Observations of the seasonality of the Antarctic microseismic signal, and its association to sea ice variability

International audienceSeismic noise spectra at all seismic stations display two peaks in the 1-20 s period band, called primary and secondary microseisms. They are caused by the coupling of ocean waves into Rayleigh waves. At most locations, microseismic power is greater during local winter (when nearby oceans are stormier) than local summer. This tendency is reversed for stations in Antarctica, where growth of local winter sea ice seems to impede microseism generation in near coastal areas. A decade of continuous data from coastal seismic stations in Antarctica show systematic seasonality in microseismic signal levels, and demonstrate associations with both broad‐ scale and local sea‐ice conditions. Primary microseisms are known to be generated at the coast and the modulation that we observe can be associated with sea‐ice variations both in the vicinity of the station and along other Antarctic coasts. The similar modulation of short‐period secondary microseisms corroborates their mostly near‐coastal origin, while the con- tinued presence of long‐period secondary microseisms sug- gests more distant source regions. These observations could be used to extend the monitoring of climate variability prior to the availability of satellite‐derived climate indicators

Characteristic atmosphere-ocean-solid earth interactions in the Antarctic coastal and marine environment inferred from seismic and infrasound recording at Syowa Station, East Antarctica

International audienceSeveral characteristic waves detected by seismographs in Antarctic stations have been recognized as originating from the physical interaction between the solid earth and the atmosphere-ocean-cryosphere system surrounding the Antarctic and may be used as a proxy for characterizing ocean wave climate. A Chaparral-type infrasound sensor was installed at Syowa Station (SYO; 39.6E, 69.0S), East Antarctica, in April 2008 during the International Polar Year (IPY2007-2008). Matching data are also available for this time period from the existing broadband seismic recorder located close by. Continuous infrasound data for 2008-2009 include background signals (microbaroms) with a broad peak in the wave period between the values of 4 and 10 s. Signals with the same period are recorded by the broadband seismograph at SYO (microseisms). This period band is identified as double-frequency microseisms/baroms (DFM). The DFM have relatively lower amplitudes during winter. We suggest that this is due to the sea-ice extent around the coast causing a decreased ocean loading effect. In contrast, the single frequency microseisms/baroms with a peak in period between 12 and 30 s are observed under storm conditions, particularly in winter. On the infrasound data, stationary signals are identified with harmonic overtones at a few Hertz to lowermost human audible band, which we suggest is due to local effects such as sea-ice cracking and vibration. Microseism measurements are a useful proxy for characterizing ocean wave climate, complementing other oceanographic and geophysical data. At SYO, continuous monitoring by both broadband seismograph and infrasound contributes to the Federation of Digital Seismographic Networks, the Comprehensive Nuclear-Test-Ban Treaty in the high southern latitudes and the Pan-Antarctic Observations System under the Scientific Committee on Antarctic Research

Numerical Modeling of Iceberg Capsizing Responsible for Glacial Earthquakes

The capsizing of icebergs calved from marine‐terminating glaciers generate horizontal forces on the glacier front, producing long‐period seismic signals referred to as glacial earthquakes. These forces can be estimated by broadband seismic inversion, but their interpretation in terms of magnitude and waveform variability is not straightforward. We present a numerical model for fluid drag that can be used to study buoyancy‐driven iceberg capsize dynamics and the generated contact forces on a calving face using the finite‐element approach. We investigate the sensitivity of the force to drag effects, iceberg geometry, calving style, and initial buoyancy. We show that there is no simple relationship between force amplitude and iceberg volume, and similar force magnitudes can be reached for different iceberg sizes. The force history and spectral content varies with the iceberg attributes. The iceberg aspect ratio primarily controls the capsize dynamics, the force shape, and force frequency, whereas the iceberg height has a stronger impact on the force magnitude. Iceberg hydrostatic imbalance generates contact forces with specific frequency peaks that explain the variability in glacial earthquake dominant frequency. For similar icebergs, top‐out and bottom‐out events have significantly different capsize dynamics leading to larger top‐out forces especially for thin icebergs. For realistic iceberg dimensions, we find contact‐force magnitudes that range between 5.6 × 1011 and 2 × 1014 kg·m, consistent with seismic observations. This study provides a useful framework for interpreting glacial earthquake sources and estimating the ice mass loss from coupled analysis of seismic signals and modeling results

Mantle upwellings and convective instabilities revealed by seismic tomography and helium isotope geochemistry beneath eastern Africa

International audienceThe relationship between intraplate volcanism and continental tectonics has been investigated for North and East Africa using a high resolution three-dimensional anisotropic tomographic model derived from seismic data of a French experiment ''Horn of Africa'' and existing broadband data. The joint inversion for seismic velocity and anisotropy of the upper 400 km of the mantle, and geochemical data reveals a complex interaction between mantle upwellings, and lithosphere. Two kinds of mantle upwellings can be distinguished: The first one, the Afar ''plume'' originates from deeper than 400 km and is characterized by enrichment in primordial 3 He and 3 He/ 4 He ratios higher than those along mid-ocean ridges (MOR). The second one, associated with other Cenozoic volcanic provinces (Darfur, Tibesti, Hoggar, Cameroon), with 3 He/ 4 He ratios similar to, or lower than MOR, is a consequence of shallower upwelling. The presumed asthenospheric convective instabilities are oriented in an east-west direction, resulting from interaction between south-north asthenospheric mantle flow, main plume head and topography on the base of lithosphere

Complex force history of a calving-generated glacial earthquake derived from broadband seismic inversion

The force applied to the Earth by the calving of two icebergs at Jakobshavn Isbrae, Greenland, has been quantified. The source force history was recovered by inversion of regional broadband seismograms without any a priori constraint on the source time function, in contrast with previous studies. For periods 10-100 s, the three-component force can be obtained from distant stations alone and is proportional to the closest station seismograms. This inversion makes it possible to quantify changes of the source force direction and amplitude as a function of time and frequency. A detailed comparison with a video of the event was used to identify four forces associated with collision, then bottom-out and top-out rotation of the first and second icebergs, and ice mélange motion. Only the two iceberg rotations were identified in previous studies. All four processes are found here to contribute to the force amplitude and variability. Such a complete time-frequency force history provides unique dynamical constraints for mechanical calving models.ERC. Grant Number: ERC-CG-2013-PE10-617472 ANR. Grant Number: ANR-11-BS01-0016 LANDQUAKES, CNCS‐UEFISCDI. Grant Number: PN-II-ID-PCE-2011-3-004

Monitoring Greenland ice sheet buoyancy-driven calving discharge using glacial earthquakes

Since the 2000s, Greenland ice sheet mass loss has been accelerating, followed by increasing numbers of glacial earthquakes (GEs) at near-grounded glaciers. GEs are caused by calving of km-scale icebergs which capsize against the terminus. Seismic record inversion allows a reconstruction of the history of GE sources which captures capsize dynamics through iceberg-to-terminus contact. When compared with a catalog of contact forces from an iceberg capsize model, seismic force history accurately computes calving volumes while the earthquake magnitude fails to uniquely characterize iceberg size, giving errors up to 1 km ³ . Calving determined from GEs recorded ateight glaciers in 1993–2013 accounts for up to 21% of the associated discharge and 6% of the Greenland mass loss. The proportion of discharge attributed to capsizing calving may be underestimated by at least 10% as numerous events could not be identified by standard seismic detections (Olsen and Nettles, 2018). While calving production tends to stabilize in East Greenland, Western glaciers have released more and larger icebergs since 2010 and have become major contributors to Greenland dynamic discharge. Production of GEs and calving behavior are controlled by glacier geometry with bigger icebergs being produced when the terminus advances in deepening water. We illustrate how GEs can help in partitioning and monitoring Greenland mass loss and characterizing capsize dynamics

Infragravity waves: From driving mechanisms to impacts

Infragravity (hereafter IG) waves are surface ocean waves with frequencies below those of wind-generated “short waves” (typically below 0.04 Hz). Here we focus on the most common type of IG waves, those induced by the presence of groups in incident short waves. Three related mechanisms explain their generation: (1) the development, shoaling and release of waves bound to the short-wave group envelopes (2) the modulation by these envelopes of the location where short waves break, and (3) the merging of bores (breaking wave front, resembling to a hydraulic jump) inside the surfzone. When reaching shallow water (O(1–10 m)), IG waves can transfer part of their energy back to higher frequencies, a process which is highly dependent on beach slope. On gently sloping beaches, IG waves can dissipate a substantial amount of energy through depth-limited breaking. When the bottom is very rough, such as in coral reef environments, a substantial amount of energy can be dissipated through bottom friction. IG wave energy that is not dissipated is reflected seaward, predominantly for the lowest IG frequencies and on steep bottom slopes. This reflection of the lowest IG frequencies can result in the development of standing (also known as stationary) waves. Reflected IG waves can be refractively trapped so that quasi-periodic along-shore patterns, also referred to as edge waves, can develop. IG waves have a large range of implications in the hydro-sedimentary dynamics of coastal zones. For example, they can modulate current velocities in rip channels and strongly influence cross-shore and longshore mixing. On sandy beaches, IG waves can strongly impact the water table and associated groundwater flows. On gently sloping beaches and especially under storm conditions, IG waves can dominate cross-shore sediment transport, generally promoting offshore transport inside the surfzone. Under storm conditions, IG waves can also induce overwash and eventually promote dune erosion and barrier breaching. In tidal inlets, IG waves can propagate into the back-barrier lagoon during the flood phase and induce large modulations of currents and sediment transport. Their effect appears to be smaller during the ebb phase, due to blocking by countercurrents, particularly in shallow systems. On coral and rocky reefs, IG waves can dominate over short-waves and control the hydro-sedimentary dynamics over the reef flat and in the lagoon. In harbors and semi-enclosed basins, free IG waves can be amplified by resonance and induce large seiches (resonant oscillations). Lastly, free IG waves that are generated in the nearshore can cross oceans and they can also explain the development of the Earth's “hum” (background free oscillations of the solid earth)

Etude du manteau terrestre sous les Amériques (approche par tomographie régionale et fonctions récepteurs)

PARIS-BIUSJ-Thèses (751052125) / SudocPARIS-BIUSJ-Sci.Terre recherche (751052114) / SudocSudocFranceF

Mantle plumes: Thin, fat, successful, or failing? Constraints to explain hot spot volcanism through time and space

International audienceDensity heterogeneities in the mantle influence the dynamics of mantle upwellings and therefore modify plume characteristics. Using analog laboratory experiments, we explore the dynamics of ``thermo-chemical'' plumes containing both thermal and chemical density anomalies inherited from a stratified boundary layer at the base of the mantle. Because all plumes cool by thermal diffusion as they rise, a chemically composite thermal plume will eventually attain a level of neutral buoyancy, at which it will begin to ``fail''. Separation within the plume will occur, whereby the chemically denser material will start to sink back while the heated surrounding mantle keeps rising. It more generally implies that 1) mantle plumes are not necessarily narrow and continuous throughout the mantle but can be fat and patchy such as Iceland, 2) a hot mantle region may not be buoyant and rising, but on contrary may be sinking, and 3) mantle plumes dynamics are strongly time-dependent