7,326 research outputs found
Properties of Dynamic Earthquake Ruptures With Heterogeneous Stress Drop
Earthquake rupture is a notoriously complex process, at all observable scales.
We introduce a simplified semi-dynamic crack model to investigate the connection
between the statistical properties of stress and those of macroscopic source
parameters such as rupture size, seismic moment, apparent stress drop and radiated
energy. Rupture initiation is treated consistently with nucleation on a linear slip-weakening
fault, whereas rupture propagation and arrest are treated according to
the Griffith criterion. The available stress drop is prescribed as a spatially correlated
random field and is shown to potentially sustain a broad range of magnitudes. By
decreasing the amplitude of the stress heterogeneities or increasing their correlation
length the distribution of earthquake sizes presents a transition from Gutenberg-
Richter to characteristic earthquake behavior. This transition is studied through a
mean-field analysis. The bifurcation to characteristic earthquake behavior is sharp,
reminiscent of a first-order phase transition. A lower roll-off magnitude observed
in the Gutenberg-Richter regime is shown to depend on the correlation length of the
available stress drop, rather than being a direct signature of the nucleation process.
More generally, we highlight the possible role of the stress correlation length scale
on deviations from earthquake source self-similarity. The present reduced model
is a building block towards understanding the effect of structural and dynamic
fault heterogeneities on the scaling of source parameters and on basic properties
of seismicity
Non-equilibrium work fluctuations for oscillators in non-Markovian baths
We study work fluctuation theorems for oscillators in non-Markovian heat
baths. By calculating the work distribution function for a harmonic oscillator
with motion described by the generalized Langevin equation, the Jarzynski
equality (JE), transient fluctuation theorem (TFT), and Crooks' theorem (CT)
are shown to be exact. In addition to this derivation, numerical simulations of
anharmonic oscillators indicate that the validity of these nonequilibrium
theorems do not depend on the memory of the bath. We find that the JE and the
CT are valid under many oscillator potentials and driving forces whereas the
TFT fails when the driving force is asymmetric in time and the potential is
asymmetric in position.Comment: 7 pages, 3 figure
Magnetic Fields Recorded by Chondrules Formed in Nebular Shocks
Recent laboratory efforts (Fu et al., 2014) have constrained the remanent
magnetizations of chondrules and the magnetic field strengths at which the
chondrules were exposed to as they cooled below their Curie points. An
outstanding question is whether the inferred paleofields represent the
background magnetic field of the solar nebula or were unique to the
chondrule-forming environment. We investigate the amplification of the magnetic
field above background values for two proposed chondrule formation mechanisms,
large-scale nebular shocks and planetary bow shocks. Behind large-scale shocks,
the magnetic field parallel to the shock front is amplified by factors , regardless of the magnetic diffusivity. Therefore, chondrules melted in
these shocks probably recorded an amplified magnetic field. Behind planetary
bow shocks, the field amplification is sensitive to the magnetic diffusivity.
We compute the gas properties behind a bow shock around a 3000 km-radius
planetary embryo, with and without atmospheres, using hydrodynamics models. We
calculate the ionization state of the hot, shocked gas, including thermionic
emission from dust, and thermal ionization of gas-phase potassium atoms, and
the magnetic diffusivity due to Ohmic dissipation and ambipolar diffusion. We
find that the diffusivity is sufficiently large that magnetic fields have
already relaxed to background values in the shock downstream where chondrules
acquire magnetizations, and that these locations are sufficiently far from the
planetary embryos that chondrules should not have recorded a significant
putative dynamo field generated on these bodies. We conclude that, if melted in
planetary bow shocks, chondrules probably recorded the background nebular
field.Comment: 17 pages, 11 figures, accepted for publication in Ap
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Multiple Scales in the Fine Structure of the Isoscalar Giant Quadrupole Resonance in ^{208}Pb
The fine structure of the isoscalar giant quadrupole resonance in ^{208}Pb,
observed in high-resolution (p,p') and (e,e') experiments, is studied using the
entropy index method. In a novel way, it enables to determine the number of
scales present in the spectra and their magnitude. We find intermediate scales
of fluctuations around 1.1 MeV, 460 keV and 125 keV for an excitation energy
region 0 - 12 MeV. A comparison with scales extracted from second RPA
calculations, which are in good agreement with experiment, shows that they
arise from the internal mixing of collective motion with two particle-two hole
components of the nuclear wavefunction.Comment: 14 pages including 6 figures (to be published in Phys. Lett. B
Local Temperature and Universal Heat Conduction in FPU chains
It is shown numerically that for Fermi Pasta Ulam (FPU) chains with
alternating masses and heat baths at slightly different temperatures at the
ends, the local temperature (LT) on small scales behaves paradoxically in
steady state. This expands the long established problem of equilibration of FPU
chains. A well-behaved LT appears to be achieved for equal mass chains; the
thermal conductivity is shown to diverge with chain length N as N^(1/3),
relevant for the much debated question of the universality of one dimensional
heat conduction. The reason why earlier simulations have obtained
systematically higher exponents is explained.Comment: 4 pages, 3 figures, revised published versio
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