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High-pressure fluid injections cause transient pore pressure changes over large distances, which may induce seismicity. The zone of influence for such an injection was studied at high spatial resolutions in six decameter-scaled fluid injection experiments in crystalline rock. Pore pressure time series revealed two distinct responses based on the lag time and magnitude of pressure change, namely, a near- and far-field response. The near-field response is due to pressure diffusion. In the far-field, the fast response time and decay of pressure changes are produced by effective stress changes in the anisotropic stress field. Our experiments confirm that fracture fluid pressure perturbations around the injection point are not limited to the near field and can extend beyond the pressurized zone., Geophysical Research Letters, 48 (6), ISSN:0094-8276, ISSN:1944-8007

The transport of chemically reactive fluids through fractured clay-rich rocks is fundamental to many subsurface engineering technologies. Here, we present results of direct-shear laboratory experiments with simultaneous imaging by X-ray Computed Tomography in Opalinus claystone with subsequent fluid injection to unravel the interplay between mechanical fracture deformation, fluid sorption, and flow. Under constant radial stress (σc = 1.5 MPa), the average mechanical aperture (Formula presented.) increases with shear displacement. Upon brine injection, (Formula presented.) is reduced by 40% relative to initial conditions ((Formula presented.) μm) and fluid-sorption induces a divergent displacement of the two sample halves (Δh = ±50 − 170 μm) quantified by digital image correlation. None of these changes are observed in a control experiment with decane, indicating that creep is subordinate to swelling in sealing the fracture. Swelling-induced changes in permeability within the fracture are heterogeneous and largely affect the fracture flow field, as computed using numerical simulations., Geophysical Research Letters, 48 (5), ISSN:0094-8276, ISSN:1944-8007

Geophysical Research Letters, 45 (5), ISSN:0094-8276, ISSN:1944-8007

Geophysical Research Letters, 45 (19), ISSN:0094-8276, ISSN:1944-8007

We spatially map the shallow to deep b value ratio in the crust in California. Previous studies of the frequency magnitude distribution, as a function of depth, for selected crustal regions indicated that b decreases from b > 1.1 in the 0–5 km depth range to b < 0.8 in the depth range 7–15 km. Our detailed mapping confirms that this pattern can be established at the 99% significance level for about 32% of the entire seismically active crust. About 2% of the crust displays the opposite b-gradient. One such area is the San Francisco Bay Area. We speculate that differences in stress levels are the main factor controlling the depth dependency of b. These results confirm that the b value should not always be considered a constant in studies such as seismic hazard estimations.

We use a finite element model incorporating plate motion boundary conditions, fault constraints, and space geodetic velocities to calculate eastern Mediterranean plate kinematics and to estimate the motion of the region's major faults. We then use subsets of these constraints to generate models testing different scenarios of driving forces (slab roll-back at the Hellenic arc, Arabian plate push, gravitational collapse of W. Anatolia) to match the expected pattern of deformation. We find that models combining only trench rollback and Arabian push are unable to provide a good match to the observed deformation across western Anatolia and the Aegean and that we need to introduce the effects of mantle corner flow and gravitational collapse to fit observations.

We examined the source characteristics of the 1994 deep earthquakes of Bolivia (June 9) and Fiji (March 9) over a period range of 1–1000 s using moment tensor and body wave analyses. Moment tensor inversions from 100–1000 s reveal that these two earthquakes were essentially point sources at very long period with insignificant isotropic and non-double couple components with scalar moments Mo = 2.56 × 1021 Nm for the Bolivia and Mo = 0.26 × 1021 Nm for the Fiji earthquakes. Body wave analysis for the Bolivia event reveals a 45 s duration, a nearly linear E-NE 1 km/s rupture on the subhorizontal focal mechanism plane giving a stress drop Δσ = 114 MPa. For the Fiji earthquake we find a change in focal mechanism and a linear 3 km/s rupture plunging shallowly to the north on the steep dipping nodal plane with a duration of at least 20 s and Δσ = 14 MPa. Both ruptures propagated parallel their null (σ2) axis and suggests that their width is controlled by the thickness of the seismogenic volume in deep slabs.

The June 9, 1994 Bolivia earthquake (Mw = 8.2) occurred in the pronounced bend in the morphology of the deep South American slab, where several dynamic arguments can be put forward to explain the apparently anomalous mechanism and the horizontal fault plane; the size of the event is huge but expected when the regional seismicity of the whole century is considered. The March 9, 1994 Fiji earthquake (Fiji = 7.5) suggests that a new mode of seismic deformation is active in the northernmost termination of the Tonga-Fiji slab, cutting across the dense cluster of seismicity of the last 30 years; the size of the event is probably close to the largest to be expected over a century in this area, having the steepest frequency-moment distribution of all global deep seismicity.

[1] Inverted models of the deep mantle show a decorrelation between maps of shear VS and compressional VP wave velocities, an anti-correlation between the bulk sound velocity Vϕ and VS and a much larger variability of VS with respect to VP, expressed by large values of the ratio of their relative lateral variations. We carried out synthetic tests to verify if these features could be artifacts, explained by limits in tomographic resolution: synthetic data are calculated for an “input” model, and linearly inverted, as in tomography, to find an “output” model. Comparing the values of the aforementioned parameters for two different chemically homogeneous input models with the associated reconstructed output ones, we found that artifacts caused by realistic data noise and the nonuniform distribution of seismic sources and stations over the globe are not sufficient to introduce the features previously described. We confirm that compositional effects are required to explain them.

We develop a method for the rapid determination of the moment tensor of large worldwide earthquakes, by inversion of long-period data recorded by the MedNet regional network, and we apply it to the most significant events of 1990: March 3 (Fiji) and 25 (Costa Rica), May 12 (Sakhalin), 20 (Sudan) and 30 (Romania), June 20 (Iran), July 16 (Luzon) and December 30 (New Britain). The comparison with CMTs and moment tensors by USGS shows that an advanced regional seismic network is capable of providing reliable and rapid control of the seismic source for worldwide seismicity.

[1] We invert a global phase-anomaly database of intermediate- to long-period Rayleigh waves, recently updated with increased coverage in the European-Mediterranean region, on a global scale with a higher resolution parameterization in the region of interest. We first compare phase-velocity inversions based on ray and finite-frequency theory and derive for each a corresponding set of local phase-velocity dispersion curves (one per model pixel) between 35 s to 300 s period. Effects of the two different theories on the three-dimensional upper-mantle structure are investigated by inverting each dispersion curve for radial shear-velocity profiles. The combination of a gradient-descent method and a random-Monte-Carlo model search provides an estimated shear-velocity model with associated uncertainties for depths between 40 km to 400 km. While differences between ray-theoretical and finite-frequency models are small compared to model uncertainty, comparisons with independent models favor the finite-frequency one.

[1] To improve the fidelity of European tomographic images, we implement variable resolution parameterization, finite-frequency and ray theory, and improved data coverage in new tomographic inversions. We supplement an existing high-quality database of global teleseismic phase velocity observations of both Love and Rayleigh waves with a set of new phase delay observations recorded by a dense array of stations in the region. We invert these data to find global phase velocity maps, characterized by a more densely parameterized Europe and Mediterranean. We compare our seismic images with laterally variable independent geophysical observables, including crustal thickness, gravity, and heat flow and find correlations between 0.25 and 0.6. While at most periods finite-frequency and ray-theory yield almost coincident results, we find some possibly significant differences in the phase velocity of 150s Rayleigh waves. Our results represent significant progress toward high-resolution modeling of upper mantle S-velocity structure, with the possibility of also constraining seismic anisotropy.

[1] We present new evidence for the existence of the Love-Rayleigh discrepancy in the Mediterranean region and constrain the average polarization anisotropic structure of the Mediterranean mantle. We analysed regional Rayleigh and Love waveforms recorded at 3-component broadband seismic stations. None of the 3-component seismograms could be fit with a single 1D isotropic and smooth velocity model. However, satisfactory fits for individual Rayleigh and Love waveforms could often be obtained using realistic 1D velocity models. We used these 1D path-average velocity structures to derive 3D S-velocity models for the Mediterranean region. Our results show that Love waves require higher velocities (about 200 m/s) compared to Rayleigh waves between 30 and 120 km depth. We relate the observed anisotropy to lattice-preferred orientation of crystallographic axes of elastically anisotropic minerals such as olivine.

[1] Southern Italy is a key area for understanding the tectonic processes in the Africa-Eurasia collision zone. We analyze new GPS measurements carried out between 1994 and 2001. The results are presented in terms of time series, trajectories and velocities of crustal motion, as well as a geodetic strain rate field. While central Italy, Corsica, Sardinia and the Tyrrhenian Sea move like the Eurasian plate, the overall motion of the Sicily Rift Zone region matches African plate motion. Unexpected are the north-northwest directed motions of northeastern Sicily and the Eolian islands. Most striking on the deformation field are a north-south oriented compression along the northern Sicilian coast, compression between Apulia and northwestern Greece as well as extension in the Sicily Rift Zone and the interior of Sicily.