Mechanisms of geometrical seismic attenuation

In several recent reports, we have explained the frequency dependence of the apparent seismic quality-factor (Q) observed in many studies according to the effects of geometrical attenuation, which was defined as the zerofrequency limit of the temporal attenuation coefficient. In particular, geometrical attenuation was found to be positive for most waves traveling within the lithosphere. Here, we present three theoretical models that illustrate the origin of this geometrical attenuation, and we investigate the causes of its preferential positive values. In addition, we discuss the physical basis and limitations of both the conventional and new attenuation models. For waves in media with slowly varying properties, geometrical attenuation is caused by variations in the wavefront curvature, which can be both positive (for defocusing) and negative (for focusing). In media with velocity/density contrasts, incoherent reflectivity leads to geometrical-attenuation coefficients which are proportional to the mean squared reflectivity and are always positive. For «coherent» reflectivity, the geometrical attenuation is approximately zero, and the attenuation process can be described according to the concept of «scattering Q». However, the true meaning of this parameter is in describing the mean reflectivity within the medium, and not that of the traditional resonator quality factor known in mechanics. The general conclusion from these models is that non-zero and often positive levels of geometrical attenuation are common in realistic, heterogeneous media, both observationally and theoretically. When transformed into the conventional Q-factor form, this positive geometrical attenuation leads to Q values that quickly increase with frequency. These predictions show that the positive frequency-dependent Q observed in many datasets might represent artifacts of the transformations of the attenuation coefficients into Q

Mechanisms of geometrical seismic attenuation

Abstract In several recent papers, we explained the frequency dependence of the apparent seismic quality-factor (Q) observed in many studies by the effects of geometrical attenuation (GA), which was defined as the zero-frequency limit of the temporal attenuation coefficient. In particular, GA was found to be positive for most waves traveling within the lithosphere. Here, we present three theoretical models illustrating the origin of such GA, and investigate the causes of its preferential positive values. In addition, we discuss the physical basis and limitations of both the conventional and new attenuation models. For waves in media with slowly varying properties, GA is caused by variations of wavefront curvatures, which can be both positive (for defocusing) and negative (for focusing). In media with velocity/density contrasts, incoherent reflectivity leads to GA coefficients which are proportional to the mean squared reflectivity and always positive. For "coherent" reflectivity, the GA is approximately zero, and the attenuation process can be described by the concept of "scattering Q." However, the true meaning of this parameter is in describing the mean reflectivity within the medium and not that of the traditional resonator quality factor known in mechanics. The general conclusion from these models is that non-zero and often positive levels of GA are common in realistic, heterogeneous media both observationally and theoretically. When transformed into the conventional Q-factor form, such positive GA leads to Q values quickly increasing with frequency. These predictions show that the positive frequency dependent Q observed in many datasets may represent artifacts of the 3 transformations of the attenuation coefficients into Q

Mechanisms of geometrical seismic attenuation

Abstract In several recent papers, we explained the frequency dependence of the apparent seismic quality-factor (Q) observed in many studies by the effects of geometrical attenuation (GA), which was defined as the zero-frequency limit of the temporal attenuation coefficient. In particular, GA was found to be positive for most waves traveling within the lithosphere. Here, we present three theoretical models illustrating the origin of such GA, and investigate the causes of its preferential positive values. In addition, we discuss the physical basis and limitations of both the conventional and new attenuation models. For waves in media with slowly varying properties, GA is caused by variations of wavefront curvatures, which can be both positive (for defocusing) and negative (for focusing). In media with velocity/density contrasts, incoherent reflectivity leads to GA coefficients which are proportional to the mean squared reflectivity and always positive. For "coherent" reflectivity, the GA is approximately zero, and the attenuation process can be described by the concept of "scattering Q." However, the true meaning of this parameter is in describing the mean reflectivity within the medium and not that of the traditional resonator quality factor known in mechanics. The general conclusion from these models is that non-zero and often positive levels of GA are common in realistic, heterogeneous media both observationally and theoretically. When transformed into the conventional Q-factor form, such positive GA leads to Q values quickly increasing with frequency. These predictions show that the positive frequency dependent Q observed in many datasets may represent artifacts of the transformations of the attenuation coefficients into Q

Prion proteins: evolution and preservation of secondary structure

AbstractPrions cause a variety of neurodegenerative disorders that seem to result from a conformational change in the prion protein (PrP). Thirty-two PrP sequences have been subjected to phylogenetic analysis followed by reconstruction of the most probable evolutionary spectrum of amino acid replacements. The replacement rates suggest that the protein does not seem to be very conservative, but in the course of evolution amino acids have only been substituted within the elements of the secondary structure by those with very similar physico-chemical properties. Analysis of the full spectrum of single-step amino acid substitutions in human PrP using secondary structure prediction algorithms shows an over-representation of substitutions that tend to destabilize α-helices

Molecular Cloning of a Bovine Immunoglobulin Lambda Chain cDNA

A cDNA library of the bovine mammary gland constructed in pBR322 was screened by mRNA hybrid-selected translation and by differential hybridization. Several immunoglobulin (Ig) λ light-chain clones were identified and sequenced. Nucleotide sequence comparison of bovine and human Ig λ chains showed a high degree of homology for constant regions and for J regions. The amino acid (aa) sequence encoded by the constant region of the bovine Ig λ chain cDNA contains 107 aa with differences at 24 aa positions from the human Ig A chain. Three complementarity-determining regions (CDR1,2,3) characteristic of the variable region of bovine Ig λ chain cDNA can be distinguished. The bovine and human sequences display good homology in the framework region 3 (FR3) but only patches of homology throughout the FR2 region. The 5′ end of the bovine Ig λ chain cDNA fragment of clone 1-14E contains five stop codons: two in CDR1, one in FR1 and two in the hydrophobic prepeptide region. These data suggest that the Igλ mRNA of clone 1-14E is transcribed from the Vλ pseudogene