Imaging deep-mantle plumbing beneath La Réunion and Comores hot spots: Vertical plume conduits and horizontal ponding zones.

Whether the two large low-shear velocity provinces (LLSVPs) at the base of Earth's mantle are wide compact structures extending thousands of kilometers upward or bundles of distinct mantle plumes is the subject of debate. Full waveform shear wave tomography of the deep mantle beneath the Indian Ocean highlights the presence of several separate broad low-velocity conduits anchored at the core-mantle boundary in the eastern part of the African LLSVP, most clearly beneath La Réunion and Comores hot spots. The deep plumbing system beneath these hot spots may also include alternating vertical conduits and horizontal ponding zones, from 1000-km depth to the top of the asthenosphere, reminiscent of dyke and sills in crustal volcanic systems, albeit at a whole-mantle scale.

Evidence of reactivation of a hydrothermal system from seismic anisotropy changes.

Seismic velocity measurements have revealed that the Tohoku-Oki earthquake affected velocity structures of volcanic zones far from the epicenter. Using a seismological method based on ambient seismic noise interferometry, we monitored the anisotropy in the Mount Fuji area during the year 2011, in which the Tohoku-Oki earthquake occurred (Mw = 9.0). Here we show that even at 400 km from the epicenter, temporal variations of seismic anisotropy were observed. These variations can be explained by changes in the alignment of cracks or fluid inclusions beneath the volcanic area due to stress perturbations and the propagation of a hydrothermal fluid surge beneath the Hakone hydrothermal volcanic area. Our results demonstrate how a better understanding of the origin of anisotropy and its temporal changes beneath volcanoes and in the crust can provide insight into active processes, and can be used as part of a suite of volcanic monitoring and forecasting tools.

Observations and modeling of the elastogravity signals preceding direct seismic waves.

After an earthquake, the earliest deformation signals are not expected to be carried by the fastest (P) elastic waves but by the speed-of-light changes of the gravitational field. However, these perturbations are weak and, so far, their detection has not been accurate enough to fully understand their origins and to use them for a highly valuable rapid estimate of the earthquake magnitude. We show that gravity perturbations are particularly well observed with broadband seismometers at distances between 1000 and 2000 kilometers from the source of the 2011, moment magnitude 9.1, Tohoku earthquake. We can accurately model them by a new formalism, taking into account both the gravity changes and the gravity-induced motion. These prompt elastogravity signals open the window for minute time-scale magnitude determination for great earthquakes.

Prompt gravity signal induced by the 2011 Tohoku-Oki earthquake.

Transient gravity changes are expected to occur at all distances during an earthquake rupture, even before the arrival of seismic waves. Here we report on the search of such a prompt gravity signal in data recorded by a superconducting gravimeter and broadband seismometers during the 2011 Mw 9.0 Tohoku-Oki earthquake. During the earthquake rupture, a signal exceeding the background noise is observed with a statistical significance higher than 99% and an amplitude of a fraction of µGal, consistent in sign and order of magnitude with theoretical predictions from a first-order model. While prompt gravity signal detection with state-of-the-art gravimeters and seismometers is challenged by background seismic noise, its robust detection with gravity gradiometers under development could open new directions in earthquake seismology, and overcome fundamental limitations of current earthquake early-warning systems imposed by the propagation speed of seismic waves.

Global surface wave tomography using seismic hum.

The development of global surface wave tomography using earthquakes has been crucial to exploration of the dynamic status of Earth's deep. It is naturally believed that only large earthquakes can generate long-period seismic waves that penetrate deep enough into Earth for such exploration. The discovery of seismic hum, Earth's background free oscillations, which are randomly generated by oceanic and/or atmospheric disturbances, now provides an alternative approach. We present results of global upper-mantle seismic tomography using seismic hum and without referring to earthquakes. At periods of 100 to 400 seconds, the phase-velocity anomalies of Rayleigh waves are measured by modeling the observed cross-correlation functions between every pair of stations from among 54 globally distributed seismic stations. The anomalies are then inverted to obtain the three-dimensional S-wave velocity structure in the upper mantle. Our technique provides a new means for exploring the three-dimensional structure of the interior of terrestrial planets with an atmosphere and/or oceans, particularly Mars.