A Time\Dependent Probabilistic Seismic Hazard Model For The Central Apennines (Italy) (original) (raw)
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Bulletin of The Seismological Society of America, 2009
We produce probabilistic seismic hazard assessments for the Central Apennines, Italy, using time-dependent models that are characterized using a Brownian Passage Time (BPT) recurrence model. Using aperiodicity parameters of 0.3, 0.5, and 0.7, we examine the sensitivity of the probabilistic ground motion and its deaggregation to these parameters. For the seismic source model we incorporate both smoothed historical seismicity and geological information on faults. We use the maximum magnitude model for the fault sources together with a uniform probability of rupture along the fault (floating fault model) for faults where earthquakes cannot be correlated with known geologic structural segmentation.
Layered Seismogenic Source Model and Probabilistic Seismic-Hazard Analyses in Central Italy
Bulletin of The Seismological Society of America, 2006
We defined a seismogenic model for central Italy based on three layers of sources and computed the relative seismic-hazard maps. One layer is constituted by individual structures liable to generate major earthquakes (M Ն5.5). We defined them as seismogenic boxes by using geological information in terms of plan projection of active faults; the seismicity rates associated with an individual source are based on the geometry and kinematics of the fault; the recurrence model is controlled by the earthquake-source association, and, when possible, we defined the occurrence time of the last major event, using it in a time-dependent approach. Another layer is given by the instrumental seismicity analysis of the past two decades, which allows us to evaluate the background seismicity (M ϳϽ5.5); using a sliding-window selection of events, we defined a model of regular adjacent cells of variable a and b values of the Gutenberg-Richter relation. The last layer utilizes all the instrumental earthquakes and the historical events not correlated to known structures (4.5 Ͻ M ϳϽ6), by separating them into seismotectonic provinces shaped on a geological-structural basis. The seismic-hazard computations first use this layered model in a traditional probabilistic scheme. The results indicate a narrow belt of peak ground acceleration (PGA) higher than 0.30g (with standard deviation in attenuation functions) in the axial part of the Apennine chain, with a maximum spot of PGA Ͼ0.40g southeast of the area damaged by the 1997-1998 Umbria-Marche sequence (PGA expected not to be exceeded in 50 years at 90% probability level). The background seismicity gives a nonnegligible contribution to the hazard, at least for first damage levels. Then, a simplified time-dependent hypothesis has been introduced for the individual sources alone, computing the conditional probability of occurrence of characteristic earthquakes for each source by Brownian passage time distributions. Adopting equivalent fictitious seismicity rates, we obtained maps referring to the next 50 years by using traditional codes. These results show that the contribution of the recently active sources vanishes, and the most hazardous sites are now located south of L'Aquila and in the Sulmona area. We consider that the methodology and results obtained are useful for seismic risk reduction strategies.
In the frame of the activities promoted by the National Group for the Defence against Earthquakes, a four-year national project has been funded on the topic of determining priorities in seismic risk mitigation at the national scale. This paper describes the efforts to introduce individual faults and time-dependent issues in the seismic hazard assessment developed within this project. The elaborations refer to the national scale and use some original data produced and released for the project's purposes, namely, the Database of Italy's Seismogenic Sources and a compilation of the Italian instrumental earthquakes from 1981 until 2002. An integrated seismic hazard model that combines the individual earthquake sources recognised by geological and seismological studies with the information obtained by the national seismographic network is proposed and applied to the seismic hazard assessment. Individual sources are assumed to follow the characteristic earthquake model and their rates of occurrence derived from geometric and kinematic considerations. The minor seismic activity defined in terms of background sources is represented by Gutenberg-Richter relationships calibrated on the instrumental dataset. Then, the conditional probability of occurrence of characteristic earthquakes for each individual source is modelled by the Brownian-Passage-Time distribution. The simple time-dependent hypotheses introduced are used to derive equivalent fictitious seismicity rates: they can be entered into traditional seismic hazard codes for having maps that are referred to the time when the analysis has been performed. The results are heavily controlled by some arbitrary choices like the regional distribution of slip rate (applied to all individual sources lacking detailed information), or the uncertainties a priori attributed to the mean recurrence time. Nevertheless, the maps of conditional probability of earthquake occurrence and the seismic hazard maps, under Poisson and time-dependent hypotheses, enhance the role of moderate earthquakes in driving the seismic hazard. The databases have been updated during the life of the project and the new versions became public after the project ended. The elaborations presented here refer to the data available during the project and were not updated to be consistent with the final products released by the project. The results obtained must therefore, be considered mainly for their methodological approach to the problem, so their application to seismic protection strategies has to be done with great care.
Updated area-source seismogenic model for seismic hazard of Italy
2022
A new probabilistic seismic hazard model, called MPS19, has been recently proposed for the Italian territory, as a result of the efforts of a large national scientific community. This model is based on 11 groups of earthquake rupture forecast inputs and, particularly, on 5 area-source seismogenic models, including the so-called "MA4" model. Data-driven procedures were followed in MA4 to evaluate seismogenic parameters of each area source, such as upper and lower seismogenic depth, hypocentral depth distributions and nodal planes. In a few cases, expert judgment or ad-hoc assumptions were necessary due to the scarcity of data. MA4 consists of 20 seismicity models that consider epistemic uncertainty in the estimations of the completeness periods of the earthquake catalogue, of maximum magnitude values and of seismicity rates. In particular, 5 approaches were adopted to calculate the rates, in the form of truncated Gutenberg-Richter frequency-magnitude distribution. The first approach estimated seismicity rates using earthquakes located in each source zone, while the other approaches firstly calculated the a and b values of the truncated Gutenberg-Richter relation for groups of zones considered tectonically homogeneous, and successively partitioned in different ways the a values to the zones forming each group. The results obtained in terms of seismic hazard estimates highlighted that the uncertainty explored by the 20 seismicity models of MA4 is at least of the same order of magnitude of the uncertainty due to alternative ground motion models.
An attempt to evaluate seismic hazard in Central-Southern Italy
Natural Hazards, 1990
A method for the evaluation of seismic hazard in a given zone, taking into account both the spreading of macroseismic effects and seismic catalogue information, is applied. A databank of some 500 digitized isoseisms of earthquakes having occurred in Italy between 1542 and 1986 is used. The isoseismical maps are digitized considering for each degree of intensity the length of 24 spreading rays starting from the macroseismic epicenter or barycentre of the megaseismic area. These rays are separated from each other by the same angle, i.e. every isoseism is divided into 24 equal circular sectors. The year 1542 is taken as the beginning of the time span, since this is when the first seismic event occurred for which reliable isoseismal maps are available. The epicentral intensities considered lie between the VI and XI degrees of the Mercalli-Cancani-Sieberg scale (MCS). This digitized databank is analyzed to achieve, for each homogeneous seismogenetic zone that has been recognized, the mean azimuthal spreadings of effects for each degree of intensity as a function of the epicentral intensity. Once a mean propagation model is obtained for each zone, this is applied to seismic events of the same zone, the isoseismal maps of which are not available. A geographic grid is defined to cover the analyzed area, and for each cell of this grid it is then possible to count the number of felt events and their degree. These effects have been evaluated either on the basis of the isoseismal maps (when available) or on the basis of the mean propagations of the zone in which the single event occurred. Moreover, an index summarizing the seismic information was computed for each cell of the previous grid. All the events producing effects and their provenance are stored on files, allowing the main seismogenic zones influencing this cell to be identified. This methodology has been applied to central and southern Italy in an area between the latitudes 40.6 and 43.3 N. In particular, attention is focussed on the sample areas of Rome (given the historical and political importance of the city) and of the Sannio-Matese and Irpinia zone (in which some of the strongest earthquakes of the Apennine chain have occurred). Finally, in order to evaluate the maximum expected magnitude, extreme value statistics (Gumbel III-type) are applied to the Colli Albani area, which represents the seismogenic zone nearest to Rome. For the Sannio-Matese and Irpinia area, considering the more dangerous zone as a 'unicum', the Weibull distribution has been hypothesized to determine the mean return time for events with an intensity greater than or equal to IX.
The new Italian seismic hazard model (MPS19)
Annals of Geophysics, 2021
We describe the main structure and outcomes of the new probabilistic seismic hazard model for Italy, MPS19 [Modello di Pericolosità Sismica, 2019]. Besides to outline the probabilistic framework adopted, the multitude of new data that have been made available after the preparation of the previous MPS04, and the set of earthquake rate and ground motion models used, we give particular emphasis to the main novelties of the modeling and the MPS19 outcomes. Specifically, we (i) introduce a novel approach to estimate and to visualize the epistemic uncertainty over the whole country; (ii) assign weights to each model components (earthquake rate and ground motion models) according to a quantitative testing phase and structured experts’ elicitation sessions; (iii) test (retrospectively) the MPS19 outcomes with the horizontal peak ground acceleration observed in the last decades, and the macroseismic intensities of the last centuries; (iv) introduce a pioneering approach to build MPS19_clust...