Comparison of Time Series and Random-Vibration Theory Site-Response Methods

The random-vibration theory (RVT) approach to equivalent-linear site-response analysis is often used to simulate site amplification, particularly when large numbers of simulations are required for incorporation into probabilistic seismic-hazard analysis. The fact that RVT site-response analysis does not require the specification of input-time series makes it an attractive alternative to other site-response methods. However, some studies have indicated that the site amplification predicted by RVT site-response analysis systematically differs from that predicted by time-series approaches. This study confirms that RVT site-response analysis predicts site amplification at the natural site frequencies as much as 20%-50% larger than time-series analysis, with the largest overprediction occurring for sites with smaller natural frequencies and sites underlain by hard rock. The overprediction is caused by an increase in duration generated by the site response, which is not taken into account in the RVT calculation. Correcting for this change in duration brings the RVT results within 20% of the time-series results. A similar duration effect is observed for the RVT shear-strain calculation used to estimate the equivalent-linear strain-compatible soil properties. An alternative to applying a duration correction to improve the agreement between RVT and time-series analysis is the modeling of shear-wave velocity variability. It is shown that introducing shear-wave velocity variability through Monte Carlo simulation brings the RVT results consistently within +/- 20% of the time-series results.Nuclear Regulatory Commission NRC-04-07-122Civil, Architectural, and Environmental Engineerin

Secondary prevention of heart disease – knowledge among cardiologists and Ω-3 (Omega-3) fatty acid prescribing behaviors in Karachi, Pakistan

<p>Abstract</p> <p>Background</p> <p>The use of omega-3 fatty acids is a currently proven strategy for secondary prevention of heart disease. The prescription practices for this important nutraceutical is not currently known. It is imperative to assess the knowledge of cardiologists regarding the benefits of omega-3 fatty acids and to determine the frequency of its prescription. The aim of the study was to determine the practices and associations of dietary fish prescribing among cardiologists of Karachi and to assess their knowledge of fish oil supplementation and attitudes toward dietary practices.</p> <p>Methods</p> <p>A cross sectional survey was conducted during the period of January to March, 2008. A self report questionnaire was employed. All practicing cardiologists of Karachi were included in the study. Multiple logistic regression analysis was performed to determine the independent factors associated with high fish prescribers.</p> <p>Results</p> <p>The sample comprised of a total of 163 cardiologists practicing in Karachi, Pakistan. Most (73.6%) of the cardiologists fell in the age range of 28 – 45 years and were male (90.8%). High fish prescribers only comprised 36.2% of the respondents. After adjusting for age and gender, multivariate analysis revealed that only the variable of knowledge about fish oil's role in reducing sudden cardiac death was independently associated with high fish prescribers OR = 6.38 [95% CI 2.58–15.78].</p> <p>Conclusion</p> <p>The level of knowledge about the benefits of omega-3 fatty acids is high and the cardiologists harbor a favorable attitude towards dispensing dietary fish advice. However, the prescription practices are less than optimal and not concordant with recommendations of organisations such as the American Heart Association and National Heart Foundation of Australia. The knowledge of prevention of sudden cardiac death in CVD patients has been identified as an important predictor of high fish prescription. This particular life-saving property of omega-3 fatty acids should be the focus of any implemented educational strategy targeted to improve secondary CVD prevention via omega-3 fatty acid supplementation.</p

Loci influencing blood pressure identified using a cardiovascular gene-centric array

Blood pressure (BP) is a heritable determinant of risk for cardiovascular disease (CVD). To investigate genetic associations with systolic BP (SBP), diastolic BP (DBP), mean arterial pressure (MAP) and pulse pressure (PP), we genotyped 50 000 single-nucleotide polymorphisms (SNPs) that capture variation in 2100 candidate genes for cardiovascular phenotypes in 61 619 individuals of European ancestry from cohort studies in the USA and Europe. We identified novel associations between rs347591 and SBP (chromosome 3p25.3, in an intron of HRH1) and between rs2169137 and DBP (chromosome1q32.1 in an intron of MDM4) and between rs2014408 and SBP (chromosome 11p15 in an intron of SOX6), previously reported to be associated with MAP. We also confirmed 10 previously known loci associated with SBP, DBP, MAP or PP (ADRB1, ATP2B1, SH2B3/ATXN2, CSK, CYP17A1, FURIN, HFE, LSP1, MTHFR, SOX6) at array-wide significance (P 2.4 10(6)). We then replicated these associations in an independent set of 65 886 individuals of European ancestry. The findings from expression QTL (eQTL) analysis showed associations of SNPs in the MDM4 region with MDM4 expression. We did not find any evidence of association of the two novel SNPs in MDM4 and HRH1 with sequelae of high BP including coronary artery disease (CAD), left ventricular hypertrophy (LVH) or stroke. In summary, we identified two novel loci associated with BP and confirmed multiple previously reported associations. Our findings extend our understanding of genes involved in BP regulation, some of which may eventually provide new targets for therapeutic intervention.</p

Impact of input ground motions and site variability on seismic site response

Seismic site response analysis allows an engineer to assess the effect of local soil conditions on the ground motions expected during an earthquake. In seismic site response analysis, an input ground motion on rock is propagated through a site specific soil column. The computed response at the surface is dependent on both the input ground motion and the soil properties that characterize the site. However, there is uncertainty in both the input ground motion and the soil properties, as well as natural variability in the soil properties across a site. To account for the uncertainty in the input ground motions, engineers use a suite of motions that are selected and scaled to fit a scenario input motion. This study introduces a semi-automated method to select and scale the input motions to fit a target input motion and its variability. The proposed method is intended to replace tedious trials of combinations by hand with combinations performed by a computer. However, as in the traditional selection methods, the final selection of the combination is done by the engineer.The effect of the selected ground motion combination on the computed surface response spectrum from the site response analysis, and its variability, was investigated in this study. The results show by using a combination with as few as five motions, the median surface response spectrum can be predicted with an error of 10%. Additionally, the manner used to scale the input motions does not impact the accuracy of the median surface response spectrum, as long as the median response spectrum of the input combination agrees with the target input response spectrum. However, if the standard deviation of the surface response spectrum is to be considered (e.g., to develop median plus one standard deviation spectra), a input combination of at least 20 motions is recommended and the combination must be scaled such that the standard deviation of the input combination matches the standard deviation of the input target spectrum. Monte Carlo simulations were used to assess the impact of soil property variability on surface spectra computed by seismic site response. The results from this study indicate that by accounting for the variability of the shear-wave velocity profile of a site can cause a significant decrease in the median surface response spectrum, as well as a slight increase in the standard deviation of the surface response spectrum at periods less than the site period. By considering the variability of the nonlinear properties (shear modulus reduction and damping ratio) the median response spectrum decreased only slightly, but the standard deviation increased in a manner similar to the increase observed when considering the variability of the shear-wave velocity profile. Simultaneously considering the variability of the shear-wave velocity profile and nonlinear properties resulted in a median surface response spectrumsimilar to the median surface response spectrumcomputed with considering the variability of the shear-wave velocity alone. However, the standard deviation of the surface response spectrum was larger than the standard deviation computed by independent consideration of the variability of the shear-wave velocity or nonlinear properties.Civil, Architectural, and Environmental Engineerin

Development of geologic site classes for seismic site amplification for central and eastern North America

The time-averaged shear wave velocity in the upper 30 m of a site (Vs30) is the most common site parameter used in ground motion prediction equations for the evaluation of seismic site response. It is often the case that Vs30 is not available at sites with earthquake recordings; for example in the NGA-East site database only 45 of 1149 sites have measured values of Vs30. Accordingly, estimates of Vs30 are often made on the basis of available proxies that are widely available such as ground slope, geomorphic terrain categories, and surface geology. We compile a database of 1930 measured and inferred Vs30 values in Central and Eastern North America (CENA) to test slope and geomorphology-based proxy methods. The results indicate that these existing proxy methods are biased for sites with Vs30greater than 400 m/s. Based on a careful review of geological conditions in the CENA, we propose nineteen geologic classes based on setting (i.e., glaciated or non-glaciated), age, and depositional environmental that can form the basis for geology-based proxy estimates of Vs30 as well as for simplified stratigraphic columns

Reference Rock Site Condition for Central and Eastern North America

The reference rock site condition has two important applications for ground-motion prediction in the stable continental region of central and eastern North America (CENA). (1) It represents the site condition for which ground motions are computed using semiempirical ground-motion prediction equations. In addition, (2) it represents the site condition to which site amplification factors, which are used to modify ground-motion intensity measures for softer site condition, are referenced (i. e., site amplification is unity for reference rock). We define reference rock by its shear (S)and compression (P)-wave velocities, as well as a site attenuation parameter (kappa(0)), which is used in stochastic ground-motion simulation methods. Prior definitions of reference rock conditions in CENA were based mostly on indirect large-scale crustal velocity inversions and judgment. We compile and interpret a unique database of direct velocity measurements to develop criteria for assessing the presence of reference rock site condition based on measured seismic velocities and their gradient with respect to depth. We apply the criteria to available profiles and perform rigorous statistical analysis from which we recommend S-and P-wave velocities of 3000 and 5500 m/s, respectively, for the reference rock condition. We recommend that, for practical applications, use ranges of reference S-and P-wave velocities of 2700-3300 m/s and 5000-6100 m/s, respectively. The ranges are based on a +/- 5% change in amplification using quarterwavelength theory. We do not find evidence for regional dependence of the reference velocities, which are derived principally from three general geographic regions: (1) Atlantic coast, (2) continental interior, and (3) Appalachian Mountains. Our data do not provide reference velocities for the Gulf Coast region. The recommended velocity-compatible reference rock site kappa is 0.006 s.ope

Subsurface Shear Wave Velocity Characterization Using P-Wave Seismograms in Central and Eastern North America

The time-averaged shear (S) wave velocity in the upper 30 meters of sediment (V-S30) is a widely used site parameter for ground motion prediction. When unavailable from measurements, as is often the case at accelerograph stations in Central and Eastern North America (CENA), V-S30 is typically estimated from proxies. We propose an alternative for CENA based on a theoretical relationship between S-wave velocity and the ratio of radial to vertical components of the compressional (P)-wave-dominated portion of the velocity time series. This method is applied to 31 CENA accelerograph sites having measured S-wave velocity profiles. Time-averaged S-wave velocities to depth z (V-SZ) from the proposed method agree well with those from measurements. We develop linear relationships between V-SZ and V-S30 using CENA S-wave velocity profile data. Values of V-S30 established from the proposed method (including depth extrapolation) have lower dispersion relative to data (sigma(ln) (V) = 0.43) than do estimates from available CENA proxies.ope