Description: Recently, an earthquake sequence formed by a series of subsequences characterized by large earthquakes (three events M ≥ 6.0 and five events M ≥ 5.0) occurred in the Central Apennines in 6–7 months during 2016–2017. Several studies interpreted this cascading type of earthquake occurrence as associated with fluid movements through an interconnected network of cracks around the active faults that triggered the subsequences in different time and location. In the present study we aim to verify if the presence of fluids is confirmed by the pattern of seismic wave attenuation. To do so we separately estimate the space-averaged parameters intrinsic Qi−1 and scattering Qs−1, proportional to the correspondent attenuation parameters, by applying a modified Multiple Lapse Time Window Analysis (MLTWA), in order to establish reference background values for the study region. MLTWA measures the decay of S-wave energy with distance and lapse time and fit it to the Energy Transport Equation solution for scattering, yielding the seismic albedo, B0 = Qs−1/Qt−1 and the extinction Length, Le,∝Qt−1 parameters as a function of frequency. The knowledge of the separated attenuation parameters allows determining which process of seismic energy dissipation dominates in the region. In the present study B0 results to be around0.6 (at 1.5 Hz) and decreases with increasing frequency down to 0.2 indicating that the attenuation of the seismic energy carried by S-wave is controlled by intrinsic dissipation rather than by scattering. Moreover, the intrinsic attenuation is frequency dependent (f −0.9 for the uniform model and f −0.4 for the crust/Moho model). This behavior well matches with the presence of fluids in the faults and fracture zones in the central Apennines, in agreement with results from other independent studies in the region. Finally, a comparison among the attenuation parameters estimated for this area and those measured in several areas of Italy and other parts of the world is presented together with a discussion on the comparison between the present MLTWA attenuation estimates and homologous results obtained for the same area with different approaches.

Description: Published

Description: 106498

Description: 7T. Variazioni delle caratteristiche crostali e precursori sismici

Description: JCR Journal

Description: We performed geophysical investigations in the northwestern sector of the island of Malta to reconstruct velocity-depth models and provide shear-wave velocity profiles. We have chosen two sites, one located in Rabat (Malta) and another in the Golden Bay area. We used both active (seismic and electrical 2D-tomography, Multichanel Analysis of Surface Waves – MASW) and passive (2D arrays and single-station measurements using ambient noise) geophysical methods. Consistently with previous studies performed in this part of Malta, we have found that both sites are characterised by site resonance in the frequency range 1-2 Hz as an effect of the local lithostratigraphic succession that shows an impedance contrast at about 60-90 m depth. This resonance effect can have important implications on both seismic hazard as well as seismic risk evaluation of the region since the amplified frequency range coincides with the resonance frequencies typical of 5–10 storey buildings which are very diffuse in the Maltese Islands, especially after intense recent urbanization.

Description: Published

Description: 41-48

Description: 4T. Sismologia, geofisica e geologia per l'ingegneria sismica

Description: JCR Journal

Description: Central Apennines is one of the most seismically active areas in Italy, where a long history of earthquake (69 events with magnitude 5.0 or greater, and 7 with magnitude exceeding 6.0) has strongly influenced the development of earthquake-resistant structural design. The Amatrice earthquake with a magnitude of 6.0 has occurred on August 24th, 2016 at 1:36:32 UTC and heavily damaged the villages of Amatrice and Accumoli. In following five months nine seismic events with magnitude M〉5.0 occurred in the area. The largest event of the sequence (M6.5) occurred on October 30th at 06:40:18 UTC with the epicenter located in the vicinity of the town of Norcia. The sequence in progress together with that of L'Aquila (6 April 2009) provides a massive set of seismological data (3 events with magnitude 6.0, up to a maximum magnitude 6.5. The main objective of this study is to provide the quantification of the ground motion induced by a given earthquake that may have effects on the territory. Therefore, we obtain the scaling relationships for the high frequency ground motion in the region and regressions were carried out over 91,000 selected waveforms recorded during 672 events with magnitude ranging from Mw3.0 to Mw 6.5. For sequence we both use accelerometric and seismometric data recorded by the seismic stations of the National Accelerometric Network (RAN) and the National INGV Seismic Network (RSN). Regional attenuation and source scaling are parameterized to describe the observed ground motions as function of distance, frequency and magnitude. Peak ground velocities are measured in the selected narrow frequency bands from 0.25 to 20.0 Hz; observed peaks are regressed to define a regional attenuation function, a set of excitation terms and a set of site response terms. Results are then modeled through the random vibration theory. Finally, these set of parameters (frequency dependent attenuation, source and site related spectral parameters) are performed in order to predict the earthquake-induced ground motion in the region and then validated against recordings. We also compare our predicted ground motion parameters with recent global and regional ground motion prediction equations and reveal the importance of the retrieving specific regional seismic parameters for the ground motion predictive equations.

Description: Unpublished

Description: Washington - USA

Description: 4T. Sismicità dell'Italia

Description: 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.

Description: Unpublished

Description: Vienna - Austria

Description: 6T. Studi di pericolosità sismica e da maremoto

Description: This article has been accepted for publication in Geophysical Journal International. Published by Oxford University Press on behalf of the Royal Astronomical Society.

Description: The classical procedure of the Probabilistic Seismic Hazard Analysis (PSHA) requires a Poissonian distribution of earthquakes. Seismic catalogs follow a Poisson distribution just after the application of a declustering algorithm that leaves only one earthquake for each seismic sequence (usually the stronger, i.e. the main shock). Removing earthquakes from the seismic catalogs leads to underestimation of the annual rates of the events and consequently associate with low seismic hazard as indicated by several studies. In this study, we aim investigating the performance of two declustering methods on the Italian instrumental catalog and the impact of declustering on estimation of the b-value and on the seismic hazard analysis. To this end, first the spatial variation in the seismicity rate was estimated from the declustered catalogs using the adaptive smoothed seismicity approach, considering small earthquakes (Mw≥3.0). We then corrected the seismicity rates using new approach that allows for counting all events in the complete seismic catalog by simply changing the magnitude frequency distribution. The impact of declustering on seismic hazard analysis is illustrated using PSHA maps in terms of peak ground acceleration (PGA) and spectral acceleration (SA) in 2 s, with 10% and 2% probability of exceedance in 50 years, for Italy. We observed that the hazard calculated from the declustered catalogs was always lower than the hazard computed using the complete catalog. These results are in agreement with previous results obtained in different parts of the world.

Description: Published

Description: 1174–1187

Description: 6T. Studi di pericolosità sismica e da maremoto

Description: JCR Journal

Description: Questa iniziativa è stata rivolta a soddisfare l'esigenza di una mappa di pericolosità sismica di riferimento per l'individuazione delle zone sismiche, formulata dall'ordinanza PCM 20 marzo 2003 n.3274, All.l, che ne fissa criteri e scadenza (aprile 2004). In particolare, l'istituto Nazionale di Geofisica e Vulcanologia, raccogliendo l'auspicio della Commissione Grandi Rischi del Dipartimento della Protezione Civile, ha promosso nel luglio 2003 la redazione della mappa coinvolgendo nella sua redazione esperti del mondo scientifico oltre che propri ricercatori, consegnando una prima versione in tempi molto brevi (novembre 2003). Pur condizionata in modo significativo dalla scadenza, questa ricerca ha utilizzato e elaborato un gran numero di dati e conoscenze prodotti di recente. In particolare: i) è stata elaborata una nuova zonazione sismogenetica, denominata ZS9, a partire da un sostanziale ripensamento della precedente zonazione ZS4 (Meletti et al., 2000) alla luce delle evidenze di tettonica attiva e delle valutazioni sul potenziale sismogenetico acquisite negli ultimi anni. ZS9 è corredata, per ogni ZS, da un meccanismo focale prevalente e da un valore di profondità, determinati nella prospettiva di utilizzo con le relazioni di attenuazione descritte più sotto; ii) è stata prodotta una versione aggiornata del catalogo CPTI (Gdl CPTI, 1999), detta CPTI2, mediante: i) la revisione dei parametri dei terremoti della zona etnea; ii) la determinazione di valori di Mw e ML per tutti gli eventi; iii) la ricompilazione ex-novo della porzione 1981-1992 e la sua estensione al 2002, utilizzando gli studi macrosismici e strumentali resi disponibili a partire dal 1999; iii) sono state verificate alla luce dei dati dei terremoti più recenti le relazioni di attenuazione di amax definite a scala nazionale e europea, utilizzando distanze epicentrali calcolate in modo appropriato e le modifiche per i meccanismi focali prevalenti introdotte da Bommer et al. (2003). Inoltre, a partire da leggi di scala ricavate da dati strong- e weak-motion, sono state calibrate tre relazioni di attenuazione regionali, valide per tre macrozone; con approccio analogo è stata sviluppata una nuova relazione utilizzabili per le zone vulcaniche. iv) sono stati determinati, con approcci storici e statistici, due insiemi di intervalli di completezza dei dati del catalogo CPTI2. Secondo i criteri in uso nell'ambito scientifico internazionale, le procedure adottate per la compilazione della prima versione della mappa di amax con probabilità di superamento del 10% in 50 anni (Gruppo di Coordinamento, 2003) sono state sottoposte alla revisione di un gruppo di esperti di area europea, che ha fornito valutazioni e suggerimenti in modo sia collegiale che individuale. Conseguentemente, dati di ingresso e procedure sono stati migliorati in accordo con tali indicazioni. La mappa definitiva, presentata nella pagina successiva e descritta nel seguito, è stata ottenuta come mediana di 16 mappe corrispondenti ad altrettanti rami di un albero logico. Ciascun ramo, cui viene attribuito un peso, esplora alternative riguardanti: i) le modalità di valutazione della completezza del catalogo; ii) le modalità di determinazione dei tassi di sismicità; iii) le relazioni di attenuazione del moto del suolo. La mappa è corredata da una misura dell'incertezza, espressa in termini di distribuzione dei valori dell'84mo percentile relativi al campione delle 16 mappe.

Description: Istituto Nazionale di Geofisica e Vulcanologia (INGV)

Description: Published

Description: 4T. Sismicità dell'Italia

Description: 5T. Sismologia, geofisica e geologia per l'ingegneria sismica

Description: 6T. Studi di pericolosità sismica e da maremoto

Description: 4IT. Banche dati

Description: The increase in the wealth of information on the seismotectonic structure of the Marmara region after two devastating earthquakes (M7.6 Izmit and M7.2 Duzce events) in the year 1999 opened the way for the reassessment of the probabilistic seismic hazard in the light of new datasets. In this connection, the most recent findings and outputs of different national and international projects concerning seismicity and fault characterization in terms of geometric and kinematic properties are exploited in the present study to build an updated seismic hazard model. A revised fault segmentation model, alternative earthquake rupture models under a Poisson and renewal assumptions, as well as recently derived global and regional ground motion prediction equations (GMPEs) are put together in the present model to assess the seismic hazard in the region. Probabilistic seismic hazard assessment (PSHA) is conducted based on characteristic earthquake modelling for the fault segments capable of producing large earthquakes and smoothed seismicity modelling for the background smaller magnitude earthquake activity. The time-independent and time-dependent seismic hazard results in terms of spatial distributions of three ground-shaking intensity measures (peak ground acceleration, PGA, and 0.2 s and 1.0 s spectral accelerations (SA) on rock having 10% and 2% probabilities of exceedance in 50 years) as well as the corresponding hazard curves for selected cities are shown and compared with previous studies.

Description: Published

Description: id 489

Description: 6T. Studi di pericolosità sismica e da maremoto

Description: JCR Journal

Description: On 23 October 2011, an M w 7.1 earthquake struck the Van Lake region of eastern Turkey; causing vast damage in the cities of Van and Erciş. The mainshock was followed by a large number of aftershocks, which define a 60–70 km long and 30–35 km wide northeast–southwest-trending structure, in agreement with the source rupture mod- els derived for the main event. In this paper, we take advantage of this large data set to examine the spatial and temporal properties of the Van earthquake aftershock activity. We derive the spatial distribution of b-value of the Gutenberg–Richter law, as well as com- plementary seismicity parameters, along the surface projection of the fault plane. Recent studies have been published on the same issue, presenting controversial and sometimes opposite results. With respect to previous studies, we rely on a possibly higher quality catalog of relocated earthquakes. Furthermore, we adopt a more conservative approach, excluding from the analysis the first few days of data, until the M c reaches a stable com- pleteness threshold; finally, we conduct statistical tests in order to check the significance of the spatial and temporal variation of b-value across the fault plane. Calculations are made for the complete catalog and for two independent aftershock subcatalogs, after which a stable magnitude of completeness Mc is reached. For each catalog, we correlate the observed b-value patterns with slip distribution models of the mainshock obtained through the inversion of seismological and geodetic data. Overall, the b-values vary from 0.9 to 1.5 along the Van rupture fault zone. The higher b-values (〉 1:1) are observed around the epicenter of the mainshock characterizing the higher coseismic slip area on the fault projection. Low b-values are concentrated at the periph- eral portion of the fault, away from high-slip patches. Moreover, the b-value distribution over the fault plane undergoes significant variation throughout the aftershock sequence.

Description: Published

Description: 913–930

Description: 7T. Variazioni delle caratteristiche crostali e precursori sismici

Description: JCR Journal

Description: On May 20 2012, an event of Ml 5.9 (Mw 5.6) stuck the southem edge of the Po river plain (Pianura Padana). The earthquake was preceded by a foreshock of Ml 4.1 (Mw 3.8), less than 3 hours before the Mw 5.6 main. Hypocentral depths were 6.3 km for both events. Centroid depths were 5 and 6 km, respectively. The activated fault was a reverse one, dipping to the south. Then a complex seismic sequence started, in which more than six earthquakes with Ml greater than 5 stuck the area, the last one on June 3, 2012. Aftershocks delineated a 50 km long and 10-15 km wide zone, approximately elongated in the WE direction. More than 2100 events were located between May 19 and June 25 2012 by the INGV National Seismic Network, 80 of them with Ml greater than 3.5. The damage due to the Ml 5+ earthquakes was widespread, as they severely hit historical towns and industrial infrastructures. However, a striking inconsistency exists between the relatively small moment magnitudes and the corrisponding high level of damage. In order to define a velocity structure for the crust beneath the Pianura Padana, to be used for waveform inversion of moment tensors, we gathered all the geophysical and geological information available for the area. The model is characterized by very thick and shallow Quaternary sediments, to be used for the inversion of broadband waveforms for moment tensor (MT) solutions, in the frequency band between 0.02-0.1 Hz. We calculated moment tensors for 20 events down to Mw~3.2. We demonstrate how surface waves dominate the seismograms in the region, which may have played a major role in enhancing the damage to industrial structures observed in the epicentral area. Synthetic seismograms computed using the developed model well reproduced the anomalous durations of the ground motion observed in Pianura Padana, also highlighting important implications for the seismic hazard in the entire area. The present seismic hazard assessment as well as the size of the historical earthquakes in the region (and so their recurrence times), may need to be re-evaluated in the light of this new results.

Description: Published

Description: Roma

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

Description: open