690 research outputs found
Laboratory observations of slow earthquakes and the spectrum of tectonic fault slip modes
Slow earthquakes represent an important conundrum in earthquake physics. While regular
earthquakes are catastrophic events with rupture velocities governed by elastic wave speed,
the processes that underlie slow fault slip phenomena, including recent discoveries of tremor,
slow-slip and low-frequency earthquakes, are less understood. Theoretical models and sparse
laboratory observations have provided insights, but the physics of slow fault rupture remain
enigmatic. Here we report on laboratory observations that illuminate the mechanics of
slow-slip phenomena. We show that a spectrum of slow-slip behaviours arises near the
threshold between stable and unstable failure, and is governed by frictional dynamics via the
interplay of fault frictional properties, effective normal stress and the elastic stiffness of the
surrounding material. This generalizable frictional mechanism may act in concert with other
hypothesized processes that damp dynamic ruptures, and is consistent with the broad range
of geologic environments where slow earthquakes are observed
The role of deformation bands in dictating poromechanical properties of unconsolidated sand and sandstone
Cataclastic shear bands in sands and sandstones are typically stronger, stiffer, and exhibit lower permeability than the surrounding matrix, and therefore act as barriers to fluid flow. Previous work has quantified the reduction in permeability associated with these features; however, little is known about the role of shear band structure in controlling the way they impact permeability and elastic properties. Here, we report on a suite of laboratory measurements designed to measure the poromechanical properties for host material and natural shear bands, over effective stresses from 1â65 MPa. In order to investigate the role of host material properties in controlling poromechanical evolution with stress, we sampled shear bands from two well-studied sandstones representing structurally distinct end-members: a poorly cemented marine terrace sand from the footwall of the McKinleyville thrust fault in Humboldt County, California, and a strongly-cemented sandstone from the hanging wall of the Moab Fault in Moab, Utah. The permeability-porosity trends are similar for all samples, with permeability decreasing systematically with increasing effective stress and decreasing porosity. The permeability of the host material is consistently >1 order of magnitude greater than the shear bands for both localities. For the unconsolidated case, shear bands are less permeable and stiffer than the host material, whereas for the consolidated case, shear bands are slightly less permeable, and wave speeds are slower than in the host. We attribute the differences between the McKinleyville and Moab examples to changes in structure of the nearby host material that accompanied formation of the shear band
Evolution of elastic and mechanical properties during fault shear. The roles of clay content, fabric development, and porosity
Phyllosilicates weaken faults due to the formation of shear fabrics. Although the impacts of clay abundance and fabric on frictional strength, sliding stability, and porosity of faults are well studied, their influence on elastic properties is less known, though they are key factors for fault stiffness. We document the role that fabric and consolidation play in elastic properties and show that smectite content is the most important factor determining whether fabric or porosity controls the elastic response of faults. We conducted a suite of shear experiments on synthetic smectite-quartz fault gouges (10â100 wt% smectite) and sediment incoming to the Sumatra subduction zone. We monitored Vp, Vs, friction, porosity, shear and bulk moduli. We find that mechanical and elastic properties for gouges with abundant smectite are almost entirely controlled by fabric formation (decreasing mechanical and elastic properties with shear). Though fabrics control the elastic response of smectite-poor gouges over intermediate shear strains, porosity is the primary control throughout the majority of shearing. Elastic properties vary systematically with smectite content: High smectite gouges have values of Vp ~ 1,300â1,800 m/s, Vs ~ 900â1,100 m/s, K ~ 1â4 GPa, and G ~ 1â2 GPa, and low smectite gouges have values of Vp ~ 2,300â2,500 m/s, Vs ~ 1,200â1,300 m/s, K ~ 5â8 GPa, and G ~ 2.5â3 GPa. We find that, even in smectite-poor gouges, shear fabric also affects stiffness and elastic moduli, implying that while smectite abundance plays a clear role in controlling gouge properties, other fine-grained and platy clay minerals may produce similar behavior through their control on the development of fabrics and thin shear surfaces
Frictional and lithological controls on shallow slow slip at the Northern Hikurangi Margin
Slow slip events (SSEs) have been identified at subduction zones globally as an important link in the continuum between elastodynamic ruptures and stable creep. The northern Hikurangi margin is home to shallow SSEs which propagate to within 2 km of the seafloor and possibly to the trench, providing insights into the physical conditions conducive to SSE behavior. We report on a suite of friction experiments performed on protolith material entering the SSE source region at the Hikurangi margin, collected during the International Ocean Discovery Program Expedition 375. We performed velocity stepping and slide-hold-slide experiments over a range of fault slip rates, from plate rate (5 cm/yr or 1.6 Ă 10â9 m/s) to âŒ1 mm/s (10â3 m/s) and quantified the frictional velocity dependence and healing rates for a range of lithologies atdifferent stresses. The frictional velocity dependence (a-b) and critical slip distance DC increase with fault slip rate in our experiments. We observe atransition from velocity weakening to strengthening at slip rates of âŒ0.3 ”m/s. This velocity dependence of DC could be due to a combination of dilatant strengthening and a widening of the active shear zone at higher slip rates. We document low healing rates in the clay-rich volcaniclastic conglomerates, which lie above the incoming plate basement at least locally, and relatively higher healing rates in the chalk lithology. Finally, our experimental constraints on healing rates in different input lithologies extrapolated to timescales of 1â10 years are consistent with the geodetically inferred low stress drops and healing rates characteristic of the Hikurangi SSEs
HD 69686: A Mysterious High Velocity B Star
We report on the discovery of a high velocity B star, HD 69686. We estimate
its space velocity, distance, surface temperature, gravity, and age. With these
data, we are able to reconstruct the trajectory of the star and to trace it
back to its birthplace. We use evolutionary tracks for single stars to estimate
that HD 69686 was born 73 Myr ago in the outer part of our Galaxy (
kpc) at a position well below the Galactic plane ( kpc), a very
unusual birthplace for a B star. Along the star's projected path in the sky, we
also find about 12 other stars having similar proper motions, and their
photometry data suggest that they are located at the same distance as HD 69686
and probably have the same age. We speculate on the origin of this group by
star formation in a high velocity cloud or as a Galactic merger fragment.Comment: 28 pages, 6 figures, accepted for publication in Ap
First Kepler results on compact pulsators II: KIC 010139564, a new pulsating subdwarf B (V361 Hya) star with an additional low-frequency mode
We present the discovery of nonradial pulsations in a hot subdwarf B star
based on 30.5 days of nearly continuous time-series photometry using the
\emph{Kepler} spacecraft. KIC 010139564 is found to be a short-period pulsator
of the V361 Hya (EC 14026) class with more than 10 independent pulsation modes
whose periods range from 130 to 190 seconds. It also shows one periodicity at a
period of 3165 seconds. If this periodicity is a high order g-mode, then this
star may be the hottest member of the hybrid DW Lyn stars. In addition to the
resolved pulsation frequencies, additional periodic variations in the light
curve suggest that a significant number of additional pulsation frequencies may
be present. The long duration of the run, the extremely high duty cycle, and
the well-behaved noise properties allow us to explore the stability of the
periodic variations, and to place strong constraints on how many of them are
independent stellar oscillation modes. We find that most of the identified
periodicities are indeed stable in phase and amplitude, suggesting a rotation
period of 2-3 weeks for this star, but further observations are needed to
confirm this suspicion.Comment: 10 pages, accepted for publication in MNRA
A pressure vessel for true-triaxial deformation and fluid flow during frictional shear
see Abstract Volum
The Unseen Population of F to K-type Companions to Hot Subdwarf Stars
We present a method to select hot subdwarf stars with A to M-type companions
using photometric selection criteria. We cover a wide range in wavelength by
combining GALEX ultraviolet data, optical photometry from the SDSS and the
Carlsberg Meridian telescope, near-infrared data from 2MASS and UKIDSS. We
construct two complimentary samples, one by matching GALEX, CMC and 2MASS, as
well as a smaller, but deeper, sample using GALEX, SDSS and UKIDSS. In both
cases, a large number of composite subdwarf plus main-sequence star candidates
were found. We fit their spectral energy distributions with a composite model
in order to estimate the subdwarf and companion star effective temperatures
along with the distance to each system. The distribution of subdwarf effective
temperature was found to primarily lie in the 20,000 - 30,000 K regime, but we
also find cooler subdwarf candidates, making up ~5-10 per cent. The most
prevalent companion spectral types were seen to be main-sequence stars between
F0 and K0, while subdwarfs with M-type companions appear much rarer. This is
clear observational confirmation that a very efficient first stable Roche-lobe
overflow channel appears to produce a large number of subdwarfs with F to
K-type companions. Our samples thus support the importance of binary evolution
for subdwarf formation.Comment: 30 pages, 10 figures, 11 tables. Accepted for publication in MNRA
Modeling the System Parameters of 2M1533+3759: A New Longer-Period Low-Mass Eclipsing sdB+dM Binary
We present new photometric and spectroscopic observations for 2M 1533+3759 (=
NSVS 07826147). It has an orbital period of 0.16177042 day, significantly
longer than the 2.3--3.0 hour periods of the other known eclipsing sdB+dM
systems. Spectroscopic analysis of the hot primary yields Teff = 29230 +/- 125
K, log g = 5.58 +/- 0.03 and log N(He)/N(H) = -2.37 +/- 0.05. The sdB velocity
amplitude is K1 = 71.1 +/- 1.0 km/s. The only detectable light contribution
from the secondary is due to the surprisingly strong reflection effect. Light
curve modeling produced several solutions corresponding to different values of
the system mass ratio, q(M2/M1), but only one is consistent with a core helium
burning star, q=0.301. The orbital inclination is 86.6 degree. The sdB primary
mass is M1 = 0.376 +/- 0.055 Msun and its radius is R1 = 0.166 +/- 0.007 Rsun.
2M1533+3759 joins PG0911+456 (and possibly also HS2333+3927) in having an
unusually low mass for an sdB star. SdB stars with masses significantly lower
than the canonical value of 0.48 Msun, down to as low as 0.30 Msun, were
theoretically predicted by Han et al. (2002, 2003), but observational evidence
has only recently begun to confirm the existence of such stars. The existence
of core helium burning stars with masses lower than 0.40--0.43 Msun implies
that at least some sdB progenitors have initial main sequence masses of
1.8--2.0 Msun or more, i.e. they are at least main sequence A stars. The
secondary is a main sequence M5 star.Comment: 47 pages, 7 figure
A Spectroscopic Analysis of Blue Stragglers, Horizontal Branch and Turn-Off Stars in Four Globular Clusters
We present a spectroscopic analysis of HST/STIS and FOS low- and
intermediate-resolution spectroscopy of 55 stars (turn-off stars, horizontal
branch stars and blue stragglers) in four globular clusters (47 Tucanae, M3,
NGC6752, and NGC6397). Stars were analyzed with non-Local Thermodynamic
Equilibrium model atmospheres, and values for their effective temperatures and
gravities and some rotation rates were obtained. Using photometric fluxes, we
also obtained radii, luminosities and spectroscopic masses.Comment: 71 pages, 28 figures. Electronic figures only in the published
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