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From seismic interferometry, we investigate the strain sensitivity to seismic velocity variations related to a seismic swarm activity that occurred in 2013 along the Alto Tiberina low angle normal fault. We compute daily auto‐correlation functions of ambient noise recorded at seismic stations located in the vicinity of the fault over the course of 10 years. Using the stretching technique, we compute daily velocity variations smoothed over a period of 100 days with a time lapse approach. Through the application of an optimization procedure based on synthetic modeling, we separate the non‐tectonic, thermoelastic and rain induced velocity variations, from the tectonic components. Consequently, we unravel a significant velocity drop of 0.033% coinciding with the swarm occurring at seismogenic depth (3–5 km). Additionally, the time evolution of the velocity changes shows a direct relationship with the strain rate rather than the strain indicating a non‐linear behavior of the crust induced by aseismic slip. The deduced strain sensitivity, exhibiting an order of magnitude comparable to that observed within volcanic settings, confirms this non‐linear behavior and suggests the presence of pressurized fluids at depth. Plain Language Summary: The driving mechanism of seismic swarms is still unclear. Seismic swarms are bursts of small earthquakes without a clear triggering mainshock. Understanding the physical processes driving these sequences is crucial for earthquake hazard assessment. To that scope, we focus in this study on a small seismic swarm that occurred in 2013 in northern Apennines, Italy. We monitor the properties of crustal rocks using continuous recordings of ambient seismic noise recorded at seismic stations located in the vicinity of the Alto Tiberina low angle normal fault over the course of 10 years. With this approach, we demonstrate the ability to retrieve quantitative information about the fault's state and to indirectly infer that the triggering of the swarm is mainly related to the presence of fluids in the crust. Key Points: Velocity variations from seismic coda wave interferometrySeparation of thermoelastic and hydrological stress induced velocity variation from tectonic changesSeismic swarm triggered by aseismic slip favored by the presence of fluids [ABSTRACT FROM AUTHOR]

This study reports the combination of remote sensing and ground geophysical techniques to locate an abandoned and hidden municipal solid waste landfill located in a fluvial plain in the French Western Alps. Following earthworks and further floods that eroded into the river bank, wastes made of a mixture of plastic, metal and soil/earth, were uncovered and some of them flowed into the river. The existence of an abandoned landfill, several decades-old, was known, but the knowledge of its exact location was forgotten. Historic aerial photographs back to 1948 allowed delineation in space and time of the location of a platform that was used for landfill operations between around 1973 and 1983. A LiDAR DEM acquired in 2012 allowed was used to locate topographic depressions 0.1 to 0.4 m in depth, notably inside the platform. These depressions are interpreted as resulting from differential compaction originating from the presence of compressible wastes. Geophysical mapping techniques (magnetic and electromagnetic) confirmed the presence of anomalies inside the identified platform. Geophysical imaging techniques (ground-penetrating radar, electrical resistivity tomography) provided a quantitative evaluation of the width and depth of the individual pits. The combination of the different techniques allowed for estimating the first-order volume of waste. The methodology adopted in this work is applicable to detect landfills exhibiting differential compaction and physical contrasts. [ABSTRACT FROM AUTHOR]

Among mountainous permafrost landforms, rock glaciers are mostly abundant in periglacial areas, as tongue-shaped heterogeneous bodies. Passive seismic monitoring systems have the potential to provide continuous recordings sensitive to hydro-mechanical parameters of the subsurface. Two active rock glaciers located in the Alps (Gugla, Switzerland, and Laurichard, France) have been instrumented with seismic networks. Here, we analyze the spectral content of ambient noise to study the modal sensitivity of rock glaciers, which is directly linked to the system's elastic properties. For both sites, we succeed in tracking and monitoring resonance frequencies of specific vibrating modes of the rock glaciers over several years. These frequencies show a seasonal pattern characterized by higher frequencies at the end of winters and lower frequencies in warm periods. We interpret these variations as the effect of the seasonal freeze–thawing cycle on elastic properties of the medium. To assess this assumption, we model both rock glaciers in summer, using seismic velocities constrained by active seismic acquisitions, while bedrock depth is constrained by ground-penetrating radar surveys. The variations in elastic properties occurring in winter due to freezing were taken into account thanks to a three-phase Biot–Gassmann poroelastic model, where the rock glacier is considered a mixture of a solid porous matrix and pores filled by water or ice. Assuming rock glaciers to be vibrating structures, we numerically compute the modal response of such mechanical models by a finite-element method. The resulting modeled resonance frequencies fit well the measured ones over seasons, reinforcing the validity of our poroelastic approach. This seismic monitoring allows then a better understanding of the location, intensity and timing of freeze–thawing cycles affecting rock glaciers. [ABSTRACT FROM AUTHOR]

We developed a high-resolution MIKE SHE/MIKE 11 model of a 231.3 ha headwater catchment in the granitic uplands of the French Massif Central to estimate the contribution of groundwater upwelling to the water balance of the Dauges mire, an acidic valley mire of international importance for nature conservation. We estimated that groundwater upwelling from the underlying weathered granite formations – mostly an approximately 55 m deep fissured zone – provides 27.1 % of total long-term inflows to the mire. This contribution increases to 37.2 % in September when total inflows are small. Overland boundary inflow accounts for an average of 40.2 % of total inflows. However, most of this originates from groundwater seepage through mineral soils along the mire margins or in small non-channelised valleys upslope of the mire. A sensitivity analysis showed that model performance in terms of the simulation of mire groundwater levels was most sensitive to parameters describing the mineral soils and weathered granite formations rather than the overlying peat layer. Variation partitioning demonstrated that groundwater upwelling was the most important factor driving simulated monthly groundwater table depth within the mire. Sustained groundwater upwelling maintains the mire water table close to or at ground level for most of the year. As a result, precipitation and overland boundary inflows quickly leave the wetland as saturation-excess runoff. There was close agreement between the observed distribution of mire habitats and areas where the simulated long-term groundwater seepage rate was larger than zero in September. Our results demonstrate that, contrary to the assumed small contribution of groundwater to the hydrology of hard-rock regions, groundwater upwelling from underlying weathered formations can be a quantitatively important and functionally critical element of the water balance of valley mires in granitic headwater catchments. These results have important legal and management implications. [ABSTRACT FROM AUTHOR]

A novel traveltime tomographic approach is applied to anisotropic media, limited to 2D geometry at present. A general anisotropic Eikonal solver based on a discontinuous Galerkin method is combined with an efficient adjoint formulation for multiparameter least‐squares inversion. This new approach is tested considering synthetic crosshole ground‐penetrating radar data. The configuration of the ground‐penetrating radar survey is inspired by a real experiment done on layered carbonate media disturbed by the presence of a deep gallery, which induces a localized high‐electromagnetic contrast. This made it possible to define a well‐adapted general workflow in this context. We notably show that, under the elliptical anisotropic assumption, the parametrization based on vertical and horizontal velocities provides less biased results than those obtained by considering the vertical velocity and the relevant Thomsen parameter ε. The initial vertical and horizontal velocity models are identical and built from an isotropic inversion. The presence of the high‐contrast gallery generates a weak diffraction pattern, which is taken into account in our tomography approach. It also creates potential artefacts due to the model discretization, which are mitigated by a model regularization term within the definition of the misfit function. This general workflow is then applied to the real experiment dataset. The vertical and horizontal velocity images provide similar structures as those previously obtained by isotropic full waveform inversion, complemented by an image of a rather weak elliptical anisotropy. [ABSTRACT FROM AUTHOR]

A network of seismometers has been installed on the Gugla rock glacier since October 2015 to estimate seismic velocity changes and detect microseismicity. These two processes are related to mechanical and structural variations occurring within the rock glacier. Seismic monitoring thus allows a better understanding of the dynamics of rock glaciers throughout the year. We observed seasonal variations in seismic wave velocity and microseismic activity over the 3 yr of the study. In the first part of our analysis, we used ambient noise correlations to compute daily changes of surface wave velocity. In winter, seismic wave velocities were higher, probably due to refreezing of the permafrost active layer and cooling of the uppermost permafrost layers, leading to increased overall rigidity of the medium. This assumption was verified using a seismic model of wave propagation that estimates the depth of P - and S -wave velocity changes from 0 down to 10 m. During melting periods, both a sudden velocity decrease and a decorrelation of the seismic responses were observed. These effects can probably be explained by the increased water content of the active layer. In the second part of our study, we focused on detecting microseismic signals generated in and around the rock glacier. This seismic activity (microquakes and rockfalls) also exhibits seasonal variations, with a maximum in spring and summer, which correlates principally with an exacerbated post-winter erosional phase of the front and a faster rock glacier displacement rate. In addition, we observed short bursts of microseismicity, both during snowfall and during rapid melting periods, probably due to pore pressure increase. [ABSTRACT FROM AUTHOR]

The contribution of groundwater to the hydrology of hard rock regions has long been assumed to be small. This is being progressively challenged and conceptual hydrological models of headwater wetlands in these regions may need to be revised. We developed a high-resolution MIKE SHE / MIKE 11 model of a 231.3 ha headwater catchment in the granitic uplands of the French Massif Central to estimate the contribution of groundwater upwelling to the water balance of the Dauges mire, an acidic valley mire of international importance for nature conservation. We estimated that groundwater upwelling from the underlying granite weathering formations - mostly an approximately 55m deep fissured zone - provides 27.1 % of total long-term inflows to the mire. This contribution increases to 37.2 % in September when total inflows are small. Overland boundary inflow accounts for an average of 40.2 % of total inflows. However most of this originates from groundwater seepage through mineral soils along the mire margins or in small unchannelized valleys upslope of the mire. A sensitivity analysis showed that model performance in terms of the simulation of mire groundwater levels was most sensitive to parameters describing the mineral soils and granite weathered formations rather than the overlying peat layer. Variation partitioning showed that groundwater upwelling was the most important factor driving simulated monthly groundwater table depth within the mire. Sustained groundwater upwelling maintains the mire water table close to or at ground level for most of the year. As a result, precipitation and overland boundary inflows are mostly evacuated as saturation-excess runoff. There was close agreement between the observed distribution of mire habitats and areas where the simulated long-term groundwater seepage rate was larger than zero in September. Groundwater upwelling from the underlying weathered formations can be a quantitatively important and functionally critical element of the water balance of valley mires in granitic headwater catchments. These results have important legal and management implications. [ABSTRACT FROM AUTHOR]

Field data are needed for a better understanding of sea ice decline in the context of climate change. The rapid technological and methodological advances of the last decade have led to a reconsideration of seismic methods in this matter. In particular, passive seismology has filled an important gap by removing the need to use active sources. We present a seismic experiment where an array of 247 geophones was deployed on sea ice, in the Van Mijen fjord near Sveagruva (Svalbard). The array is a mix of 1C and 3C stations with sampling frequencies of 500 and 1000 Hz. They recorded continuously the ambient seismic field in sea ice between 28 February and 26 March 2019. Data also include active acquisitions on 1 and 26 March with a radar antenna, a shaker unit, impulsive sources, and artificial sources of seismic noise. This data set is of unprecedented quality regarding sea ice seismic monitoring, as it also includes thousands of microseismic events recorded each day. By combining passive seismology approaches with specific array processing methods, we demonstrate that the multimodal dispersion curves of sea ice can be calculated without an active source and then used to infer sea ice properties. We calculated an ice thickness, Young's modulus, and Poisson's ratio with values h=54±3 cm, E=3.9±0.15 GPa, and ν=0.34±0.02 on 1 March, and h=58±3 cm, E=4.4±0.15 GPa, and ν=0.32±0.02 on 5 March. These values are consistent with in situ field measurements and observations. Key Points: An array of 247 geophones was deployed on sea ice for 27 days during winter 2019Broadband dispersion of several guided wave modes propagating in sea ice were extracted from passive seismic dataSea ice thickness and elastic properties were estimated from guided wave dispersion [ABSTRACT FROM AUTHOR]

The Mw 7.8 2016 Kaikoura earthquake ruptured the Kekerengu-Needle fault resulting in the loading of its eastern continuation, the Wairarapa fault. Since the most recent earthquake on Wairarapa occurred in 1855 and is one of the strongest continental earthquakes ever observed, it is critical to assess the seismic potential of the Wairarapa fault, which might be prone to break. Using Lidar data, we examine its bare-earth morphology and reveal ~650 mostly undiscovered offset geomorphic markers. Using a code we developed in earlier work, we automatically measure the lateral and vertical offsets of these markers providing more than 7000 well constrained measurements. The data document the lateral and vertical slip profiles of the 1855 earthquake for the first time and show its total slip reached ~20 m at surface. Modeling the entire offset dataset reveals 7 prior earthquakes ruptured the entire fault, each similarly producing 16.9 ± 1.4 m dextral slip and ~0.6 m vertical slip at surface in the same central bend zone of the fault. Thus, the Wairarapa fault repeatedly produced giant earthquakes and is likely able to produce a similarly strong forthcoming event. The extreme large size of the Wairarapa earthquakes questions our understanding of earthquake physics. [ABSTRACT FROM AUTHOR]

The METAFORET experiment was designed to demonstrate that complex wave physics phenomena classically observed at the meso- and microscales in acoustics and in optics also apply at the geophysics scale. In particular, the experiment shows that a dense forest of trees can behave as a locally resonant metamaterial for seismic surface waves. The dense arrangement of trees anchored into the ground creates anomalous dispersion curves for surface waves, which highlight a large frequency band-gap around one resonant frequency of the trees, at ∼45 Hz. This demonstration is carried out through the deployment of a dense seismic array of ∼1000 autonomous geophones providing seismic recordings under vibrating source excitation at the transition between an open field and a forest. Additional geophysical equipment was deployed (e.g. ground-penetrating radar, velocimeters on trees) to provide essential complementary measurements. Insights and interpretations on the observed seismic wavefield, including the attenuation length, the intensity ratio between the field and the forest and the surface wave polarization, are validated with 2-D numerical simulations of trees over a layered half-space. [ABSTRACT FROM AUTHOR]

In this paper, a simple analysis is done to explain the observed increase of resonance frequency of City-Hall building in Grenoble (France), a 12-story reinforced concrete building. This period corresponds also to the observed variation of the resonance frequency of the Grenoble's sedimentary basin. The postulated hypothesis is that the frequency increase reflects the stiffness increase of the soil-structure system related to the cold period that hit Western Europe in 2012. To explore this hypothesis we have processed continuous recording during the early 2012 recorded at the roof level and at a close free-field accelerometric station. The variation of site effect is monitored by the horizontal-to-vertical spectral ratio of seismic noise, and the variation of apparent and system frequencies of the building by the random decrement technique. Apparent frequency is computed by deconvolution method between roof and basement. The maximum freezing penetration is 0.75 m and the horizontal relative motion stiffness of the foundation is strongly sensitive to the modification of the upper soil layer. The results suggest a variation (<1 %) larger than twice the standard deviation of the natural wandering of resonance frequency observed at City-Hall building for normal weather conditions, and question on the development of realistic models developed for the detection of damage and for the physical interpretation of such frequency variations observed in actual buildings. [ABSTRACT FROM AUTHOR]

Seismo-electromagnetic phenomena in porous media arise from seismic wave-induced fluid motion in the pore space, which perturbs the equilibrium of the electric double layer. This paper describes with details the original experimental apparatus built within the ultra-shielded chamber of the Low Noise Underground Laboratory of Rustrel (France). We measured seismo-magnetic conversions in moist sand using two induction magnetometers, and a pneumatic seismic source to generate the seismic wave propagation. We ensured to avoid the magnetometer vibrations, which could induce strong disturbances from induction origin. Interpretation of the data is improved by an analytical description of phase velocities for fast ( P f) and slow longitudinal modes, transverse mode ( S) as well as the extensional mode due to the cylindrical geometry of the sample. The purpose of this paper is to provide elements to measure correctly coseismic seismomagnetic fields and to specify their amplitude. The seismic arrivals recorded in the sample showing a 1200–1300 m s−1 velocity have been associated to P and extensional waves. The measured seismo-magnetic arrivals show a velocity of about 800 m s−1 close to the calculated phase velocity of S waves. Therefore, we show that the seismo-magnetic field is associated to the transverse part of the propagation, as theoretically predicted by Pride (1994) , but never measured up to now. Moreover, the combined experimental and analytical approaches lead us to the conclusion that the measured seismo-magnetic field is probably about 0.035 nT for a 1 m s−2 seismic source acceleration (0.1 g). [ABSTRACT FROM AUTHOR]

The H/V spectral ratio method based on seismic noise (HVSRN) was used in the Grenoble Basin (France), an Alpine valley characterized by a small apex ratio. The resonance frequencies obtained in the experiments were compared to the thickness of the sediments deduced from a microgravimetric survey and to the 1-D theoretical assessment of site responses. Given the abundance of data on the sediments and depth of the basin, the values of the theoretical resonance frequency f o can be determined quite accurately. However, it has been observed that the effects of basin geometry can disturb f o measurements using the HVSR method, in particular for a case like the Grenoble Basin, which has a small apex ratio (w/H) and strong suspected 2-D and/or 3-D effects. Interpretation of f o values in terms of bedrock depth gives rise to estimation errors of about 10% in certain cases, with the most significant errors (>50%) occurring on the edges of the basin, where subsurface layers are characterised by larger heterogeneities and where the basin topography is accentuated. This study suggests that great care must be taken when using the HVSRN method as an exploration tool, at least in valleys with a small apex ratio. [ABSTRACT FROM AUTHOR]

The electromagnetic fields that are generated as a spherical seismic wave (either P or S) traverses an interface separating two porous materials are numerically modeled both with and without the generation of Biot slow waves at the interface. In the case of an incident fast-P wave, the predicted electric-field amplitudes when slow waves are neglected can easily be off by as much as an order of magnitude. In the case of an incident S wave, the error is much smaller (typically on the order of 10% or less) because not much S-wave energy gets converted into slow waves. In neglecting the slow waves, only six plane waves (reflected and transmitted fast-P, S, and EM waves) are available with which to match the eight continuity conditions that hold at each interface. This overdetermined problem is solved by placing weights on the eight continuity conditions so that those conditions that are most important for obtaining the proper response are emphasized. It is demonstrated that when slow waves are neglected, it is best to also neglect the continuity of the Darcy flow and fluid pressure across an interface. The principal conclusion of this work is that to properly model the electromagnetic (EM) fields generated at an interface by an incident seismic wave, the full Biot theory that allows for generation of slow waves must be employed. © 2002 Acoustical Society of America. [ABSTRACT FROM AUTHOR]