Philippe Lognonné

Seismic Detection of the Lunar Core

Reinterpreted Apollo-era seismic data from the Moon reveal a solid inner core and a fluid outer core. Despite recent insight regarding the history and current state of the Moon from satellite sensing and analyses of limited Apollo-era seismic data, deficiencies remain in our understanding of the deep lunar interior. We reanalyzed Apollo lunar seismograms using array-processing methods to search for the presence of reflected and converted seismic energy from the core. Our results suggest the presence of a solid inner and fluid outer core, overlain by a partially molten boundary layer. The relative sizes of the inner and outer core suggest that the core is ~60% liquid by volume. Based on phase diagrams of iron alloys and the presence of partial melt, the core probably contains less than 6 weight % of lighter alloying components, which is consistent with a volatile-depleted interior.

A new seismic model of the Moon: implications for structure, thermal evolution and formation of the Moon

Abstract The seismic determinations of the crustal thickness and mantle velocities are key parameters for most geophysical and geochemical lunar studies. We determine a new seismic model of the Moon after a complete independent reprocessing of the Apollo lunar seismic data with determination of arrival times of about 60 natural and artificial lunar quakes, as well as travel times of converted phases at the crust–mantle interface below the Apollo 12 landing site. On the near side in the Procellarum KREEP Terrane, the only major discontinuity compatible with the crust–mantle boundary is located around 30 km deep. In this terrane, seismic constraints on the crust and mantle lead to a 30 km thick anorthositic crust and a pyroxenite cold mantle, with a bulk composition of 6.4% Al 2 O 3 , 4.9% CaO and 13.3% FeO. Mantle temperatures are in accordance with profiles obtained from the observed electrical conductivity and exclude a liquid Fe core, while being compatible with a Fe–S liquid core. Our Moon model might be explained by a mixture of a primitive Earth with tholeiitic crust and depleted upper mantle, together with a chondritic enstatitic parent body for the impactor planet. It provides mixture coefficients comparable to those obtained by impact simulation as well as an estimate of bulk U of about 28 ppb, in accordance with the U budget in a 40 km mean thick crust, 700 km thick depleted mantle and a lower undepleted primitive mantle.

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

[1] The 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 monitoring.

Normal modes modelling of post‐seismic ionospheric oscillations

Since 1960, several experiments have shown strong perturbations of the ionosphere after earthquakes. For the biggest quakes, Doppler ionospheric soundings have revealed displacements of several tens of meter of the ionized layers E and F. An accurate modelling of these phenomena was however never described. We present here synthetic displacements of such oscillations and compare it with experimental data. Realistic models of the atmosphere including viscosity are used, as well as realistic Earth and seismic source model. These ionospheric oscillations are computed by normal modes summation taking explicitly into account the coupling between the solid Earth and the open atmosphere. They are associated to the Rayleigh fundamental surface waves. We obtain a good agreement between synthetics and observations, so it should be possible to use this method to study the ionospheric post-seismic perturbations and possibly the high altitude profile of the density and the viscosity of the atmosphere.

Normal modes and seismograms in an anelastic rotating Earth

In order to account for rotation and anelastic effects in the normal modes of the Earth, a structure of the space of normal modes different from those generally used in the elastic self-adjoint case is necessary. This can be done with a duality relation between the eigenproblem and the one obtained simply by reversing the Earths rotation. This leads to new biorthonormality relations between modes and dual modes. Seismograms can then be expressed in terms of a normal mode expansion. The normal modes of an anelastic rotating Earth can be computed with perturbation theory. In order to take into account the coupling terms between different dispersion curves, as well as between toroidal and spheroidal modes, the perturbations start from an anelastic, non-rotating Earth rather than from an elastic one. The secular terms of the perturbation series, due to coupling effects between modes of the same multiplet, can then be removed. This ensures that higher order perturbation theory converges to the anelastic modes with sufficient accuracy, and gives, up to third order, expressions for the eigenmodes and eigenfrequencies. These expressions can be used to compute modes and seismograms of an anelastic realistic Earth model, neglecting neither rotation, anelasticity, anisotropy or lateral heterogeneities.

The French Pilot Experiment OFM-SISMOBS: first scientific results on noise level and event detection

Abstract The seismic data recorded during the French Pilot Experiment OFM-SISMOBS (Observatoire Fond de Mer) were carefully analyzed. This experiment was successfully conducted between 28 April and 11 May 1992. Notwithstanding the technical goal of the experiment, which was to show the feasibility of installing and recovering two sets of three-component broadband seismometers (one inside an Ocean Drilling Program (ODP) borehole and another inside an ocean-bottom seismograph (OBS) sphere in the vicinity of the hole), the second goal of the experiment was to obtain for the first time the seismic noise level in the broadband range 0.5–3600 s, to conduct a comparative study of broadband noise on the sea-floor, downhole and on a continent, and to determine the detection threshold of seismic events. After the installation of both sets of seismometers, seismic signals were recorded continuously for 10 days. The analysis of these signals shows that the seismic noise is smaller in the period range 4–30 s for both sea-floor seismometers (OFM; Observatoire Fond de Mer) and downhole seismometers (OFP; Observatoire Fond de Puits) than at a typical broadband continental station such as GEOSCOPE Station SSB. The noise is smaller than that at SSB up to 600 s for OFM. The noise on vertical components is much smaller than on the horizontal ones. This difference might be explained by instrument settling. It was also observed that the noise level tends to decrease with time for both OFM and OFP, which means that the equilibrium stage was not attained by the end of the experiment. The patterns of microseismic noise in oceanic and continental areas are completely different. The background microseismic noise is shifted towards shorter periods for OFM and OFP compared with a continental station. This might be related to the difference in the crustal structure between oceans and continents. The low level of seismic noise implies that the detection threshold of earthquakes is very low, and it has been possible to record correctly teleseismic earthquakes of magnitude as small as 5.3. It was also possible to extract the Earth tide oceanic signal. Therefore, the experiment was a complete technical and scientific success, and demonstrated that the installation of a permanent broadband seismic and geophysical observatory on the sea-floor is now possible and can provide the scientific community with high-quality seismic data.

First seismic receiver functions on the Moon

We applied the S receiver function technique [Farra and Vinnik, 2000] to the recordings of deep moon-quakes at seismograph station Apollo 12 in order to detect phases converted (Sp) and reflected beneath the station. We detected Sp phases from the base of the surficial low-velocity zone and from the mantle-crust boundary. The average P velocity in the surficial layer 1 km thick should be a few times higher than in reference model [Toksoz et al., 1974]. The observed time, amplitude and waveform of Sp phase from the mantle-crust boundary are close to those predicted by the reference model but with a modified surficial layer. The S wavetrains within the first 10 s may contain waves scattered in the mantle. This scattering is stronger than in the Earth at comparable depths. The polarized component in the coda waves that we observe is another previously unknown phenomenon.

ULTRA BROAD BAND SEISMOLOGY ON INTERMARSNET

Abstract Very broad band seismometers, which are included in the baseline of the InterMarsNet payload cover the whole frequency band from tidal frequencies up to 50 Hz. By using the results of field tests performed on Earth with the deployment scenario of the InterMarsNet seismic experiment, it is shown that it should probaly be possible to reach very low micro-seismic noise level, possibly less than a spectral amplitude of 10 9 ms −2 Hz − 1 2 on the vertical component. Despite the fact that only three stations will be available for the network, sufficient information should be available in the three-component short period records to determine the location of Marsquake foci and the origin time to a level that will enable studies of the seismicity, interior velocity structure, and tectonic activity of the planet. Despite the fact that the most current signals will be recorded in the frequency band 10mHz–10 Hz, it is shown that the detection of a quake of moment 1018 Nm or the stack of the InterMarsNet records of quakes with a moment greater than 1017 Nm will allow normal mode frequency measurement in the range 5–20mHz for a noise amplitude of 10 −9 ms −2 Hz − 1 2 . Observation in the range 3–5 mHz cannot also be ruled out. These normal mode frequencies will have a high scientific return for determining the deep Mars structure. At longer periods, the detection of the tides of Phobos will provide information on the deep internal structure even if no strong quakes are recorded. It is shown that the amplitude of the Phobos tide fundamental after the analysis of a 2 year time series from a single station, for a realistic and conservative estimate of the long period noise may be measured with a precision of 1%. For less noise scenario, and/or by using the stack of all the records, information from the higher Phobos tidal harmonics will also be obtained.

The seismic OPTIMISM experiment

Abstract The study of the deep interior of Mars suffers from the very limited amount of data available, particularly seismological data. The objective of the OPTIMISM seismic experiment, lost with the failure of the Mars 96 mission, was to perform a seismic reconnaissance of Mars, to constrain the level of martian seismic noise and its level of seismicity. The seismometer was expected to operate during one year, with a sensitivity one hundred times higher than the Viking seismometer. Observation of relatively frequent low magnitude marsquakes, as well as a few large magnitude quakes might then be probably achieved. The OPTIMISM experiment might then, as a seismic ‘path-finder’, open a new field in Mars exploration and a new era in our present knowledge of the interior of Mars. A seismic experiment on Mars, especially performed by a network of stations, remains as the necessary experiment for the determination of the internal structure of the planet.

First tsunami gravity wave detection in ionospheric radio occultation data

After the 11 March 2011 earthquake and tsunami off the coast of Tohoku, the ionospheric signature of the displacements induced in the overlying atmosphere has been observed by ground stations in various regions of the Pacific Ocean. We analyze here the data of radio occultation satellites, detecting the tsunami-driven gravity wave for the first time using a fully space-based ionospheric observation system. One satellite of the Constellation Observing System for Meteorology, Ionosphere and Climate (COSMIC) recorded an occultation in the region above the tsunami 2.5 h after the earthquake. The ionosphere was sounded from top to bottom, thus providing the vertical structure of the gravity wave excited by the tsunami propagation, observed as oscillations of the ionospheric Total Electron Content (TEC). The observed vertical wavelength was about 50 km, with maximum amplitude exceeding 1 total electron content unit when the occultation reached 200 km height. We compared the observations with synthetic data obtained by summation of the tsunami-coupled gravity normal modes of the Earth/Ocean/atmosphere system, which models the associated motion of the ionosphere plasma. These results provide experimental constraints on the attenuation of the gravity wave with altitude due to atmosphere viscosity, improving the understanding of the propagation of tsunami-driven gravity waves in the upper atmosphere. They demonstrate that the amplitude of the tsunami can be estimated to within 20% by the recorded ionospheric data.