Luigi Burlini

Fault-induced seismic anisotropy by hydration in subducting oceanic plates

The variation of elastic-wave velocities as a function of the direction of propagation through the Earth’s interior is a widely documented phenomenon called seismic anisotropy. The geometry and amount of seismic anisotropy is generally estimated by measuring shear-wave splitting, which consists of determining the polarization direction of the fast shear-wave component and the time delay between the fast and slow, orthogonally polarized, waves. In subduction zones, the teleseismic fast shear-wave component is oriented generally parallel to the strike of the trench, although a few exceptions have been reported (Cascadia and restricted areas of South America). The interpretation of shear-wave splitting above subduction zones has been controversial and none of the inferred models seems to be sufficiently complete to explain the entire range of anisotropic patterns registered worldwide. Here we show that the amount and the geometry of seismic anisotropies measured in the forearc regions of subduction zones strongly depend on the preferred orientation of hydrated faults in the subducting oceanic plate. The anisotropy originates from the crystallographic preferred orientation of highly anisotropic hydrous minerals (serpentine and talc) formed along steeply dipping faults and from the larger-scale vertical layering consisting of dry and hydrated crust–mantle sections whose spacing is several times smaller than teleseismic wavelengths. Fault orientations and estimated delay times are consistent with the observed shear-wave splitting patterns in most subduction zones.

Characterizing an unstable mountain slope using shallow 2D and 3D seismic tomography

As transport routes and population centers in mountainous areas expand, risks associated with rockfalls and rockslides grow at an alarming rate. As a consequence, there is an urgent need to delineate mountain slopes susceptible to catastrophic collapse in a safe and noninvasive manner. For this purpose, we have developed a 3D tomographic seismic refraction technique and applied it to an unstable alpine mountain slope, a significant segment of which is moving at 0.01–0.02 m∕year toward the adjacent valley floor. First arrivals recorded across an extensive region of the exposed gneissic rock mass have extraordinarily low apparent velocities at short (0.2 m) to long (>100 m) shot-receiver offsets. Inversion of the first-arrival traveltimes produces a 3D tomogram that reveals the presence of a huge volume of very-low-quality rock with ultralow to very low P-wave velocities of 500–2700 m∕s . These values are astonishingly low compared to the average horizontal P-wave velocity of 5400 m∕s determined from labora...

Volcanic Symphony in the Lab

Analysis of acoustic signals from lab samples links rapid pressure drops of pore fluids with low-frequency volcanic earthquakes.