Frechet derivatives of coupled seismograms with respect to an anelastic rotating earth

International audienceA theory, based on higher-order perturbations, is given and used to express the partial derivatives of the seismic waveform with respect t o perturbations of density, anelasticity and anisotropy. Frechet derivatives are expressed with respect to a general aspherical model by using modulation functions, which are already used for the computation of aspherical seismograms. A direct solution method (DSM) that optimizes other previously proposed DSMs is proposed for the computation of the modulation functions. Numerical tests point out significant differences between the general FrCchet derivatives and those used for more classical approaches (great-circle average or frozen-path approximation), as well as important focusing/defocusing effects. This theory will enable future imaging inversions of the small-scale heterogeneities of the Earth

Diffraction of long period Rayleigh waves effects of mode coupling by a slab: effects of mode coupling

International audienceWe compute seismograms of the fundamental Rayleigh waves propagating through a slab structure , with either a lateral variation in seismic velocities , or in attenuation. At periods of 100 sec, we show that the phase delay is strongly reduced by the surface waves Fresnel zone, and that coupling must be considered far along the dispersion branch, up to at least oe-• 25. Limiting the coupling to fewer modes produces a signal associated to a ghost structure at the antipode of the slab. We also show that the amplitude perturbations produced by the diffraction and the attenuation of the slab are comparable in size. Future waveform studies, especially those associated to global waveform inversions, must then carefully consider these effects

Seismometer Detection of Dust Devil Vortices by Ground Tilt

We report seismic signals on a desert playa caused by convective vortices and dust devils. The long-period (10-100s) signatures, with tilts of ~10$^{-7}$ radians, are correlated with the presence of vortices, detected with nearby sensors as sharp temporary pressure drops (0.2-1 mbar) and solar obscuration by dust. We show that the shape and amplitude of the signals, manifesting primarily as horizontal accelerations, can be modeled approximately with a simple quasi-static point-load model of the negative pressure field associated with the vortices acting on the ground as an elastic half space. We suggest the load imposed by a dust devil of diameter D and core pressure {\Delta}Po is ~({\pi}/2){\Delta}PoD$^2$, or for a typical terrestrial devil of 5 m diameter and 2 mbar, about the weight of a small car. The tilt depends on the inverse square of distance, and on the elastic properties of the ground, and the large signals we observe are in part due to the relatively soft playa sediment and the shallow installation of the instrument. Ground tilt may be a particularly sensitive means of detecting dust devils. The simple point-load model fails for large dust devils at short ranges, but more elaborate models incorporating the work of Sorrells (1971) may explain some of the more complex features in such cases, taking the vortex winds and ground velocity into account. We discuss some implications for the InSight mission to Mars.Comment: Contributed Article for Bulletin of the Seismological Society of America, Accepted 29th August 201

Computation of seismograms and atmospheric oscillations by normal-mode summation for a spherical earth model with realistic atmosphere

We describe a theory to compute seismograms and atmospheric disturbances such as ionospheric oscillations or pressure variations in a realistic spherical earth model with atmosphere. This theory is valid for a source located either in the solid earth or in the atmosphere. Solid earth and atmospheric normal modes are computed for a radiation boundary condition that models the dissipation of acoustic signals in the high atmosphere of the Earth. We show that the coupling between ground and atmosphere occurs at a set of frequencies related to fundamentals and harmonics of atmospheric modes. Spheroidal modes near these frequencies have up to 0.04 per cent of their energy in the atmosphere, and thus may be strongly excited by atmospheric sources. This theory can be used for more accurate modelling of the seismic data from meteoritic events or volcanic eruptions as well as for the analysis of barograms or ionograms recorded after large earthquakes

Normal modes and long period seismograms in a 3D anelastic elliptical rotating Earth

International audienceWe present results of a direct method to compute realistic long period (>200 s in this example) synthetic seismograms using higher order perturbation theory (up to the 3rd order). Normal modes are computed for the Earth, taking into account rotation, ellipticity, 3D elastic lateral heterogeneities (SAW12D) for the whole mantle, and anelastic lateral variations in the upper mantle (QR19). Coupling between different modes and branches is included in the computation. We study the sensitivity of the waveform to 3D anelasticity and the biases between 3D elastic and 3D anelastic variations. The resulting seismograms can be computed to higher frequency and may then be used to perform whole mantle joint inversions for elastic and anelastic structure

Present-day Mars' seismicity predicted from 3-D thermal evolution models of interior dynamics

©2018. American Geophysical UnionThe Interior Exploration using Seismic Investigations, Geodesy and Heat Transport mission, to be launched in 2018, will perform a comprehensive geophysical investigation of Mars in situ. The Seismic Experiment for Interior Structure package aims to detect global and regional seismic events and in turn offer constraints on core size, crustal thickness, and core, mantle, and crustal composition. In this study, we estimate the present‐day amount and distribution of seismicity using 3‐D numerical thermal evolution models of Mars, taking into account contributions from convective stresses as well as from stresses associated with cooling and planetary contraction. Defining the seismogenic lithosphere by an isotherm and assuming two end‐member cases of 573 K and the 1073 K, we determine the seismogenic lithosphere thickness. Assuming a seismic efficiency between 0.025 and 1, this thickness is used to estimate the total annual seismic moment budget, and our models show values between 5.7 × 1016 and 3.9 × 1019 Nm

Three-dimensional waveform modeling of ionospheric signature induced by the 2004 Sumatra tsunami

International audienceThe Sumatra, December 26th, 2004, tsunami produced internal gravity waves in the neutral atmosphere and large disturbances in the overlying ionospheric plasma. To corroborate the tsunamigenic hypothesis of these perturbations, we reproduce, with a 3D numerical modeling of the ocean-atmosphere-ionosphere coupling, the tsunami signature in the Total Electron Content (TEC) data measured by the Jason-1 and Topex/Poseidon satellite altimeters. The agreement between the observed and synthetic TEC shows that ionospheric remote sensing can provide new tools for offshore tsunami detection and monitorin

Ionospheric detection of gravity waves induced by tsunamis

Tsunami waves propagating across long distances in the open-ocean can induce atmospheric gravity waves by dynamic coupling at the surface. In the period range 10 to 20 minutes, both have very similar horizontal velocities, while the gravity wave propagates obliquely upward with a vertical velocity of the order of 50 m s^(−1), and reaches the ionosphere after a few hours. We use ionospheric sounding technique from Global Positioning System to image a perturbation possibly associated with a tsunami-gravity wave. The tsunami was produced after the M_w= 8.2 earthquake in Peru on 2001 June 23, and it reached the coast of Japan some 22 hours later. We used data from the GEONET network in Japan to image small-scale perturbations of the Total Electron Content above Japan and up to 400 km off shore. We observed a short-scale ionospheric perturbation that presents the expected characteristics of a coupled tsunami-gravity wave. This first detection of the gravity wave induced by a tsunami opens new opportunities for the application of ionospheric imaging to offshore detection of tsunamis

An exploration of the internet’s embodied narratives through software dial-up modems: atemporality, neutrality, immateriality

My research-creation thesis is not about the internet itself. It is about the way we frame and understand that network. It is, more specifically about its narratives: metaphors and discourses that are developed and engineered by internet stakeholders in order to reinforce their position and, ultimately, gain control of our data. Starting from the ubiquitous metaphor of the internet cloud that depicts the internet as a non-tangible and harmless object we observe from far, my thesis triangulates between three sub-discourses emerging from this narrative. These are the illusion that our internet processes are 1) atemporal, 2) neutral and 3) immaterial. As a research-creation thesis, it moreover inquires into these beliefs by engaging with a specific technology that I argue as foundational for deconstructing these narratives: the dial-up modem that was used during the early internet era to create an internet connection. Contextualising this technology inside a web of artistic, theoretical and methodological references, my thesis guides the reader through four design projects where these analog modems are used in a critical and speculative way. In opposition to the way internet processes are crafted by engineers as purely operational, optimized and seamless, these projects serve as a way to reflect on the internet’s materiality and embodied discourses. Moreover, they serve as frameworks to make explicit the temporal, political and material characteristics of the infrastructure

Estimations of the Seismic Pressure Noise on Mars Determined from Large Eddy Simulations and Demonstration of Pressure Decorrelation Techniques for the Insight Mission

The atmospheric pressure fluctuations on Mars induce an elastic response in the ground that creates a ground tilt, detectable as a seismic signal on the InSight seismometer SEIS. The seismic pressure noise is modeled using Large Eddy Simulations (LES) of the wind and surface pressure at the InSight landing site and a Green’s function ground deformation approach that is subsequently validated via a detailed comparison with two other methods: a spectral approach, and an approach based on Sorrells’ theory (Sorrells,Geophys. J. Int. 26:71–82, 1971; Sorrells et al., Nat. Phys. Sci. 229:14–16, 1971). The horizontal accelerations as a result of the ground tilt due to the LES turbulence-induced pressure fluctuations are found to be typically ∼ 2–40 nm/s2 in amplitude, whereas the direct horizontal acceleration is two orders of magnitude smaller and is thus negligible in comparison. The vertical accelerations are found to be ∼ 0.1–6 nm/s2 in amplitude. These are expected to be worst-case estimates for the seismic noise as we use a half-space approximation; the presence at some (shallow) depth of a harder layer would significantly reduce quasi-static displacement and tilt effects. We show that under calm conditions, a single-pressure measurement is representative of the large-scale pressure field (to a distance of several kilometers), particularly in the prevailing wind direction. However, during windy conditions, small-scale turbulence results in a reduced correlation between the pressure signals, and the single-pressure measurement becomes less representative of the pressure field. The correlation between the seismic signal and the pressure signal is found to be higher for the windiest period because the seismic pressure noise reflects the atmospheric structure close to the seismometer. In the same way that we reduce the atmospheric seismic signal by making use of a pressure sensor that is part of the InSight Auxiliary Payload Sensor Suite, we also the use the synthetic noise data obtained from the LES pressure field to demonstrate a decorrelation strategy. We show that our decorrelation approach is efficient, resulting in a reduction by a factor of ∼ 5 in the observed horizontal tilt noise (in the wind direction) and the vertical noise. This technique can, therefore, be used to remove the pressure signal from the seismic data obtained on Mars during the InSight mission