Broad-band near-field ground motion simulations in 3-dimensional scattering media

The heterogeneous nature of Earth's crust is manifested in the scattering of propagating seismic waves. In recent years, different techniques have been developed to include such phenomenon in broad-band ground-motion calculations, either considering scattering as a semi-stochastic or purely stochastic process. In this study, we simulate broad-band (0-10Hz) ground motions with a 3-D finite-difference wave propagation solver using several 3-D media characterized by von Karman correlation functions with different correlation lengths and standard deviation values. Our goal is to investigate scattering characteristics and its influence on the seismic wavefield at short and intermediate distances from the source in terms of ground motion parameters. We also examine scattering phenomena, related to the loss of radiation pattern and the directivity breakdown. We first simulate broad-band ground motions for a point-source characterized by a classic ω2 spectrum model. Fault finiteness is then introduced by means of a Haskell-type source model presenting both subshear and super-shear rupture speed. Results indicate that scattering plays an important role in ground motion even at short distances from the source, where source effects are thought to be dominating. In particular, peak ground motion parameters can be affected even at relatively low frequencies, implying that earthquake ground-motion simulations should include scattering also for peak ground velocity (PGV) calculations. At the same time, we find a gradual loss of the source signature in the 2-5Hz frequency range, together with a distortion of the Mach cones in case of super-shear rupture. For more complex source models and truly heterogeneous Earth, these effects may occur even at lower frequencies. Our simulations suggests that von Karman correlation functions with correlation length between several hundred metres and few kilometres, Hurst exponent around 0.3 and standard deviation in the 5-10percent range reproduce the available observation

The role of topography and lateral velocity heterogeneities on near-source scattering and ground-motion variability

The scattering of seismic waves travelling in the Earth is not only caused by random velocity heterogeneity but also by surface topography. Both factors are known to strongly affect ground-motion complexity even at relatively short distance from the source. In this study, we simulate ground motion with a 3-D finite-difference wave propagation solver in the 0-5Hz frequency band using three topography models representative of the Swiss alpine region and realistic heterogeneous media characterized by the Von Karman correlation functions. Subsequently, we analyse and quantify the characteristics of the scattered wavefield in the near-source region. Our study shows that both topography and velocity heterogeneity scattering may excite large coda waves of comparable relative amplitude, especially at around 1Hz, although large variability in space may occur. Using the single scattering model, we estimate average QC values in the range 20-30 at 1Hz, 36-54 at 1.5Hz and 62-109 at 3Hz for constant background velocity models with no intrinsic attenuation. In principle, envelopes of topography-scattered seismic waves can be qualitatively predicted by theoretical back-scattering models, while forward- or hybrid-scattering models better reproduce the effects of random velocity heterogeneity on the wavefield. This is because continuous multiple scattering caused by small-scale velocity perturbations leads to more gentle coda decay and envelope broadening, while topography abruptly scatters the wavefield once it impinges the free surface. The large impedance contrast also results in more efficient mode mixing. However, the introduction of realistic low-velocity layers near the free surface increases the complexity of ground motion dramatically and indicates that the role of topography in elastic waves scattering can be relevant especially in proximity of the source. Long-period surface waves can form most of the late coda, especially when intrinsic attenuation is taken into account. Our simulations indicate that both topography and velocity heterogeneity scattering may result in large ground-motion variability, characterized by standard deviation values in the range 0.2-0.5 also at short distance from the source. We conclude that both topography and velocity heterogeneity should be considered to correctly assess the ground-motion variability in earthquake scenario studies even at intermediate frequenc

Sensitivity of broad-band ground-motion simulations to earthquake source and Earth structure variations: an application to the Messina Straits (Italy)

In this paper, we investigate ground-motion variability due to different faulting approximations and crustal-model parametrizations in the Messina Straits area (Southern Italy). Considering three 1-D velocity models proposed for this region and a total of 72 different source realizations, we compute broad-band (0-10 Hz) synthetics for Mw 7.0 events using a fault plane geometry recently proposed. We explore source complexity in terms of classic kinematic (constant rise-time and rupture speed) and pseudo-dynamic models (variable rise-time and rupture speed). Heterogeneous slip distributions are generated using a Von Karman autocorrelation function. Rise-time variability is related to slip, whereas rupture speed variations are connected to static stress drop. Boxcar, triangle and modified Yoffe are the adopted source time functions. We find that ground-motion variability associated to differences in crustal models is constant and becomes important at intermediate and long periods. On the other hand, source-induced ground-motion variability is negligible at long periods and strong at intermediate-short periods. Using our source-modelling approach and the three different 1-D structural models, we investigate shaking levels for the 1908 Mw 7.1 Messina earthquake adopting a recently proposed model for fault geometry and final slip. Our simulations suggest that peak levels in Messina and Reggio Calabria must have reached 0.6-0.7 g during this earthquak

Prevalence of heavy smokers in the year 2000 in the province of Varese, Italy.

ABSTRACT: Prevalence of heavy smokers in the year 2000 in the Province of Varese, Italy. A. Imperatori, N. Rotolo, V. Conti, D. Di Natale, V. Tropeano, W. Mantovani. Background. Knowing the prevalence of heavy smokers (HS) by gender and age is a pre-requisite for bringing into effect public health measures against smoking-related diseases. Smoking prevalence data is available for the Italian Regions, however it is generally unknown for the Italian Provinces. Methods. In the year 2000 a survey of smoking prevalence was conducted by 47 general practitioners (GPs), by personal interview, in a large sample of the Varese Province population 45-74 years of age (28,034 subjects; 13,528 men, 14,506 women). Each surveyed subject was categorised either as ever HS (current/former smoker of at least 10 pack-years) or as non HS. The information on smoking habit collected by the GPs was anonymously pooled for analysis. Prevalence figures of smoking were tabulated by gender and by 5-year age-strata. Results. In the population 45-74 years of age the percentage of ever HS overall was 22.3% (34.4% of men; 11.0% of women). The prevalence of ever HS in both sexes combined progressively decreased with advancing age, from 23.6% (45-49 year stratum) to 19.5% (70-74 year stratum). Current HS were 24.5% of men and 9.5% of women. Conclusions. The year 2000 survey on smoking habit, showing 22.3% prevalence of ever HS in age range 45-74 years, is the first conducted in the Varese Province using a large population sample. The data on heavy cigarette smoking presented in this paper, stratified by gender and age, may be used to monitor changes in the smoking habit and in the incidence of smoking-related illnesses at the provincial level

Self-selection effects in smokers attending lung cancer screening: a 9.5-year population-based cohort study in Varese, Italy.

BACKGROUND:: We hypothesize that mortality risk profile of participants and nonparticipants in nonrandomized lung cancer (LC) screening of smokers may be different. METHODS:: In 1997, a population-based cohort of 5815 smokers of Varese Province was invited to nonrandomized LC screening by annual chest x-ray examination for 4 years. LC risk factors and screening participation rate were recorded. Except for screening, the whole cohort received usual care. After 9.5-year observation, we compared mortality of participants versus nonparticipants by assessing age-standardized all-cause mortality rate ratio (MRR) and disease group-specific MRR with 95% confidence intervals (95% CI). RESULTS:: Self-selected screening participants were 21% of cohort. Participants were younger (p < 0.001), were more frequently current smokers (p = 0.019), had more pack-years of smoking (p < 0.0001), and had higher rate of LC family history (p < 0.0001) and of occupational LC risk (p < 0.0001) relative to nonparticipants. In logistic regression analysis familial LC, occupational risk and pack-years smoked were significant predictors of participation in screening and of developing LC. Participants displayed a healthy effect, as shown by all-cause MRR = 0.67 (95% CI, 0.53-0.84), all cancers except LC MRR = 0.61 (95% CI, 0.41-0.91), cardiovascular diseases MRR = 0.38 (95% CI, 0.22-0.63), and noncancer disease other than cardiovascular or respiratory MRR = 0.57 (95% CI, 0.34-0.92). The LC mortality (MRR = 1.40; 95% CI, 1.03-1.91) was higher in participants relative to nonparticipants (p = 0.031). CONCLUSION:: The selection effect in LC screening participants was dual: healthy effect and higher LC mortality. In assessing the overall effectiveness of LC screening on a population level, a higher LC mortality risk in participants should be considered

Cost of a population-based programme of chest x-ray screening for lung cancer.

Background. After the implementation of a population- based programme of chest x-ray (CXR) screening on smokers in Varese, Italy, lung cancer (LC) mortality was significantly reduced. Analysis of the incremental costs due to this type of screening programme is needed to evaluate its economic impact on the healthcare system. Methods. In July 1997 a population-based cohort, consisting of all high-risk smokers (n=5,815) identified among 60,000 adult residents from the Varese province, was invited to a LC screening programme (an annual CXR for five years) in a general practice setting, and was observed through 2006. Invitees received National Health Service (NHS) usual care, with the addition of CXRs in screening participants. At the end of observation, among the 245 LCs diagnosed in the entire screening-invited cohort the observed LC deaths were 38 fewer than expected. To estimate the incremental direct cost due to screening in the invited cohort for the period July 1997-2006, we compared the direct cost of screening administration, CXR screens and LC management in the invited cohort and in the uninvited and unscreened controls in NHS usual care setting. Results. Over the 9.5 years, the total incremental direct healthcare costs (including screening organization/administration, CXR screens, additional procedures prompted by false-positive tests, overdiagnosed LCs) were estimated to range from \u20ac 607,440 to \u20ac 618,370 (in euros as of 2012), equating to between \u20ac 15,985- \u20ac 16,273 per patient out of the 38 LC deaths averted. Conclusions. In a general practice setting, the incremental cost for a CXR screening programme targeted at all high-risk smokers in a population of 60,000 adults was estimated to be about \u20ac65,000 per annum, approx. \u20ac16,000 for each LC death averted