Author Archives: Maeva Pourpoint

Coming soon, a seismometer on Mars

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Insight, standing for Interior Exploration using Seismic Investigations, Geodesy and Heat Transport, is a NASA funded mission to place a single geophysical robotic lander on Mars. The lander is planned for launch on March 2016 and will be equiped with a seismometer and a heat flow probe. The main objective of this mission will be to study Mars’ deep interior and early geological evolution bringing a better understanding of the Solar System’s terrestrial planets and their evolutionary process. The seismometer will help determine whether there is any seismic activity in Mars as well as the size, thickness, density, velocity and overall structure of Mars’ crust, mantle and core. The seismometer is a broad-band instrument and is designed to detect sources such as quakes but also seismic ambient noise generated by atmospheric excitation and tidal forces from Mars’ satellite. However if any seismic activity is recorded, its source won’t be located because at least three seismometers are needed to locate the source of a quake. So besides the obvious answer of cost, I was wondering why they don’t plan on sending more seismometers. Would anyone have an idea ?

You can find more information on this topic on the NASA webpage dedicated to this mission : http://insight.jpl.nasa.gov/home.cfm

Origin of the Earth’s hum

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Earthquakes can cause the Earth to vibrate over period of days to months. However even in the absence of earthquakes, the Earth keeps vibrating at very low frequencies. This continuous vibration generated by very slow seismic waves with periods greater than 50 s was first discovered in the late 1990s and since then several explanations involving ocean wave propagation have been proposed to explain the origin of this phenomenon. One of the theories suggests that this continuous hum is generated by the constructive interference of microseismic waves that are created during the collision of ocean waves moving in opposite directions. However the period of these microseismic waves is lower than 13 s. So the origin of this Earth’s hum remained unexplained until recently. In February 2015, a group of french researchers proposed the interaction of long ocean waves with the seafloor as a possible explanation and by integrating this new theory into their models, were able to generate microseismic signals with periods ranging from 13 to 300 s. They concluded that both the collision of opposing ocean waves and in a larger extent the movement of long ocean waves over the ocean bottom are responsible for the Earth’s hum.

To learn more about this study, please see the following paper: Ardhuin, F., Gualtieri, L., & Stutzmann, E. (2015). How ocean waves rock the Earth: two mechanisms explain microseisms with periods 3 to 300 s. Geophysical Research Letters.

Reference : http://blogs.agu.org/geospace/2015/04/07/new-study-explains-source-of-earths-mysterious-ringing/

Counteracting the effects of an earthquake with a seismic metamaterial

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In 2013, a team of scientists at a french construction firm published a paper describing a new way of counteracting the destructive effects of an earthquake using a seismic metamaterial. In this paper, they explain how they tried to attenuate the amplitude of seismic waves at the free surface by modifying the energy distribution. Their experiment consisted in simulating an earthquake and using a metamaterial made of a grid of vertical and empty cylindrical columns bored into soil near the earthquake source to attenuate the energy released by the surface waves. They were able to reflect the energy of the incoming surface waves and hence significantly dampen their energy.

To learn more about the details and the results of this experiment, please see their paper:http://arxiv.org/pdf/1301.7642v1.pdf

Seismic wave attenuation: geometrical spreading, anelasticity, multipathing and scattering

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As we discussed in class, seismic waves can lose energy through reflection, geometrical spreading and intrinsic attenuation, also referred as anelasticity.

Geometrical spreading depends on the distance r the wave has propagated from the source. In a uniform material, seismic waves propagate away from their source as spherical wave front of increasing area. Because of the conservation of energy, the energy per unit area of wave front decreases as the distance from the source increases. For surface waves, in the case of a homogeneous flat earth, the energy per unit area of wave front decreases as 1/r and hence the amplitude, which is proportional to the root square of the energy, decreases as 1/√r. For body waves, the energy per unit area of wave front decreases as 1/r2 and hence the amplitude decreases as 1/r.

On the other hand, anelasticity reduces seismic wave amplitudes by converting part of their kinetic energy to frictional heat by permanent deformation of the medium. Anelasticity is characterized by the frequency-dependent quality factor Q, which is a measure of the the energy lost per oscillation of the seismic wave : Q = 2πE/∆E . So the smaller Q, the larger the energy loss.The loss of energy will lead to exponential decay of the seismic wave amplitude : A(t) = A0*e-πft/Q . The smaller Q and the larger the frequency (i.e. more oscillations per second), the larger the attenuation and the seismic wave amplitude decay.

Two other processes can also reduce seismic wave amplitudes: multipathing and scattering. Multipathing and scattering can be thought of as elastic processes. They conserve energy and decrease or increase the amplitude of an incoming wave by shifting its energy to an earlier or later arrival.

Seismic wave multipathing is caused by velocity variations within the medium of propagation. According to Fermat’s principle, seismic waves follow the least-time path of propagation between two points in a medium. Lateral velocity variations in the medium will then cause seismic waves to focus in high velocity regions and defocus in low velocity regions.The spacing between seismic rays in a region represents the energy density in this region. The further apart the rays are, the lower the amplitudes of the recorded wave. By contrast, the closer the rays are, the larger the wave amplitudes. So the seismic waves arriving at a station have usually followed different ray paths in addition to the ideal, direct path and the region of the earth they sampled forms a volume called Fresnel zone. Multipathing can be a significant attenuation effect because most seismic activity occur at plate boundaries and velocity heterogeneities are important in these regions.

Likewise, heterogeneities within the propagation medium cause a propagating wave field to be scattered. These heterogeneities can be velocity anomalies but also material heterogeneities such as mineral boundaries, pore edges, cracks… Scattering will cause part of the energy released by an earthquake to arrive later at a receiver (i.e. after the initial pulse) as a coda (i.e. tail of incoherent energy that decays over a few seconds to a few minutes). Whether a seismic wave will be scattered or not when encountering a heterogeneity depends on the ratio of the heterogeneity size to the wavelength and the propagation distance in the heterogeneous medium. If the heterogeneity is large compared to the wavelength, the seismic energy will follow a different ray path (i.e. multipathing effect). However if the heterogeneity and the wavelength have the same order of magnitude, the seismic energy will be scattered. Heterogeneities much smaller than the wavelength will just change the medium’s “bulk” properties. Scattering can be significant in the continental crust because of the presence of many small-scale geologic structures that can significantly affect short wavelength waves (i.e. tens of kilometers or smaller).

Reference : Stein, S., & Wysession, M. (2009). An introduction to seismology, earthquakes, and earth structure. John Wiley & Sons.