ALBERT

Description: The muographic imaging of volcanoes relies on the measured transmittance of the atmospheric-muon flux through the target. An important bias affecting the result comes from background contamination mimicking a higher transmittance. The MU-RAY and TOMUVOL collaborations measured independently in 2013 the atmospheric muon flux transmitted through the Puy de Dôme volcano using their early prototype detectors, based on plastic scintillators and on Glass Resistive Plate Chambers, respectively. These detectors had three (MU-RAY) or four (TOMUVOL) detection layers of 1 m 2 each, tens (MU-RAY) or hundreds (TOMUVOL) of ns time resolution, a few mm position resolution, an energy threshold of few hundreds MeV and no particle identification capabilities. The prototypes were deployed about 1.3 km away from the summit, where they measured, behind rock depths larger than 1000 m, remnant fluxes of 1.83±0.50(syst)±0.07(stat) m −2 day −1 deg −2 (MU-RAY) and 1.95±0.16(syst)±0.05(stat) m −2 day −1 deg −2 (TOMUVOL), that roughly correspond to the expected flux of high-energy atmospheric muons crossing 600 metres water equivalent (m.w.e) at 18° elevation. This implies that imaging depths larger than 500 m.w.e from 1 km away using such prototype detectors suffers from an overwhelming background. These measurements confirm that a new generation of detectors with higher momentum threshold, time-of-flight measurement and/or particle identification is needed. The MU-RAY and TOMUVOL collaborations expect shortly to operate improved detectors, suitable for a robust muographic-imaging of kilometre scale volcanoes.

Description: Gravimetric methods are expected to play a decisive role in geophysical modeling of the regional crustal structure applied to geoneutrino studies. GIGJ (GOCE Inversion for Geoneutrinos at JUNO) is a 3-D numerical model constituted by ~46 × 103 voxels of 50 × 50 × 0.1 km, built by inverting GOCE (Gravity field and steady-state Ocean Circulation Explorer) gravimetric data over the 6° × 4° area centered at the JUNO (Jiangmen Underground Neutrino Observatory) experiment, currently under construction in the Guangdong Province (China). The a priori modeling is based on the adoption of deep seismic sounding profiles, receiver functions, teleseismic P wave velocity models, and Moho depth maps, according to their own accuracy and spatial resolution. The inversion method allowed for integrating GOCE data with the a priori information and some regularization conditions through a Bayesian approach and a stochastic optimization. GIGJ fits the highly accurate and homogeneously distributed GOCE gravity data with a ~1 mGal standard deviation of the residuals, compatible with the observation accuracy. GIGJ provides a site-specific subdivision of the crustal layers masses, of which uncertainties include estimation errors, associated to the gravimetric solution, and systematic uncertainties, related to the adoption of a fixed sedimentary layer. A consequence of this local rearrangement of the crustal layer thicknesses is a ~21% reduction and a ~24% increase of the middle and lower crust geoneutrino signal, respectively. The geophysical uncertainties of geoneutrino signals at JUNO produced by unitary uranium and thorium abundances distributed in the upper, middle, and lower crust are reduced by 77%, 55%, and 78%, respectively. The numerical model is available at this site (http://www.fe.infn.it/radioactivity/GIGJ).

Description: Abstract Layer 2A, the porous and permeable uppermost igneous oceanic crust, permits the circulation of fluid within the crust, the exchange of dissolved mineral species between the ocean and crust, and the convective dissipation of heat from the crust. We examine the presence, temporal extent, thickness, and evolution of layer 2A using multichannel seismic data collected at 30°S in the South Atlantic across crustal age ranges of 0–70 Ma and half spreading rates of 12–31 mm/year. We observe the layer 2A/2B boundary in 0–48 Myr old crust but not in crust older than ~48 Ma. The thickness of layer 2A in the South Atlantic has substantial variability, with a mean of 760 m and a standard deviation of 290 m. Layer 2A has no systematic change in thickness with age in the South Atlantic, and thickness does not correlate with spreading rate. The crust in the South Atlantic is never fully sealed by sediment cover, which implies that the fluid circulation system in the upper crust never becomes fully closed and the thickness of layer 2A can work as a proxy for the depth at which significant circulation can occur. The disappearance of the layer 2A/2B boundary in older crust implies that fluid circulation within the upper crust continues to occur for at least ~48 Myr after crustal formation in the South Atlantic, after which layer 2A becomes indistinguishable from layer 2B in reflection images.

Description: This work arises from the field observations made during the civil protection emergency period connected to the 2007 Stromboli eruption. We observed changes in the shallow feeding system of the volcano to which we give a volcanological interpretation and the relative implications. Here we describe the processes occurred in the upper feeding system from the end of the 2007 effusive eruption on April 3 until the renewal of the strombolian explosive activity at the summit craters (June 30), interpreted using multidisciplinary data. We used thermal camera data collected both from helicopter and from a fixed station at 400 m to retrieve the evolving summit crater activity. These data, compared with seismic signals and published geochemical records, allowed us to detail the shifting of the degassing activity within the crater terrace from NE to SW, occurred between April 15 and 25, 2007 prior to the resumption of the strombolian activity. In particular, from mid-April a gradual SW displacement in the maximum apparent temperatures was recorded at the vents within the summit craters, together with a change in the VLP location and confirmed by variations in geochemical indicators (CO 2 /SO 2 plume ratios and CO 2 fluxes) from literature. The shallow feeding system experienced a major readjustment after the end of the effusive activity, determining variations in the pressure leakage of the source, slowly deepening and shifting toward SW. All these data, together with the framework supplied by previous structural surveys, allowed us to propose that the compaction of debris accumulated in the uppermost conduit by inward crater collapses, occurred in early March, produced the observed anomalies. At Stromboli, major morphology changes, taking place in the following years, were anticipated by these small and apparently minor processes occurred in the upper feeding system. Other studies are relating similar changes to modifications of the eruptive activity also at other open-conduit volcanoes, so we believe that it may be important to have a constant monitoring of these phenomena in order to better understand their shallow feeding systems.

Description: Abstract Plastic deformation of olivine at relatively low temperatures (i.e., low‐temperature plasticity) likely controls the strength of the lithospheric mantle in a variety of geodynamic contexts. Unfortunately, laboratory estimates of the strength of olivine deforming by low‐temperature plasticity vary considerably from study to study, limiting confidence in extrapolation to geological conditions. Here we present the results of deformation experiments on olivine single crystals and aggregates conducted in a deformation‐DIA at confining pressures of 5 to 9 GPa and temperatures of 298 to 1473 K. These results demonstrate that, under conditions in which low‐temperature plasticity is the dominant deformation mechanism, fine‐grained samples are stronger at yield than coarse‐grained samples, and the yield stress decreases with increasing temperature. All samples exhibited significant strain hardening until an approximately constant flow stress was reached. The magnitude of the increase in stress from the yield stress to the flow stress was independent of grain size and temperature. Cyclical loading experiments revealed a Bauschinger effect, wherein the initial yield strength is higher than the yield strength during subsequent cycles. Both strain hardening and the Bauschinger effect are interpreted to result from the development of back stresses associated with long‐range dislocation interactions. We calibrated a constitutive model based on these observations, and extrapolation of the model to geological conditions predicts that the strength of the lithosphere at yield is low compared to previous experimental predictions but increases significantly with increasing strain. Our results resolve apparent discrepancies in recent observational estimates of the strength of the oceanic lithosphere.

Description: Abstract Beneath southwestern Colombia, intermediate‐depth earthquakes in the Cauca cluster locate in the subducting Nazca plate and in two columns extending ~40‐km into the mantle wedge above the slab. To investigate the cluster, we determine focal mechanisms for 69 small‐to‐moderate‐sized (2.3 ≤ Ml ≤ 4.7) earthquakes in the cluster by fitting short‐period body wave waveforms using the cut‐and‐paste method. The focal mechanisms have various faulting types and variably oriented nodal planes. We invert the focal mechanisms of the intraslab earthquakes for the intraslab stress field but cannot fit the region with a homogeneous stress tensor. We find that the principal stress axes rotate with the slab geometry, which has a concave shape and increases in dip angle from north to south. The northern region has slab normal compression and similar‐magnitude maximum and intermediate principal stresses. The minimum stress axis is oriented ~41° counterclockwise from the downdip direction. In the steeper southern region, the intermediate stress axis orients in the downdip direction. Deviation from a typical downdip extensional stress field may result from a buoyant young slab, an eastward mantle flow push, and/or along‐strike compression from the concave shape of the slab. This stress field would allow slip along preexisting faults of various orientations, such as the trench‐perpendicular seafloor features presently observed offshore, and contribute to the apparent heterogeneity of the intraslab stress field. The mantle wedge earthquakes also have various focal mechanisms but tend to have a subvertical nodal plane that aligns with the earthquake locations.

Description: Abstract We investigate the 4 April 2010 Mw 7.2 El Mayor‐Cucapah (Mexico) earthquake using three‐dimensional surface deformation computed from preevent and postevent airborne lidar topography. By profiling the E‐W, N‐S, and vertical displacement fields at densely sampled (∼300 m) intervals along the multisegment rupture and computing fault offsets in each component, we map the slip vector along strike. Because the computed slip vectors must lie on the plane of the fault, whose local strike is known, we calculate how fault dip changes along the rupture. A principal goal is to resolve the discrepancy between field‐based inferences of widespread low‐angle (〈30°) oblique‐normal slip beneath the Sierra Cucapah, and geodetic and/or seismological models which support steeper (50°–75°) faulting in this area. Our results confirm that low‐angle slip occurred along a short (∼2 km) stretch of the Paso Superior fault—where the three‐dimensional rupture trace is also best fit by gently inclined planes—as well as along shorter (∼1 km) section of the Paso Inferior fault. We also characterize an ∼8‐km fault crossing the Puerta accommodation zone as dipping ∼60°NE with slip of ∼2 m. These results indicate that within the northern Sierra Cucapah, deep‐seated rupture of steep faults (resolved by coarse geodetic models) transfers at shallower depths onto low‐angle structures. We also observe a statistically significant positive correlation between fault dip and slip, with slip pronounced along steep sections of fault and inhibited along low‐angle sections. This highlights the important role of local structural fabric in controlling the surface expression of large earthquakes.

Description: Abstract The Laguna del Maule (LdM) volcanic field comprises the greatest concentration of post‐glacial rhyolite in the Andes, and includes the products of ~40 km3 of explosive and effusive eruptions. Recent observations at LdM by InSAR and GPS geodesy have revealed inflation at rates exceeding 20 cm/yr since 2007, capturing an ongoing period of growth of a potentially large upper crustal magma reservoir. Moreover, magnetotelluric and gravity studies indicate the presence of fluids and/or partial melt in the upper crust near the center of inflation. Petrologic observations imply repeated, rapid extraction of rhyolitic melt from crystal mush stored at depths of 4‐6 km during at least the past 26 ka. We utilize multiple types of surface wave observations to constrain the location and geometry of low velocity domains beneath LdM. We present a 3D shear‐wave velocity model that delineates a ~450 km3 shallow magma reservoir (~2 to 8 km below surface) with an average melt fraction of ~5%. Interpretation of the seismic tomography in light of existing gravity, magnetotelluric, and geodetic observations supports this model and reveals variations in melt content and a deeper magma system feeding the shallow reservoir in greater detail than any of the geophysical methods alone. Geophysical imaging of the LdM magma system today is consistent with the petrologic inferences of the reservoir structure and growth during the past 20‐40 kyr. Taken together with the ongoing unrest, a future rhyolite eruption of at least the scale of those common during the Holocene is a reasonable possibility.

Description: Abstract We performed a series of deformation experiments on synthetic magnetite aggregates to characterize the high‐temperature rheological behavior of this mineral under nominally dry and hydrous conditions. Grain growth laws for magnetite were additionally determined from a series of static annealing tests. Synthetic magnetite aggregates were formed by hot isostatic pressing of fine‐grained magnetite powder at 1100° C temperature and 300 MPa confining pressure for 20 h, resulting in polycrystalline material with a mean grain size around 40μm and containing 2–4% porosity. Samples were subsequently deformed to axial strains of up to 10% under constant load conditions at temperatures between 900 and 1150° C in a triaxial deformation apparatus under 300 MPa confining pressure at applied stresses in the range of 8–385 MPa or in a uniaxial creep rig at atmospheric pressure with stresses of 1–15 MPa. The aggregates exhibit typical power‐law creep behavior with a mean stress exponent of 3 at high stresses, indicating a dislocation creep mechanism, and a transition to near‐Newtonian creep with a mean stress exponent of 1.1 at lower stresses. The presence of water in the magnetite samples resulted in significantly enhanced static grain growth and strain rates. Best‐fit flow laws to the data indicate activation energies of around 460 and 310 kJ/mol for dislocation and diffusion creep of nominally dry magnetite, respectively. Based on the experimentally determined flow laws, magnetite is predicted to be weaker than most major silicate phases in relatively dry rocks such as oceanic gabbros during high‐temperature crustal deformation.