, 15 June 1995 Ms=6.2 Aigion one. With respect to the stable Europe, we find for Peloponnessos an average displacement rate of 30 mm/yr in the N215° direction, similar to that found in previous studies. Our results show that most of the deformation in the Corinthian Gulf is localizes off-shore, in a narrow band, in the central part of the Gulf. The extension rate measured over 10 years is 11 mm/yr in the N185° direction in the middle of the Gulf (Xiloxastro) and 16 mm/yr in the N185° direction in its western part (Aigion). The southern block appears un-deformed, except the region of Aigion event. Using CNES DIAPASON software, we derived 85 interferograms of the Corinthian Gulf from 38 raw ERS SAR images acquired between 1992 and 1999. The interferograms sampling the 1995 earthquake show a clear coseismic signal reaching 250 +/- 15 mm at Psaromita cape, a value consistent with the GPS measurements. No post-seismic motion, within the error bars of SAR interferometry (+/- 15 mm), is observed during the 1995-1999 period

Ground deformation monitoring of the eruption offshore Mayotte

In May 2018, the Mayotte island, located in the Indian Ocean, was affected by an unprecedented seismic crisis, followed by anomalous on-land surface displacements in July 2018. Cumulatively from July 1, 2018 to December 31, 2021, the horizontal displacements were approximately 21 to 25 cm eastward, and subsidence was approximately 10 to 19 cm. The study of data recorded by the on-land GNSS network, and their modeling coupled with data from ocean bottom pressure gauges, allowed us to propose a magmatic origin of the seismic crisis with the deflation of a deep source east of Mayotte, that was confirmed in May 2019 by the discovery of a submarine eruption, 50 km offshore of Mayotte ([Feuillet et al., 2021]). Despite a non-optimal network geometry and receivers located far from the source, the GNSS data allowed following the deep dynamics of magma transfer, via the volume flow monitoring, throughout the eruption

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

On November 11, 2019, a Mw 4.9 earthquake hit the region close to Montelimar (lower Rhône Valley, France), on the eastern margin of the Massif Central close to the external part of the Alps. Occuring in a moderate seismicity area, this earthquake is remarkable for its very shallow focal depth (between 1 and 3 km), its magnitude, and the moderate to large damages it produced in several villages. InSAR interferograms indicated a shallow rupture about 4 km long reaching the surface and the reactivation of the ancient NE-SW La Rouviere normal fault in reverse faulting in agreement with the present-day E-W compressional tectonics. The peculiarity of this earthquake together with a poor coverage of the epicentral region by permanent seismological and geodetic stations triggered the mobilisation of the French post-seismic unit and the broad French scientific community from various institutions, with the deployment of geophysical instruments (seismological and geodesic stations), geological field surveys, and field evaluation of the intensity of the earthquake. Within 7 days after the mainshock, 47 seismological stations were deployed in the epicentral area to improve the Le Teil aftershocks locations relative to the French permanent seismological network (RESIF), monitor the temporal and spatial evolution of microearthquakes close to the fault plane and temporal evolution of the seismic response of 3 damaged historical buildings, and to study suspected site effects and their influence in the distribution of seismic damage. This seismological dataset, completed by data owned by different institutions, was integrated in a homogeneous archive and distributed through FDSN web services by the RESIF data center. This dataset, together with observations of surface rupture evidences, geologic, geodetic and satellite data, will help to unravel the causes and rupture mechanism of this earthquake, and contribute to account in seismic hazard assessment for earthquakes along the major regional Cévenne fault system in a context of present-day compressional tectonics