Waveform complexity in teleseismic broadband SH displacements: Slab diffractions or deep mantle reflections?

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/95458/1/grl4356.pd

Analysis of amplitude and travel‐time anomalies for short‐period P‐waves from NTS explosions

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/156474/1/gji00006.pd

Holographic Screens and Transport Coefficients in the Fluid/Gravity Correspondence

We consider in the framework of the fluid/gravity correspondence the dynamics of hypersurfaces located in the holographic radial direction at r = r_0. We prove that these hypersurfaces evolve, to all orders in the derivative expansion and including all higher curvature corrections, according to the same hydrodynamics equations with identical transport coefficients. The analysis is carried out for normal fluids as well as for superfluids. Consequently, this proves the exactness of the bulk viscosity formula derived in arXiv:1103.1657 via the null horizon dynamics.Comment: 4 pages; v2: added clarifying comments and references; v3: added comment about dependence of bulk viscosity formula on full equilibrium solution, not only horizon dat

Assessing citizen science participation skill for altruism or university course credit: a case study analysis

peer-reviewedA common challenge in citizen science projects is gaining and retaining participants. At the same time, the tertiary education sector is constantly being challenged to provide more meaningful and practical work for students. Can participation in citizen science projects be used as coursework with real practical experiential-learning benefits, without affecting the citizen science project outcomes? We seek to begin to answer this question via a case study analysis with Cyclone Center (CC), which asks participants to classify tropical cyclone characteristics through analysis of infrared satellite imagery. Skill of individual users has previously been shown to be obtainable once classifiers have looked at approximately 200 images using an expectation-maximisation likelihood approach. We use skill scores to determine if participation for course credit or altruism influenced skill for volunteers and students from two universities under three increasingly complex categories of classifications (eye or no eye; stronger, weaker, or the same; and which of six fundamental storm types). A bootstrap resampling approach was used to account for discrepancies between sample sizes. Overall, there is limited evidence for substantive differences in classification performance between credit awarded and altruistic participants, with only one finding of significance at <p = 0.05 (Maynooth University showing lower mean agreement with the volunteer consensus on eye vs. no-eye). There is evidence that integrating participation into a larger assessment that requires the students to show understanding of the project may reduce a low-skill student tail. Furthermore, students’ perceptions of the coursework compared to more traditional assignments were overall favourable. These findings, if replicated for other citizen science projects, open up possible avenues to more generally increasing participation in, and exploitation of, citizen science projects in the academic secto

Contending cultures of counterterrorism: transatlantic divergence or convergence?

Terrorist attacks on the United States, Spain and the United Kingdom have underlined the differing responses of Europe and the United States to the 'new terrorism'. This article analyses these responses through the prism of historically determined strategic cultures. For the last four years the United States has directed the full resources of a 'national security' approach towards this threat and has emphasized unilateralism. Europe, based on its own past experience of terrorism, has adopted a regulatory approach pursued through multilateralism. These divergences in transatlantic approaches, with potentially major implications for the future of the relationship, have appeared to be mitigated by a revised American strategy of counterterrorism that has emerged during 2005. However, this article contends that while strategic doctrines may change, the more immutable nature of strategic culture will make convergence difficult. This problem will be compounded by the fact that neither Europe nor America have yet addressed the deeper connections between terrorism and the process of globalization

The core shadow zone boundary and lateral variations of the P velocity structure of the lowermost mantle

The recent determination of high-quality, short-period P-wave amplitude profiles near the core shadow zone for three source-receiver combinations allows an exploration of lateral variations in P velocity structure at the base of the mantle. Various radially symmetric models are tested by comparison of the data with amplitudes measured from generalized ray theory synthetics. The assumption necessary to justify one-dimensional modeling--that each of the profiles is most sensitive to scales of heterogeneity larger than the discrete regions sampled--is well supported by the coherence of the individual profiles. The observed amplitude versus distance profiles exhibit significant regional variations of the apparent shadow zone boundary, with as much as a 5[deg] shift in onset distance, but it is possible to model the overall behavior using simple, regionally varying, positive P velocity gradients in the lowermost mantle. Velocity models with pervasive negative velocity gradients in the D" layer are not consistent with the data. The modeling indicates that D" velocities beneath the North Pole are ~ 2% slower than those beneath the central Pacific, while velocities beneath the North Pacific are ~ 1% faster. Although the simplest class of successful models begins to deviate from the PREM reference Earth model as much as 690 km above the core-mantle boundary, these models do not violate global mantle velocity constraints, and the theoretical slowness values calculated for the three models are consistent with slowness measurements for the same general regions. More complex, multiple-gradient lower mantle velocity structures with a thinner zone of lateral heterogeneity may be compatible with the P wave data, but such detailed structures cannot be resolved by our modeling (though bounds can be placed on viable structures). The strong lateral variations required by the data support the presence of compositional heterogeneity in D", and the data require that at least in several locations the predominant P velocity gradients in D" are positive.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/27976/1/0000408.pd

The Incompressible Navier-Stokes Equations From Black Hole Membrane Dynamics

We consider the dynamics of a d+1 space-time dimensional membrane defined by the event horizon of a black brane in (d+2)-dimensional asymptotically Anti-de-Sitter space-time and show that it is described by the d-dimensional incompressible Navier-Stokes equations of non-relativistic fluids. The fluid velocity corresponds to the normal to the horizon while the rate of change in the fluid energy is equal to minus the rate of change in the horizon cross-sectional area. The analysis is performed in the Membrane Paradigm approach to black holes and it holds for a general non-singular null hypersurface, provided a large scale hydrodynamic limit exists. Thus we find, for instance, that the dynamics of the Rindler acceleration horizon is also described by the incompressible Navier-Stokes equations. The result resembles the relation between the Burgers and KPZ equations and we discuss its implications.Comment: 5 pages; v2: expanded discussion to clarify a few points, title slightly change

Changing forest structure across the landscape of the Sierra Nevada, CA, USA, since the 1930s

Understanding the dynamics of forest structure aids inference regarding future forests and their distributions around the world. Over the last few decades, several papers have addressed changing forest structure in the Sierra Nevada, CA, USA, but these studies were limited in scope. We carried out a broad comparison of forest density and composition in the 1930s versus the 2000s for the west slope of the central and northern Sierra Nevada, using the two most extensive data sets available. Forests in this region have endured a long, complex history of human disturbance, and are now experiencing climatic shifts. We subdivided the landscape into elevation and latitude zones and compared historical and modern tree densities within each zone. We compared densities in historical plots to burned and unburned modern plots, as well as densities of individual tree species in historical vs. modern plots for their entire elevational distribution. Density of small trees (10.2-30.4 cm dbh) was significantly higher in the modern data set for all elevations and all latitudes, ranging from 20 to 148% higher. However, density of large trees (61.0 cm) was lower in the modern data set for most elevations and latitudes, ranging from 41% to 60% lower in most zones. Density difference of mid-sized trees (30.5-60.9 cm) was mixed, but was generally higher in modern plots. The pattern of more small trees but fewer large trees held for most individual species as well, but with notable exceptions. Our comparison of burned and unburned plots strongly implicates fire suppression as a driver of increased density of small trees in low- to mid-elevation forests. However, modern high-elevation (.2500 m) forests, where fire suppression impacts should be minimal, were also significantly denser than historical plots. Changing climatic conditions may be driving increased densities of small trees in high elevations, as well as decreased densities of large trees across the region

Evidence for a shear velocity discontinuity in the lower mantle beneath India and the Indian Ocean

SH and sSH seismograms are modeled to determine the shear velocity structure in the D" region beneath India and the Indian Ocean. The signals show waveform complexities similar to those observed in data sampling the D" region beneath Alaska, the Caribbean, and Eurasia (Lay and Helmberger), which have been attributed to a 2.7% shear velocity discontinuity ~ 280 km above the core-mantle boundary. The new data set consists of long-period tangential component recordings at WWSSN stations in Africa, the Middle East, and Europe for 11 intermediate and deep focus Indonesian earthquakes. In the distance range 70-82[deg] the waveforms show an arrival between SH and ScSH with systematic moveout. From 89 to 94[deg] there is a strong distortion of the SH waveforms, indicating the arrival of several phases closely spaced in time. The relative time shifts of similar complexity in the corresponding sSH phases requires a deep mantle origin. The depth dependence and moveout of the interference effects are well-predicted for both SH and sSH phases by a model with a lower mantle discontinuity. Alternative explanations of the interference as resulting from receiver reverberations, SKS contamination, multiple source complexity, or near source multipathing are ruled out by systematic tests. While it is apparent that lateral variations in the lower mantle velocity structure prevent any single model from fitting all of the data, synthetic waveform modeling (using generalized ray theory and reflectivity) shows that the data can be well-fit by a model with a discontinuity similar in size and depth to that proposed for the previously investigated regions (Lay and Helmberger), but with a negative velocity gradient within the D" layer.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/26508/1/0000046.pd

Origin of two-band chorus in the radiation belt of Earth.

Naturally occurring chorus emissions are a class of electromagnetic waves found in the space environments of the Earth and other magnetized planets. They play an essential role in accelerating high-energy electrons forming the hazardous radiation belt environment. Chorus typically occurs in two distinct frequency bands separated by a gap. The origin of this two-band structure remains a 50-year old question. Here we report, using NASA's Van Allen Probe measurements, that banded chorus waves are commonly accompanied by two separate anisotropic electron components. Using numerical simulations, we show that the initially excited single-band chorus waves alter the electron distribution immediately via Landau resonance, and suppress the electron anisotropy at medium energies. This naturally divides the electron anisotropy into a low and a high energy components which excite the upper-band and lower-band chorus waves, respectively. This mechanism may also apply to the generation of chorus waves in other magnetized planetary magnetospheres