ALBERT

Description: The ground-motion prediction equation (GMPE) is a basic component for probabilistic seismic-hazard analysis. There is a wide variety of GMPEs that are usually obtained by means of inversion techniques of datasets containing ground motions recorded at different stations. However, to date there is not yet a commonly accepted procedure to select the best GMPE for a specific case; usually, a set of GMPEs is implemented (more or less arbitrarily) in a logic-tree structure, in which each GMPE is weighted by experts, mostly according to gut feeling. Here, we discuss a general probabilistic framework to numerically score and weight GMPEs, highlighting features, limitations, and approximations; finally, we put forward a numerical procedure to score GMPEs, taking into account their forecasting performances, and to merge them through an ensemble modeling. Then, we apply the procedure to the Italian territory; in addition to illustrating how the procedure works, we investigate other relevant aspects (such as the importance of the focal mechanism) of the GMPEs to different site conditions. Online Material: Figures showing regression analysis for peak ground acceleration (PGA) values and location map, and earthquake catalog and summary table of parameters corresponding to each ground-motion prediction equation (GMPE) implemented.

Description: The relative seismic velocity variations possibly associated to large earthquakes can be readily monitored via cross-correlation of seismic noise. In a recently published study, more than 2 yr of continuous seismic records have been analysed from three stations surrounding the epicentre of the 2009 April 6, M w 6.1 L'Aquila earthquake, observing a clear decrease of seismic velocities likely corresponding to the co-seismic shaking. Here, we extend the analysis in space, including seismic stations within a radius of 60 km from the main shock epicentre, and in time, collecting 5 yr of data for the six stations within 40 km of it. Our aim is to investigate how far the crustal damage is visible through this technique, and to detect a potential post-seismic recovery of velocity variations. We find that the co-seismic drop in velocity variations extends up to 40 km from the epicentre, with spatial distribution (maximum around the fault and in the north–east direction from it) in agreement with the horizontal co-seismic displacement detected by global positioning system (GPS). In the first few months after L'Aquila earthquake, the crust's perturbation in terms of velocity variations displays a very unstable behaviour, followed by a slow linear recovery towards pre-earthquake conditions; by almost 4 yr after the event, the co-seismic drop of seismic velocity is not yet fully recovered. The strong oscillations of the velocity changes in the first months after the earthquake prevent to detect the fast exponential recovery seen by GPS data. A test of differently parametrized fitting curves demonstrate that the post-seismic recovery is best explained by a sum of a logarithmic and a linear term, suggesting that processes like viscoelastic relaxation, frictional afterlip and poroelastic rebound may be acting concurrently.

Description: The ground-motion prediction equation (GMPE) is a basic component for probabilistic seismic-hazard analysis. There is a wide variety of GMPEs that are usually obtained by means of inversion techniques of datasets containing ground motions recorded at different stations. However, to date there is not yet a commonly accepted procedure to select the best GMPE for a specific case; usually, a set of GMPEs is implemented (more or less arbitrarily) in a logic-tree structure, in which each GMPE is weighted by experts, mostly according to gut feeling. Here, we discuss a general probabilistic framework to numerically score and weight GMPEs, highlighting features, limitations, and approximations; finally, we put forward a numerical procedure to score GMPEs, taking into account their forecasting performances, and to merge them through an ensemble modeling. Then, we apply the procedure to the Italian territory; in addition to illustrating how the procedure works, we investigate other relevant aspects (such as the importance of the focal mechanism) of the GMPEs to different site conditions. Online Material: Figures showing regression analysis for peak ground acceleration (PGA) values and location map, and earthquake catalog and summary table of parameters corresponding to each ground-motion prediction equation (GMPE) implemented.

Description: Since August 2016, central Italy has been struck by one of the most important seismic sequences ever recorded in the country. In this study, a strong-motion data set, consisting of nearly 10,000 waveforms, has been analyzed to gather insights about the main features of ground motion, in terms of regional variability, shaking intensity, and near-source effects. In particular, the shake maps from the three main events in the sequence have been calculated to evaluate the distribution of shaking at a regional scale, and a residual analysis has been performed, aimed at interpreting the strong-motion parameters as functions of source distance, azimuth, and local site conditions. Particular attention has been dedicated to near-source effects (i.e., hanging wall/footwall, forward-directivity, or fling-step effects). Finally, ground-motion intensities in the near-source area have been discussed with respect to the values used for structural design. In general, the areas of maximum shaking appear to reflect, primarily, rupture complexity on the finite faults. Large ground-motion variability is observed along the Apennine direction (northwest–southeast) that can be attributed to source-directivity effects, especially evident in the case of small-magnitude aftershocks. Amplifications are observed in correspondence to intramountain basins, fluvial valleys, and the loose deposits along the Adriatic coast. Near-source ground motions exhibit hanging-wall effects, forward-directivity pulses, and permanent displacement.

Description: 〈span〉〈div〉Summary〈/div〉We investigate the temporal changes of crustal velocity associated to the seismic sequence of 2016-2017, which struck central Italy with a series of moderate to large earthquakes. We cross-correlate continuous recordings of two years of ambient seismic noise from a network of 28 stations within a radius of 90 km around Amatrice town. We then map the spatio-temporal evolution of the velocity perturbations under the effect of subsequent earthquakes. Coinciding with each of the three mainshocks of the sequence we observe a sudden drop of seismic velocity which tends to quickly recover in the short term. After the end of the strongest activity of the sequence, the coseismic velocity changes display gradual healing towards pre-earthquake conditions following a quasi-linear trend, such that by the end of 2017 about 75 per cent of the perturbation is recovered. The spatial distribution of the velocity drop fluctuates with time, and the area that shows the most intense variations beyond the ruptured fault system elongates in the NE direction. This zone roughly corresponds to a region of foredeep sedimentary deposits consisting of highly hydrated and porous sandstones, which respond to the passage of seismic waves with increased pore pressure and crack number, leading to a reduction of the effective relative velocity.〈/span〉

Description: 〈span〉〈div〉Abstract〈/div〉This work describes a procedure to configure U.S. Geological Survey (USGS)‐ShakeMap for a given region. The procedure is applied to Italy to update and improve the ShakeMap service provided by Istituto Nazionale di Geofisica e Vulcanologia (INGV). The new configuration features (1) the adoption of recently developed ground‐motion models (GMMs) and of an updated map of VS30 for the local site effects and (2) the adoption of the newly developed USGS‐ShakeMap version 4 (v.4) software (see 〈a href="https://pubs.geoscienceworld.org/srl#sc10Data%20and%20Resources"〉Data and Resources〈/a〉). We have used the same subdivision in tectonic regimes adopted for the GMMs for the new Italian seismic hazard model (MPS19, 〈a href="https://pubs.geoscienceworld.org/srl#rf35"〉Meletti 〈span〉et al.〈/span〉, 2017〈/a〉) and selected the most appropriate GMMs after application of a ranking procedure consisting of statistical tests. A cross‐validation technique has been applied to test the goodness of the selected configuration and to compare the ShakeMaps obtained with the old (〈a href="https://pubs.geoscienceworld.org/srl#rf36"〉Michelini 〈span〉et al.〈/span〉, 2008〈/a〉) and the new settings. Finally, the INGV ShakeMap workflow has been renovated to exploit the data and analysis chain implemented at INGV from real‐time data streams acquisition to analyst revised waveforms including additional data (e.g., revised location, fault geometry) that may become available days after the event occurrence.〈/span〉

Description: 〈span〉〈div〉ABSTRACT〈/div〉This work describes a procedure to configure U.S. Geological Survey (USGS)‐ShakeMap for a given region. The procedure is applied to Italy to update and improve the ShakeMap service provided by Istituto Nazionale di Geofisica e Vulcanologia (INGV). The new configuration features (1) the adoption of recently developed ground‐motion models (GMMs) and of an updated map of VS30 for the local site effects and (2) the adoption of the newly developed USGS‐ShakeMap version 4 (v.4) software (see 〈a href="https://pubs.geoscienceworld.org/srl#sc10Data%20and%20Resources"〉Data and Resources〈/a〉). We have used the same subdivision in tectonic regimes adopted for the GMMs for the new Italian seismic hazard model (MPS19, 〈a href="https://pubs.geoscienceworld.org/srl#rf35"〉Meletti 〈span〉et al.〈/span〉, 2017〈/a〉) and selected the most appropriate GMMs after application of a ranking procedure consisting of statistical tests. A cross‐validation technique has been applied to test the goodness of the selected configuration and to compare the ShakeMaps obtained with the old (〈a href="https://pubs.geoscienceworld.org/srl#rf36"〉Michelini 〈span〉et al.〈/span〉, 2008〈/a〉) and the new settings. Finally, the INGV ShakeMap workflow has been renovated to exploit the data and analysis chain implemented at INGV from real‐time data streams acquisition to analyst revised waveforms including additional data (e.g., revised location, fault geometry) that may become available days after the event occurrence.〈/span〉

Description: Since the late 1960s - early 1970s, seismologists started studying the elastic properties of the Earth crust looking for signals from the Earth interior indicating that a large earthquake is coming. To be useful for prediction a signal needs to: 1) occur before most large earthquakes and 2) occur only before large earthquakes. Up to now, no one has ever found such a signal, but since the beginning of the search, seismologists developed theories that included variations of the elastic property of the Earth crust prior to the occurrence of a large earthquake. The most popular is the theory of the dilatancy: when a rock is subject to stress, the rock grains are shifted generating micro-cracks, thus the rock itself increases its volume. Inside the fractured rock, fluid saturation and pore pressure play an important role in earthquake nucleation, by modulating the effective stress. Thus measuring the variations of wave speed and of anisotropic parameter in time can be highly informative on how the stress leading to a major fault failure builds up. In 1980s and 1990s such kind of research on earthquake precursors slowed down and the priority was given to seismic hazard and ground motions studies, which are very important since these are the basis for the building codes in many countries. Today we have dense and sophisticated seismic networks to measure wave-fields characteristics: we archive continuous waveform data recorded at three components broad-band seismometers, we almost routinely obtain highresolution earthquake locations. Therefore we are ready to start to systematically look at seismic-wave propagation properties to possibly reveal short-term variations in the elastic properties of the Earth crust. One seismological quantity which, since the beginning, is recognized to be diagnostic of the level of fracturation and/or of the pore pressure in the rock, hence of its state of stress, is the ratio between the compressional (P-wave) and the shear (S-wave) seismic velocities: Vp/Vs. Variations of this ratio have been recently observed and measured during the preparatory phase of a major earthquake. In active fault areas and volcanoes, tectonic stress variation influences fracture field orientation and fluid migration processes, whose evolution with time can be monitored through the measurement of the anisotropic parameters. Through the study of S waves anisotropy it is therefore potentially possible to measure the presence, migration and state of the fluid in the rock traveled by seismic waves, thus providing a valuable route to understand the seismogenic phenomena and their precursors. On the other hand, only in the very recent times with the availability of the continuous seismic records, many authors have shown how it is possible to estimate the relative variations in the wave speed through the analysis of the crosscorrelation of the ambient seismic noise. In this paper we first analyze in detail these two seismological methods: shear wave splitting and seismic noise cross correlation, presenting a short historical review, their theoretical bases, the problems, learning, limitations and perspectives. We, then, compare the main results in terms of temporal trends of the observables retrieved applying both methods to the Pollino area (southern Apennines, Italy) case study.

Description: Published

Description: 257-274

Description: 4T. Fisica dei terremoti e scenari cosismici

Description: JCR Journal

Description: restricted

Description: The relative seismic velocity variations possibly associated to large earthquakes can be readily monitored via cross-correlation of seismic noise. In a recently published study, more than 2 yr of continuous seismic records have been analysed from three stations surrounding the epicentre of the 2009 April 6, Mw 6.1 L’Aquila earthquake, observing a clear decrease of seismic velocities likely corresponding to the co-seismic shaking. Here, we extend the analysis in space, including seismic stations within a radius of 60 km from the main shock epicentre, and in time, collecting 5 yr of data for the six stations within 40 km of it. Our aim is to investigate how far the crustal damage is visible through this technique, and to detect a potential post-seismic recovery of velocity variations. We find that the co-seismic drop in velocity variations extends up to 40 km from the epicentre, with spatial distribution (maximum around the fault and in the north– east direction from it) in agreement with the horizontal co-seismic displacement detected by global positioning system (GPS). In the first few months after L’Aquila earthquake, the crust’s perturbation in terms of velocity variations displays a very unstable behaviour, followed by a slow linear recovery towards pre-earthquake conditions; by almost 4 yr after the event, the co-seismic drop of seismic velocity is not yet fully recovered. The strong oscillations of the velocity changes in the first months after the earthquake prevent to detect the fast exponential recovery seen by GPS data. A test of differently parametrized fitting curves demonstrate that the post-seismic recovery is best explained by a sum of a logarithmic and a linear term, suggesting that processes like viscoelastic relaxation, frictional afterlip and poroelastic rebound may be acting concurrently.

Description: Published

Description: 604-6011

Description: 4T. Fisica dei terremoti e scenari cosismici

Description: JCR Journal

Description: restricted

Description: Naples is one of the most vulnerable cities in the world because it is threatened by several natural and man-made hazards: earthquakes, volcanic eruptions, tsunamis, landslides, hydrogeological disasters, and morphologic alterations due to human interference. In addition, the risk is increased by the high density of population (Naples and the surrounding area are among the most populated in Italy), and by the type and condition of buildings and monuments. In light of this, it is crucial to assess the ground shaking suffered by the city. To create a ShakeMap atlas for the region and to reconstruct the seismic history of the city from historical to recent times, we gather information from the most reliable and complete databases of macroseismic intensity records dating back to the eleventh century. The events felt in Naples cover a time span ranging from 1293 to 1999. The first event (Mw 5.8) was an earthquake in 1293, located in the southern Apennines, at a distance of 100 km from Naples. The most recent event was an earthquake of moderate magnitude in 1999, located beneath Vesuvius (Fig. 1). In the previous release of the macroseismic databases, two additional events associated with the volcanic activity of Vesuvius in 62 and 79 A.D. were included. They are not included in the new release of the databases because they occurred before 1000 A.D., and likewise they have been not included in this atlas because they are too ancient to be incorporated into any time and magnitude window of completeness. For instrumental events (e.g., after 1980), we merge these macroseismic records with strong-motion data. Basically, we integrate information from five Italian databases and catalogs. This gives us the opportunity to explore several sources of information, expanding the completeness of our data set in both time and magnitude. A total of 84 earthquakes have been analyzed. For each event, we compute the shakemap set (Wald et al., 1999; Michelini et al., 2008; Worden et al., 2010) using an ad hoc implementation developed for this application, with (1) specificground-motion prediction equations (GMPEs) accounting for the different attenuation properties in volcanic areas compared with the tectonic ones, and (2) detailed local microzonation to include the site effects. These shakemaps are provided in terms of Mercalli–Cancani–Sieberg intensity (MCS hereinafter) and peak ground acceleration (PGA). For PGA, the maps are provided in terms of median values and 16th and 84th percentiles, to quantify the epistemic uncertainties associated with the ground-motion measurements. In our prospective, the ShakeMap atlas has a dual application. On one hand, it is an important instrument in seismic risk management because it quantifies the level of shaking suffered by a city during its history, and it could be implemented to the quantification of the number of people exposed to certain degrees of shaking (Allen et al., 2009). Intensity data provide the evaluation of the damage caused by earthquakes; the damage is closely connected with the ground shaking, building type, and vulnerability, and it is not possible to separate these contributions. On the other hand, the atlas can be used as starting point for Bayesian estimation of seismic hazard. This technique allows for the merging of the more standard approach adopted, for example, in the compilation of the national hazard map of Italy used in this Bayesian framework as prior mode, with the site-type approach to the purpose of likelihood function (Selva and Sandri, 2013). The site-type technique is based on ground shaking data recorded in a given area; because the majority of earthquakes occurred when no seismometers were available, site data are mainly from macroseismic evaluation, that is, the felt effect is reconstructed from historical documents. The first two sections of the paper describe the databases and catalogs used, and the specific shakemap configuration applied. In the final section, we analyse the completeness of the atlas in terms of time for different magnitude/intensity thresholds, adopting and comparing two different strategies, one based mainly on historical analysis and the other on statistical evaluation.

Description: Published

Description: 963-972

Description: 3T. Pericolosità sismica e contributo alla definizione del rischio

Description: JCR Journal

Description: restricted