ALBERT

Beschreibung: Combined synthesis experiments and first-principles calculations show that MgSiO 3 -perovskite with minor Al or Fe does not incorporate significant OH under lower mantle conditions. Perovskite, stishovite, and residual melt were synthesized from natural Bamble enstatite samples (Mg/(Fe + Mg) = 0.89 and 0.93; Al 2 O 3  〈 0.1 wt% with 35 and 2065 ppm wt H 2 O, respectively) in the laser-heated diamond anvil cell at 1600–2000 K and 25–65 GPa. Combined Fourier transform infrared (FTIR) spectroscopy, x-ray diffraction, and ex-situ transmission electron microscopy (TEM) analysis demonstrates little difference in the resulting perovskite as a function of initial water content. Four distinct OH vibrational stretching bands are evident upon cooling below 100 K (3576, 3378, 3274, and 3078 cm −1 ), suggesting 4 potential bonding sites for OH in perovskite with a maximum water content of 220 ppm wt H 2 O, and likely no more than 10 ppm wt H 2 O. Complementary, Fe-free, first-principles calculations predict multiple potential bonding sites for hydrogen in perovskite, each with significant solution enthalpy (0.2 eV/defect). We calculate that perovskite can dissolve less than 37 ppm wt H 2 O (400 ppm H/Si) at the top of the lower mantle, decreasing to 31 ppm wt H 2 O (340 ppm H/Si) at 125 GPa and 3000 K in the absence of a melt or fluid phase. We propose that these results resolve a long-standing debate of the perovskite melting curve and explain the order of magnitude increase in viscosity from upper to lower mantle.

Beschreibung: We analyze the influence of ocean tides on the triggering of low frequency earthquakes (LFEs) in northern Cascadia using 3 LFE catalogues for southern Vancouver Island and Washington state from episodic tremor and slip events between 2003-2013. Sensitivities of LFE families to tidally induced fault-normal stress, up-dip shear stress (UDSS) and corresponding time derivatives are computed and their geographic variability is mapped. We find localized areas showing higher sensitivity to UDSS than their surroundings suggesting that tidal sensitivity depends on laterally heterogeneous physical properties such as variable pore fluid pressures and frictional properties along the plate interface. We observe that sensitivity of LFEs to UDSS rises dramatically from near zero on the first day of strong activity to a maximum ~4 days later. In addition, the peak LFE rate transitions from a correlation with peak tidal shear stress rate to a correlation with peak tidal shear stress through large slow slip events. We identify 64 Rapid-Tremor-Reversals (RTRs) that start a few days after the main slip front. The RTRs have an average stress drop of ~0.8 kPa and a majority (72%) occur during periods of large positive UDSS. The combined observations imply that RTRs play an important role in slow slip processes and that modulation of creep rate due to tidal stress and tidal triggering of secondary events are jointly responsible for the observed tidal sensitivity.

Beschreibung: A seven-year time series of satellite radar images over Unimak Island, Alaska-site of Westdahl Volcano, Fisher Caldera and Shishaldin Volcano-was processed using a model-free Persistent Scatterer Interferometry (PSI) technique assisted by Numerical Weather Prediction (NWP) model. The deformation-only signals were optimally extracted from atmosphere-contaminated phase records. The reconstructed deformation time series maps are compared with campaign and continuous Global Positioning System (GPS) measurements as well as Small Baseline Subset (SBAS) InSAR results for quality assessment and geophysical interpretation. We observed subtle surface inflation at Westdahl Volcano that can be fit by a Mogi source located at approximately 3.6 km north of Westdahl peak and at depth of about 6.9 km that is consistent to the GPS estimated depth for the 1998 to 2001 time period. The magma chamber volume change decays during the period of 2003 to 2010. The deformation field over Fisher Caldera is steadily subsiding over time. Its best-fit analytical model is a sill source that is about 7.9 km in length, 0.54 km in width, and locates at about 5.5 km BSL underneath the center of Fisher Caldera with strike angle of N52°E. Very little deformation was detected near Shishaldin peak, however, a region approximately 15 km east of Shishaldin, as well as an area at the Tugamak range at about 30 km northwest of Shishaldin, show evidence for movement towards the satellite, with a temporal signature correlated with the 2004 Shishaldin eruption. The cause of these movements is unknown.

Beschreibung: We carried out measurements of the magnetic field vector at two sites during Integrated Ocean Drilling Program (IODP) Expedition 330 to the Louisville Seamount Chain. The aim was to impose constraints on the magnetization direction and to contribute to the reconstruction of possible hotspot motion. The measurements were conducted using the Göttingen Borehole Magnetometer (GBM). It comprises three fiber optic gyros (FOG) that can be used to reorient the magnetic field data. To improve accuracy, we are using a new algorithm that combines FOG data and data of two inclinometers. As can be evaluated by comparing downlog and uplog of the measurements, the three dimensional magnetic field data obtained is of good quality. An interpretation of the magnetic field data using a state of the art method based on horizontal layers yields results inconsistent with measurements of the natural remanent magnetization (NRM) of drill core samples. In the following, we define the magnetization from the horizontal layer as apparent magnetization and develop a new interpretation method based on dipping layers. Our method includes a new approximate forward modelling algorithm and considerably improves the consistency of the borehole measurements and the NRM data. We show that a priori information about the geometry of a layer is required to constrain the inclination and declination of magnetization. Especially the azimuth of a layer and the declination of magnetization cannot be determined separately. Using azimuth and layer dip information from borehole images, we obtain constraints on inclination and declination for one particular layer.

Beschreibung: During the 3 May 1887 M W 7.5 Sonora earthquake (surface rupture end-to-end length: 101.8 km), an array of three north–south striking Basin-and-Range Province faults (from north to south Pitáycachi, Teras, and Otates) slipped sequentially along the western margin of the Sierra Madre Occidental Plateau. This detailed field survey of the 1887 earthquake rupture zone along the Pitáycachi fault includes mapping the rupture scarp and measurements of surface deformation. The surface rupture has an endpoint-to-endpoint length of ≥41.0 km, dips ~70°W, and is characterized by normal − left-lateral extension. The maximum surface offset is 487 cm and the mean offset 260 cm. The rupture trace shows a complex pattern of second-order segmentation. However, this segmentation is not expressed in the 1887 along-rupture surface-offset profile, which indicates that the secondary segments are linked at depth into a single coherent fault surface. The Pitáycachi surface rupture shows a well-developed bipolar branching pattern suggesting that the rupture originated in its central part, where the polarity of the rupture bifurcations changes. Most likely the rupture first propagated bilaterally along the Pitáycachi fault. The southern rupture front likely jumped across a step-over to the Teras fault and from there across a major relay zone to the Otates fault. Branching probably resulted from the lateral propagation of the rupture after breaching the seismogenic part of the crust, given that the much shorter ruptures of the Otates and Teras segments did not develop branches.

Beschreibung: In this article, we calculate the seismic anisotropy resulting from melt redistribution during pure and simple shear deformation. Deformation strongly modifies the geometry of melts initially occupying three grain junctions. The initially isotropic fractional area of intergranular contact, contiguity, becomes anisotropic due to deformation. Consequently, the component of contiguity evaluated on the plane parallel to axis of maximum compressive stress decreases. During both modes of deformation, the trace of the contiguity tensor remains nearly unchanged. In the companion article [ Drombosky and Hier-Majumder , 2015, labeled DHM], we outline the numerical methods and present the synthetic micrographs from our numerical deformation experiments. In pure shear deformation, the principal contiguity directions remain stationary while they rotate during simple shear. The ratio between the principal components of the contiguity tensor decrease from 1 in an undeformed aggregate to 0.1 after 45% shortening in pure shear and to 0.3 after a shear strain of 0.75 in simple shear. In both pure and simple shear experiments anisotropy in the shear wave velocity increases with the strain in a strongly nonlinear fashion. In pure shear deformation, the steady-state microstructure produces nearly 3% anisotropy between shear waves vibrating perpendicular and parallel to the planes of melt films.

Beschreibung: The North Aegean core complexes developed in middle Eocene soon after the end of continental block convergence and piling-up of the Hellenic Thrust Wedge. They formed during back-arc extension, driven by the Hellenic slab rollback, at the back of the thrust wedge. A series of scaled laboratory experiments were performed to test whether the gravity spreading of a thrust wedge is a suitable process for the development of the North Aegean core complexes during back-arc extension. Wedge-shaped sand-silicon models with variable boundary displacement velocities and different geometries of the upper sand layer were used to study the effects of variations in wedge rheology on the pattern of extension. The models exemplify that extension, either distributed (wide rift mode) or localised (core complex mode), is always located at the wedge rear. Core complex development was favoured in models with thicker brittle layer (higher frictional strength) and low stretching rate (lower ductile strength). Both core complex location at the wedge rear and detachment location and dip are interdependent and intrinsically related to the initial wedge shape of the extending system. The experimental model displays striking similarities with the extensional pattern of the North Aegean in terms of: i) location, size and shape of core complexes as well as their sequence of development, and ii) detachment location and dip. We conclude that it is the initial wedge geometry of the system and the weak nature of the crust at the onset of extension that controlled the extensional pattern of the North Aegean Domain.

Beschreibung: Static stress changes following large earthquakes are known to affect the rate and distribution of aftershocks, yet this process has not been thoroughly investigated for nano and pico seismicity at centimeter length scales. Here we utilize a unique dataset of M ≥ −3.4 earthquakes following a M W 2.2 earthquake in Mponeng gold mine, South Africa that was recorded during a quiet interval in the mine to investigate if rate and state based modeling is valid for shallow, mining-induced seismicity. We use Dieterich's [1994] rate and state dependent formulation for earthquake productivity, which requires estimation of four parameters: (1) Coulomb stress changes due to the mainshock, (2) the reference seismicity rate, (3) frictional resistance parameter, and (4) the duration of aftershock relaxation time. Comparisons of the modeled spatio-temporal patterns of seismicity based on two different source models with the observed distribution show that while the spatial patterns match well, the rate of modeled aftershocks is lower than the observed rate. To test our model, we used three metrics of the goodness-of-fit evaluation. The null hypothesis, of no significant difference between modeled and observed seismicity rates, was only rejected in the depth interval containing the mainshock. Results show that mining-induced earthquakes may be followed by a stress relaxation expressed through aftershocks located on the rupture plane and in regions of positive Coulomb stress change. Furthermore, we demonstrate that the main features of the temporal and spatial distribution of very small, mining-induced earthquakes can be successfully determined using rate- and state-based stress modeling.

Beschreibung: No abstract is available for this article.

Beschreibung: Field observations show that hydraulic fracture growth in naturally fractured formations like shale is complex. Pre-existing discontinuities in shale, including natural fractures and bedding, act as planes of weakness that divert fracture propagation. To investigate the influence of weak planes on hydraulic fracture propagation, we performed Semi-Circular Bend (SCB) tests on Marcellus shale core samples containing calcite-filled natural fractures (veins). The approach angle of the induced fracture to the veins and the thickness of the veins have a strong influence on propagation. As the approach angle becomes more oblique to the induced fracture plane, and as the vein gets thicker, the induced fracture is more likely to divert into the vein. Microstructural analysis of tested samples shows that the induced fracture propagates in the middle of the vein but not at the interface between vein and the rock matrix. Cleavage planes and fluid inclusion trails in the vein cements exert some control on the fracture path. Combining the experimental results with theoretical fracture-mechanics arguments, the fracture toughness of the calcite veins was estimated to range from 0.24 MPa∙m 1/2 to 0.83 MPa∙m 1/2 , depending on the value used for the Young's modulus of the calcite vein material. Measured fracture toughness of unfractured Marcellus shale was 0.47 MPa∙m 1/2 .