1,823 research outputs found

    Normal and Reverse Faulting Driven by the Subduction Zone Earthquake Cycle in the Northern Chilean Forearc

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    Despite its location in a convergent tectonic setting, the Coastal Cordillera of northern Chile between 21°S and 25°S is dominated by structures demonstrating extension in the direction of plate convergence. In some locations, however, normal faults have been reactivated as reverse faults, complicating the interpretation of long-term strain. In order to place these new observations in a tectonic context, we model stress changes induced on these faults by the subduction earthquake cycle. Our simulations predict that interseismic locking on the plate boundary encourages normal slip on fore-arc faults, which may result from elastic rebound due to interplate earthquakes or from seismic or aseismic motion that takes place within the interseismic period. Conversely, stress generated by strong subduction zone earthquakes, such as the 1995 Mw = 8.1 Antofagasta event, provides a mechanism for the reverse reactivation we document here. Upper plate fault slip in response to the low-magnitude stress changes induced by the subduction earthquake cycle suggests that the absolute level of stress on these faults is very low. Furthermore, seismic hazard analysis for northern Chile requires consideration of not only the plate boundary earthquake cycle but also the cycle on fore-arc faults that may or may not coincide with the interplate pattern. Though the relationships between permanent strain and deformation calculated using elastic models remain unclear, the compatibility of modeled stress fields with the distribution of fore-arc faulting suggests that interseismic strain accumulation and coseismic deformation on the subduction megathrust both play significant roles in shaping structural behavior in the upper plate

    Slip Distribution of the 2014 Mw=8.1 Pisagua, Northern Chile, Earthquake Sequence Estimated From Coseismic Fore-Arc Surface Cracks

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    The 2014 Mw = 8.1 Iquique (Pisagua), Chile, earthquake sequence ruptured a segment of the Nazca-South America subduction zone that last hosted a great earthquake in 1877. The sequence opened \u3e3700 surface cracks in the fore arc of decameter-scale length and millimeter-to centimeter-scale aperture. We use the strikes of measured cracks, inferred to be perpendicular to coseismically applied tension, to estimate the slip distribution of the main shock and largest aftershock. The slip estimates are compatible with those based on seismic, geodetic, and tsunami data, indicating that geologic observations can also place quantitative constraints on rupture properties. The earthquake sequence ruptured between two asperities inferred from a regional-scale distribution of surface cracks, interpreted to represent a modal or most common rupture scenario for the northern Chile subduction zone. We suggest that past events, including the 1877 earthquake, broke the 2014 Pisagua source area together with adjacent sections in a throughgoing rupture

    Surface Cracks Record Long-Term Seismic Segmentation of the Andean Margin

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    Understanding the long-term patterns of great earthquake rupture along a subduction zone provides a framework for assessing modern seismic hazard. However, evidence that can be used to infer the size and location of past earthquakes is typically erased by erosion after a few thousand years. Meter-scale cracks that cut the surface of coastal areas in northern Chile and southern Peru preserve a record of earthquakes spanning several hundred thousand years owing to the hyperarid climate of the region. These cracks have been observed to form during and/or shortly after strong subduction earthquakes, are preserved for long time periods throughout the Atacama Desert, demonstrate evidence for multiple episodes of reactivation, and show changes in orientation over spatial scales similar to the size of earthquake segments. Our observations and models show that crack orientations are consistent with dynamic and static stress fields generated by recent earthquakes. While localized structural and topographic processes influence some cracks, the strong preferred orientation over large regions indicates that cracks are primarily formed by plate boundary–scale stresses, namely repeated earthquakes. We invert the crack-based strain data for slip along the well-known Iquique seismic gap segment of the margin and find consistency with gravity anomaly–based inferences of long-term earthquake slip patterns, as well as the magnitude and location of the November 2007 Tocopilla earthquake. We suggest that the meter-scale cracks can be used to map characteristic earthquake rupture segments that persist over many seismic cycles, which encourages future study of cracks and other small-scale structures to better constrain the persistence of asperities in other arid, tectonically active regions

    High Genetic Diversity and Low Differentiation of Michelia coriacea (Magnoliaceae), a Critically Endangered Endemic in Southeast Yunnan, China

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    Michelia coriacea, a critically endangered tree, has a restricted and fragmented distribution in Southeast Yunnan Province, China. The genetic diversity, genetic structure and gene flow in the three extant populations of this species were detected by 10 inter-simple sequence repeat (ISSR) markers and 11 simple sequence repeat (SSR) markers. Examination of genetic diversity revealed that the species maintained a relatively high level of genetic diversity at the species level (percentage of polymorphic bands) PPB = 96.36% from ISSRs; PPL (percentage of polymorphic loci) = 95.56% from SSRs, despite several fragmental populations. Low levels of genetic differentiation among the populations of M. coriacea were detected by Nei’s Gst = 0.187 for ISSR and Wright’s Fst = 0.090 for SSR markers, which is further confirmed by Bayesian model-based STRUCTURE and PCoA analysis that could not reveal a clear separation between populations, although YKP was differentiated to other two populations by ISSR markers. Meanwhile, AMOVA analysis also indicated that 22.84% and 13.90% of genetic variation existed among populations for ISSRs and SSRs, respectively. The high level of genetic diversity, low genetic differentiation, and the population, structure imply that the fragmented habitat and the isolated population of M. coriacea may be due to recent over-exploitation. Conservation and management of M. coriacea should concentrate on maintaining the high level of genetic variability through both in and ex-situ conservation actions

    Aligning the CMS Muon Endcap Detector with a System of Optical Sensors

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    The positions and orientations of one sixth of 468 large cathode strip chambers in the endcaps of the CMS muon detector are directly monitored by several hundred sensors including 2-D optical sensors with linear CCDs illuminated by cross-hair lasers. Position measurements obtained by photogrammetry and survey under field-off conditions show that chambers in the +Z endcap have been placed on the yoke disks with an average accuracy of 1\approx 1 mm in all 3 dimensions. We reconstruct absolute ZCMS_{CMS} positions and orientations of chambers at B=0T and B=4T using data from the optical alignment system. The measured position resolution and sensitivity to relative motion is about 60 μm\mu m. The precision for measuring chamber positions taking into account mechanical tolerances is \mbox{270μm\approx 270 \mu m}. Comparing reconstruction of optical alignment data and photogrammetry measurements at B=0T indicates an accuracy of \approx 680 μm\mu m currently achieved with the hardware alignment system. Optical position measurements at B=4T show significant chamber displacements of up to 13 mm due to yoke disk deformation

    Design and Performance of the Alignment System for the CMS Muon Endcaps

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    The alignment system for the CMS Muon Endcap detector employs several hundred sensors such as optical 1-D CCD sensors illuminated by lasers and analog distance- and tilt-sensors to monitor the positions of one sixth of 468 large Cathode Strip Chambers. The chambers mounted on the endcap yoke disks undergo substantial deformation on the order of centimeters when the 4T field is switched on and off. The Muon Endcap alignment system is required to monitor chamber positions with \mbox{75-200 μ\mum} accuracy in the Rϕ\phi plane, \approx400 μ\mum in the radial direction, and \approx1 mm in the z-direction along the beam axis. The complete alignment hardware for one of the two endcaps has been installed at CERN. A major system test was performed when the 4T solenoid magnet was ramped up to full field for the first time in August 2006. We present the overall system design and first results on disk deformations, which indicate that the measurements agree with expectations

    Results from a Fermilab neutrino beam dump experiment

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    The flux of prompt neutrinos from a beam dump has been measured in an experiment at the Fermi National Accelerator Laboratory (E613). Assuming that the charm production has a linear dependence on atomic number and varies as (1−‖×‖)5 e−2mT, a model dependent cross section of 27±5μb/nucleon can be derived. For neutrino energies greater than 20 GeV, the flux of electron neutrinos with respect to muon neutrinos is 0.78±0.19. For neutrinos with energy greater than 30 GeV and p⟂ greater than 0.2, the flux of ν̄u compared to νμ is 0.96±0.22.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/87363/2/100_1.pd

    Prompt Neutrino Results from Fermi Lab

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    Results from a Fermi lab experiment to study prompt neutrino production are presented. Assuming the prompt neutrinos come from the decay of charmed mesons we find a total DD production cross section of approx. 20 μb/nucleon, in good agreement with previous CERN results. We find a ν/ν ratio and a νe/νμ of approx. 1.0. The energy and pT spectra of the prompt neutrinos are consistent with those expected from DD production. Limits on the production of supersymmetric particles have also been obtained.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/87356/2/262_1.pd

    Measurement of inclusive D*+- and associated dijet cross sections in photoproduction at HERA

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    Inclusive photoproduction of D*+- mesons has been measured for photon-proton centre-of-mass energies in the range 130 < W < 280 GeV and a photon virtuality Q^2 < 1 GeV^2. The data sample used corresponds to an integrated luminosity of 37 pb^-1. Total and differential cross sections as functions of the D* transverse momentum and pseudorapidity are presented in restricted kinematical regions and the data are compared with next-to-leading order (NLO) perturbative QCD calculations using the "massive charm" and "massless charm" schemes. The measured cross sections are generally above the NLO calculations, in particular in the forward (proton) direction. The large data sample also allows the study of dijet production associated with charm. A significant resolved as well as a direct photon component contribute to the cross section. Leading order QCD Monte Carlo calculations indicate that the resolved contribution arises from a significant charm component in the photon. A massive charm NLO parton level calculation yields lower cross sections compared to the measured results in a kinematic region where the resolved photon contribution is significant.Comment: 32 pages including 6 figure
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