Rayleigh wave dispersion measurements reveal low-velocity zones beneath the new crust in the Gulf of California

Rayleigh wave tomography provides images of the shallow mantle shear wave velocity structure beneath the Gulf of California. Low-velocity zones (LVZs) are found on axis between 26 and 50 km depth beneath the Guaymas Basin but mostly off axis under the other rift basins, with the largest feature underlying the Ballenas Transform Fault. We interpret the broadly distributed LVZs as regions of partial melting in a solid mantle matrix. The pathway for melt migration and focusing is more complex than an axis-centered source aligned above a deeper region of mantle melt and likely reflects the magmatic evolution of rift segments. We also consider the existence of solid lower continental crust in the Gulf north of the Guaymas Basin, where the association of the LVZs with asthenospheric upwelling suggests lateral flow assisted by a heat source. These results provide key constraints for numerical models of mantle upwelling and melt focusing in this young oblique rift

Seismic structure beneath the Gulf of California: a contribution from group velocity measurements

Rayleigh wave group velocity dispersion measurements from local and regional earthquakes are used to interpret the lithospheric structure in the Gulf of California region. We compute group velocity maps for Rayleigh waves from 10 to 150 s using earthquakes recorded by broad-band stations of the Network of Autonomously Recording Seismographs in Baja California and Mexico mainland, UNM in Mexico, BOR, DPP and GOR in southern California and TUC in Arizona. The study area is gridded in 120 longitude cells by 180 latitude cells, with an equal spacing of 10 × 10 km. Assuming that each gridpoint is laterally homogeneous, for each period the tomographic maps are inverted to produce a 3-D lithospheric shear wave velocity model for the region. Near the Gulf of California rift axis, we found three prominent low shear wave velocity regions, which are associated with mantle upwelling near the Cerro Prieto volcanic field, the Ballenas Transform Fault and the East Pacific Rise. Upwelling of the mantle at lithospheric and asthenospheric depths characterizes most of the Gulf. This more detailed finding is new when compared to previous surface wave studies in the region. A low-velocity zone in northcentral Baja at ∼28ºN which extends east–south–eastwards is interpreted as an asthenospheric window. In addition, we also identify a well-defined high-velocity zone in the upper mantle beneath central-western Baja California, which correlates with the previously interpreted location of the stalled Guadalupe and Magdalena microplates. We interpret locations of the fossil slab and slab window in light of the distribution of unique post-subduction volcanic rocks in the Gulf of California and Baja California. We also observe a high-velocity anomaly at 50-km depth extending down to ∼130 km near the southwestern Baja coastline and beneath Baja, which may represent another remnant of the Farallon slab

Normal faults and thrusts re-activated by deep fluids: the 6 April 2009 Mw 6.3 L’Aquila earthquake, central Italy.

On April 6 2009, a Mw=6.3 earthquake occurred in the central Apennines (Italy) damaging L’Aquila city and the surrounding country. We relocate the October 2008-April 6 2009 foreshocks and about 2000 aftershocks occurred between April 6 and April 30 2009, by applying a double-difference technique and determine the stress field from focal mechanisms. The events concentrate in the upper 15 km of the crust. Three main NW-SE to NNW-SSE striking, 30°-45° and 80°-90° dipping faults activate during the seismic sequence. Among these, a normal fault and a thrust were re-activated with dip-slip movements in response to NE-SW extension. The structural maturity of the seismogenic fault system is lower than that displayed by other systems in southern Apennines, because of the lower strain rate of the central sector of the chain with respect to the southern one. VP/VS increases progressively from October 2008 to the April 6 2009 mainshock occurrence along a NW-SE strike due to an increment in pore fluid pressure along the fault planes. Pore pressure diffusion controls the space-time evolution of aftershocks. A hydraulic diffusivity of 80 m2/s and a seismogenic permeability of about 10-12 m2 suggest the involvement of gas-rich (CO2) fluids within a highly fractured medium. Suprahydrostatic, high fluid pressure (about 200 MPa at 10 km of depth) within overpressurized traps, bounded by pre-existing structural and/or lithological discontinuities at the lower-upper crust boundary, are required to activate the April 2009 sequence. Traps are the storage zone of CO2-rich fluids uprising from the underlying, about 20 km deep, metasomatized mantle wedge. These traps easily occur in extensional regimes like in the axial sector of Apennines, but are difficult to form in strike-slip regimes, where sub-vertical faults may cross the entire crust. In the Apennines, fluids may activate faults responsible for earthquakes up to Mw=5-6. Deep fluids more than tectonic stress may control the seismotectogenesis of accretionary wedges

Mantle wedge dynamics vs crustal seismicity in the Apennines (Italy)

In the Apennines subduction (Italy), earthquakes mainly occur within overriding plate, along the chain axis. The events concentrate in the upper 15 km of the crust above the mantle wedge and focal solutions indicate normal faulting. In the foreland, the seismogenic volume affects the upper 35 km of the crust. Focal solutions indicate prevailing reverse faulting in the northern foreland and strike-slip faulting in the southern one. The deepening of the seismogenic volume from the chain axis to the foreland follows the deepening of the Moho and isotherms. The seismicity above the mantle wedge is associated with uplift of the chain axial zone, volcanism, high CO2 flux, and extension. The upward pushing of the asthenospheric mantle and the mantle-derived, CO2-rich fluids trapped within the crust below the chain axis causes this seismicity. All these features indicate that the axial zone of Apennines is affected by early rifting processes. In northern Italy, the widespread and deeper seismicity in the foreland reflects active accretion processes. In the southern foreland, the observed dextral strike-slip faulting and the lack of reverse focal solutions suggest that accretion processes are not active at present. In our interpretation of the Apennines subduction, the shallower seismicity of the overriding plate is due to the dynamics (uprising and eastward migration) of the asthenospheric wedge

Optics Solutions for the Collimation Insertion of LHC

While the two collimation insertions in the LHC must have similar basic layouts and match to almost identical dispersion suppressors to respect the geometry of the existing tunnel, their different roles impose opposite requirements on the normalized dispersion within them. For betatron collimation it must be near zero, while for momentum collimation it must have a peak at the location of the primary collimator, immediately after the dispersion suppressor. The insertion lattice solution found for the latter case requires up to 30% asymmetry in the quadrupole gradients (in line with the current trend in LHC lattice development to break the exact antisymmetry within insertions). To achieve this using twin-aperture warm quadrupoles, the modules making up each quadrupole will be wired in such a way that the two beams still see the same sequence of focusing fields. We describe the optimum setup, exibility and collimation quality for the two insertions

The Partial Visibility Representation Extension Problem

For a graph $G$, a function $\psi$ is called a \emph{bar visibility representation} of $G$ when for each vertex $v \in V(G)$, $\psi(v)$ is a horizontal line segment (\emph{bar}) and $uv \in E(G)$ iff there is an unobstructed, vertical, $\varepsilon$-wide line of sight between $\psi(u)$ and $\psi(v)$. Graphs admitting such representations are well understood (via simple characterizations) and recognizable in linear time. For a directed graph $G$, a bar visibility representation $\psi$ of $G$, additionally, puts the bar $\psi(u)$ strictly below the bar $\psi(v)$ for each directed edge $(u,v)$ of $G$. We study a generalization of the recognition problem where a function $\psi'$ defined on a subset $V'$ of $V(G)$ is given and the question is whether there is a bar visibility representation $\psi$ of $G$ with $\psi(v) = \psi'(v)$ for every $v \in V'$. We show that for undirected graphs this problem together with closely related problems are \NP-complete, but for certain cases involving directed graphs it is solvable in polynomial time.Comment: Appears in the Proceedings of the 24th International Symposium on Graph Drawing and Network Visualization (GD 2016

Cold AGS Snake Optimization by Modeling

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Neighborhoods of trees in circular orderings

In phylogenetics, a common strategy used to construct an evolutionary tree for a set of species X is to search in the space of all such trees for one that optimizes some given score function (such as the minimum evolution, parsimony or likelihood score). As this can be computationally intensive, it was recently proposed to restrict such searches to the set of all those trees that are compatible with some circular ordering of the set X. To inform the design of efficient algorithms to perform such searches, it is therefore of interest to find bounds for the number of trees compatible with a fixed ordering in the neighborhood of a tree that is determined by certain tree operations commonly used to search for trees: the nearest neighbor interchange (nni), the subtree prune and regraft (spr) and the tree bisection and reconnection (tbr) operations. We show that the size of such a neighborhood of a binary tree associated with the nni operation is independent of the tree’s topology, but that this is not the case for the spr and tbr operations. We also give tight upper and lower bounds for the size of the neighborhood of a binary tree for the spr and tbr operations and characterize those trees for which these bounds are attained

Rayleigh wave dispersion measurements reveal low-velocity zones beneath the new crust in the Gulf of California

Rayleigh wave tomography provides images of the shallow mantle shear wave velocity structure beneath the Gulf of California. Low-velocity zones (LVZs) are found on axis between 26 and 50 km depth beneath the Guaymas Basin but mostly off axis under the other rift basins, with the largest feature underlying the Ballenas Transform Fault. We interpret the broadly distributed LVZs as regions of partial melting in a solid mantle matrix. The pathway for melt migration and focusing is more complex than an axis-centered source aligned above a deeper region of mantle melt and likely reflects the magmatic evolution of rift segments. We also consider the existence of solid lower continental crust in the Gulf north of the Guaymas Basin, where the association of the LVZs with asthenospheric upwelling suggests lateral flow assisted by a heat source. These results provide key constraints for numerical models of mantle upwelling and melt focusing in this young oblique rift