Deciphering the Whisper of Volcanoes: Monitoring Velocity Changes at Kamchatka's Klyuchevskoy Group With Fluctuating Noise Fields

Volcanic inflation and deflation often precede eruptions and can lead to seismic velocity changes (dv/v $dv/v$) in the subsurface. Recently, interferometry on the coda of ambient noise‐cross‐correlation functions yielded encouraging results in detecting these changes at active volcanoes. Here, we analyze seismic data recorded at the Klyuchevskoy Volcanic Group in Kamchatka, Russia, between summer of 2015 and summer of 2016 to study signals related to volcanic activity. However, ubiquitous volcanic tremors introduce distortions in the noise wavefield that cause artifacts in the dv/v $dv/v$ estimates masking the impact of physical mechanisms. To avoid such instabilities, we propose a new technique called time‐segmented passive image interferometry. In this technique, we employ a hierarchical clustering algorithm to find periods in which the wavefield can be considered stationary. For these periods, we perform separate noise interferometry studies. To further increase the temporal resolution of our results, we use an AI‐driven approach to find stations with similar dv/v $dv/v$ responses and apply a spatial stack. The impacts of snow load and precipitation dominate the resulting dv/v $dv/v$ time series, as we demonstrate with the help of a simple model. In February 2016, we observe an abrupt velocity drop due to the M7.2 Zhupanov earthquake. Shortly after, we register a gradual velocity increase of about 0.3% at Bezymianny Volcano coinciding with surface deformation observed using remote sensing techniques. We suggest that the inflation of a shallow reservoir related to the beginning of Bezymianny's 2016/2017 eruptive cycle could have caused this local velocity increase and a decorrelation of the correlation function coda

Conduire le changement en bibliothèque

Comment conduire le changement de manière efficace dans le contexte très particulier introduit par les révolutions du numérique et des réseaux dans les bibliothèques ? Comment assurer le bon fonctionnement d’un équipement lorsque le rythme des changements à conduire s’accélère ? Ce volume apporte des éléments de réponses pratiques, tirées de réalisations concrètes et propose une réflexion plus générale sur les modalités du management en bibliothèque, centrée sur la notion émergente, dans les institutions publiques, d’organisation apprenante

Pour une géographie des plaisirs urbains

La revue Urbanités : présentation du support La revue Urbanités, c'est le projet de quatre doctorants de l'ENS de Lyon : ils sont géographes et ont pour horizon commun, malgré leur différente spécialité, la ville et l'urbain. Ce projet est né du constat qu'il n'y avait très peu de revues consacrées aux études urbaines. À travers un nouvel angle et un nouveau support, ils ont abordé ce projet éditorial avec enthousiasme et une démarche particulière : proposer des numéros thématiques avec des a..

Monitoring groundwater dynamics in a mountain ridge using seismic interferometry: Influence of topography, local subsurface structure and meteorological conditions

International audienceGroundwater storage monitoring is now one of the most promising application of seismic interferometry techniques. In steep mountain environments, where drilling wells is particularly challenging, the use of seismic stations to retrieve relative seismic velocity changes could fundamentally advance our understanding of groundwater dynamics. However, very few studies have looked at seismic velocity variations at the scale of a single steep topography unit. Here, we estimate velocity variations from six stations covering a distance of 3.5 km on a single mountain ridge in the county of Hualien, Taiwan. One station was placed at the top of a ridge (900m elevation), two at the mid-slope of the topography and two others at the bottom (200m elevation), near the river banks. The aim is twofold: Determining how homogenous these velocity changes are and understanding the possible impact of topography on groundwater variations in a mountainous setting. Results from auto-correlations and cross-correlations are compared with meteorological data and other geophysical analysis. We identify the average hydrological dynamics of the ridge unit and connect the residual velocity changes to local site characteristics and upstream weather conditions

Deciphering the Whisper of Volcanoes: Monitoring Velocity Changes at Kamchatka's Klyuchevskoy Group With Fluctuating Noise Fields

Volcanic inflation and deflation often precede eruptions and can lead to seismic velocity changes (dv/v $dv/v$) in the subsurface. Recently, interferometry on the coda of ambient noise‐cross‐correlation functions yielded encouraging results in detecting these changes at active volcanoes. Here, we analyze seismic data recorded at the Klyuchevskoy Volcanic Group in Kamchatka, Russia, between summer of 2015 and summer of 2016 to study signals related to volcanic activity. However, ubiquitous volcanic tremors introduce distortions in the noise wavefield that cause artifacts in the dv/v $dv/v$ estimates masking the impact of physical mechanisms. To avoid such instabilities, we propose a new technique called time‐segmented passive image interferometry. In this technique, we employ a hierarchical clustering algorithm to find periods in which the wavefield can be considered stationary. For these periods, we perform separate noise interferometry studies. To further increase the temporal resolution of our results, we use an AI‐driven approach to find stations with similar dv/v $dv/v$ responses and apply a spatial stack. The impacts of snow load and precipitation dominate the resulting dv/v $dv/v$ time series, as we demonstrate with the help of a simple model. In February 2016, we observe an abrupt velocity drop due to the M7.2 Zhupanov earthquake. Shortly after, we register a gradual velocity increase of about 0.3% at Bezymianny Volcano coinciding with surface deformation observed using remote sensing techniques. We suggest that the inflation of a shallow reservoir related to the beginning of Bezymianny's 2016/2017 eruptive cycle could have caused this local velocity increase and a decorrelation of the correlation function coda.Plain Language Summary: Before eruptions, volcanoes inflate due to the rising magma from below. Previous studies have found that these deformations can lead to small changes in the properties of the surrounding rock. We use passive image interferometry, a method that relies on the omnipresent background vibration of the Earth—mostly induced by the oceans, to measure these changes at the Klyuchevskoy Volcanic Group in Kamchatka, Russia. However, in Kamchatka, this background noise is masked and distorted by small earthquakes and tremors originating from the volcanoes themselves. We combine machine learning techniques with established monitoring methods to find times when these tremors remain similar. Afterward, we use data from these time periods in the conventional way to observe changes in the soil and the rock. Our results show that rain‐ and snowfall and the thickness of the snow cover exert the strongest influence on the properties of the rocks. Additionally, we found that a large magnitude 7.2 earthquake, which struck Kamchatka during our study, caused a slight weakening of the rocks due to microstructural damage. We register changes shortly before an eruption and suggest a connection to the beginning of an eruptive cycle in 2016.Key Points: Fluctuating noise conditions lead to distortions in noise interferometry studies, which we avoid with the help of machine learning. The seismic velocity on Kamchatka is affected by numerous mechanisms, amongst them environmental, tectonic, and volcanic events. We observe a velocity increase at Bezymianny during February 2016 and link it to the beginning of the eruptive cycle.German Research Foundationhttps://doi.org/10.14470/K47560642124https://doi.org/10.24381/cds.e2161bachttps://doi.org/10.5880/GFZ.2.4.2022.002https://doi.org/10.5281/zenodo.748193

Seismic velocity recovery following the 2015 Mw 7.8 Gorkha earthquake, Nepal: Towards a coupled vision of damage and hydrological-induced velocity variations

International audienceFollowing the passage of seismic waves, most geomaterials experience non-linear mesoscopic elasticity (NLME). This is described by a drop in elastic moduli that precedes a subsequent recovery of physical properties over a relaxation timescale. Thanks to the development of seismic interferometry techniques that allows for the continuous monitoring of relative seismic velocity changes δv in the subsurface, observations of NLME (δvNLME) in the field are now numerous. In parallel, a growing community uses seismic interferometry to monitor velocity changes induced by seasonal hydrological variations (δvhydro). Monitoring of these variations are often independently done and a linear superposition of both effects is mostly assumed when decomposing the observed δv signal (δv = δvNLME + δvhydro). However, transient hydrological behaviour following co-seismic ground shaking has been widely reported in boreholes measurements and streamflow, which suggests that δvhydro may be impacted by the transient variation of material properties caused by NLME. In this presentation, we attempt to characterize the relative seismic velocity variations δv retrieved from a small dense seismic array in Nepal that was deployed in the aftermath of the 2015 Mw 7.8 Gorkha earthquake and that is prone to highly variable hydrological conditions. We first investigated the effect of aftershocks in computing δv at a 10-minute resolution centered around significant ground shaking events. After correcting δv for NLME caused by the Gorkha earthquake and its subsequent aftershocks, we test whether the corresponding residuals are in agreement with the background hydrological behaviour which we inferred from a calibrated hydrological model. This is not the case and we find that transient hydrological properties improve the data description in the early phase after the mainshock. We report three distinct relaxation time scales that are relevant for the recovery of seismic velocity at our field site: 1. A long time scale activated by the main shock of the Gorkha earthquake (~1 year) 2. A relatively short timescale (1-3 days) that occurs after moderate aftershocks. 3. An intermediate timescale (4-6 months) during the 2015 monsoon season that corresponds to the recovery of the hydrological system. This timescale could correspond to an enhanced permeability caused by Gorkha ground shaking. Our study demonstrates the capability of seismic interferometry to monitor transient hydrological properties after earthquakes at a spatial scale that is not available with classical hydrological measurements. This investigation demands calibrated hydrological models and a framework in which the different forcing of δv are coupled

Supplementary Information files for Sediment flux-driven channel geometry adjustment of bedrock and mixed gravel‐bedrock rivers

Supplementary Information files for Sediment flux-driven channel geometry adjustment of bedrock and mixed gravel‐bedrock riversSediment supply (Qs) is often overlooked in modelling studies of landscape evolution, despite sediment playing a key role in the physical processes that drive erosion and sedimentation in river channels. Here, we show the direct impact of the supply of coarse-grained, hard, sediment on the geometry of bedrock channels from the Rangitikei river, New Zealand. Channels receiving a coarse bedload sediment supply are systematically (up to an order of magnitude) wider than channels with no bedload sediment input for a given discharge. We also present physical model experiments of a bedrock river channel with a fixed water discharge (1.5 l/min) under different Qs (between 0 and 20 g/l) that allow the quantification of the role of sediment in setting the width and slope of channels and the distribution of shear stress within channels. The addition of bedload sediment increases the width, slope, and width-to-depth ratio of the channels, and increasing sediment loads promote emerging complexity in channel morphology and shear stress distributions. Channels with low Qs are characterised by simple in-channel morphologies with a uniform distribution of shear stress within the channel while channels with high Qs are characterised by dynamic channels with multiple active threads and a non-uniform distribution of shear stress. We compare bedrock channel geometries from the Rangitikei and the experiments to alluvial channels and demonstrate that the behaviour is similar, with a transition from single thread and uniform channels to multiple threads occurring when bedload sediment is present. In the experimental bedrock channels, this threshold Qs is when the input sediment supply exceeds the transport capacity of the channel. Caution is required when using the channel geometry to reconstruct past environmental conditions or to invert for tectonic uplift rates, because multiple configurations of channel geometry can exist for a given discharge, solely due to input Qs.<br

Seismic Velocity Recovery in the Subsurface: Transient Damage and Groundwater Drainage Following the 2015 Gorkha Earthquake, Nepal

International audienceShallow earthquakes frequently disturb the hydrological and mechanical state of the subsurface, with consequences for hazard and water management. Transient post-seismic hydrological behavior has been widely reported, suggesting that the recovery of material properties (relaxation) following ground shaking may impact groundwater fluctuations. However, the monitoring of seismic velocity variations associated with earthquake damage and hydrological variations are often done assuming that both effects are independent. In a field site prone to highly variable hydrological conditions, we disentangle the different forcing of the relative seismic velocity variations δv retrieved from a small dense seismic array in Nepal in the aftermath of the 2015 Mw 7.8 Gorkha earthquake. We successfully model transient damage effects by introducing a universal relaxation function that contains a unique maximum relaxation timescale for the main shock and the aftershocks, independent of the ground shaking levels. Next, we remove the modeled velocity from the raw data and test whether the corresponding residuals agree with a background hydrological behavior we inferred from a previously calibrated groundwater model. The fitting of the δv data with this model is improved when we introduce transient hydrological properties in the phase immediately following the main shock. This transient behavior, interpreted as an enhanced permeability in the shallow subsurface, lasts for ∼6 months and is shorter than the damage relaxation (∼1 yr). Thus, we demonstrate the capability of seismic interferometry to deconvolve transient hydrological properties after earthquakes from non-linear mechanical recovery

Toward Using Seismic Interferometry to Quantify Landscape Mechanical Variations after Earthquakes

International audienceIn mountainous terrain, large earthquakes often cause widespread coseismic landsliding as well as hydrological and hydrogeological disturbances. A subsequent transient phase with high landslide rates has also been reported for several earthquakes. Separately, subsurface seismic velocities are frequently observed to drop coseismically and subsequently recover. Consistent with various laboratory work, we hypothesize that the seismic-velocity changes track coseismic damage and progressive recovery of landscape substrate, which modulate landslide hazard and hydrogeological processes, on timescales of months to years. To test this, we analyze the near-surface seismic-velocity variations, obtained with single-station high-frequency (0.5–4 Hz) passive image interferometry, in the epicentral zones of four shallow earthquakes, for which constraints on landslide susceptibility through time exist. In the case of the 1999 Chi-Chi earthquake, detailed landslide mapping allows us to accurately constrain an exponential recovery of landslide susceptibility with a relaxation timescale of about 1 yr, similar to the pattern of recovery of seismic velocities. The 2004 Niigata, 2008 Iwate, and 2015 Gorkha earthquakes have less-resolved constraints on landsliding, but, assuming an exponential recovery, we also find matching relaxation timescales, from ∼0.1 to ∼0.6 yr, for the landslide and seismic recoveries. These observations support our hypothesis and suggest that systematic monitoring of seismic velocities after large earthquakes may help constrain and manage the evolution of landslide hazard in epicentral areas. To achieve this goal, we end by discussing several ways to improve the link between seismic velocity and landscape mechanical properties, specifically by better constraining time-dependent near-surface strength and hydrogeological changes. Hillslopes displaying coseismic surface fissuring and displacement may be an important target for future geotechnical analysis and coupled to passive geophysical investigations

Amphimedonoic acid and psammaplysene E, novel brominated alkaloids from Amphimedon sp.

International audienceExamination of the CH2Cl2-MeOH (1:1) extract from the Madagascan sponge Amphimedon sp. highlighted two new brominated alkaloids, amphimedonoic acid (1) and psammaplysene E (2), along with the known 3,5-dibromo-4-methoxybenzoic acid (3). Their structures were elucidated by 1D and 2D NMR spectroscopy and HRESIMS data