Temporal changes in the lunar soil from correlation of diffuse vibrations

International audienceIt was recently demonstrated that one can reconstruct the impulse response between passive sensors, by cross-correlating diffuse waves or ambient noise. Using seismic waves recorded on the moon, we show here that not only direct waves can be retrieved but also late arrivals that have been scattered before reaching the seismometers. As these late arrivals propagate for longer time, they are more sensitive to weak perturbations of the medium such as velocity changes. This high sensitivity of scattered waves is used to monitor periodic velocity changes in the lunar soil by measuring small delays of the passively retrieved coda waves. The velocity changes result from temperature variations due to periodic heating of the lunar surface by the sun

Dynamics of Piton de la Fournaise volcano observed by passive image interferometry with multiple references

AbstractActivity of Piton de la Fournaise (PdF) volcano in La Réunion Island modifies the seismic velocities within the edifice. Using the 2010 and 2011 data from a network of 21 seismic stations in the vicinity of PdF, changes of seismic velocities are investigated using passive image interferometry, i.e. interferometry of seismic noise correlations. As noise correlations change significantly over time in response to volcanic activity, a method is presented that allows us to measure continuous long term velocity changes with high and constant accuracy by using multiple periods as reference. A long term velocity increase is found that averages about 0.25% per year. This trend is superimposed by short term changes that exhibit a clear connection with summit seismo-tectonic earthquakes indicating the effect of volcanic activity. Characteristic signatures of velocity changes are identified for post-eruptive periods of deflation that show an increase of velocity associated with subsidence observed by GPS. Periods of pre-eruptive inflation are characterized by decreasing velocity. Seismic crises can be associated with either increasing or decreasing velocity depending on whether the magma movement leads to deflation due to an eruption emptying the shallow plumbing system or to inflation caused by a non-eruptive intrusion. With a simple assumption about the spatial sensitivity of the measurements both processes are found to have the strongest effect in the central summit area of the volcano which also shows the strongest surface displacements during the time investigated here. We do not observe a dependence of the velocity change on the location of the erupting fissures, instead the distribution of changes for the three inflation periods and the two eruptions are similar indicating that the velocity changes observed here reflect the dynamics of a shallow magma reservoir rather than the effect of the eruption at the surface

Deciphering the Whisper of Volcanoes: Monitoring Velocity Changes at Kamchatka's Klyuchevskoy Group With Fluctuating Noise Fields

Volcanic inflation and deflation often precede eruptions and can lead to seismic velocity changes (dv/v $dv/v$) in the subsurface. Recently, interferometry on the coda of ambient noise‐cross‐correlation functions yielded encouraging results in detecting these changes at active volcanoes. Here, we analyze seismic data recorded at the Klyuchevskoy Volcanic Group in Kamchatka, Russia, between summer of 2015 and summer of 2016 to study signals related to volcanic activity. However, ubiquitous volcanic tremors introduce distortions in the noise wavefield that cause artifacts in the dv/v $dv/v$ estimates masking the impact of physical mechanisms. To avoid such instabilities, we propose a new technique called time‐segmented passive image interferometry. In this technique, we employ a hierarchical clustering algorithm to find periods in which the wavefield can be considered stationary. For these periods, we perform separate noise interferometry studies. To further increase the temporal resolution of our results, we use an AI‐driven approach to find stations with similar dv/v $dv/v$ responses and apply a spatial stack. The impacts of snow load and precipitation dominate the resulting dv/v $dv/v$ time series, as we demonstrate with the help of a simple model. In February 2016, we observe an abrupt velocity drop due to the M7.2 Zhupanov earthquake. Shortly after, we register a gradual velocity increase of about 0.3% at Bezymianny Volcano coinciding with surface deformation observed using remote sensing techniques. We suggest that the inflation of a shallow reservoir related to the beginning of Bezymianny's 2016/2017 eruptive cycle could have caused this local velocity increase and a decorrelation of the correlation function coda

Hot Upper Mantle Beneath the Tristan da Cunha Hotspot From Probabilistic Rayleigh-Wave Inversion and Petrological Modeling

Understanding the enigmatic intraplate volcanism in the Tristan da Cunha region requires knowledge of the temperature of the lithosphere and asthenosphere beneath it. We measured phase-velocity curves of Rayleigh waves using cross-correlation of teleseismic seismograms from an array of ocean-bottom seismometers around Tristan, constrained a region-average, shear-velocity structure, and inferred the temperature of the lithosphere and asthenosphere beneath the hotspot. The ocean-bottom data set presented some challenges, which required data-processing and measurement approaches different from those tuned for land-based arrays of stations. Having derived a robust, phase-velocity curve for the Tristan area, we inverted it for a shear wave velocity profile using a probabilistic (Markov chain Monte Carlo) approach. The model shows a pronounced low-velocity anomaly from 70 to at least 120 km depth. VS in the low velocity zone is 4.1-4.2 km/s, not as low as reported for Hawaii (∼4.0 km/s), which probably indicates a less pronounced thermal anomaly and, possibly, less partial melting. Petrological modeling shows that the seismic and bathymetry data are consistent with a moderately hot mantle (mantle potential temperature of 1,410-1,430°C, an excess of about 50-120°C compared to the global average) and a melt fraction smaller than 1%. Both purely seismic inversions and petrological modeling indicate a lithospheric thickness of 65-70 km, consistent with recent estimates from receiver functions. The presence of warmer-than-average asthenosphere beneath Tristan is consistent with a hot upwelling (plume) from the deep mantle. However, the excess temperature we determine is smaller than that reported for some other major hotspots, in particular Hawaii

Elastic-wave propagation and the Coriolis force

In a coordinate system fixed with respect to the rotating Earth, the Coriolis force deflects an object sideways relative to its direction of motion. A beautiful demonstration of that effect is the Foucault pendulum, illustrated in figure 1a. As the long pendulum rocks back and forth, the Coriolis force deflects it the same way on both the forward and reverse swings—to the right in the Northern Hemisphere and to the left in the Southern Hemisphere. The net result is that the pendulum’s plane of oscillation rotates clockwise in the Northern Hemisphere, a change evidenced in figure 1a by the little cylinders that the pendulum has knocked down. The time rate of change of the oscillation direction is given by ϕ̇=−Ωcosθϕ̇=−Ωcosθ, where, as illustrated in figure 1b, Ω is the rotation rate of Earth and θθ is the colatitude—that is, the angle between the local vertical and Earth’s rotation axis. The minus sign arises because the pendulum maintains its oscillation direction as Earth rotates under it. As a consequence, the pendulum’s direction of oscillation rotates in the sense opposite that of Earth’s rotation, a result most readily visualized by imagining the pendulum to be at the North Pole

Sensitivity kernels for static and dynamic tomography of scattering and absorbing media with elastic waves: a probabilistic approach

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