Aurélien Mordret

A large magmatic sill complex beneath the Toba caldera

Imaging before an unimaginable eruption Volcanic super-eruptions eject thousands of times the volume of the largest documented eruptions over human history. Several “supervolcanoes” capable of this type of unimaginable devastation dot the surface of Earth today. Jaxybulatov et al. use seismic background noise to estimate the size and maturity of one of the worlds largest volcanic reservoirs, the Toba caldera in northern Sumatra. Magma storage under the caldera occurs slowly over time, in the form of horizontal layers of magma injected into the crust. These magmatic sills are documented to 20 kilometers below the surface, but more sills could exist even deeper. The characterization of magma storage in large volcanic systems may help us to prepare for future volcanic super-eruptions. Science, this issue p. 617 Ambient noise tomography images the magma distribution of a large volcanic reservoir under the Toba caldera. An understanding of the formation of large magmatic reservoirs is a key issue for the evaluation of possible strong volcanic eruptions in the future. We estimated the size and level of maturity of one of the largest volcanic reservoirs, based on radial seismic anisotropy. We used ambient-noise seismic tomography below the Toba caldera (in northern Sumatra) to observe the anisotropy that we interpret as the expression of a fine-scale layering caused by the presence of many partially molten sills in the crust below 7 kilometers. This result demonstrates that the magmatic reservoirs of present (non-eroded) supervolcanoes can be formed as large sill complexes and supports the concept of the long-term incremental evolution of magma bodies that lead to the largest volcanic eruptions.

Three-dimensional shear velocity anisotropic model of Piton de la Fournaise Volcano (La Réunion Island) from ambient seismic noise

We cross correlate 4 years of seismic noise from the seismic network of Piton de la Fournaise Volcano (La Reunion Island) to measure the group velocity dispersion curves of Rayleigh and Love waves. We average measurements from vertical and radial components to obtain 577 Rayleigh wave dispersion curves. The transverse components provided 395 Love wave dispersion curves. We regionalize the group velocities measurements into 2-D velocity maps between 0.4 and 8 s. Finally, we locally inverted these maps for a pseudo 3-D anisotropic shear-velocity model down to 3 km below the sea level using a Neighborhood Algorithm. The 3-D isotropic shear-wave model shows three distinct high-velocity anomalies surrounded by a low-velocity ring. The anomaly located below the present “Plaine des Sables” could be related to an old intrusive body at the location of the former volcanic center before it migrated toward its present location. The second high-velocity body located below the summit of the volcano likely corresponds to the actual preferential dyke intrusion zone as highlighted by the seismicity. The third high-velocity anomaly located below the “Grandes Pentes” and the “Grand Brule” areas and is an imprint of the solidified magma chamber of the dismantled “Les Alizes” Volcano. Radial anisotropy shows two main anomalies: positive anisotropy above sea level highlighting the recent edifice of Piton de la Fournaise with an accumulation of horizontal lava flows and the second one below the sea level with a negative anisotropy corresponding to the ancient edifice of Piton de la Fournaise dominated by intrusions of vertical dykes.

Monitoring southwest Greenland’s ice sheet melt with ambient seismic noise

Researchers monitor southwest Greenland’s ice sheet mass changes by measuring seismic velocity variations in Greenland’s crust. The Greenland ice sheet presently accounts for ~70% of global ice sheet mass loss. Because this mass loss is associated with sea-level rise at a rate of 0.7 mm/year, the development of improved monitoring techniques to observe ongoing changes in ice sheet mass balance is of paramount concern. Spaceborne mass balance techniques are commonly used; however, they are inadequate for many purposes because of their low spatial and/or temporal resolution. We demonstrate that small variations in seismic wave speed in Earth’s crust, as measured with the correlation of seismic noise, may be used to infer seasonal ice sheet mass balance. Seasonal loading and unloading of glacial mass induces strain in the crust, and these strains then result in seismic velocity changes due to poroelastic processes. Our method provides a new and independent way of monitoring (in near real time) ice sheet mass balance, yielding new constraints on ice sheet evolution and its contribution to global sea-level changes. An increased number of seismic stations in the vicinity of ice sheets will enhance our ability to create detailed space-time records of ice mass variations.

Continuous Monitoring of High‐Rise Buildings Using Seismic InterferometryContinuous Monitoring of High‐Rise Buildings Using Seismic Interferometry

The linear seismic response of a building is commonly extracted from ambient vibration measurements. Seismic deconvolution interferometry performed on ambient vibrations can be used to estimate the dynamic characteristics of a building, such as its shear-wave velocity and its damping. The continuous nature of the ambient vibrations allows us to measure these parameters repeatedly and to observe their temporal variations. We used 2 weeks of ambient vibrations, recorded by 36 accelerometers that were installed in the Green Building at the Massachusetts Institute of Technology campus, to monitor the shear wavespeed and the apparent attenuation factor of the building. Because of the low strain of the ambient vibrations, we observed small speed changes followed by recoveries. We showed that measuring the velocity variations for the deconvolution functions, filtered around the fundamental-mode frequency, is equivalent to measuring the wandering of the fundamental frequency in the raw ambient vibration data. By comparing these results with local weather parameters, we showed that the air humidity is the dominating factor in the velocity variations of the waves in the Green Building, as well as the main force behind the wandering of the fundamental mode. The one-day periodic variations are affected by both the temperature and the humidity. The apparent attenuation, measured as the exponential decay of the fundamental-mode waveforms, is strongly biased due to the amplitude of the raw vibrations and shows a more complex behavior with respect to the weather measurements. We have also detected normal-mode nonlinear interaction for the Green Building, likely due to heterogeneity or anisotropy of its structure. We found that the temporal behavior of the frequency singlets may be used for monitoring.