Point source

About: Point source is a research topic. Over the lifetime, 5077 publications have been published within this topic receiving 94091 citations.

A framework for fast probabilistic centroid-moment-tensor determination—inversion of regional static displacement measurements

Abstract: The determination of earthquake source parameters is an important task in seismology. For many applications, it is also valuable to understand the uncertainties associated with these determinations, and this is particularly true in the context of earthquake early warning (EEW) and hazard mitigation. In this paper, we develop a framework for probabilistic moment tensor point source inversions in near real time. Our methodology allows us to find an approximation to p(md), the conditional probability of source models (m) given observations (d). This is obtained by smoothly interpolating a set of random prior samples, using Mixture Density Networks (MDNs)-a class of neural networks which output the parameters of a Gaussian mixture model. By combining multiple networks as 'committees', we are able to obtain a significant improvement in performance over that of a single MDN. Once a committee has been constructed, new observations can be inverted within milliseconds on a standard desktop computer. The method is therefore well suited for use in situations such as EEW, where inversions must be performed routinely and rapidly for a fixed station geometry. To demonstrate the method, we invert regional static GPS displacement data for the 2010 MW 7.2 El Mayor Cucapah earthquake in Baja California to obtain estimates of magnitude, centroid location and depth and focal mechanism. We investigate the extent to which we can constrain moment tensor point sources with static displacement observations under realistic conditions. Our inversion results agree well with published point source solutions for this event, once the uncertainty bounds of each are taken into account.

Flattening-filter-based empirical methods to parametrize the head scatter factor.

Abstract: Parametrizing the collimator scatter factor, S c (or head scatter factor), of a linear accelerator by the side of the equivalent square of the collimator setting at the isocenter does not accurately predict the change in S c when the width and length of a rectangular field are exchanged. We have studied two methods based on measurements of square fields to predict S c ’s of rectangular fields more accurately. The first method parametrizes S c by the side of the equivalent square of the flattening filter region visible from the point of calculation. The S c ’s of rectangular fields were predicted to an accuracy of 1% from measurements with square fields. The second method computes S c of rectangular configurations by integrating radiation that can reach the point of calculation from a point source at the target and a second extended source at the flattening filter. The radial distribution of the extended source at the level of the flattening filter is computed from S c of square fields measured at the isocenter. Effects of extended distance are modeled by separately performing inverse square law corrections for the two sources. This method also predicted the measured values to within 1% accuracy.

CAL 83 - A puzzling X-ray source in the Large Magellanic Cloud

Abstract: Spectroscopic observations of the X-ray point source no. 83 in the Columbia Astrophysics Laboratory Einstein survey of the Large Magellanic Cloud have been accumulated over 4 yr. The optical spectrum shows no stellar absorption features, but only emission lines typical of an accretion disk. Radial velocity measurements reveal a small (K = 30 km/s) velocity variation modulated at 0.93 d, which appears to be the orbital period. No strong constraints can be put on the mass of the collapsed object, but its companion must be a low-mass, evolved star. Evidence for a precessing disk with a possible period of 69 days is presented. Exosat X-ray observations reveal no short-term (less than 6 hr) periodicities, although erratic, random variations were observed. The X-ray spectrum is very soft, reminiscent of some of the candidate black-hole sources. IUE ultraviolet spectra show only weak emission lines of N V and He II. The UV flux is variable with a mean effective temperature of about 19,000 K. Optical B, V photometry during November 1985 showed random variations of about 0.2 mag with a mean V = 17.3 mag.

Dose rate distributions around 60Co, 137Cs, 198Au, 192Ir, 241Am, 125I (models 6702 and 6711) brachytherapy sources and the nuclide 99Tcm.

Abstract: Simple analytical functions derived from our point source Monte Carlo calculations on the combined attenuation and scatter factor, B exp(-mu r), for 60Co, 137Cs, 198Au, 192Ir, 241Am, 125I (models 6702 and 6711) brachytherapy sources and the nuclide 99Tcm, for water spherical geometries of radii R = 15 and 20 cm, are presented. Our results for the broadly used 60Co, 137Cs, 198Au and 192Ir brachytherapy sources can be compared directly and found in excellent agreement with the widely accepted data of Meisberger et al in the limited distance range for which the latter are valid. Our data, however, can be used with high accuracy outside this distance range. Many discrepancies observed among different data sets available in recent literature are attributable to differences in geometries used. The results for the recently introduced 241Am source are very dissimilar to those produced by any other currently used brachytherapy source. Dose rate distributions, based on the above simple functions, are proposed in accordance with the recommendations for calibration of the brachytherapy sources in terms of reference air kerma rate and were found to be in good agreement with data available in the literature. Our calculations for 125I sources (models 6702 and 6711), provided that the characteristic x-rays from titanium encapsulation are taken into account, support recent experimental and theoretical dose rate distributions indicating that currently accepted values for 125I may be overestimated.

Synthesis of body wave seismograms from point sources in anisotropic media

Abstract: A method of synthetic seismogram calculation in arbitrary anisotropic media is developed. A solution in the frequency domain is derived in the form of a double integral with respect to horizontal slowness and azimuthal angle. Approximation of the influence of azimuthal anisotropy by Fourier series expansions makes it possible to carry out the integration over azimuthal angle analytically using series coefficients derived from simultaneous linear equations. As a result the double contour integral is reduced to a Fourier-Bessel integral over horizontal slowness which retains information about azimuthal anisotropy. Numerical evaluation of this integral enables us to compute body wave seismograms with allowance for nongeometrical effects. This approach can be applied to reflection-transmission problems or to the calculation of individual wavelet propagation in multilayered anisotropic media. The influence of anisotropy on point source radiation in a homogeneous medium is analyzed. Comparison of numerical results with high frequency asymptotic solution provides a reasonable physical interpretation for wave propagation anomalies. Synthetic data for a medium of orthorhombic symmetry show that the radiation pattern and the polarization of shear waves are very sensitive to the presence of anisotropy. The most pronounced amplitude distortions are caused by focusing of energy near velocity maxima and by defocusing near velocity minima. Numerical modeling reveals significant nongeometrical phenomena in anisotropic media. In some cases deviations from geometrical seismics are substantial even in the far field.