1,721,078 research outputs found
Depth-Dependent Shear-Wave Attenuation in Central Apennines, Italy
No full textWe used 1029 earthquakes, with magnitudes ranging from M 3.0 to M 6.5, located in central Apennines, Italy, and recorded by 414 local stations to study the variation of the quality factor QS of shear waves with depth. We first determined average nonparametric attenuation functions in the frequency band from 0.5 to 20 Hz and hypocenter distances less than 155 km to correct the observed acceleration spectra for attenuation effects. Then, we separated source and site effects from the corrected spectral records to determine the changes of QS with depth. We used a 1D local shear-wave velocity model to calculate the travel times of the source-station paths, and we inverted the observed spectra to determine QS in three different depth intervals (0–4 km, 4–10 km and 10–15 km) and five frequencies (0.5, 1, 4, 10 and 20 Hz). We found that QS increases with frequency at all depths considered and tends to have lower values at shallow depths. The average value of QS is consistent with previous studies made in central Italy and can be approximated by QS = 43f0.94. To describe the frequency dependence of QS with depth (H), we determine the following relations: QS = 5.5f1.39, 0.5 ≤ f ≤ 10 Hz and QS = 151.5, f > 10 Hz for 0–4 km, QS = 52f0.87 for 4 < H < 10 km and QS = 51f0.92 for 10 ≤ H ≤ 15 km. We conclude that the Q-depth-dependent model can be useful to improve estimates of source parameters and ground motion prediction in the central Apennines region of Italy
Between-event and between-station variability observed in the Fourier and response spectra domains: Comparison with seismological models
No full textAnomalously deep earthquakes in northwestern Italy
No full textIt is usually assumed that earthquakes in intraplate regions occur in the upper crust, and northwestern Italy is generally assigned to this kind of 'normal' seismicity. In this work, the depth distribution of the events localized in this area by the Istituto Geofisico Geodetico (IGG) seismic network in the period 1991-1997 is analyzed in detail. In particular, the location capability of the network is discussed, adopting as reference quarry blasts (for the epicentral position) and the locations obtained from a dense temporary network (for the depth estimate). Within the so-obtained error limits, the depth distribution of events show a characteristic pattern: while for most of the area covered by the network the well-located seismicity lies within the first 20 km of depth, in a band following the inner arc of the Western Alps, numerous events have anomalously large focal depths, reaching a maximum of 114 km. These depth determinations cannot be attributed to instabilities of the location procedure: different choices of the propagation models used for the hypocentral determination led to very similar depth values, always significantly larger than the standard values for the surrounding areas. A strong correlation has been found between the 3-dimensional distribution of these foci and the P-wave propagation anomalies obtained from tomographic studies, suggesting a direct link between elastic and rheological properties of lower crust and upper mantle in this area
New integrated events location procedure in vast and irregular networks in a quasi real-time mode
No full textOnsite earthquake early warning: Predictive models for acceleration response spectra considering site effects
No full textOnsite earthquake early warning (EEW) systems exploit predictive models relating features extracted over the P-wave window to S-wave ground-motion parameters. These models are usually calibrated considering recordings from multiple stations and combining datasets from different regions under the ergodic assumption. Here, we show that the local-site conditions can play a significant role in determining the performance of onsite EEW predictive models in terms of rates of false or missed alerts. Interestingly, if partially nonergodic models are implemented, as done in probabilistic seismic hazard analysis, the negative impact of local-site amplifications can be mitigated. We explore the influence of site effects for onsite EEW predictive models calibrated between the peak displacement (Pd) and integral squared velocity (Iv2) measured over a 3 s P-wave window, and the acceleration response spectra (RSA) at nine different periods T (T 0:1, 0.15, 0.2, 0.3, 0.5, 0.75, 1.0, 1.5, and 2.0 s). We consider 58 earthquakes with magnitudes between Mw 3.7 and 6.5, belonging to the 2016–2017 central Italy seismic sequence that have been recorded by 100 accelerometer stations at hypocentral distances lesser than 150 km. We implement a mixed-effects regression analysis to explore the variability of the ground motion in terms of RSA predicted at different sites by considering two different group levels: in the first, each station is considered separately; in the second, we consider the Eurocode 8 (EC8, 2004) soil classification. Considering a probabilistic alert decision module applied to data from two selected stations, we show that the predictive models including site effects provide more reliable alerts, reducing the false alarms from 2.6% to 0.53% and the missed alarms from 10.1% to 4.8%. The residuals analysis shows that including a site-specific random effect in the predictive model contributes to reducing the apparent aleatory variability, whereas grouping data by EC8 classification does not provide significant benefit for EEW purposes
Linking crustal tomographic images and surface geology around the Giudicarie and Schio-Vicenza fault systems (Southern Alps, Italy)
No full textAttenuation relationship for low magnitude earthquakes using standard seismometric records
No full textReliability of source parameters for small events in central italy: Insights from spectral decomposition analysis applied to both synthetic and real data
No full textWe apply a spectral decomposition approach to isolate the source spectra from propagation and site effects and, in turn, to estimate the source parameters of small-to-moderate earthquakes that occurred in central Italy. The data set is composed of about 400,000 waveforms relevant to 4111 earthquakes in the moment magnitude range 1.5–6.5, recorded by a high-density network of stations installed in the study area. We first investigate the reliability of the source parameters for small magnitudes through numerical simulations. We generate synthetic spectra for different source scaling models and near-surface attenuation effects, considering the source–station geometry and the data availability of the central Italy data set. Our analysis with synthetics shows that the spectral decomposition is effective in isolating the source contributions from other factors. Moreover, the analysis of the residual distributions suggests that moment magnitude 1.8 is the lower bound for the retrieval of reliable Brune’s source parameters, although we observe an increase of residual’s variability below magnitude 3, and the estimated source parameters could be biased below magnitude 2.3. Remarkably, the assessment of the stress drop Δσ for small events is strongly hampered by site-specific attenuation near the surface. In view of the results with synthetics, we analyze the source parameters of earthquakes recorded in central Italy. The corner frequency versus seismic moment relationship describes a source scaling in which Δσ increases with increasing moment magnitude Mw, the mean Δσ varying from 0.1 MPa for Mw 5. In particular, Δσ increases mainly for Mw in the ranges 2.5–3 and 4.5–5.2. The corner frequencies estimated from the apparent source spectra do not show any dependence on hypocentral distance and magnitude, confirming that uncorrected anelastic attenuation effects do not significantly bias the results
Ground Shaking Scenarios at the Basilica with the World’s Largest Elliptical Dome in Vicoforte, Northern Italy
No full textDissociation between exhaled nitric oxide and hyperresponsiveness in children with mild intermittent asthma.
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