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We use seismic ambient noise correlation and coda wave interferometry to estimate velocity variations at high temporal resolution, during the pre-eruptive period and the onset of the 2018 eruption of Kilauea volcano. A progressive velocity increase is observed from March to the end of April. It is followed by rapid decrease starting a few days before the onset of the East Rift Zone (ERZ) eruption and then by sharp velocity drop when the eruption started. The change of trend from velocity increase to decrease is progressively delayed by a few days from the summit caldera toward the ERZ. The location of the velocity perturbations shows a migration of the sources of velocity changes from the summit caldera toward the ERZ before the eruption. Using a model of pressure source, we show that the simultaneous caldera inflation and velocity increase probably result from an anisotropic distribution of fault and crack orientations. The velocity decrease could be due to damaging processes above the shallow reservoir and to plastic deformations around the caldera. We introduce a forward model of rock damage associated with the volcano-tectonic seismicity to calculate the velocity decrease. The good agreement between the calculated and the observed velocity variations shows that a large part of the velocity decrease results from damage of the medium. The delayed onsets of velocity decrease and the source migration of velocity perturbations are probably related to progressive fault openings in the Southern and Eastern parts of the caldera and to magma transfer toward the ERZ.

This file contains the data, results and software for the article: PRE-ERUPTIVE DAMAGE, WEAKENING AND MAGMA-EDIFICE COUPLING AT PITON DE LA FOURNAISE VOLCANO submitted to Journal of Geophysical Research - Solid Earth - version of Dec. 15, 2022.

International audience; Identifying the causes of flank destabilization of active volcanic edifices is key to prevent catastrophic events. The persistent seismicity recorded below the eastern flank of Piton de la Fournaise shield volcano (Réunion Island), both in between and during eruptive events, may give indications on the mechanical stability of this edifice. Whether this asymmetric "cup" shaped seismicity is linked to magma injections and whether it sparks a gravitational flank slide motivates this study. Here we model the elasto-plastic behavior of this volcanic edifice at crustal scale, with the 3D finite-element code Adeli. First, we test the influence of tensile failure, recently implemented in combination to a Drucker-Prager shear failure criterion; a pressurized cavity below a flat top surface triggers shear failure in general, with tensile failure restricted to the surface and cavity tip. Then we include the topography of Piton de la Fournaise in the gravity field. Considering first only elasticity, deviatoric stresses attain about 35 MPa below the volcanic edifice and displacements are maximum in the horizontal east-west direction, reaching 30 m near sea-level. Introducing plastic behavior produces a rather symmetric cup shape plastic domain around the volcano's summit, that extends at depth with reducing bedrock {\it effective} friction (which acts is a proxy for reduced standard friction due to pore fluid pressurization). An asymmetric listric shear zone develops down to -3 km (bsl) only if the tensile strength, cohesion and friction angle are set as low as 1.5 MPa, 3 MPa and 3°, respectively; these values hence provide a lower bound for the edifice's effective strength. The second part of this study explores the influence of an internal overpressure, which is either applied as a vertical inflation source located about 500 m below the surface of the eastern flank, simulating a distal dike, or from a deeper ellipsoid simulating the magma reservoir located at depth ca. 0 km (near sea level) below the summit. The resulting strain pattern forms a cup-shaped shear zone dipping down below the eastern flanks of the edifice, reaching depth -2 km ({\it bsl}) if effective friction angle is $ \leq 5^\circ$. Whereas the deep base of the dike and the eastern edge of the magma reservoir coincide geometrically in the models, the inflating dike produces a shear zone 1 km shallower than does the inflating magma reservoir, the latter coinciding better with the shape of the observed seismic cup. Hence, we propose that this structure is a mechanical consequence of continuous magma supply in the reservoir, coherent with previous interpretations. This means that at least originally it did not need to form as a pre-existing weak zone or a magma-filled structure. However, this shear zone delimits an underlying domain in dilatation relative to a constricted hanging-wall; it may thus promote magma sills. It also branches to the surface with planar radial shear zones comparable to some observed eruptive fissures. The 3D kinematics of this shear zone does not rule out the possibility of a giant flank slide, although it does not appear today as imminent.

The Ubaye Region is a seismically active region in the Western Alps (France), regularly struck by seismic swarms characterized by a high number of small to moderate earthquakes, such as in 2003–2004 or 2012–2015. While some earthquakes could be associated with known faults, the character of the observations (high seismicity – low deformation rate) requires complex driving processes beyond local or regional tectonics. Most conceptual models involve fluids present down to depths of several km, and/or long-range transport.Magnetotellurics (MT) is known to be an efficient imaging method sensitive to crustal fluids. During 2020/21, a data set of 30 MT sites was acquired, covering a signal period ranging between 10-4 to 104 s, with generally all 5 components measured. Data quality was generally satisfactory up to 3 s and sometimes up to 100 s. Major problems were related to topography (including logistics), and to the presence of electromagnetic noise, only to be mitigated by advanced processing methods (FFMT). For the 3-D inversion required by the data (phase tensors, WAL, topography), we have chosen a joint inversion of induction vectors, phase tensors and off-diagonal impedances (previously corrected for static shift with help of phase tensor inversion). This allowed us to obtain the best 3-D model using the ModEM inversion code, explaining all three data types reasonably well.The main findings from this investigation include (a) a prominent conductor (down to 20 Ωm) located along the axis of the swarm zone, though generally above it; (b) a regional dominance of the Penninic Front in the East and the overridden Mesozoic (Dauphinoise) sediments in the West, both not fully covered by the current survey; (c) strike directions that agree well with most of the mapped faults and focal mechanisms of the strongest seismic events.Uncertainties associated with the insufficient data coverage in some of the most interesting zones were studied by analysing the sensitivities provided by the inversion and direct forward modelling of significant model features. In general, this led to the conclusion that our sensitivity does reach the border of the seismic swarm activity, but does not cover its depth extent. Due to the gap in data in the central area of interest, the geometry and connectivity of the main conductor remains a hypothesis. Thus, a truly quantitative characterization of prominent identified structures is not currently possible and should be better assessed with additional measurement sites. The different conceptual models proposed for the origin of the seismic swarm activity will be discussed in the light of the MT imaging, and the associated uncertainties.

Building on previous work aimed at identifying and characterizing the potential mechanical trigger controlling eruptions and destabilization at Piton de la Fournaise, we study the mechanical behavior of the volcanic edifice on a crustal scale. Do the recurrent earthquake pattern correspond to a destabilization structure, precursor of a large-scale flank sliding? Or instead to a reactivated area of magma storage (partially crystallized “sill”)? To answer these questions, we design numerical models which estimate the stress field associated with the volcanic complex. We use the ADELI finite element method in three dimensions, which handles elasto-visco-plastic rheologies. In these models, we take into account 1) the topographic load, 2) the major density and resistance heterogeneities within the volcano obtained from previous studies, and 3) the overpressure induced by the intrusion of a dike of arbitrary geometry.The modeled dike injection generates deformation and stress fields such that their isocontours highlight an ellipsoidal cup structure extending from the central cone to a depth close to 0 and reaching the ends of the eastern flank. This zone could be assimilated to the zone of seismicity observed and described previously. Together with several systematic test cases, we will discuss the significance of these results, such as whether it reveals a rheological delimitation zone of the hydrothermalized bedrock, resulting from the combined influence of the topographic load and that of a magmatic injection.

Continuous monitoring of seismicity and surface displacement of active volcanoes can reveal important features of the eruptive cycle. Here high-quality GPS and earthquake data recorded at Grimsvotn volcano by the Icelandic Meteorological Office during the 2004-2011 intereruptive period are analyzed. These showed a characteristic pattern, with an initial similar to 2 year long exponential decay followed by similar to 3 year long constant surface displacement inflation rate. We model it by using a one magma reservoir model in an elastic damaging edifice, with incompressible magma and constant pressure at the base of the magma conduit. Seismicity rate and damage were first modeled, and simple analytical expressions were derived for the magma reservoir overpressure and surface displacement as functions of time. Very good fits of the seismicity and surface displacement data were obtained by fitting only three phenomenological parameters. Characteristic time and power strain show maxima from which reference times were inferred that split the intereruptive period into five periods. After the pressurization periods, damage occurring in the third period induced weakly nonlinear variations in magma overpressure and flow, and surface displacement. During the fourth period, the damage dominated and variations became more strongly nonlinear, the reservoir overpressure decreased, and magma flow increased. This process lasted until the power strain reached its second maximum, where instability was generalized. This maximum is a physical limit, the occurrence of which shortly precedes rupture and, eventually, eruption. This analysis allows characterization of the state of the volcanic edifice during the intereruptive period and supports medium-term prediction of rupture and eruption.

The accuracy of elastic analytical solutions and numerical models, widely used in volcanology to interpret surface ground deformation, depends heavily on the Young’s modulus chosen to represent the medium. The paucity of laboratory studies that provide Young’s moduli for volcanic rocks, and studies that tackle the topic of upscaling these values to the relevant lengthscale, has left volcano modellers ill-equipped to select appropriate Young’s moduli for their models. Here we present a wealth of laboratory data and suggest tools, widely used in geotechnics but adapted here to better suit volcanic rocks, to upscale these values to the scale of a volcanic rock mass. We provide the means to estimate upscaled values of Young’s modulus, Poisson’s ratio, shear modulus, and bulk modulus for a volcanic rock mass that can be improved with laboratory measurements and/or structural assessments of the studied area, but do not rely on them. In the absence of information, we estimate upscaled values of Young’s modulus, Poisson’s ratio, shear modulus, and bulk modulus for volcanic rock with an average porosity and an average fracture density/quality to be 5.4 GPa, 0.3, 2.1 GPa, and 4.5 GPa, respectively. The proposed Young’s modulus for a typical volcanic rock mass of 5.4 GPa is much lower than the values typically used in volcano modelling. We also offer two methods to estimate depth-dependent rock mass Young’s moduli, and provide two examples, using published data from boreholes within Kīlauea volcano (USA) and Mt. Unzen (Japan), to demonstrate how to apply our approach to real datasets. It is our hope that our data and analysis will assist in the selection of elastic moduli for volcano modelling. To this end, our new publication (Heap et al., 2019), which outlines our approach in detail, also provides a Microsoft Excel© spreadsheet containing the data and necessary equations to calculate rock mass elastic moduli that can be updated when new data become available. The selection of the most appropriate elastic moduli will provide the most accurate model predictions and therefore the most reliable information regarding the unrest of a particular volcano or volcanic terrain.Heap, M.J., Villeneuve, M., Albino, F., Farquharson, J.I., Brothelande, E., Amelung, F., Got, J.L. and Baud, P., 2019. Towards more realistic values of elastic moduli for volcano modelling. Journal of Volcanology and Geothermal Research, https://doi.org/10.1016/j.jvolgeores.2019.106684.

International audience; Structures developing above long‐term growing shallow‐level magma reservoirs, such as resurgent domes, may contain information on the reservoir itself. To understand the formation of such tectonic features, we have investigated the deformation process around a shallow pressurized magma reservoir embedded in a damaging elastic volcanic edifice. Our model allows evidencing the effect of the progressive damage in producing the fault pattern associated to tectonic surface deformation. Damage is first isotropic around the cavity and constitutes a damaged zone. Then the free‐surface effect appears and an anisotropic shear strain develops from the boundary of the damaged zone; it localizes on reverse faults that propagate upward to the surface. When the surface deformation is sufficient, normal faulting appears. Finally, the complete structure shows an undeformed wedge above the damaged zone. This structure is similar to those found by analogue modelling and from field geologic observations. From this model, we found a relation to estimate the reservoir radius and depth from the graben and dome widths. From limit analysis, we deduced an analytical expression of the magma reservoir pressure which provides a better understanding of the magma pressure build‐up during doming. The dip of reverse faults limiting the dome can be inferred from the minimal pressure required to rupture the crust around the reservoir. Finally, the magma reservoir overpressure, the dip of the faults, the reservoir depth and the damaged zone radius is inferred from three parameters: the ratio ρR computed from the dome and graben widths, the cohesion and the friction angle.

We monitored the displacement and seismic activity of an unstable vertical rock slice in a natural limestone cliff of the southeast Vercors massif, southeast France, during the months preceding its collapse. Displacement measurements showed an average acceleration of the movement of its top, with clear increases in the displacement velocity and in the discrete seismic event production rate during periods where temperature falls, with more activity when rainfall or frost occurs. Crises of discrete seismic events produce high amplitudes in periodograms, but do not change the high frequency base noise level rate. We infer that these crises express the critical crack growth induced by water weakening (from water vapor condensation or rain) of the rock strength rather than to a rapid change in applied stresses. Seismic noise analysis showed a steady increase in the high frequency base noise level and the emergence of spectral modes in the signal recorded by the sensor installed on the unstable rock slice during the weeks preceding the collapse. High frequency seismic noise base level seems to represent subcritical crack growth. It is a smooth and robust parameter whose variations are related to generalized changes in the rupture process. Drop of the seismic noise amplitude was concomitant with the emergence of spectral modes – that are compatible with high-order eigenmodes of the unstable rock slice – during the later stages of its instability. Seismic noise analysis, especially high frequency base noise level analysis may complement that of inverse displacement velocity in early-warning approaches when strong displacement fluctuations occur.

Seismic activity has increased in the Kanto region, Japan, following the 2011 M9.0 Tohoku earthquake. We here reassess this increase up to June 2014, to show that normal, Omori-like relaxation characterizes the activity on crustal faults as well as on the Philippine Sea plate, but not on the deeper Pacific plate. There repeating earthquakes display a twofold rate of occurrence (still ongoing in June 2014) as compared to the pre-Tohoku rate, suggesting enhanced creep. We compute the Coulomb stress changes on the upper locked portion of the Philippine Sea plate, which last ruptured in 1923. We find that this fault was little affected by either the coseismic, the postseismic, the accelerated creep, or the 2011 Boso silent slip event.

International audience; Monitoring of large basaltic volcanoes, such as Piton de la Fournaise (La Réunion Island, France), has revealed preeruptive accelerations in surface displacements and seismicity rate over a period of between 1 h and several weeks before magma reaches the surface. Such eruptions are attributed to ruptures of pressurized magma reservoirs. Elastic models used to describe surface deformation would assume that accelerations in surface deformation are due to increases in reservoir pressure. This assumption requires changes in magma or pressure conditions at the base of the magma feeding system that are unrealistic over the observed timescale. Another possible cause for these accelerations is magma pressure in the reservoir weakening the volcanic edifice. In the present study, we modeled such weakening by progressive damage to an initially elastic edifice. We used an incremental damage model, with seismicity as a damage variable with daily increments. Elastic moduli decrease linearly with each damage increment. Applied to an initially elastic edifice with constant pressure at the base of the system, this damage model reproduces surface displacement accelerations quite well when damage is sufficient. Process dynamics is controlled by the damage parameter, taken as the ratio between the incremental rupture surface and the surface to be ruptured. In this case, edifice strength and magma reservoir pressure decrease with decreasing elastic moduli, whereas surface displacement accelerates. We discuss the consequences of pressure decreases in magma reservoirs.

[1] From 2003 to 2007, eruptive activity at Piton de la Fournaise was shown to follow cycles, comprising many summit/proximal eruptions and finishing by a distal eruption. GPS measurements evidenced striking asymmetric deformation between its western and eastern flanks. Horizontal displacements recorded during interdistal periods showed a characteristic amplitude at the top of the eastern flank. Displacements recorded at the base of the summit cone showed a bimodal distribution, with low amplitudes during interdistal periods and large ones during distal eruptions. To account for displacement asymmetry, characteristic amplitude, and large flank displacement, we modeled the volcanic edifice using a Drucker-Prager elastoplastic rheology. Friction angles of 15° and >30° were needed to model the displacements respectively during distal eruptions and interdistal periods; this change shows that strain weakening occurred during distal events. Large plastic displacement that occurred in the eastern flank during distal eruptions relaxed the horizontal elastic stress accumulated during interdistal periods; it triggered summit deflation, horizontal magma transfer, and distal flank eruption and reset the eruptive cycle. Our elastoplastic models also show that simple source geometries may induce large eastern flank displacements that would be explained by a complex geometry in a linear elastic edifice. Magma supply is often thought to control volcano's eruptive activity, with surface deformation reflecting changes in magma supply rate, the volcano's response being linear. Our results bring some evidences that on Piton de la Fournaise time-space discretization of magma transfer may be the result of the edifice's nonlinear response, rather than changes in magma supply.

SUMMARY To understand the mechanisms of intraplate earthquakes, well-documented seismological and geodynamical studies are needed, to provide information for adequate mechanical modelling. This paper describes a detailed analysis of the earthquake sequence associated with the Rambervillers (France) M ∼ 5.4 earthquake (2003 February 22), which occurred in the western Vosges massif (France), some tens of kilometres to the north of the sites of the Epinal (1973–1974) and Remiremont (1984–1985) earthquake sequences. We computed the location of the mainshock and focal mechanism, together with the double-difference locations of 419 aftershocks of the subsequent earthquake sequence, which included 195 well-recorded similar events (earthquakes with similar waveforms). We combined pP phases recorded by remote dense seismic networks with waveform modelling, to accurately determine the mainshock depth (12.5 ± 1.5 km). Computations of focal mechanisms from regional waveform inversions showed a normal fault plane with a N315 ◦ ± 10 ◦ strike and a 45 ◦ ± 15 ◦ dip. A detailed space–time analysis allowed two earthquake sequences to be identified: a classic mainshock–aftershock sequence and a secondary sequence that began 250 days after the mainshock. More events were recorded during the secondary sequence than during the main sequence. Very few similar events were recorded during the first 2 days after the mainshock, although data were recorded continuously. Double-difference locations from traveltime differences show that these immediate, non-similar aftershocks occurred in a 4 km × 2k m subhorizontal area at a depth of about 12 km. Similar events were far more numerous during the secondary earthquake sequence. Double-difference relocation of these similar events revealed a N315 ◦ striking, 65 ◦ dipping fault plane that is compatible with the mainshock source mechanism. During this later sequence, seismicity oscillated over a vertical range of 2–3 km around a mean depth of approximately 12 km. This observation strongly suggests fluid migration. Normal faulting, subhorizontal faulting and fluid transfer at about 12 km depth can be interpreted in terms of the crust’s response (mainshock and post-seismic relaxation) to flexural stresses induced by the alpine compression. This relaxation involves weakening and collapse of the crust, and fluid transfer.

International audience; Resurgence, defined as the post-collapse long-term uplift of a caldera floor, is commonly attributed to a renewed rise of magma. The Yenkahe dome (Vanuatu) exhibits a common morphology - elongated with a graben on top - among resurgent domes, and is also one of the most active structures of the kind. In this study, we performed a joint analysis based on analogue and finite element numerical models to (1) constrain the width and depth of the long-term deformation intrusive source of the Yenkahe dome and (2) discuss the close association between the Yenkahe dome and the active Yasur cone. We consider the resurgent deformation at the surface to be driven by the uplift of a magma reservoir roof in depth. As the edifice deformation response depends on the medium and the source properties, the mechanical behavior of the upper crust and the nature of the source are modeled using two very different sets of hypotheses. Analogue modeling uses silicone putty, an analogue for a large viscous magma body, intruding a sand-plaster mixture reproducing a Mohr-Coulomb behavior for the crust. Numerical models consider the vertical displacement of a rigid indenter, allowing the conservation of a flat-shaped roof, into an elastoplastic material. Numerical and analogue models show different resurgent dome structures at depth but similar dome and graben morphologies in the surface. Inverse faults - or equivalent shearing zones - delimiting the dome provide an explanation for the confined nature of resurgent doming and the persistent volcanic activity on the dome border represented by the Yasur volcano. Analogue and numerical models together provide an estimation range of 1-1.8 km for the intrusive deformation source depth, and 1.3-2 km for its width. The proposed association between the Yenkahe dome and the Yasur volcano is compatible with such a shallow depth of the magma reservoir, and argues for a discontinuous resurgence process. (C) 2015 Elsevier B.V. All rights reserved.

SUMMARY We present here a detailed analysis of a seismic data set recorded by a dense seismological network installed over 6 months in the southeast of France. This experiment was set-up at the boundary between the Ligurian basin and the southern subalpine thrust belt (the Nice arc), which is a complex tectonic region that undergoes low to moderate seismicity. We recorded more than 500 microearthquakes, among which 348 occurred exactly in the centre of the network during a very active seismic sequence that lasted mainly over 2 months. We performed an absolute location of all of the events and calculated the magnitudes. Then we applied a cross-correlation technique to gather similar events and to relocate relatively few of them. This method revealed a very clear alignment of 19 events in a direction N120 ◦ oblique to the N20 ◦ general trend of seismicity. Focal mechanisms were determined for the four largest events and composite solutions for 32 smaller ones. Both the alignments of the earthquakes and the focal solutions revealed that two oblique segments of the fault were activated during the crisis. The main segment (8 km long) that was oriented NNE with a left-lateral strike slip movement is called the Blausasc fault. Taking into account the tectonic evolution and the relationships between surface structures and the distribution of earthquakes, and through a paleaoreconstruction of the tectonic evolution, we propose that the Blausasc fault is the hidden root of the Peille-Laghet fault, which has a mapped length of at least 15 km. The smaller segment (0.6 km long) that was activated during the crisis could be interpreted as an antithetic Riedel fracture. The active Blausasc fault is located in a densely populated zone, at only 10 km from the crowded cities of Monaco and Nice. It is thus particularly interesting to analyse it for hazard assessment. In the last section, we present a simulation that is aimed at predicting what the ground motion in the city of Nice would be like if an earthquake of magnitude 5.7 occurs on this fault. For this, we used the recordings of the largest event of the seismic sequence (Ml = 3.2) and an empirical Green’s function summation scheme to simulate the ground motion at two stations situated in urban environments. The values obtained show that especially on soft soil sites, the effects of such an earthquake would be considerable in the city of Nice.

A real-time procedure has been set up to select and relocate relatively similar events recorded by the U.S. Geological Survey Hawaiian Volcano Observatory (HVO, Hawaii) seismic network at the Observatory. It allows the relocation of such similar events with an uncertainty whose order of magnitude is 50 m horizontally and 100 m vertically. As a first step, a systematic exploration of the volcano to find similar events within the seismicity recorded between years 1988 and 2000 has been undertaken and allows events to be gathered into large multiplets. A generalized relative relocation of these similar events has been performed, and a database has been built from the result of this relocation. Incoming earthquakes are routinely compared to this database and relocated, and the database is updated. Preliminary results are presented and discussed. Manuscript received 21 May 2001.

The current tectonics of north Iceland are characterised by max M 6–7 earthquakes in the transform zone between the northern rift zone and the Kobleinsey Ridge. The last rifting period (1975–1984) was associated with an opening of up to 9 m along the Krafla fissure swarm. This period was followed by the disappearance of microseismicity along the eastern part of Husavik-Flatey Fault, the main structure of the transform zone. A GPS network was occupied in 1995 and again in 1997 to study present-day deformation on both sides of the on-land part of HFF and around its junction with the northern rift zone. The main deformational features observed are: (1) in Tjornes, the HFF is locked above the ductile/brittle boundary below which ductile creep is occurring, and (2) aseismic opening along the Theystareykir and Krafla fissure swarms and along a N 120°E superficial fault that marks the southern boundary of Tjornes.

Crustal faults that produce most of their slip aseismically typically generate large numbers of small earthquakes. These events have generally been interpreted as coming from localized patches of the fault that undergo unstable (stick–slip) sliding, surrounded by larger regions of stable sliding (creep). In published catalogues the microearthquakes often appear to be distributed over large portions of the fault surface. By accurately locating large numbers of microearthquakes from faults of different orientations in California and Hawaii, we show here that instead the locations define highly concentrated streaks that are characteristically aligned in the direction of fault slip. The underlying cause of this structural organization of the fault surface remains to be determined.

In January 1983, a dike intrusion/fissure eruption generated a swarm of 375 magnitude 1 to 3 earthquakes along a 16-km segment of Kilauea's Middle East Rift Zone. We searched the Hawaiian Volcano Observatory catalog for multiplets of similar events from this region from 1980 through 1985 and obtained precise relative locations by waveform cross correlation. Over 150 of the intrusion earthquakes could be grouped into 14 multiplets of five or more events with sufficient similarity for accurate relocation. Some multiplets were active for only a few minutes during the downrift migration phase of the seismic swarm, consistent with generation near the propagating dike tip, while others were active for several days. The two multiplets nearest the origin of the seismic swarm include events from the preceding days and months. Most multiplets span only 50 to 100 m following relocation, are located at about 3 to 4 km depth, and appear to deepen downrift. The catalog depths of those earthquakes in multiplets and those not in multiplets are similar, suggesting that most of the recorded seismicity may have come from a very limited depth interval despite the fact that the dike breached the surface. By analogy with a mechanical model used to explain a similar clustering of background seismicity in the Upper East Rift in 1991, we infer that the earthquakes are generated in regions of high stress concentration immediately above Kilauea's deforming deep rift body. This conclusion is consistent with the depth of the top of the deep rift body inferred from geodetic data and with numerical calculations suggesting that a significant ambient differential stress is required for dikes to produce earthquakes larger than magnitude 1.

We study the S wave coda of microearthquake multiplets (earthquakes showing similar seismograms) recorded on the Kilauea volcano, and we analyze the direction of emission for wavelets composing the coda. Our data set consists of 71 small magnitude, well-relocated, clustered events that occurred in an area of about 1 km2 in the southern flank of Kilauea, recorded at hypocentral distances of 10 to 20 km. For each station, we apply a cross-spectral moving-window technique to each pair of multiplet records to estimate the travel time delays due to source location offsets. These delays are computed for consecutive windows, starting at the P wave arrival time and ending at 4 times the S wave travel time. They are next used to estimate the slowness vector for each window along the seismograms. Our analysis shows (1) that scattered waves are emitted anisotropically along some preferred directions as late as 4 times the S wave travel time, and (2) that for some stations, the azimuthal plane rotates during the seismogram with a clear travel time delay signature. Our analysis shows that travel time delay variation in the coda can be purely due to geometrical propagation effects. This conclusion is of special interest when trying to use travel time delays to infer temporal changes in the crust. Our results do not support a coda diffracted isotropically in the crust, in the magnitude range and at the hypocentral distances we investigated.

[1] The direction of propagation is an important factor that affects the pattern of ground motion generated by an earthquake. Characterizing factors favoring a potential rupture propagation direction is thus an important task. Here we analyze the earthquake directivity of repeating earthquake sequences located on the San Andreas fault near Parkfield, California. All earthquakes of a sequence have very similar waveforms and have overlapping surface ruptures. We show that subtle variations of the transfer function between earthquakes of a common sequence can be interpreted as a change of apparent rupture duration. Relative apparent rupture durations are computed for all pairs of events at all available stations and for each sequence. We invert these measurements to obtain an estimation of the apparent rupture duration for each individual event of the sequence relative to a reference event. Variation of apparent rupture duration with azimuth attests for the rupture directivity. We show that the majority of analyzed microearthquakes presents a rupture in the south-east direction. We also show that, on a given repeating sequence, most earthquakes tend to show the same rupture direction.

The precise relocation of micro-earthquakes using waveform cross-correlation is an efficient tool for defining the spatial characteristics of seismogenic structures in volcanic areas. To be applied and to provide meaningful results, the technique requires, however, the presence of sufficient amounts of similar events. We examined about 870 earthquakes recorded at the Campi Flegrei (CF) caldera by a temporary digital network during the end of the 1982-1984 bradiseismic crisis. The classification of these earthquakes indicates a surprising low amount of similar events, especially according to the clustered aspect of the seismicity. The relocation technique, allows only to identify 3 elongated patterns that may be interpreted as small faults which are dipping inward toward the center of the caldera. To determine up to which point this low similarity rate is unusual, we examined the rates for several data sets coming from various volcanic areas: Hawaii, Mount Pinatubo (Philippines) and Piton de la Fournaise (La Reunion Island). The results show for all areas, percentages of similar events significantly higher than for CF and indicate that the low rate observed there is not just a matter of low detection threshold or network characteristics. At CF, most of the seismicity appears to be diffused and generated along small fractures rather than along well defined fault structures. The low similarity rate is in good agreement with a seismicity generated in a fractured medium by overpressure in the hydrothermal system and low deviatoric local stress. (C) 2010 Elsevier B.V. All rights reserved.

13p.; International audience; We study changes in effective stress (normal stress minus pore pressure) that occurred in the French Alps during the 2003-2004 Ubaye earthquake swarm. Two complementary data sets are used. First, a set of 974 relocated events allows us to finely characterize the shape of the seismogenic area and the spatial migration of seismicity during the crisis. Relocations are performed by a double-difference algorithm. We compute differences in travel times at stations both from absolute picking times and from cross-correlation delays of multiplets. The resulting catalog reveals a swarm alignment along a single planar structure striking N130°E and dipping 80°W. This relocated activity displays migration properties consistent with a triggering by a diffusive fluid overpressure front. This observation argues in favor of a deep-seated fluid circulation responsible for a significant part of the seismic activity in Ubaye. Second, we analyze time series of earthquake detections at a single seismological station located just above the swarm. This time series forms a dense chronicle of +16,000 events. We use it to estimate the history of effective stress changes during this sequence. For this purpose we model the rate of events by a stochastic epidemic-type aftershock sequence model with a nonstationary background seismic rate λ0(t). This background rate is estimated in discrete time windows. Window lengths are determined optimally according to a new change-point method on the basis of the interevent times distribution. We propose that background events are triggered directly by a transient fluid circulation at depth. Then, using rate-and-state constitutive friction laws, we estimate changes in effective stress for the observed rate of background events. We assume that changes in effective stress occurred under constant shear stressing rate conditions. We finally obtain a maximum change in effective stress close to −8 MPa, which corresponds to a maximum fluid overpressure of about 8 MPa under constant normal stress conditions. This estimate is in good agreement with values obtained from numerical modeling of fluid flow at depth, or with direct measurements reported from fluid injection experiments.

SUMMARY Accurate estimates of spectral ratios of two doublets (i.e. similar seismic events) occurring along the San Andreas Fault, in the Hollister region have been computed using a cross-spectral method. Both events of each doublet occurred within a 24 hr interval. Although both waveforms of a doublet are remarkably similar, computed spectral ratios exhibit strong, but steady variations with frequency in the first arrivals. the attenuation process can explain neither the sign nor the order of magnitude of these variations. On the contrary, they are closely related to the source mechanism: we show that the slope computed from the linear and coherent part of the spectral ratio is an estimation of the variation in the apparent rupture duration between the two events. One doublet exhibits an azimuthal distribution of the spectral-ratio variations that is interpreted as the result of a change in the position of the hypocentres, relative to the rupture area. For this doublet, location of hypocentres varies only from 10 to 30 per cent of the source length: this points out that small changes in rupture kinematics can lead to significant variations in the spectral ratio of earthquake doublets, i.e. in the amplitude of each event. For the other doublet, the variations are related to a change in source length. We also have computed spectral ratios all along the seismograms, using a moving window technique. When an azimuthal distribution of the spectral ratio is observed in the first arrivals, we notice a decrease of the spectral ratio all along the seismograms. the doublet with strong, but non-azimuthal variations of the spectral ratio in the first arrivals, does not indicate such a decrease of the spectral ratio along the seismogram. This implies that the variations of the coda decay are related not only to temporal or spatial changes of attenuation but also to source parameters. These variations have the same order of magnitude as those provoked by a temporal change in coda Q of 10 per cent. They can occult time changes of spectral ratio, for instance in case the attenuation in the crust would vary before a strong earthquake.

International audience; Piton de la Fournaise is a highly active basaltic volcano located on La Réunion island. With the aim of gaining information on its 3D internal structure, we carried out a 3D tomographic inversion based on P-wave first arrival times for earthquakes recorded by the local seismic monitoring network of the Observatoire Volcanologique du Piton de la Fournaise (OVPF).We used data recorded between 1996 and 1999 during intereruptive periods as well as during the pre-eruptive swarms which preceded the eruptions in March 1998, July and September 1999. Volcano-tectonic activity below the volcano is mostly located at shallow depth, above sea level and below the central cone. However, the seismic crisis which preceded the March 9, 1998 eruption included a large number of events at greater depth providing an unprecedented data set which sheds light on the deep structure of the volcano. The tomographic technique which we used is based on an accurate finitedifference travel-time computation and a simultaneous probabilistic inversion of both velocity models and earthquake locations. The inversion is carried out using an improved technique which allows to well constrain the inversion parameters. This processing provides high quality stable tomographic images. The obtained P-wave velocity model confirms the presence of a high-velocity plug above sea level, under the summit craters. This anomaly is interpreted as corresponding to an intrusive, solidified dyke-and-sill complex with little fluid magma storage. The high number of hypocenters in the zone above sea level accounts for massive rock fracturing when magma rises toward the surface. The shallow high-velocity plug is surrounded by a lowvelocity ring known as being fractured and vesicular lavas or scorias located on volcano flanks. Two low-velocity anomalies are found below the summit caldera. The most superficial spreads from 1 km to 0 a.s.l.. The second is located below 1 km b.s.l.. These volumes may correspond to magma storage systems. They are separated by a relatively high-velocity volume at sea level with a strong velocity gradient reaching 0.6 km/s per km

International audience; Hawaiian volcanoes are formed by the eruption of large quantities of basaltic magma related to hot-spot activity below the Pacific Plate1,2. Despite the apparent simplicity of the parent process—emission of magma onto the oceanic crust—the resulting edifices display some topographic complexity3–5. Certain features, such as rift zones and large flank slides, are common to all Hawaiian volcanoes, indicating similarities in their genesis; however, the underlying mechanism controlling this process remains unknown6,7. Here we use seismological investigations and finite element mechanical modelling to show that the load exerted bylarge Hawaiian volcanoes can be sufficient to rupture the oceanic crust. This intense deformation, combined with the accelerated subsidence of the oceanic crust and the weakness of the volcanic edifice/oceanic crust interface, may control the surface morphology of Hawaiian volcanoes, especially the existence of their giant flank instabilities8–10. Further studies are needed to determine whether such processes occur in other active intraplate volcanoes.

Doublets or multiplets are earthquakes with nearly identical waveforms. First observed on volcanoes, doublets are found in tectonic environments. Doublets can be relocated relatively with a precision of a few meters. Very good doublets separated by a large time lapse are essential for detecting slow temporal variations of crustal properties. We present basic techniques for selecting and processing doublets. In a seismic database, coherency between all pairs of seismograms is computed and high coherency pairs are candidate doublets. Time delays between waveforms are measured by cross-correlation techniques; a precision of 1 ms is common for good pairs sampled at 100 Hz. Several techniques can relocate one event relatively to the other and P and S delay residuals are obtained. Clock precision remains critical when searching for a few millisecond anomalies. Delays in the coda are analyzed by a cross-spectral moving window (CSMW) or a cross-correlation moving window (CCMW) analysis. Time delay measured in the coda shows variations even when the time elapsed between the two events of a doublet is extremely short. These variations are due to hypocenter separation and to changes in the waves which form the early coda. When seismic velocity is changing homogeneously in the propagation medium, the delay of the coda is proportional to lapse time. Thus, the slope α of the delay in the coda is a very precise (up to 10−4) measurement of the change in S-wave velocity ΔVS/VS, α = −ΔVS/VS. Relative changes of delay on the horizontal components can detect temporal variations in S-wave splitting and anisotropy. Temporal changes in coda Q may be reflected in the coda amplitude ratio measured in several frequency bands. However, minor changes in sources induce variations in early coda (the coda that just follows the S wave) amplitude ratios, comparable to those due to attenuation changes. Therefore, the interpretation of coda amplitude ratios in terms of coda Q changes should be undertaken in the late coda only, using a statistical approach. Good doublets are seldom, so we present a technique that creates “virtual doublets” from the correlation of seismic noise long sequences. Temporal variations of physical properties in surface layers are recovered by a CCMW analysis of these “virtual doublets.” This is an interesting method for measuring strain variations preceding volcanic eruptions. Many other applications should blossom in the near future. At last, we present teleseismic doublets which are a tool for measuring the rate of rotation of the inner core of the Earth. The goal of this chapter is to show that doublet processing is elementary but that the detection of temporal variations of velocity or attenuation remains quite difficult. Excellent doublets should be selected to study temporal variations and such very good natural doublets are few in most seismic regions.

International audience; Double-difference tomographic methods use directly accurate time delays computed between similar signals. Such methods are designed to image very heterogeneous media, such as volcanoes or fault zones. In seismological applications, similar signals are recorded at a given station from earthquakes sharing similar and close-by sources. In seismic exploration experiments, similar signals are often recorded at neighbouring receivers. After a brief presentation of the tomographic algorithm used, a seismological application is summarized. The potential and limits of double-difference tomographic methods are explored using various numerical experiments. They show that two effects are competing in double-difference tomography: (i) the degradation of the stability of the inversion due to the geometrical proximity of the rays used in the differentiation and (ii) the decrease in modelling error, which allows improving the stability of the inversion and using smaller quantities of a priori information when data are sufficiently accurate. The best resolution is obtained for an optimal value of the inter-source or inter-receiver distance. For optimal values of these distances and a priori information, tomography using traveltime differences provides significantly better resolved results than using traveltimes.

International audience; Seismic velocity parameters in limited, but heterogeneous volumes can be inferred using a double-difference tomographic algorithm, but to obtain meaningful results accuracy must be maintained at every step of the computation. MONTEILLER et al. (2005) have devised a double-difference tomographic algorithm that takes full advantage of the accuracy of cross-spectral time-delays of large correlated event sets. This algorithm performs an accurate computation of theoretical travel-time delays in heterogeneous media and applies a suitable inversion scheme based on optimization theory. When applied to Kilauea Volcano, in Hawaii, the double-difference tomography approach shows significant and coherent changes to the velocity model in the well-resolved volumes beneath the Kilauea caldera and the upper east rift. In this paper, we first compare the results obtained using MONTEILLER et al.'s algorithm with those obtained using the classic travel-time tomographic approach. Then, we evaluated the effect of using data series of different accuracies, such as handpicked arrival-time differences (``picking differences''), on the results produced by double-difference tomographic algorithms. We show that picking differences have a non-Gaussian probability density function (pdf). Using a hyperbolic secant pdf instead of a Gaussian pdf allows improvement of the double-difference tomographic result when using picking difference data. We completed our study by investigating the use of spatially discontinuous time-delay data.

International audience; Improving our understanding of crustal processes requires a better knowledge of the geometry and the position of geological bodies. In this study we have designed a method based upon double-difference relocation and tomography to image, as accurately as possible, a heterogeneous medium containing seismogenic objects. Our approach consisted not only of incorporating double difference in tomography but also partly in revisiting tomographic schemes for choosing accurate and stable numerical strategies, adapted to the use of cross-spectral time delays. We used a finite difference solution to the eikonal equation for travel time computation and a Tarantola-Valette approach for both the classical and double-difference three-dimensional tomographic inversion to find accurate earthquake locations and seismic velocity estimates. We estimated efficiently the square root of the inverse model's covariance matrix in the case of a Gaussian correlation function. It allows the use of correlation length and a priori model variance criteria to determine the optimal solution. Double-difference relocation of similar earthquakes is performed in the optimal velocity model, making absolute and relative locations less biased by the velocity model. Double-difference tomography is achieved by using high-accuracy time delay measurements. These algorithms have been applied to earthquake data recorded in the vicinity of Kilauea and Mauna Loa volcanoes for imaging the volcanic structures. Stable and detailed velocity models are obtained: the regional tomography unambiguously highlights the structure of the island of Hawaii and the double-difference tomography shows a detailed image of the southern Kilauea caldera–upper east rift zone magmatic complex.

[1] The 15 June 1991 climactic eruption of Mount Pinatubo (Philippines) was followed by intense seismicity that remained at a high level for several months. We located 10,839 events recorded between 1 July and mid-December 1991. In contrast to the preeruptive seismicity which was focused in two groups below the summit area, posteruptive events were widely distributed below and around the volcano. The classification of the events indicates the presence of several large multiplets, and the application of relative relocation techniques to the similar events by calculating high-precision delays between traces outlines a number of clear seismogenic structures. We used different methods to confirm the validity of our results; these tests indicate that reliable features can be detected with a small monitoring network. While the main cluster of activity can be attributed to an intrusive process starting from below the 15 June crater, the volcanic origin of the seismic activity in the other areas is more difficult to establish. Away from the summit, relocations define streaks or planes which are oriented predominantly southwest-northeast, with in several cases the presence of northwest-southeast conjugate structures. Most of the composite focal mechanisms that we could determine indicate predominantly strike-slip, right-lateral faulting. Our results indicate that most of the seismicity that occurred after the 15 June eruption is related to the east-west regional compressional stress field related to the subduction. We suggest that the regional stress field induces seismicity along new or preexisting faults in the medium surrounding the volcano where the stress field was locally disturbed by the volcanic eruption.

International audience; A new three-dimensional delay traveltime tomography is performed to image the intermediate structure of the western Gulf of Corinth. A large data set, collected in 1991 during a two-month passive tomographic experiment, has been reanalysed for the reconstruction of detailed Vp and Vs images. An improved tomography method, based on an accurate traveltime computation, is applied to invert simultaneously delayed P and S first-arrival traveltimes for both velocity and hypocentre parameters. We perform different synthetic tests to analyse the sensitivity of tomography results to the model parametrization and to the starting 1-D model selection. The analysis of the retrieved Vp and Vs models as well as deduced Vp/Vs and Vp·Vs images allows us to interpret and delineate the distribution of lithological variation, porosity/crack content and fluid saturation in the upper 9–11 km of the crust beneath the gulf. The tomographic models image a rather complex crustal structure, which is characterized by a vertical change in both velocity features and seismicity distribution. We identify a shallower zone of the crust (0–5 km depth), in which velocity distributions seem to be controlled by the still active N–S extensional regime and a deeper zone (7–11 km depth), which matches the seismogenic zone. The correlation between this latter and a specific unit of the Hellenic mountain structure (the Pyllite–Quartzite series) allows us to suggest a possible explanation for seismicity concentration in a narrow band at 7–9 km depth. Finally, the occurrence of clusters showing low-angle normal fault mechanisms in areas characterized by high Vp/Vs values indicates a possible role of fluids in triggering brittle creep along the identified low-angle normal faults.

[1] We investigated the microseismicity recorded in an active volcano to infer information concerning the volcano structure and long-term dynamics, by using relative relocations and focal mechanisms of microearthquakes. There were 32,000 earthquakes of the Mauna Loa and Kilauea volcanoes recorded by more than eight stations of the Hawaiian Volcano Observatory seismic network between 1988 and 1999. We studied 17,000 of these events and relocated more than 70%, with an accuracy ranging from 10 to 500 m. About 75% of these relocated events are located in the vicinity of subhorizontal decollement planes, at a depth of 8–11 km. However, the striking features revealed by these relocation results are steep southeast dipping fault planes working as reverse faults, clearly located below the decollement plane and which intersect it. If this decollement plane coincides with the pre-Mauna Loa seafloor, as hypothesized by numerous authors, such reverse faults rupture the pre-Mauna Loa oceanic crust. The weight of the volcano and pressure in the magma storage system are possible causes of these ruptures, fully compatible with the local stress tensor computed by Gillard et al. [1996]. Reverse faults are suspected of producing scarps revealed by kilometer-long horizontal slip-perpendicular lineations along the decollement surface and therefore large-scale roughness, asperities, and normal stress variations. These are capable of generating stick-slip, large–magnitude earthquakes, the spatial microseismic pattern observed in the south flank of Kilauea volcano, and Hilina-type instabilities. Rupture intersecting the decollement surface, causing its large-scale roughness, may be an important parameter controlling the growth of Hawaiian volcanoes.

[1] We present methods for improving the location of long-period (LP) events, deep and shallow, recorded below Kilauea Volcano by the permanent seismic network. LP events might be of particular interest to understanding eruptive processes as their source mechanism is assumed to directly involve fluid transport. However, it is usually difficult or impossible to locate their source using traditional arrival time methods because of emergent wave arrivals. At Kilauea, similar LP waveform signatures suggest the existence of LP multiplets. The waveform similarity suggests spatially close sources, while catalog solutions using arrival time estimates are widely scattered beneath Kilauea's summit caldera. In order to improve estimates of absolute LP location, we use the distribution of seismic amplitudes corrected for station site effects. The decay of the amplitude as a function of hypocentral distance is used for inferring LP location. In a second stage, we use the similarity of the events to calculate their relative positions. The analysis of the entire LP seismicity recorded between January 1997 and December 1999 suggests that a very large part of the LP event population, both deep and shallow, is generated by a small number of compact sources. Deep events are systematically composed of a weak high-frequency onset followed by a low-frequency wave train. Aligning the low-frequency wave trains does not lead to aligning the onsets indicating the two parts of the signal are dissociated. This observation favors an interpretation in terms of triggering and resonance of a magmatic conduit. Instead of defining fault planes, the precise relocation of similar LP events, based on the alignment of the high-energy low-frequency wave trains, defines limited size volumes.

Dense microearthquake swarms occur in the upper south flank of Kilauea, providing multiplets composed of hundreds of events. The similarity of their waveforms and the quality of the data have been sufficient to provide accurate relative relocations of their hypocenters. A simple and efficient method has been developed which allowed the relative relocation of more than 250 events with an average precision of about 50 m horizontally and 75 m vertically. Relocation of these events greatly improves the definition of the seismic image of the fault that generates them. Indeed, relative relocations define a plane dipping about 6° northward, although corresponding absolute locations are widely dispersed in the swarm. A composite focal mechanism, built from events providing a correct spatial sampling of the multiplet, also gives a well-constrained northward dip of about 5° to the near-horizontal plane. This technique thus collapses the clouds of hypocenters of single-event locations to a plane coinciding with the slip plane revealed by previous focal mechanism studies. We cannot conclude that all south flank earthquakes collapse to a single plane. There may locally be several planes, perhaps with different dips and depths throughout the south flank volume. The 6° northward-dipping plane we found is too steep to represent the overall flexure of the oceanic crust under the load of the island of Hawaii. This plane is probably an important feature that characterizes the basal slip layer below the upper south flank of Kilauea volcano. Differences in seismicity rate and surface deformations between the upper and lower south flank could be related to the geometry of this deep fault plane. The present work illustrates how high precision relative relocations of similar events in dense swarms, combined with the analysis of geodetic measurements, can help to describe deep fault plane geometry. Systematic selection and extensive relative relocation of similar earthquakes could be attempted in other well-instrumented, highly seismic areas to provide reliable basic information, especially useful for understanding of earthquake generation processes.

Basket geothermal heat exchanger buried at shallow depth and connected to heat pumps can be used to heat and/or cool buildings. Heat transfers and performance related to geothermal basket are complex due to heterogeneous petrophysical properties of the soil, freezing of ground and surface environmental conditions e.g. air temperature variations. To study heat transfers, a geophysical monitoring of electrical conductivity and induced polarization has been developed using three experiments at different scales. First, a petrophysical model was designed from laboratory measurements. This model describes at the pore scale, the variation of electrical conductivity and chargeability according to the soil temperature and frost. This model was applied to an in-situ experiment to monitor the growth of an ice-core. We have also designed an analogic experiment at small scale and under controlled conditions in order to study the heat transfers and the effect of the soil freezing according to different environmental conditions. This study shows the importance of lateral flow and the influence of freezing during system operation. The petrophysical model was tested and validated with several datasets and geophysical monitoring was able to detect the formation and melting of the ice core., Les corbeilles géothermiques sont des échangeurs thermiques hélicoïdaux implantés dans le proche sous-sol qui couplés à une pompe à chaleur, permettent de chauffer et/ou de rafraichir les bâtiments. Comprendre les transferts thermiques liés à ces systèmes et étudier leurs performances requiert de prendre en compte les propriétés pétrophysiques hétérogènes du sol, les phénomènes de changement de phase et les conditions environnementales, dont les variations de température de l’air et la pluviométrie. Dans ce but un monitoring géophysique en conductivité électrique et en polarisation provoquée a été développé à l’aide de trois expériences à différentes échelles. Tout d’abord, un modèle pétrophysique a été élaboré à partir d’une série d’expériences en laboratoire. Ce modèle décrit la variation de la conductivité électrique et de la chargeabilité en fonction de la température et du gel du sol à l’échelle porale. Ce modèle a été appliqué à une expérience in-situ pour suivre la formation d’un noyau de glace. Nous avons aussi développé une expérience analogique à échelle réduite et en conditions contrôlée pour mieux comprendre les transferts de chaleur au sein du sol et étudier l’impact du gel dans différentes conditions environnementales. Ces études ont montré l’importance des apports thermiques latéraux et l’impact du changement de phase lors du prélèvement de chaleur par la corbeille. Le modèle pétrophysique a été validé sur plusieurs jeux de données et les résultats du monitoring géophysique permettent de détecter la formation et la fonte d’un noyau glace.

On volcanoes accelerations of the cumulated number of earthquakes and sometimes of the surface displacement occur in the hours, days and weeks preceding the eruptions. Explaining an acceleration of surface displacement using an elastic model for the volcanic edifice implies an acceleration of the pressure increase in the magma reservoir and in the mantle, at the base of the magma conduit, at short timescales, that is not realistic. Another direction for research consists in considering that an eruption is due to the rupture of the magma reservoir, and that this rupture is not instantaneous - it can be preceded by progressive damage of the edifice, during which low-magnitude earthquakes occur. These ruptures provoke the progressive weakening of the edifice elastic moduli. In this work we have inverstigated the effect of this weakening on the surface displacements, on the reservoir pressure and on the magma flow. We have first searched for a damage law allowing fitting the measured surface displacements by using the cumulated number of recorded earthquakes. The law allowing this fit is a power-law of the incremental damage parameter, with the cumulated number of earthquakes at the exponent. This incremental damage parameter is the ratio between the characteristic incremental rupture length and the length to be ruptured for the eruption.This model allows us to explain weak and strong rates and accelerations of the surface displacement, with the value of the damage parameter. Bifurcation that occurs between a stable state of the edifice and an instable state leading to an eruption is also explained. The weakening of the edifice induces simultaneously a diminution of the reservoir overpressure and an acceleration of the surface displacement, overpressure decreasing more slowly than the elastic moduli, due to the magma feeding of the reservoir.This type of model can explain a wide range of pre-eruptive and eruptive behaviours on volcanoes. It allows the fitting of surface displacement accelerations recorded at the GPS summit stations of Piton de la Fournaise volcano for the March 30th, 2007 eruption, and explains the steady increase of the surface displacement recorded at the summit of Grimsvötn volcano (Iceland), between 2005 and 2011. It may be used for understanding the complex eruptive behaviours, comprising multiple episodes, as in the case of the October-November 2010 eruption of the Merapi volcano (Indonesia). The value of the characteristic time of the feeding system controls the time delay between the maximum of damage (end of the volcano-tectonic seismic crisis) and the maximum of the surface displacement or magma flow. A preliminary analysis shows that the ratio between the weakening characteristic times of the edifice strength and reservoir pressure participate to the control of the pre-eruptive and eruptive process dynamics.Damage of the volcanic edifice can not continue indefinitely, otherwise long-term magma storage would be impossible in the volcanic edifice. The volcanic edifice therefore necessarily experiments periods of strenghtening. A first approach for studying this strengthening relying on a criterion based on the volume relative variation of the edifice has been investigated. Results show that this approach is not sufficient to explain the necessary increase of the elastic moduli. Chemical strenghtening processes without volume changes should have a fundamental role in the existence of eruptive cycles. For improving our knowledge of these processes, we need a further geophysical observable, independent on the surface displacements - we propose to use shear-wave anisotropy in the volcanic edifice.We finally show that the surface dynamics imposed by strength changes of the edifice can influence the dynamics of the deep processes, and consider the possibility of a retroaction of the edifice strength changes on the deep source processes., Sur les volcans on enregistre fréquemment des accélérations du nombre cumulé de séismes et parfois du déplacement de la surface dans les heures, les jours ou les semaines précédant les éruptions. Expliquer une accélération du déplacement de surface par l'utilisation de modèles élastiques linéaires pour les édifices implique d'introduire une accélération de la pressurisation du réservoir et de la base du conduit magmatique avec des temps caractéristiques courts, ce qui est peu réaliste. Une autre voie de recherche consiste à considérer qu'une éruption est due à la rupture du réservoir magmatique, et que cette rupture n'est pas instantanée - elle peut être précédée par une phase d'endommagement progressif de l'édifice, pendant laquelle de petites ruptures ont lieu dans l'édifice. Ces ruptures provoquent l'affaiblissement des modules élastiques de l'édifice. Nous avons examiné l'effet que peut avoir l'affaiblissement des modules élastiques sur le déplacement de surface, ainsi que sur la pression dans le réservoir et le débit de magma entrant. Pour cela nous avons d'abord recherché une loi d'endommagement qui permette d'ajuster les déplacements de surface enregistrés en utilisant le nombre cumulé de séismes. La loi trouvée est une loi puissance d'un paramètre d'endommagement incrémental, qui est le rapport entre la longueur de rupture incrémentale caractéristique et la longueur à rompre pour obtenir une éruption. Cette loi fait intervenir, à l'exposant, le nombre cumulé de séismes.Ce modèle permet d'expliquer des vitesses et des accélérations du déplacement aussi bien faibles que fortes, selon la valeur prise par le paramètre d'endommagement, ainsi que la bifurcation entre un état stable de l'édifice, et un état instable qui mène à l'éruption. L'affaiblissement de l'édifice induit une diminution de la surpression et une accélération du déplacement de surface, la surpression diminuant plus lentement que les modules élastiques du fait de l'approvisionnement du réservoir en magma.Ce modèle peut expliquer une large gamme de comportements pré-éruptifs et éruptifs dans les volcans. Il permet d'ajuster simplement aussi bien les accélérations du déplacement de surface enregistré par les stations GPS sommitales du Piton de la Fournaise pour l'éruption du 30/03/2007, que les déplacements croissants de façon stationnaire du volcan Grimsvötn (Islande), entre 2005 et 2011. Il peut aussi être utilisé pour comprendre les comportements éruptifs plus complexes, comme dans le cas de l'éruption de 2010 du Mérapi (Indonésie). La valeur de la constante de temps du système d'approvisionnement contrôle le décalage temporel entre le maximum de l'endommagement (fin de la crise sismique volcano-tectonique) et le maximum de la déformation ou du flux de magma et donc la dynamique du processus. Une analyse préliminaire montre que le rapport entre les constantes de temps d'affaiblissement de la résistance et de la pression est un facteur de contrôle de la dynamique du processus pré-éruptif et éruptif.L'endommagement d'un édifice volcanique ne peut continuer indéfiniment, cela interdirait le stockage de magma dans l'édifice à long terme. L'édifice volcanique connait donc nécessairement des phases de consolidation. Une première approche d'étude de la consolidation reposant sur l'utilisation d'un critère basé sur les variations relatives de volume a été investiguée. Les résultats montrent cependant qu'une telle approche n'est pas suffisante pour expliquer la nécessaire augmentation des modules élastiques, et que les processus de consolidation chimique sans variation de volume doivent avoir une importance fondamentale dans l'existence de cycles éruptifs. Pour étudier ces processus, il faut disposer d'une observable géophysique indépendante du déplacement de surface - nous proposons d'utiliser l'anisotropie de propagation des ondes de cisaillement dans l'édifice volcanique. Ces variations de résistance de l'édifice peuvent contrôler la pression en profondeur.

In volcanic hazard assessment, it is crucial to be able to detect, as early as possible, shallow magma emplacement as well as magma migration towards the Earth's surface. Then it becomes important to be able to foresee the future eruption location, its amplitude and its potential impact on the volcanic edifice stability. In this manuscript, I present my research concerning magma migration spatio-temporal tracking by satellite radar remote sensing as well as development of models taking into account the mechanical coupling between a fluid, the magma, and a solid, the encasing medium. These models aim at providing a way to jointly interpret various observations, among which deformation data, in order to obtain predictive information of magma progression towards the surface., Dans une perspective de gestion de l'aléa volcanique, il est essentiel d'être capable de détecter, suffisamment tôt, un apport superficiel de magma et sa migration éventuelle vers la surface. Il est ensuite important de pouvoir prévoir la localisation de l'éruption à venir, son amplitude et le risque de déstabilisation de l'édifice volcanique associé. Dans ce manuscrit, je mets en perspective mes travaux de recherche qui concernent, à la fois, le suivi spatio-temporel de la migration du magma par mesure radar satellitaire et le développement de modèles mécaniques dynamiques prenant en compte le couplage entre un fluide, le magma, et un solide, le milieu encaissant. Ces modèles ont pour objectif de permettre l'interprétation conjointe de différents observables, dont les mesures de déformation, en vue d'obtenir une information prédictive sur la progression du magma en profondeur.

Rock instabilities are one of the most common natural risks encountered in high alpine relief and, despite this fact, they still remain largely uncharted in Earth Sciences. Given their characteristics, these instabilities need to be investigated and closely monitored in order to improve our knowledge of the stability of rock masses. The main purpose of this thesis was therefore to develop a multi-sensor instrumental device to form a sound basis for the continuous acquisition of the relevant parameters for monitoring the stability of rock masses. This self-contained device is capable of continuously measuring and storing high frequency (up to one kHz) and low frequency data generated by different types of sensor over long periods (several months). Moreover, it can be linked up to a remote-monitoring system for supervising the site and checking the operation of the instrumentation. The measurement station developed can also carry out pre-processing of the data collected and includes monitoring algorithms combining different criteria for an initial warning management procedure. A meshed-network transmission system and client-server oriented remote supervision system have also been developed within the context of this project. In March 2008, this surveillance system was used to monitor fifteen instability sites in France. Within the context of this thesis, particular attention was focused on the variation in deformations affecting an unstable vertical rock slice on the Chamousset site (south-east Vercors, France), up to about one week prior to its collapse and fall. Observations of various parameters (deformation, weather conditions, monitoring of seismic activity) taken over a period of about three months, revealed the following : a clear but moderate acceleration in horizontal displacement of the top of the rock slice, closely related to alternating freezing-thawing episodes; numerous seismic events, due to the progressive failure of the rock slice, some of which had a very low prevailing frequency (5 to 10Hz), probably dominated by slice resonance; individualised spectral modes at frequencies greater than 10-15 Hz on the rock slice and which, as a general rule, tended to increase with time; occurrence of seismic crises which did not degenerate towards the instability. In view of its dynamics, the experimental work carried out on site has enhanced our knowledge of the processes involved and, in particular, revealed new parameters that would be worthwhile including in the study of this type of instability. Based on these findings, it was possible to specify the instrumental characteristics to be achieved., Les instabilités rocheuses constituent l'un des risques naturels le plus souvent rencontre dans les grands reliefs alpins et restent malgré tout un domaine encore mal connu des Sciences de la Terre. Compte tenu de leurs caractéristiques, l'amélioration de nos connaissances dans les domaine de la stabilité des massifs rocheux passe par l'instrumentation et la surveillance de ces instabilités. L'objet de cette thèse a ainsi tout d'abord consisté à mettre au point un dispositif instrumental multi-capteurs, constituant une base solide pour l'acquisition en continu de paramètres pertinents pour les suivi temporel de la stabilité des massifs rocheux. Ce dispositif permet en effet de mesurer et stocker en continu et de façon autonome pendant de longues durées (plusieurs mois) des données hautes fréquences (jusqu'à 1 kHz) et lentes issues de différents types de capteurs. Par ailleurs, il peut être télémétré de façon à suivre à distance l'évolution du site surveille ainsi que les fonctionnement de l'instrumentation. La station de mesure mise au point permet également de faire un pré-traitement des données et a été agrémentée d'algorithmes de surveillance permettant de combiner différents critères pour une première gestion d'alerte. Le travail réalisé a en outre permis de mettre au point un système de transmission par réseau maillé et un système de supervision distant orienté client-serveur. En juin 2008, ce système de surveillance permet de suivre une quinzaine de sites instables en France. Dans les cadre de cette thèse, nous nous sommes plus particulièrement intéressés à l'évolution des déformations subies par une écaille rocheuse instable, sur les site de Chamousset (Sud-Est Vercors), jusqu'à environ une semaine avant son éboulement. L'observation de différents paramètres (déformation, météorologie, écoute sismique) et leur suivi temporel pendant environ 3 mois ont ainsi permis de mettre en évidence : une accélération nette mais modérée du déplacement horizontal du sommet de l'écaille, fortement liée aux épisodes de gel-dégel; de très nombreux nano-séismes, dus à la rupture progressive de l'écaille et dont certains présentent une fréquence dominante très basse (5 a 10 Hz), probablement dominée par la résonance de l'écaille;des modes spectraux s'individualisant à des fréquences supérieures à 10-15 Hz sur l'écaille et ayant tendance à augmenter en moyenne avec les temps; l'occurrence de crises sismiques qui ne dégénèrent pas vers l'instabilité. Compte tenu de sa dynamique, l'expérimentation réalisée sur ce site a permis de progresser dans la connaissance des processus mis en jeu et notamment de mettre en évidence de nouveaux paramètres dignes d'intérêt pour l'étude de ce type d'instabilités et donc de préciser les spécifications des caractéristiques instrumentales à atteindre.

This thesis is a study into the structure and functioning of hydrothermal systems. We focused on two areas, Waimangu, New Zealand, and the Phlegrean Fields - Solfatara, Italy, that we observed with several geophysical methods. The first system was chosen owing to its strong dynamics, and because it seems to be purely hydrothermal, without any magmatic influence; the second one allowed the observation and the geophysical characterization of a shallow hydrothermal plume. Our study first aimed to recognize the large scale structure of the Waimangu hydrothermal system, and to characterize its main surface expressions, namely hot lakes, geysers, and fumaroles. For this purpose, we related the self-potential and the electrical resistivity methods to measurements of temperature and CO2 flux. We then concentrated on three small-scale laboratories : Iodine vent, Inferno Crater Lake, and the Old Geyser Site. these areas are different from each other in size and also in their water chemistry, which is a key parameter in the sensitivity of electrical methods. This range enabled us to implement acoustic and electrical imaging techniques on singular systems that have their own dynamics. At Iodine Pool, the implementation of recent imaging techniques from underwater acoustics allowed us to locate hydrothermal noise sources with high accuracy. The self-potential monitoring of an intermittent vent showed cyclic fluctuations that could be explained by a change in the hydraulic head. The joint application of acoustic and electrical imaging at the Old Geyser Site showed how these two techniques complement each other in defining the location of a hydrothermal structure that matches both acoustic and electrical sources. A third step consisted in the study of the geo-electrical signature of fluid movements that relate to the Inferno cyclic activity. Electrical resistivity monitoring highlighted that the observed lake fluctuations are characterized by notable electrical resistivity changes; these variations could be due to phase changes occurring at depth, and are in good agreement with analog modellings. The experiments that have been made at the Solfatara allowed us to image the hydrothermal plume with electrical methods (3-D resistivity tomography), acoustic and thermal measurements. We present the results from both electrical resistivity tomography and temperature data that allowed us to establish the boundaries of the hydrothermal plume., Ce travail de thèse porte sur l'étude de la structure et du fonctionnement des systèmes hydrothermaux volcaniques. Nous nous sommes attachés à l'étude géophysique multi-méthodes de deux systèmes hydrothermaux : Waimangu, en Nouvelle-Zélande, et la Solfatara, en Italie. Le premier en raison de sa dynamique, et parce qu'il apparait comme une zone laboratoire purement hydrothermale, sans influence magmatique; le second pour l'observation et la caractérisation géophysique d'un panache hydrothermal accessible à la surface. Dans un premier temps, nous nous sommes intéressés à la reconnaissance de la structure de Waimangu à grande échelle, et à la caractérisation des principales composantes du système (lacs de cratères, zones fumeroliennes et geysers). Cette étude a permis d'associer les méthodes de potentiel spontané et de résistivité électrique avec des mesures ponctuelles de température et de flux de CO2. Nous avons ensuite ciblé notre étude sur des laboratoires à petite-échelle présents sur Waimangu : Iodine vent, Inferno Crater Lake et le site du Old Geyser. Ces systèmes différent les uns des autres par leur taille, mais aussi par leurs caractéristiques physico-chimiques, qui jouent un rôle determinant dans la sensibilité des méthodes géo-électriques. Sur le site de Iodine Pool, l'application de techniques récentes d'imagerie acoustique sous-marine a permis la localisation précise des sources de bruit hydrothermal. Le suivi par polarisation spontanée d'un évent intermittent a montré une cyclicité dans les signaux, qui peut être expliquée par une variation de hauteur piézométrique. L'application conjointe de l'imagerie acoustique et de l'imagerie électrique sur le site du Old Geyser a mis en évidence la complémentarité des deux techniques dans la localisation d'une structure hydrothermale qui correspond à la fois à une source acoustique et électrique. Dans un troisième temps, nous avons étudié la signature géo-électrique des mouvements de fluides qui accompagnent l'activité cyclique du Lac Inferno. Le suivi temporel de résistivité électrique montre que les variations du lac et les mouvements de fluide observés sont caractérisés par des variations importantes de résistivité dans le sous-sol qui pourraient correspondre à des changements de phase du fluide en profondeur, en accord avec des modélisations analogiques. La campagne réalisée à la Solfatare a permis d'imager le panache hydrothermal par les méthodes électriques (tomographie de résistivité en trois dimensions), acoustiques et thermiques. Nous présentons les résultats obtenus avec la tomographie de résistivité électrique et les mesures de température, qui ont permis de déterminer avec précision les limites spatiales du panache hydrothermal.

Improving our understanding of crustal processes requires a better knowledge of the geometry and the position of geological bodies. In this study we have designed a method based upon double-difference relocation and tomography to image, as accurately as possible, a heterogeneous medium containing seismogenic objects. Our approach bas been not only to introduce double-difference in tomography, but rather to partly revisit tomographic schemes for chosing accurate and adapted numerical strategies. We used a finite-difference solution to the Eikonal equation (Podvin and Lecomte, 1991) for travel-time computation and a Tarantola-Valette approach for bath the double- difference and the 3D tomographic inversion to find accurate earthquake locations and seismic velocity estimates. Though an efficient estimation of the squared root of the inverse model covariance matrix in the case of the gaussian correlation function, we use correlation length and a priori model variance criteria to determine the optimal solution. Double-difference relocation of similar earthquakes is performed in the optimal velocity model, making absolute and relative locations less biased by the velocity model. Double difference tomography is achieved by using high-accuracy time delay measurements. These algorithms have been applied to earthquake data recorded in the vicinity of Kilauea and Mauna Loa volcanoes for imaging the volcanic structures. They allow to find stable and detailed velocity models : regional tomography allows to retrieve unambiguously the structure of Hawaii island, and double difference tomography shows a detailed image of the southern Kilauea Caldera -East Rift Zone region magmatic complex., Connaître mieux l'intérieur de la Terre nécessite d'améliorer les méthodes d'investigation géophysique. Cette thèse s'est interessée à une classe importante de ces méthodes : la tomographie sismique. La tomographie sismique classique utilise des temps d'arrivée d'ondes sismiques, dont la précision dépend du pas d'échantillonnage et du pointé de l'onde. Lorsque les trains d'onde sont similaires, c'est-à-dire lorsque les sources et/ ou les récepteurs sont proches, on peut calculer, avec une précision inférieure au pas d'échantillonnage, le décalage temporel entre les trains d'ondes. Cette thèse a eu pour objectif l'utilisation de telles données en tomographie sismique. Utiliser pleinement la précision de ces données a nécessité une revisite complète des algorithmes de tomographie, tant du point de vue du problème direct que du point de vue du problème inverse. Une solution robuste et précise a été trouvée au problème direct (calcul du temps de propagation des ondes dans un milieu hétérogène). Une attention particulière a été portée au problème inverse : une approche Tarantola-Valette a été utilisée, de façon à trouver le modèle de norme minimale qui ajuste les données. La minimisation de cette norme est obtenue par l'introduction d 'une fonction de corrélation des paramètres du modèle, dont la longueur et l'amplitude sont ajustées en suivant une procédure d'optimisation. De cette façon, toute fluctuation du modèle est contrainte par les données. L'algorithme mis au point a été utilisé avec des données provenant de séismes du volcan Kilauea (Hawaii) enregistrés par le réseau local USGS-HVO (50 stations). Il a permis de retrouver le modèle de vitesse à deux échelles : (1) une échelle régionale, où l'on retrouve le système magmatique profond, sous les calderas et les rifts du Mauna Loa et du Kilauea; le modèle trouvé permet d'expliquer les déformations mises en évidence dans la croûte océanique, la distribution confinée de la sismicité et les déformations intenses du flanc Sud du Kilauea; (2) une échelle locale, où l'on met en évidence un conduit magmatique sub-vertical sous la caldera et le rift Est du Kilauea, indétectable avec d'autres données. Cette méthode, précise, permet de choisir un modèle unique et stable à l'aide un critère d'optimisation objectif.