ALBERT

Description: This paper presents an original multidisciplinary (geological-structural-geomorphological and seismological) study aimed at investigating the origin of diffused seismic damages affecting several ancient buildings in the Roman port city of Ostia. We also evaluate the possibility to relate these damages to a previously hypothesized ENE-WSW trending fault, bordering the morphological height upon which the Ostia town was founded. Aimed at this scope, we performed seismic noise measures (by using 14 seismic stations) that show no significantly different response and lack of significant ground motion differential amplifications. The coexistence of (i) no local geological heterogeneities and (ii) low amplification of spectral ratios in the recorded seismic signals seems to exclude that the observed seismic damage may be the consequence of significant site effects. When also the large distance from the strongest Apennine’s seismogenic source areas is considered, the possibility that the observed damage may be the consequence of local events should be considered. We discuss the potentiality of the ENE-WSW trending fault as the source of the observed seismic damages, highlighting the supporting evidence as well as the uncertainties of such interpretation.

Description: Published

Description: 833–851

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

Description: JCR Journal

Description: We investigate an anomalous deep seismic sequence characterized by low-frequency bursts of earthquakes (maximum magnitude 5) that occurred between December 2013 and January 2014 in the southern Apenninic chain, Italy. Previous studies (Di Luccio et al., 2018) have shown evidences of fluid involvement in the earthquake nucleation process and identified thermal anomaly in nearby aquifers where CO2 of magmatic origin dissolves. Seismic source parameters reveal important information about the rupture mechanisms and stress field and their relation with the geological-tectonics processes. It is commonly assumed seismic source as pure shear dislocation described by a double-couple model. When volumetric changes occur, we need to consider the non-double couple source component in the description of the rupture process, as in geothermal and volcanic systems where fluids play an important role. In this study we analyze the 2013-2014 seismic sequence (earthquakes larger than 3) through a full moment tensor (FMT) inversion by using the HybridMT code (Kwiateck et al., 2016). The FMT is based on computing the integral of the first P-wave ground displacement pulse that is proportional to the seismic moment. Uncertainties of estimated FMTs are expressed by the normalized root-mean-squares between theoretical and observed amplitudes. The FMT technique is done on the vertical components of the seismograms, using a detailed 1D velocity model and accurate locations of the events. After a visual inspection of the waveforms based on the signal-to-noise ratio, we compute the displacement to estimate P-wave pulse polarities and the area beneath the P-pulse for each event and each station within epicentral distance comparable to the focal depth. The inversion procedure provides Mw, seismic moment and P, T and B axis orientation. Our results show high percentages of non-double couple components that vary over time and do not depend on earthquake magnitude. The stress axis orientations are in agreement with the regional crustal stress regime. The comparison of the obtained source parameters with petrological and geological data will allow us to better understand the emplacement mechanisms of intrusive bodies and the seismicity in mountain chains.

Description: Published

Description: San Francisco, California

Description: 3T. Sorgente sismica

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 dis- cuss 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 perfor- mances, 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 mecha- nism) of the GMPEs to different site conditions.

Description: Published

Description: 720-733

Description: 5T. Modelli di pericolosità sismica e da maremoto

Description: JCR Journal

Description: In recent years it has become accepted that earthquake source can attain significant Non-Double-Couple (NDC) components. Among the driving factors of deviation from normal double-couple (DC) mechanisms there is the opening/closing of fracture networks and the activation of pre-existing faults by pore fluid pressure perturbations. This observation makes the thorough analysis of source mechanism of key importance for the understanding of withdrawal/injection induced seismicity from geothermal and hydrocarbon reservoirs, as well as of water reservoir induced seismicity. In addition to the DC component, seismic moment tensor can be decomposed into isotropic (ISO) and compensated linear vector dipole (CLVD) components. In this study we performed a careful analysis of the seismic moment tensor of induced microseismicity recorded in the Val d'Agri (Southern Apennines, Italy) focusing our attention on the NDC component. The Val d'Agri is a Quaternary extensional basin that hosts the largest onshore European oil field and a water reservoir (Pertusillo Lake impoundment) characterized by severe seasonal level oscillations. Our input data-set includes swarm-type induced micro-seismicity recorded between 2005-2006 by a high-performance network and accurately localized by a reservoir-scale local earthquake tomography. We analyze two different seismicity clusters: (i) a swarm of 69 earthquakes with 0.3 ML 1.8 induced by a wastewater disposal well of the oilfield during the initial daily injection tests (10 days); (ii) 526 earthquakes with -0.2 ML 2.7 induced by seasonal volume changes of the artificial lake. We perform the seismic moment tensor inversion by using HybridMT code. After a very accurate signal-to-noise selection and hand-made picking of P-pulses, we obtain %DC, %ISO, %CLVD for each event. DC and NDC components are analyzed and compared with the spatio-temporal distribution of seismicity, the local stress field, the injection parameters and the water level in the impoundment. We find significant NDC components and abrupt temporal variations in the DC and ISO components that appear linked to the extremely variable parameters of the injection tests into the disposal well.

Description: Published

Description: New Orleans, LA

Description: 2T. Sorgente Sismica

Description: The “2016-2018 Central-Italy seismic sequence” is characterized by a Mw 6.0 mainshock that occured close to the towns of Accumoli and Amatrice at 1:36 a.m. (UTC) of August 24th 2016 and it is still ongoing. In 20 months activity, 63905 recorded earthquakes are spatially elongated NW- SE, the aftershocks are mainly concentrated between 2 and 25 km of depth with minimum and maximum magnitude ranging from Mw 0.1 to 6.5, respectively. This study area is characterized by a complex geological and structural setting derived by multi-phased contractional and extensional deformation began in the Early Tertiary. In detail, the epicentral area is located in the central sector of Apennine chain characterized by several NW-SE oriented Quaternary fault systems and compressional fronts. Nowadays stress indicators (such as borehole breakouts, anisotropic parameters, earthquake focal mechanisms and active faults) point out that the axial part of the Apennine belt is characterized by a general extension about NE-SW oriented. Slip data from Amatrice fault structures show that roughly N–S trending leftlateral strike–slip and transtensional/normal faults are all kinematically consistent with the existence of a Late Quaternary. The remote stress field is characterized by a NE–SW-oriented minimum horizontal stress (Shmin, sigma3 or maximum extension) and by a NW–SE trending maximum horizontal stress (SHmax, sigma1 or maximum compression). In this preliminary study we define the possible correlations between shear wave splitting and moment tensor parameters during the “2016-2018 Central-Italy seismic sequence”. Shear Wave Splitting phenomenon is described by 2 parameters: fast polarization direction and delay time. Generally, fast component strikes as the orientation of the anisotropic symmetry axes, and delay time measures the anisotropic strength. In this geo-structural context, for sedimentary rocks, according to the Extensive Dilatancy Anisotropy (EDA) model and its evolution in Anisotropic Poro Elasticity (APE) the local variations of the anisotropic parameters are both time and space dependent. As a consequence the fast direction will be parallel to the SHmax direction and the delay time will measure the intensity and/or thickness of the fracture field. Moreover, SHmax corresponds to sigma1 (thrust and strike-slip faulting regime) or sigma2 (normal faulting regime) and can be related to P, T and B axes orientations of seismic moment tensors. In this assumption, we analyze the spatial variations of local stress-field in term of SHmax which is strictly related to sigma, and fast polarization direction. After computing the parameters above described, we compare them with the stress field and geologic structures of the crust to explain the tectonic processes.

Description: Published

Description: Malta

Description: 2T. Deformazione crostale attiva

Description: 3T. Sorgente sismica