François Beauducel

Volcano‐wide fringes in ERS synthetic aperture radar interferograms of Etna (1992–1998): Deformation or tropospheric effect?

Mount Etna (3300 m) is the volcano that has been first and most studied by differential synthetic aperture radar. Previous papers gave evidence for a large-scale deformation of the entire edifice consistent with unrest episodes but with a poor fit with classical elastic models. Also, atmospheric effects on mountainous areas are known to be very significant. Accordingly, interferograms may reflect both deformation and tropospheric effects. We investigate here the possibility of evaluating and correcting tropospheric effects directly from the interferograms. From 38 ERS synthetic aperture radar ascending scenes taken from 1992 to 1999, we have computed 238 interferograms. The amount of data allows us to analyze how the coherence is maintained over long periods of time and thus how it reveals the location of permanent scatters. From a simple analysis of phase-elevation relation, we give evidence for the clear difference in pattern between stratified tropospheric effects and large-scale deformation produced by a source in a Three-dimensional elastic body with topography. Using combination of inversion processes, we analyze both deformation source and tropospheric effects for the complete set of 238 interferograms. Finally time evolution of the parameters and modeling probability estimation are retrieved for each SAR image. Estimated relative change in tropospheric delay varies in time from -2.7 to +3.0 (± 1.2) fringes. They are consistent with other tropospheric estimations at Etna based on ground met and Global Positioning System observations on the same period of time. Magmatic volume variations remain below error bars and thus cannot be estimated properly. Finally, some of the interferograms may be entirely explained by the difference in tropospheric conditions between the two acquisition periods, without the need to invoking a deformation of the volcano.

Collection and three‐dimensional modeling of GPS and tilt data at Merapi volcano, Java

We study here the deformations associated with the November 1996 to March 1997 eruption period at Mount Merapi (Central Java), one of the most active volcanoes in Indonesia. This activity period includes a vertical explosion on January 17 and an increase of the lava dome volume by about 3×106 m3. Two Global Positioning System (GPS) campaigns have been carried out on a six-benchmark network at the beginning and at the end of the period. Relative displacements with respect to the reference point show an average subsidence of 6.5 cm. A multicomponent tilt station installed on the southeast flank, 3 km from the summit, recorded a tilt of 11.1 ± 0.7 μrad in the tangential direction and 0.9 ± 0.4 μrad in the radial direction. These data are interpreted using a three-dimensional (3-D) elastic model based on the mixed boundary element method and a near-neighbor Monte Carlo inversion. Interpretation of tilt data requires an accurate mesh for discretizing the 3-D topography. The final result supports a horizontal elliptic magma source located 8.5 ± 0.4 km below the summit and 2 ± 0.4 km to the east of it. In particular, the data cannot be consistent with the location of a magma chamber determined from seismic activity analysis (i.e., 2 km below the summit). The computed depth depends strongly on the source shape and cannot be constrained properly because of the small amount of data. The computed deflation of 11 ± 2×106 m3 is about 3 times larger than the observed increase in the lava dome volume. This difference is attributed to rock avalanches and pyroclastic flows on the flanks of the volcano.

Constraints on magma flux from displacements data at Merapi volcano, Java, Indonesia

The displacement field has been monitored in the vicinity of the crater rim at Mount Merapi (Indonesia) from 1993 to 1997. During this period the volcanic activity has been quasicontinuous with dome growth, explosions, and pyroclastic flows. We measured a nine-point network every year with the Global Positioning System static method. Interpretation of results is conducted with a three-dimensional elastostatic boundary elements code that takes into account topography, fractures, and complex magma source geometry. The inversion technique yields an estimate of the variation with time of the boundary conditions at the magma duct interface together with the probability associated with the best model. The Youngs modulus of the equivalent continuum is found to be very low (of the order of 1 GPa), a feature which suggests that a viscoelastic behavior may be more appropriate for this rock mass, given the observed seismic velocities for the domain of interest. A striking compatibility is outlined between observed deformations and the rate of occurrence of multiphase seismic events, once the main fractures of the structure have been taken into consideration. This suggests that the summit elastic (or viscoelastic) deformation field is controlled by the magma flux within the duct rather than by magma pressure variations. In addition, a nonelastic displacement has been identified at the westernmost point of the network. This was considered critical for the stability of the summit structure, a concern whose validity has been verified a posteriori by the July 1998 explosion.

Subsidence of Campi Flegrei (Italy) detected by SAR interferometry

Seven ERS SAR images of the Napoli area ac- quired from 1993 to 1996 were used to create six differential interferograms. Although the ambiguity height of the selected couples is high, because of the adverse characteristics of the area, the interferograms exhibit a relatively low level of c o- herence, even for couples sampling short time intervals. A c- cording to the estimated accuracy, no deformation is observed on Vesuvio volcano. The best interferogram, corresponding to the period Feb. 14, 1993 - Apr. 3, 1996, shows that the centre of the Campi Flegrei caldera, near Pozzuoli harbour, subsided by about three fringes (84 mm) during the three years interval. This result is consistent with the value of 28 ± 2 mm/year obtained from routine levelling surveys over the same period in the same area. The deflation that started af ter the 1983- 1984 seismic and inflation crisis is thus still sig nificant. The modelling of the fringe pattern, assuming a spherical deflating source within an elastic half-space m edium, predicts a source centred about 800 m Southwest of Pozzuoli at a depth of 2.7 ± 0.3 km. For the studied time wi ndow, the maximum subsidence predicted by the model is 25 mm/year.

The Mw = 6.3, November 21, 2004, Les Saintes earthquake (Guadeloupe): Tectonic setting, slip model and static stress changes

On November 21, 2004, a magnitude 6.3 earthquake occurred offshore, 10 km south of Les Saintes archipelago in Guadeloupe (French West Indies). There were more than 30000 aftershocks recorded in the following two years, most of them at shallow depth near the islands of the archipelago. The main shock and its main aftershock of February 14, 2005 (Mw = 5.8) ruptured a NE-dipping normal fault (Roseau fault), mapped and identified as active from high-resolution bathymetric data a few years before. This fault belongs to an arc-parallel en echelon fault system that follows the inner edge of the northern part of the Lesser Antilles arc, accommodating the sinistral component of oblique convergence between the North American and Caribbean plates. The distribution of aftershocks and damage (destruction and landslides) are consistent with the main fault plane location and attitude. The slip model of the main shock, obtained by inverting jointly global broadband and local strong motion records, is characterized by two main slip zones located 5 to 10 km to the SE and NW of the hypocenter. The main shock is shown to have increased the Coulomb stress at the tips of the ruptured plane by more than 4 bars where most of the aftershocks occurred, implying that failures on fault system were mainly promoted by static stress changes. The earthquake also had an effect on volcanic activity since the Boiling Lake in Dominica drained twice, probably as a result of the extensional strain induced by the earthquake and its main aftershock.

Multiscale Mapping of Completeness Magnitude of Earthquake Catalogs

We propose a multiscale method to map the spatial variations of the com- pleteness magnitude Mc of earthquake catalogs. The Gutenberg-Richter law describ- ing the earthquake frequency-magnitude distribution (FMD) might not hold over the entire magnitude range, and small areas may exhibit a specific type of seismicity, especially in volcanotectonic contexts. For these reasons, any scaling relation should be obtained by adapting the dimension of the studied zone to the range of the event magnitude. Here, we associate ranges of larger magnitudes with increasing areas for data selection based on empirical relations in seismotectonics. Then, for each point in space, we document the earthquake FMD at all length scales within the corresponding earthquake magnitude ranges. High resolution of the Mc-value is achieved through the determination of the smallest space-magnitude scale in which the Gutenberg-Richter law is verified. The multiscale procedure isolates the magnitude range that meets the best local seismicity and local record capacity. Using artificial catalogs and earthquake catalogs of the Lesser Antilles arc, this Mc-mapping method is shown to be efficient in regions with mixed types of seismicity, a variable density of epicenters, and various levels of registration.

Retrieving 65 years of volcano summit deformation from multitemporal structure from motion: The case of Piton de la Fournaise (La Réunion Island)

The structure-from-Motion photogrammetry technique enables use of historical airborne photography to achieve high-resolution 3D terrain models. We apply this method on Piton de la Fournaise volcano (La Reunion), which allows a unique opportunity to retrieve high-resolution (1.5-0.11 m) Digital Elevation Models and precise deformation maps of the volcano since 1950. Our results provide evidence that the summit volume increased throughout the study period, at a stable rate of 2.2 Mm3/yr between 1950 and 2000, increasing by a factor of four (to 8.0Mm3/yr) prior to the major 2007 eruption which was accompanied by summit caldera collapse. At the same time, summit deformation was asymmetric, with 9.2±2.5 m of eastward seaward displacement, and 1.3±2.5 m to the west during 1950-2015. Our results reveal a temporal evolution in the volcano magma influx rate and deformation. Tracking these fluxes and the long-lived preferential eastern motion is crucial to mitigate risks associated to flank destabilization.

Global coseismic deformations, GNSS time series analysis, and earthquake scaling laws

We investigate how two decades of coseismic deformations affect time series of GPS station coordinates (Global Navigation Satellite System) and what constraints geodetic observations give on earthquake scaling laws. We developed a simple but rapid model for coseismic deformations, assuming different earthquake scaling relations, that we systematically applied on earthquakes with magnitude larger than 4. We found that coseismic displacements accumulated during the last two decades can be larger than 10 m locally and that the cumulative displacement is not only due to large earthquakes but also to the accumulation of many small motions induced by smaller earthquakes. Then, investigating a global network of GPS stations, we demonstrate that a systematic global modeling of coseismic deformations helps greatly to detect discontinuities in GPS coordinate time series, which are still today one of the major sources of error in terrestrial reference frame construction (e.g., the International Terrestrial Reference Frame). We show that numerous discontinuities induced by earthquakes are too small to be visually detected because of seasonal variations and GPS noise that disturb their identification. However, not taking these discontinuities into account has a large impact on the station velocity estimation, considering todays precision requirements. Finally, six groups of earthquake scaling laws were tested. Comparisons with our GPS time series analysis on dedicated earthquakes give insights on the consistency of these scaling laws with geodetic observations and Okada coseismic approach.

Contribution of Tiltmeters and Extensometers to Monitor Piton de la Fournaise Activity

Tiltmeters and extensometers were among the first instruments used to monitor volcano ground deformation and are used worldwide. Since their implementation in the early 80’s at Piton de la Fournaise (La Reunion Island), tiltmeters and extensometers largely contributed to the knowledge and the forecast of eruptive activity on this volcano. These instruments allowed to evidence eruptive precursors at Piton de la Fournaise on two time scales (months/weeks and hours/mins), associated with reservoir pressurization and dike propagations, respectively, but also to follow eruptive/post eruptive relaxation and collapse events.

Implementation of a Multistation Approach for Automated Event Classification at Piton de la Fournaise Volcano

ABSTRACT We implemented the first operational automated seismic‐event classification system for monitoring activity at the Piton de la Fournaise volcano observatory (OVPF, La Reunion Island). Our classifier is based on the Random Forest algorithm. It distinguishes between eight classes of seismic signals: summit and deep volcano tectonic events, local, regional, and teleseismic earthquakes, T phases, rockfalls, and sound waves. It adopts a multistation approach and automatically selects the best features for each station and combination of stations from a large set of waveform‐ and spectrum‐based features. It reaches peak performance when it runs on a three‐station combination: one station on the summit of Piton de la Fournaise, one in its caldera, and one on the volcano flank. We interfaced our classification system with the observatory management interface WebObs used at OVPF.