Sequence-based hazard analysis for Italy considering a grid seismic source model

Earthquakes are usually clustered in both time and space and, within each cluster, the event of highest magnitude is conventionally identified as the mainshock, while the foreshocks and the aftershocks are the events that occur before and after it, respectively. Mainshocks are the earthquakes considered in the classical formulation of the probabilistic seismic hazard analysis (PSHA), where the contribution of foreshocks and aftershocks is usually neglected. In fact, it has been shown that it is possible to rigorously, within the hypotheses of the model, account for the effect of mainshock-aftershocks sequences by means of the sequence-based PSHA (i.e., SPSHA). SPSHA extends the usability of the homogeneous Poisson process, adopted for mainshocks within PSHA, to also describe the occurrence of clusters maintaining the same input data of PSHA; i.e., the seismic rates derived by a declustered catalog. The aftershocks’ occurrences are accounted for by means of conditional non-homogeneous Poisson processes based on the modified Omori law. The seismic source model for Italy has been recently investigated, and the objective of the study herein presented is to include and evaluate the effect of aftershocks, by means of SPSHA, based on a new grid model. In the paper, the results of PSHA and SPSHA are compared, considering the spectral and return periods that are of typical interest for earthquake engineering. Finally, a comparison with the SPSHA map based on a well- established source model for Italy is also provided

The 2016 Italian seismic hazard model

The Italian reference seismic hazard model was released in 2004, but it has been adopted for the definition of seismic zones in 2006 and for building code only in 2009. At the beginning of 2015 the Seismic Hazard Center (CPS) of INGV was commissioned to coordinate the national scientific community with the aim of elaborating a new reference seismic hazard model, mainly finalized to the update of seismic code. The CPS designed a roadmap to release within 2 years a significantly renewed model, with regard both to the updated input elements and to the strategies to follow, in order to obtain a shared and largely accepted PSHA. The main requirements of the model were discussed in meetings with the experts on earthquake engineering. A public call was opened according to a transparent procedure; we received 24 proposals from many national institutions. The activities were organized in 6 tasks: project coordination, input data, seismicity models, ground motion prediction equations, computation and rendering, validation. In the first phase, the working groups of each task worked separately; in the second phase of the project they collaborated to release a final model. During the project, many scientific aspects were carefully considered, as in many other seismic hazard projects: the use of a declustered catalogue versus a non declustered one, the adoption of the logic-tree approach instead of an ensemble modeling, the definition of objective strategies to assign the weight to each single model, and so on.PublishedSantiago Chile5T. Modelli di pericolosità sismica e da maremot

Decomposing DInSAR Time-Series into 3-D in Combination with GPS in the Case of Low Strain Rates: An Application to the Hyblean Plateau, Sicily, Italy

Differential Interferometric SAR (DInSAR) time-series techniques can be used to derive surface displacement rates with accuracies of 1 mm/year, by measuring the one-dimensional distance change between a satellite and the surface over time. However, the slanted direction of the measurements complicates interpretation of the signal, especially in regions that are subject to multiple deformation processes. The Simultaneous and Integrated Strain Tensor Estimation from Geodetic and Satellite Deformation Measurements (SISTEM) algorithm enables decomposition into a three-dimensional velocity field through joint inversion with GNSS measurements, but has never been applied to interseismic deformation where strain rates are low. Here, we apply SISTEM for the first time to detect tectonic deformation on the Hyblean Foreland Plateau in South-East Sicily. In order to increase the signal-to-noise ratio of the DInSAR data beforehand, we reduce atmospheric InSAR noise using a weather model and combine it with a multi-directional spatial filtering technique. The resultant three-dimensional velocity field allows identification of anthropogenic, as well as tectonic deformation, with sub-centimeter accuracies in areas of sufficient GPS coverage. Our enhanced method allows for a more detailed view of ongoing deformation processes as compared to the single use of either GNSS or DInSAR only and thus is suited to improve assessments of regional seismic hazard

LASSCI2009.2: layered earthquake rupture forecast model for central Italy, submitted to the CSEP project

The Collaboratory for the Study of Earthquake Predictability (CSEP) selected Italy as a testing region for probabilistic earthquake forecast models in October, 2008. The model we have submitted for the two medium-term forecast periods of 5 and 10 years (from 2009) is a time-dependent, geologically based earthquake rupture forecast that is defined for central Italy only (11-15˚ E; 41-45˚ N). The model took into account three separate layers of seismogenic sources: background seismicity; seismotectonic provinces; and individual faults that can produce major earthquakes (seismogenic boxes). For CSEP testing purposes, the background seismicity layer covered a range of magnitudes from 5.0 to 5.3 and the seismicity rates were obtained by truncated Gutenberg-Richter relationships for cells centered on the CSEP grid. Then the seismotectonic provinces layer returned the expected rates of medium-to-large earthquakes following a traditional Cornell-type approach. Finally, for the seismogenic boxes layer, the rates were based on the geometry and kinematics of the faults that different earthquake recurrence models have been assigned to, ranging from pure Gutenberg-Richter behavior to characteristic events, with the intermediate behavior named as the hybrid model. The results for different magnitude ranges highlight the contribution of each of the three layers to the total computation. The expected rates for M >6.0 on April 1, 2009 (thus computed before the L'Aquila, 2009, MW= 6.3 earthquake) are of particular interest. They showed local maxima in the two seismogenic-box sources of Paganica and Sulmona, one of which was activated by the L'Aquila earthquake of April 6, 2009. Earthquake rates as of August 1, 2009, (now under test) also showed a maximum close to the Sulmona source for MW ~6.5; significant seismicity rates (10-4 to 10-3 in 5 years) for destructive events (magnitude up to 7.0) were located in other individual sources identified as being capable of such earthquakes in the central part of this area of the Apennines

Integrating faults and past earthquakes into a probabilistic seismic hazard model for peninsular Italy

Italy is one of the most seismically active countries in Europe. Moderate to strong earthquakes, with magnitudes of up to ∼ 7, have been historically recorded for many active faults. Currently, probabilistic seismic hazard assessments in Italy are mainly based on area source models, in which seis- micity is modelled using a number of seismotectonic zones and the occurrence of earthquakes is assumed uniform. How- ever, in the past decade, efforts have increasingly been di- rected towards using fault sources in seismic hazard models to obtain more detailed and potentially more realistic patterns of ground motion. In our model, we used two categories of earthquake sources. The first involves active faults, and us- ing geological slip rates to quantify the seismic activity rate. We produced an inventory of all fault sources with details of their geometric, kinematic, and energetic properties. The associated parameters were used to compute the total seis- mic moment rate of each fault. We evaluated the magnitude– frequency distribution (MFD) of each fault source using two models: a characteristic Gaussian model centred at the max- imum magnitude and a truncated Gutenberg–Richter model. The second earthquake source category involves grid-point seismicity, with a fixed-radius smoothed approach and a his- torical catalogue were used to evaluate seismic activity. Un- der the assumption that deformation is concentrated along faults, we combined the MFD derived from the geometry and slip rates of active faults with the MFD from the spatially smoothed earthquake sources and assumed that the smoothed seismic activity in the vicinity of an active fault gradually de- creases by a fault-size-driven factor. Additionally, we com- puted horizontal peak ground acceleration (PGA) maps for return periods of 475 and 2475 years. Although the ranges and gross spatial distributions of the expected accelerations obtained here are comparable to those obtained through methods involving seismic catalogues and classical zonation models, the spatial pattern of the hazard maps obtained with our model is far more detailed. Our model is characterized by areas that are more hazardous and that correspond to mapped active faults, while previous models yield expected acceler- ations that are almost uniformly distributed across large re- gions. In addition, we conducted sensitivity tests to deter- mine the impact on the hazard results of the earthquake rates derived from two MFD models for faults and to determine the relative contributions of faults versus distributed seismic activity. We believe that our model represents advancements in terms of the input data (quantity and quality) and method- ology used in the field of fault-based regional seismic hazard modelling in Italy.Published2017-20395T. Modelli di pericolosità sismica e da maremotoJCR Journa

Preface: Linking faults to seismic hazard assessment in Europe

The title of this special issue, “Linking faults to seismic hazard assessment in Europe”, is the result of a challenging experiment that we have been carrying out for a few years: creating a working group of field geologists, fault modellers, data modellers, and seismic hazard practitioners to discuss and share ideas, promote initiatives to strengthen collaborations, and improve knowledge and practice of fault-based seismic hazard assessment. This special issue was designed in the framework of activity of the Fault2SHA Working Group (formally approved by the European Seismological Commission – ESC – at the 35th General Assembly, held in Trieste (Italy), in September 2016, http://www.fault2sha.net, last access: 9 May 2018). The key questions the Fault2SHA Working Group asked the research community with this special issue were as follows. What is the best strategy to fill in the gap in knowledge and expertise in Europe when considering faults in seismic hazard assessments? Are field geologists providing the relevant information for seismic hazard assessment? Are seismic hazard analysts interpreting field data appropriately? Is the full range of uncertainties associated with the characterization of faults correctly understood and propagated in computations? How can fault modellers contribute to a better representation of the long-term behaviour of fault networks in seismic hazard studies? Providing answers to these questions is fundamental in order to reduce the consequences of future earthquakes and improve the reliability of seismic hazard assessments.Published1349–13506T. Studi di pericolosità sismica e da maremotoJCR Journa

Seismic hazard in central Italy and the 2016 Amatrice earthquake

The Amatrice earthquake of August 24th, 2016 (Mw 6.0) struck an area that in the national reference seismic hazard model (MPS04) is characterized by expected horizontal peak ground acceleration (PGA) with 10% probability of exceedance in 50 years higher than 0.25 g. After the occurrence of moderate-to-large magnitude earthquakes with a strong impact on the population, such as the L’Aquila 2009 and Emilia 2012 ones (Mw 6.1 and 5.9, respectively), possible underestimations of the seismic hazard by MPS04 were investigated, in order to analyze and evaluate the possible need for its update. One of the most common misunderstanding is to compare recorded PGA only with PGA with 10% probability of exceedance in 50 years. Moreover, by definition, probabilistic models cannot be validated (or rejected) on the basis of a single event. However, comparisons of forecasted shakings with observed data are useful for understating the consistency of the model. It is then worth highlighting the importance of these comparisons. In fact, MPS04 is the basis for the current Italian building code to provide the effective design procedures and, thus, any modification to the seismic hazard would also affect the building code. In this paper, comparisons between recorded ground motion during the Amatrice earthquake and seismic hazard estimates are performed, showing that the observed accelerations are consistent with the values expected by the MPS04 model

Testing the seismogenic sources of the January 11th 1693 Sicilian earthquake (Io X/XI): insights from macroseismic field simulations

In January 11th 1693 an earthquake, commonly reported as the largest Italian seismic event (Io = X/XI MCS and Mw 7.4 according to CPTI04 reference catalogue), occurred in eastern Sicily, causing more than 54.000 casualties and totally destruction in the areas embracing the nowadays provinces of Catania, Siracusa and Ragusa. The entire Sicily Ionian coast was hit by a tsunami, with waves up to 8 metres high. Several geological sources differing in location, attitude and kinematics have been proposed by different authors for this earthquake: the NNW-SSE Malta Escarpment normal fault located offshore the eastern coast of Sicily, the nearly N-S Scicli strike-slip fault located in the central Hyblean plateau, the WSW-ENE Scordia-Lentini graben in the northern Hyblean region, the NW-dipping Ionian subduction plane, and lastly the NNW-dipping Sicilian Basal Thrust across the central-eastern Sicily and the Ionian offshore. In this paper, we attempt to discriminate among the above sources by applying a forward modelling technique which, starting from given fault model parameters (strike, dip, length, width, hypocentral location and magnitude) and reproducing acceleration time history above 1 Hz (the range of frequencies correlated with building damage), calculates the data point intensities at the surface. The differences between the observed and calculated macroseismic intensities, expressed as L1 norm, are discussed in order to identify the better analytical solutions. The obtained results are strongly dependent from the equivalent magnitude (Mw) attributed to the 1693 event, which in the literature ranges from Mw 6.8 to 8.0. Almost all the analysed fault models fall to reproduce the highest intensity (X/XI MCS) data points of the Hyblean region, suggesting that this area might have undergone a cumulative damage effects due to an intense foreshock activity (January 9th 1693, Mw 6.2, and January 11th 1693, morning, Mw 4.3). The portion of the macroseismic field located north of the Gela-Catania thrust front is better reproduced by the Malta Escarpment solution (Mw 7.1) and subordinately by the Sicilian Basal Thrust and by the Scordia-Lentini graben source models. The Hyblean portion of the field is better reproduced by the Ionian Subduction Plane (Mw 8) and subordinately by the Scicli line (Mw 7.4 and 7.1) source models. The entire field is better reproduced by the Scicli line related sources (Mw 7.1 and 7.4). Regional scale geological and seismotectonic considerations may help to further discriminate among the various sources