Seismic tomography | EPFL Graph Search

Seismic tomography is a technique for imaging the subsurface of the Earth with seismic waves produced by earthquakes or explosions. P-, S-, and surface waves can be used for tomographic models of different resolutions based on seismic wavelength, wave source distance, and the seismograph array coverage. The data received at seismometers are used to solve an inverse problem, wherein the locations of reflection and refraction of the wave paths are determined. This solution can be used to create 3D images of velocity anomalies which may be interpreted as structural, thermal, or compositional variations. Geoscientists use these images to better understand core, mantle, and plate tectonic processes. Theory Tomography is solved as an inverse problem. Seismic travel time data are compared to an initial Earth model and the model is modified until the best possible fit between the model predictions and observed data is found. Seismic waves would travel in straight lines if Earth was o

Chemical Characterisation of Deep Earth Reservoirs

Hélène Marie Piet

Seismic imaging of the internal Earth has emphasized the inhomogeneity of its mantle. This interface layer of nearly 3000 km surrounding the central metallic core conducts heat released from the latter, which played a significant role in shaping the actual mantle as it currently is. The present state of the deep Earth (mainly the lower mantle and the core) is also a consequence of the physical properties of the material composing them. Large-low shear velocity provinces atop the core-mantle boundary, for example, are composed of a denser material than their surrounding environment but their origin, formation and survival over billions of years to mantle convection, is still largely debated. Here we use chemical partitioning to characterise the current composition of those deep Earth reservoirs and better understand the processes leading to their formation. In the first part, experiments were conducted in the laser-heated diamond anvil cell and the latest advances in microanalysis were used to refine iron partitioning between minerals of the lower mantle. We reconcile long-standing discrepancies in previous partitioning data and show that significant variations of silicate iron content are associated to changing ferric iron incorporation. We also note that such chemical variations might significantly affect the physical properties of the silicate, which might in turn provoke mantle rheology variations. Our results and observations thus provide a mineral physics based argument for the stabilization of deep Earth reservoirs and especially large structures like large low-shear-velocity provinces (LLSVPs). In the second and numerical part of the thesis, previous results on siderophile (iron-loving) element partitioning between metal and silicate were used to constrain some aspects of core formation. We constrain the chemical and physical state of the magma ocean during core formation explaining both the siderophile composition of the mantle and the light element composition of the core. We simulate the Moon-Forming Giant Impact in the last steps of Earth accretion, where we constrain the size of the impactor to be smaller than 15% of Earth's total mass, a roughly Mars-sized object, and further show that melting of the large fractions of the mantle upon impact by the giant impactor is consistent with siderophile composition of the bulk silicate Earth, which can have great implications for the geodynamical evolution of the mantle. In conclusions, given the fact that the mantle may have been almost fully molten, further crystallization may have isolated a layer of liquid primitive material at the base of the mantle possibly explaining the seismic signature of the LLSVPs associated with a strong thermochemical contrast with the surrounding mantle. The layered composition in iron of the actual mantle also provides a chemical argument for a viscously layered mantle, which could have helped sustain LLSVPs for billions of years from efficient convection.

EPFL2016

Core formation and core composition from coupled geochemical and geophysical constraints

James Badro, Hélène Marie Piet

The formation of Earth's core left behind geophysical and geochemical signatures in both the core and mantle that remain to this day. Seismology requires that the core be lighter than pure iron and therefore must contain light elements, and the geochemistry of mantle-derived rocks reveals extensive siderophile element depletion and fractionation. Both features are inherited from metal-silicate differentiation in primitive Earth and depend upon the nature of physio-chemical conditions that prevailed during core formation. To date, core formation models have only attempted to address the evolution of core and mantle compositional signatures separately, rather than seeking a joint solution. Here we combine experimental petrology, geochemistry, mineral physics and seismology to constrain a range of core formation conditions that satisfy both constraints. We find that core formation occurred in a hot (liquidus) yet moderately deep magma ocean not exceeding 1,800 km depth, under redox conditions more oxidized than present-day Earth. This new scenario, at odds with the current belief that core formation occurred under reducing conditions, proposes that Earth's magma ocean started oxidized and has become reduced through time, by oxygen incorporation into the core. This core formation model produces a core that contains 2.7-5% oxygen along with 2-3.6% silicon, with densities and velocities in accord with radial seismic models, and leaves behind a silicate mantle that matches the observed mantle abundances of nickel, cobalt, chromium, and vanadium.

Natl Acad Sciences2015

Ambient seismic noise monitoring of a clay landslide: Toward failure prediction

Michel Jaboyedoff, Clément Michoud

Given that clay-rich landslides may become mobilized, leading to rapid mass movements (earthflows and debris flows), they pose critical problems in risk management worldwide. The most widely proposed mechanism leading to such flow-like movements is the increase in water pore pressure in the sliding mass, generating partial or complete liquefaction. This solid-to-liquid transition results in a dramatic reduction of mechanical rigidity in the liquefied zones, which could be detected by monitoring shear wave velocity variations. With this purpose in mind, the ambient seismic noise correlation technique has been applied to measure the variation in the seismic surface wave velocity in the Pont Bourquin landslide (Swiss Alps). This small but active composite earthslide-earthflow was equipped with continuously recording seismic sensors during spring and summer 2010. An earthslide of a few thousand cubic meters was triggered in mid-August 2010, after a rainy period. This article shows that the seismic velocity of the sliding material, measured from daily noise correlograms, decreased continuously and rapidly for several days prior to the catastrophic event. From a spectral analysis of the velocity decrease, it was possible to determine the location of the change at the base of the sliding layer. These results demonstrate that ambient seismic noise can be used to detect rigidity variations before failure and could potentially be used to predict landslides.

2012

Chemical Characterisation of Deep Earth Reservoirs

Hélène Marie Piet

EPFL2016