P wave velocity variations in the Coso Region, California, derived from local earthquake travel times

Inversion of 4036 P wave travel time residuals from 429 local earthquakes using a tomographic scheme provides information about three-dimensional upper crustal velocity variations in the Indian Wells Valley-Coso region of southeastern California. The residuals are calculated relative to a Coso-specific velocity model, corrected for station elevation, weighted, and back-projected along their ray paths through models defined with layers of blocks. Slowness variations in the surface layer reflect local geology, including slow velocities for the sedimentary basins of Indian Wells and Rose valleys and relatively fast velocities for the Sierra Nevada and Argus Mountains. In the depth range of 3–5 km the inversion images an area of reduced compressional velocity in western and northern Indian Wells Valley but finds no major velocity variations beneath the Coso volcanic field to the north. These results are consistent with a recent study of anomalous shear wave attenuation in the Coso region. Between 5 and 10 km depth, low-velocity areas (7% slow) appear at the southern end of the Coso volcanics, reaching east to the Coso Basin. Numerical tests of the inversion's resolution and sensitivity to noise indicate that these major anomalies are significant and well-resolved, while other apparent velocity variations in poorly sampled areas are probably artifacts. The seismic data alone are not sufficient to uniquely characterize the physical state of these low-velocity regions. Because of the Coso region's history of Pleistocene bimodal volcanism, high heat flow, geothermal activity, geodetic deformation, and seismic activity, one possibility is to link the zones of decreased P velocity to contemporary magmatic activity

Shear wave structure of a transect of the Los Angeles basin from multimode surface waves and H/V spectral ratio analysis

We use broad-band stations of the ‘Los Angeles Syncline Seismic Interferometry Experiment’ (LASSIE) to perform a joint inversion of the Horizontal to Vertical spectral ratios (H/V) and multimode dispersion curves (phase and group velocity) for both Rayleigh and Love waves at each station of a dense line of sensors. The H/V of the autocorrelated signal at a seismic station is proportional to the ratio of the imaginary parts of the Green’s function. The presence of low-frequency peaks (∼0.2 Hz) in H/V allows us to constrain the structure of the basin with high confidence to a depth of 6 km. The velocity models we obtain are broadly consistent with the SCEC CVM-H community model and agree well with known geological features. Because our approach differs substantially from previous modelling of crustal velocities in southern California, this research validates both the utility of the diffuse field H/V measurements for deep structural characterization and the predictive value of the CVM-H community velocity model in the Los Angeles region. We also analyse a lower frequency peak (∼0.03 Hz) in H/V and suggest it could be the signature of the Moho. Finally, we show that the independent comparison of the H and V components with their corresponding theoretical counterparts gives information about the degree of diffusivity of the ambient seismic field

Fluctuation Spectrum from a Scalar-Tensor Bimetric Gravity Theory

Predictions of the CMB spectrum from a bimetric gravity theory (gr-qc/0101126) are presented. The initial inflationary period in BGT is driven by a vanishingly small speed of gravitational waves v_g in the very early universe. This initial inflationary period is insensitive to the choice of scalar field potential and initial values of the scalar field. After this initial period of inflation, v_g will increase rapidly and the effects of a potential will become important. We show that a quadratic potential introduced into BGT yields an approximately flat spectrum with inflation parameters: n_s=0.98, n_t=-0.027, alpha_s=-3.2e-4 and alpha_t=-5.0e-4, with r >= 0.014.Comment: 14 pages, uses amsmath, amssym

Analysis of upper mantle structure using wave field continuation of P waves

Wave field continuation transforms seismic record section data directly into velocity-depth space, simultaneously providing an estimate of model nonuniqueness. This inversion, previously used for reflection and refraction data, converts readily to spherical earth problems through simple adjustments in each of the two linear transformations: the slant stack and downward continuation. Because the time resolution inherent in the data transforms to depth resolution in the model space, this method is extremely useful for analysis of data compatibility with preexisting models and direct comparison between data sets, as well as the complete inversion of raw data for structure. Wave field inversion demands densely sampled, digital data, and assumes source coherency and lateral homogeneity along the profile. We test this technique for upper mantle analysis using a previously studied, large, array-recorded data set representative of structure beneath the Gulf of California. We compare slant stacks and downward continuations of both synthetic and data record sections to illustrate the method's resolution capability. Wave field continuation proves particularly useful in comparing entire data sets to various models; even subtle structural differences are resolvable given good data quality

The Identification of Extreme Asymptotic Giant Branch Stars and Red Supergiants in M33 by 24 {\mu}m Variability

We present the first detection of 24 {\mu}m variability in 24 sources in the Local Group galaxy M33. These results are based on 4 epochs of MIPS observations, which are irregularly spaced over ~750 days. We find that these sources are constrained exclusively to the Holmberg radius of the galaxy, which increases their chances of being members of M33. We have constructed spectral energy distributions (SEDs) ranging from the optical to the sub-mm to investigate the nature of these objects. We find that 23 of our objects are most likely heavily self-obscured, evolved stars; while the remaining source is the Giant HII region, NGC 604. We believe that the observed variability is the intrinsic variability of the central star reprocessed through their circumstellar dust shells. Radiative transfer modeling was carried out to determine their likely chemical composition, luminosity, and dust production rate (DPR). As a sample, our modeling has determined an average luminosity of (3.8 $\pm$ 0.9) x 10$^4$ L$_\odot$ and a total DPR of (2.3 $\pm$ 0.1) x 10$^{-5}$ M$_\odot$ yr$^{-1}$. Most of the sources, given the high DPRs and short wavelength obscuration, are likely "extreme" AGB (XAGB) stars. Five of the sources are found to have luminosities above the classical AGB limit (M$_{\rm bol}$ 54,000 L$_\odot$), which classifies them as probably red supergiants (RSGs). Almost all of the sources are classified as oxygen rich. As also seen in the LMC, a significant fraction of the dust in M33 is produced by a handful of XAGB and RSG stars.Comment: 36 pages, 14 figures, 4 tables, Accepted for publication in A

Noise reduction technique for scanning tunneling microscopy

Journal ArticleNoise stemming from mechanical vibration, electronic noise, or low frequency (l / f power spectrum) inherent in the tunneling process, often limits the resolution, speed, or range of application of scanning tunneling microscopy (STM). We demonstrate a technique for minimizing the effect of these noise sources on the STM image

Scanning capacitance microscopy on a 25 nm scale

Journal ArticleA near-field capacitance microscope has been demonstrated on a 25 nm scale. A resonant circuit provides the means for sensing the capacitance variations between a sub-lGO-nra tip and surface with a sensitivity of 1X 10 19 F in a I kHz bandwidth. Feedback control is used to scan the tip at constant gap across a sample, providing a means of noncontact surface profiling. Images of conducting and nonconducting structures are presented

Measurement of in-plane magnetization by force microscopy

Journal ArticleWe present data which show that the magnetic force microscope is capable of detecting the component of the magnetic field parallel to the surface of a sample under study. Images of bits in a Co-alloy thin-film disk and of laser-written bits in a TbFe film were taken with a magnetized tip tilted at 45° with respect to the surface normal. In both cases the asymmetric part of the image of a domain is interpreted in terms of gradients in the in-plane component of the magnetic field. The bits written in the Co-alloy disk were decorated with small magnetized particles, allowing identification of the domain boundaries and the asymmetric component of the force microscope image due to in-plane magnetization

Integral throat entrance development, qualification and production for the Antares 3 nozzle

Although design analyses of a G-90 graphite integral throat entrance for the Antares 3 solid rocket motor nozzle indicated acceptable margins of safety, the nozzle throat insert suffered a thermostructural failure during the first development firing. Subsequent re-analysis using properties measured on material from the same billet as the nozzle throat insert showed negative margins. Carbon-carbon was investigated and found to result in large positive margins of safety. The G-90 graphite was replaced by SAI fast processed 4-D material which uses Hercules HM 10000 fiber as the reinforcement. Its construction allows powder filling of the interstices after preform fabrication which accelerates the densification process. Allied 15V coal tar pitch is then used to complete densification. The properties were extensively characterized on this material and six nozzles were subjected to demonstration, development and qualification firings