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Sebastiani, Giovanni
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Sebastiani, Giovanni
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- PublicationOpen AccessTime Variability of crustal attenuation during the Amatrice-Visso-Norcia earthquake sequence in the Central Apennines (Italy)(2018-12-10)
; ; ; ; ; ; ; ; ; ; ; Over the last decade our work has been mostly about reducing uncertainties over spectral measurements in seismology (e.g. Malagnini and Munafò, 2018; Malagnini and Dreger, 2016; Munafò et al., 2016; Akinci et al., 2014). Here we measure time-domain peak values from narrow bandpass-filtered time histories and transform them into spectral estimates by using the theoretical results of Random Vibration Theory (Cartwright and Longuet-Higgins, 1956) and the Parseval Theorem. We develop a novel approach to quantify time domain fluctuations of highfrequency seismic attenuation and apply it to a massive data set of seismic waveforms from the Central Apennines in Italy, which includes recordings spanning the recent earthquake sequence of Amatrice-Visso-Norcia (2016-2017). Our observations show that the crustal seismic wave propagation in the region is strongly affected by transients triggered by the main events. The time varying attenuation is probably due to the associated migration of crustal fluids, in addition to seasonal oscillations related to precipitation-induced variations of crustal stresses. We also observe oscillation periods in the attenuation time series corresponding to solid Earth tides. Sensitivity to tides is stronger in the aftermath of the mainshocks, indicating an important role played by rock damage.117 28 - PublicationOpen AccessSpatiotemporal Analysis of COVID-19 Incidence Data(1) Background: A better understanding of COVID‐19 dynamics in terms of interactions among individuals would be of paramount importance to increase the effectiveness of containment measures. Despite this, the research lacks spatiotemporal statistical and mathematical analysis based on large datasets. We describe a novel methodology to extract useful spatiotemporal information from COVID‐19 pandemic data. (2) Methods: We perform specific analyses based on mathematical and statistical tools, like mathematical morphology, hierarchical clustering, parametric data modeling and non‐parametric statistics. These analyses are here applied to the large dataset consisting of about 19,000 COVID‐19 patients in the Veneto region (Italy) during the entire Italian national lockdown. (3) Results: We estimate the COVID‐19 cumulative incidence spatial distribution, significantly reducing image noise. We identify four clusters of connected provinces based on the temporal evolution of the incidence. Surprisingly, while one cluster consists of three neighboring provinces, another one contains two provinces more than 210 km apart by highway. The survival function of the local spatial incidence values is modeled here by a tapered Pareto model, also used in other applied fields like seismology and economy in connection to networks. Model’s parameters could be relevant to describe quantitatively the epidemic. (4) Conclusion: The proposed methodology can be applied to a general situation, potentially helping to adopt strategic decisions such as the restriction of mobility and gatherings.
117 19 - PublicationOpen AccessAssessing the nature of stochastic uncertainties for ground motion predictions: the Apennines, Italy.(2019-04)
; ; ; ; ; ; ; In this study we aim to assessing the nature of stochastic uncertainties in ground-motion predictions, by including the variability of region-specific crustal attenuation in time and space in the Central/Northern Apennines (Italy), using the events occurred during 2016-2017 earthquake sequence. Spectral characteristics of excitation, attenuation and duration of ground motion are derived through a regression analysis of the peak ground velocities in the frequency range of 0.25–22 Hz. Regressions are carried out over thousands time windows before and after the Amatrice (M6.0), Visso (M5.9) and Norcia (M6.3) earthquakes, in order to evaluate the fluctuations in seismic wave attenuation induced by the largest mainshocks of the seismic sequence. Propagation terms are modeled using random vibration theory, through a grid search over the attenuation parameters. Here we show that crustal attenuation is strongly affected by transients triggered by the main events, and quantify the impact of the seismic wave attenuation variability on the ground-motion hazard in the Central/Northern Apennines. We also determine the effect of spatial variability of crustal attenuation and its contribution to stochastic uncertainties in ground motion predictions.58 11 - PublicationOpen AccessModulation of seismic attenuation at Parkfield, before and after the 2004 M6 earthquake, and one example from the Geysers geothermal field.(2019-12-11)
; ; ; ; ; ; ; ; ; ; ; Seismic attenuation is generally thought to be a constant, or a simple monotonic function of frequency, and generally not a function of time. Examples of exceptions include attenuation enhancement due to shallow earthquake-induced damage, and fluctuations due to fluid diffusion. In reality, seismic attenuation fluctuates continuously in time at all frequencies, and the presence of cracks, their density and connectivity, as well as the presence and saturation of fluids, play a central role in defining such behavior. Due to multiple mechanisms, the crack density within a fault’s damage zone varies throughout the seismic cycle. Moreover, non-tectonic stress loads, seasonal or tidal, can change the crack density of crustal rocks, and leave detectable signatures on seismic attenuation. A strong signature can also be left on the crustal attenuation by a stress transfer from a nearby fault. Here we show that attenuation time histories from the San Andreas Fault (SAF) at Parkfield are affected by seasonal loading cycles, as well as by 1.5–3 year periodic variations of creep rates, consistent with published results that documented a broad spectral peak, between 1.5 and 4 years, of the spectra calculated over the activity of repeating earthquakes, and over InSAR time series. After the Parkfield mainshock, the modulation of seismic attenuation is clearly correlated to tidal forces. Opposite attenuation trends are seen on the two sides of the fault up to the M6.5 2003 San Simeon earthquake, when attenuation changed discontinuously, in the same directions of the relative trends. Attenuation increased steadily for over one year on the SW side of the SAF, until the San Simeon earthquake, whereas it decreased steadily on the NE side of the SAF, roughly for the 6 months prior to the event. Random fluctuations are observed up to the 2004 M6 Parkfield mainshock, when rebounds in opposite directions are observed, in which attenuation decreased on the SW side, and increased on the NE side. Another example of changes of attenuation with time is given for the Geysers geothermal field, where a large data set of earthquake recordings from a dense temporal deployment are analyzed and results are given in terms of 1/Q(f,t).50 5 - PublicationOpen AccessModulation of Seismic Attenuation at Parkfield, Before and After the 2004 M6 Earthquake(2019-06-04)
; ; ; ; ; ; ; ; ; The crack density within a fault's damage zone is thought to vary as seismic rupture is approached, as well as in the postseismic period. Moreover, external stress loads, seasonal or tidal, may also change the crack density in rocks, and all such processes can leave detectable signatures on seismic attenuation. Here we show that attenuation time histories from the San Andreas Fault at Parkfield are affected by seasonal loading cycles, as well as by 1.5–3‐year periodic variations of creep rates, consistent with Turner et al. (2015, https://doi.org/10.1002/2015JB011998), who documented a broad spectral peak, between 1.5 and 4 years, of the spectra calculated over the activity of repeating earthquakes, and over InSAR time series. After the Parkfield main shock, we see a clear modulation between seismic attenuation correlated to tidal forces. Opposite attenuation trends are seen on the two sides of the fault up to the M6.5 2003 San Simeon earthquake, when attenuation changed discontinuously, in the same directions of the relative trends. Attenuation increased steadily of over one year on the SW side of the San Andreas Fault, until the San Simeon earthquake, whereas it decreased steadily on the NE side of the San Andreas Fault, roughly for the six months prior to the event. Random fluctuations are observed up to the 2004 M6 Parkfield main shock, when rebounds in opposite directions are observed, in which attenuation decreased on the SW side, and increased on the NE side.183 29 - PublicationOpen AccessAftershock patterns in recent central Appennines sequences(2019-03-18)
; ; ; ; ; During the last 20 years, three seismic sequences affected the Apenninic belt (central Italy): Colfiorito (1997–1998), L'Aquila (2009), and Amatrice Visso-Norcia Campotosto (2016–2017). They lasted for a long time, with a series of moderate-to-large earthquakes distributed over 40- to 60-km-long Apenninic-trending segments. Their closeness in space and time suggested to study their aftershock sequences to highlight similarities and differences. Aftershock space migration and the distribution of aftershock interarrival times were studied. Mathematical Morphology and nonparametric statistics were applied to reduce the effect of spatial noise. Parametric analysis in time domain and spectral analysis were performed. Two different types of aftershock sequences were found. The L'Aquila sequence presented a continuous and periodic temporal variation (period ≃120 days) of aftershock activity center along the sequence axis, while the other two sequences showed a piecewise continuous pattern and a shorter duration.We also found two different types of temporal evolution of the mean radial distance between the aftershock hypocenters and the one of a reference event corresponding to the start of a large and fast increase of daily energy release. One type was well described by a simple exponential model, while a power law model was more appropriate for the other one. Furthermore, in the first case, the aftershock interarrival time was very well fitted by an exponential model, while noticeable deviations were present in the other case. A possible explanation was provided in terms of the local geological and hydrogeological properties, which depend on the region location with respect to the Ancona-Anzio tectonic lineament.208 44 - PublicationOpen AccessExploring the relationship between the magnitudes of seismic events(2015-07-20)
; ; ;The distribution of the magnitudes of seismic events is generally assumed to be independent on past seismicity. However, by considering events in causal relation, for example mother-daughter, it seems natural to assume that the magnitude of a daughter event is conditionally dependent on the one of the corresponding mother event. In order to find experimental evidence supporting this hypothesis, we analyze different catalogs, both real and simulated, in two different ways. From each catalog, we obtain the law of triggered events' magnitude by kernel density. The results obtained show that the distribution density of triggered events' magnitude varies with the magnitude of their corresponding mother events. As the intuition suggests, an increase of mother events' magnitude induces an increase of the probability of having "high" values of triggered events' magnitude. In addition, we see a statistically significant increasing linear dependence of the magnitude means.111 19 - PublicationRestrictedMagnitude-dependent epidemic-type aftershock sequences model for earthquakes(2016-04)
; ; ;We propose a version of the pure temporal epidemic type aftershock sequences (ETAS) model: the ETAS model with correlated magnitudes. As for the standard case, we assume the Gutenberg-Richter law to be the probability density for the magnitudes of the background events. Instead, the magnitude of the triggered shocks is assumed to be probabilistically dependent on that of the relative mother events. This probabilistic dependence is motivated by some recent works in the literature and by the results of a statistical analysis made on some seismic catalogs [Spassiani and Sebastiani, J. Geophys. Res. 121, 903 (2016)10.1002/2015JB012398]. On the basis of the experimental evidences obtained in the latter paper for the real catalogs, we theoretically derive the probability density function for the magnitudes of the triggered shocks proposed in Spassiani and Sebastiani and there used for the analysis of two simulated catalogs. To this aim, we impose a fundamental condition: averaging over all the magnitudes of the mother events, we must obtain again the Gutenberg-Richter law. This ensures the validity of this law at any event's generation when ignoring past seismicity. The ETAS model with correlated magnitudes is then theoretically analyzed here. In particular, we use the tool of the probability generating function and the Palm theory, in order to derive an approximation of the probability of zero events in a small time interval and to interpret the results in terms of the interevent time between consecutive shocks, the latter being a very useful random variable in the assessment of seismic hazard.123 2 - PublicationOpen AccessVaccination Criteria Based on Factors Influencing COVID-19 Diffusion and Mortality(2020-12-15)
; ; ; ; ; ; ; SARS-CoV-2 is highly contagious, rapidly turned into a pandemic, and is causing a relevant number of critical to severe life-threatening COVID-19 patients. However, robust statistical studies of a large cohort of patients, potentially useful to implement a vaccination campaign, are rare. We analyzed public data of about 19,000 patients for the period 28 February to 15 May 2020 by several mathematical methods. Precisely, we describe the COVID-19 evolution of a number of variables that include age, gender, patient's care location, and comorbidities. It prompts consideration of special preventive and therapeutic measures for subjects more prone to developing life-threatening conditions while affording quantitative parameters for predicting the effects of an outburst of the pandemic on public health structures and facilities adopted in response. We propose a mathematical way to use these results as a powerful tool to face the pandemic and implement a mass vaccination campaign. This is done by means of priority criteria based on the influence of the considered variables on the probability of both death and infection.113 12 - PublicationOpen AccessForecasting the Next Parkfield Mainshock on the San Andreas Fault (California)A physical model was recently proposed to describe the phenomenon of coupling erosion that took place in the Japan Trough between 1998 and 2009, and the subsequent dynamic rupture occurred during the 2011 M9.1 Tohoku-Oki earthquake. Although 75% of the coupled area of the Japanese subduction was eroded away before nucleation, coseismic slip displaced both the locked (velocity weakening) and the eroded (velocity-strengthening) parts of the asperity. Here we show that a similar phenomenon of erosion repeatedly takes place at Parkfield on a NW patch of the SAF close to the asperity responsible for the repeating M6 earthquakes. We consider the variance of the spatial center of daily seismic activity along the SAF fault calculated on a moving time window. Initially the variance linearly grows due to increasing frictional engagement up to a maximum value. Then a process of erosion of the coupled area of the fault linearly reduces the variance until the stress is transferred onto the adjacent asperity, leading to failure. When halted due to a stress perturbation from the 1983 Coalinga earthquake, the process promptly resumes a virtually unchanged increasing trend. The stable and regular decrease of the variance started in early 1988 allows a very accurate retrospective prediction of the time of occurrence of the 2004 main shock. The process is repeating itself during the current seismic cycle, which, if undisturbed, will produce another mainshock in mid-2024.
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