Central Italy Seismic Sequence Research Articles

The Influence of Lithology and Fault Source Volume on the Magnitude–Frequency‐Distribution of Earthquakes

AbstractThe earthquake Magnitude‐Frequency‐Distribution is usually modeled with the Gutenberg‐Richter law, where the b‐value controls the relative rate of small and large earthquakes. b‐value shows an inverse dependence on differential stress, it increases with fault roughness and in areas with fluid involvement in faulting. b‐value analyses have been also applied to infer temporal evolution of the stress state along active faults or to discriminate between foreshocks and aftershocks. For the Mw 6.5 2016–2017 Central Italy seismic sequence, we show that: (a) away from the major earthquake faults, b‐values are controlled by lithology and style of deformation; (b) the absolute number of the b‐values depends on the adopted magnitude scale and catalog, but differences induced by lithology are preserved; (c) the selection of the fault source volume can strongly influence b‐value changes in time, highlighting some complexities on the applicability of the b‐value as fault stress meter.

Correction: Salvini et al. Ground Displacements Estimation through GNSS and Geometric Leveling: A Geological Interpretation of the 2016–2017 Seismic Sequence in Central Italy. Geosciences 2022, 12, 167

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Damage assessment of Apennine masonry churches severely damaged during the 2016 Central Italy seismic sequence

Damage assessment of Apennine masonry churches severely damaged during the 2016 Central Italy seismic sequence

Empirical fragility curves for masonry churches and their macro-elements using a large database: Proposal of a new likelihood function

Several earthquakes have highlighted over time the high vulnerability of masonry churches to horizontal actions, so that more and more research is being devoted to the assessment of their seismic response and to the design of strengthening interventions to guarantee their preservation.This paper analyses in detail the damage suffered by a large sample of 2870 masonry churches hit by the 2016–17 Central Italy seismic sequence, using the damage data extracted from the DaDO Web GIS. Fragility curves based on the empirical/observational approach are elaborated starting from the damage probability matrices (DPMs) in terms of global damage, both for the whole database and for some subsets of churches with homogeneous characteristics, in order to analyze the influence of some typological features. In particular, it is highlighted that the subset of small churches with rectangular plan and single nave show a generally lower vulnerability compared with those of the whole database.Moreover, the damage observed in the most common macro-elements detectable in masonry churches is examined by defining specific damage indices for macro-elements. The fragility curves for the macro-elements are, then, compared with those referring to the global damage, evidencing some interesting aspects useful both in the survey phase and for planning specific interventions. One of the main outcomes is that the seismic responses of the ‘Façade’ and ‘Principal nave’ macro-elements may closely approximate the global response of the church.Finally, the paper presents a new fitting procedure of the observational damage data aimed to build fragility curves for subsets of churches characterized by a reduced number of data falling in some PGA ranges. A ‘weighting’ procedure is, indeed, proposed in order to give less relevance to small groups of churches falling in the less populated PGA ranges.

Development of a new seismic vulnerability model for churches based on simple typological features

The sequence of seismic events occurred in last decades in Italy highlighted the high vulnerability of churches. Since the Umbria–Marche earthquake in 1997, a systematic series of post-earthquake surveys has been carried out. The data collected during those surveys were used to propose first sets of vulnerability and fragility curves. After the three last major earthquakes (L’Aquila 2009, Emilia 2012, Central Italy 2016–2017), other significant series of on-site surveys led to the collection of a large amount of data, both on the damage suffered by churches and their typological characteristics. In this study, a data set of 1127 churches damaged by the 2009 L’Aquila earthquake and the 2016–2017 Central Italy seismic sequence was analyzed. This article presents a new simplified approach to determine vulnerability indexes of churches, in order to obtain vulnerability curves, and then a set of fragility curves, depending on the typological characteristics of the single church. This allows to have a single set of curves with the entire sample and without a typological subdivision. This article first analyzes in detail the available database records to define their representativeness and influence on vulnerability. Then, a series of vulnerability modifiers are defined, pointing out which parameters effectively affect the overall vulnerability. These observations also allow identifying which parameters deserve to be collected onsite, improving the efficiency of future survey campaigns. In conclusion, starting from the vulnerability modifiers, a set of vulnerability and fragility curves are presented.

Multidisciplinary Study of Mud Emissions Following the 2016 Norcia Earthquake

We report composition, grain size, and rheological data related to the mud emitted as a consequence of the maximum moment magnitude (Mw max = 6.5) on 30 October 2016, commonly referred to as the Norcia earthquake (central Italy), and on the activity of pre-existent mud volcanoes affected by the central Italy seismic sequence started on 24 August 2016. The emission sites were located at Monteleone di Fermo and Santa Vittoria in Matenano, two municipalities near the town of Fermo (Marche Region, Italy). We sampled, measured, and analyzed the products of mud emissions 3 days after the mainshock to characterize the mud by geochemical, mineralogical, and rheological analyses. The muds’ geochemical composition and low electrical conductivity suggest a continental origin, likely belonging to the Colombacci Formation. The collected muds are silt–sand–water-rich suspensions characterized by a Bigham rheology with viscosity values between 6.3∙107 and 6.9∙105 Pa∙s. The calculated minimum fluidization velocity of the mud suspensions is between 0.05 m/s (grain size of 2 μm) and 0.74 m/s (grain size of 8 μm). Water-rich mud suspensions flowing on a slope move faster as the water content increases up to 30 wt.%. At higher values, the velocity remains almost constant due to the disaggregation of bonds among the solid particles in the mixtures.

Scattering Attenuation Images of the Control of Thrusts and Fluid Overpressure on the 2016–2017 Central Italy Seismic Sequence

AbstractDeep fluid circulation likely triggered the large extensional events of the 2016–2017 Central Italy seismic sequence. Nevertheless, the connection between fault mechanisms, main crustal‐scale thrusts, and the circulation and interaction of fluids with tectonic structures controlling the sequence is still debated. Here, we show that the 3D temporal and spatial mapping of peak delays, proxy of scattering attenuation, detects thrusts and sedimentary structures and their control on fluid overpressure and release. After the mainshocks, scattering attenuation drastically increases across the hanging wall of the Monti Sibillini and Acquasanta thrusts, revealing fracturing and fluid migration. Before the sequence, low‐scattering volumes within Triassic formations highlight regions of fluid overpressure, which enhances rock compaction. Our results highlight the control of thrusts and paleogeography on the sequence and hint at the monitoring potential of the technique for the seismic hazard assessment of the Central Apennines and other tectonic regions.

A Reliable Procedure to Estimate the Rupture Propagation Directions from Source Directivity: The 2016–2018 Central Italy Seismic Sequence

A Reliable Procedure to Estimate the Rupture Propagation Directions from Source Directivity: The 2016–2018 Central Italy Seismic Sequence

A Reliable Procedure to Estimate the Rupture Propagation Directions from Source Directivity: The 2016–2018 Central Italy Seismic Sequence

Abstract We present a new approach to estimate the predominant direction of rupture propagation during a seismic sequence. A fast estimation of the rupture propagation direction is essential to know the azimuthal distribution of shaking around the seismic source and the associated risks for the earthquake occurrence. The main advantage of the proposed method is that it is conceptually reliable, simple, and fast (near real time). The approach uses the empirical Green’s function technique and can be applied directly to the waveforms without requiring the deconvolution of the instrumental response and without knowing a priori the attenuation model and the orientation of the activated fault system. We apply the method to the 2016–2017 Amatrice-Visso-Norcia high-energy and long-lasting earthquake series in central Italy, which affected a large area up to 80 km along strike, with more than 130,000 events of small-to-moderate magnitude recorded until the end of August 2022. Most of the selected events analyzed in this study have a magnitude greater than 4.4 and only four seismic events have a magnitude in the range of 3.3–3.7. Our results show that the complex activated normal fault system has a rupture direction mainly controlled by the pre-existing normal faults and by the orientation of the reactivated faults. In addition, the preferred direction of rupture propagation is also controlled by the presence of fluid in the pre-existing structural discontinuities. We discuss the possible role of fluids as a cause of bimaterial interface. Another important finding from our analysis is that the spatial evolution of seismicity is controlled by the directivity.

Instantaneous limit equilibrium back analyses of major rockslides triggered during the 2016–2017 central Italy seismic sequence

Abstract. Among the almost 1400 landslides triggered by the shocks of the 2016–2017 central Italy seismic sequence, only a limited number, all classifiable as rockslides, involved volumes larger than 1000 m3. Four of these failures, including the three largest among the documented landslides, were described in terms of structural and geomechanical investigations in a previous paper. In this study, the estimated acceleration time histories at the rockslide sites were evaluated through a 2D simplified numerical model accounting for the attenuation phenomena and for the topographic effect of the rock cliffs from which the slide detached. Instantaneous stability analyses were carried out to obtain insights into the variability of the instantaneous margin of safety along the motion, over the entire spectrum of mechanisms that could be activated. Finally, some general suggestions on the pseudo-static verification method for 3D cases are proposed, which represent useful indications to hazard evaluation at local and regional scales.

The Unlocking Process Leading to the 2016 Central Italy Seismic Sequence

AbstractApproximately 23,000 well‐located earthquakes from 2009 to 2016 are used as templates to recover seismic activity preceding the 2016 Central Italy seismic sequence. The resulting spatiotemporal pattern is analyzed by additional ∼91,000 newly detected events. In the 8 years before the sequence onset, seismicity (ML ≤ 3.7) develops at the hangingwall of the 2016 normal faults and along a sub‐horizontal shear zone, bounding the active extensional system at depth. This activity, mainly organized in foreshock‐mainshock and swarm‐like clusters, migrates toward the nucleation area of the first Mw 6.0 mainshock of the sequence (24th of August in Amatrice). We propose an unlocking process based on variable temporal clustering of the seismicity, including repeaters, identifying fault portions with different degrees of coupling. Such a progressive localization of the seismic activity at the fault edges induces a weakening of the locked patch of the Amatrice mainshock.

Operational critical issues filling in the Italian form for the post-earthquake damage assessment of churches (A-DC 2006)

In the emergency phase following a severe seismic event, it is essential to quickly proceed with the assessment of the damage suffered by buildings, both to assess which structures are still usable and to promptly intervene on damaged structures in order to prevent them from further damage. Damage data is also very important for the scientific community as it allows the creation of fragility curves for groups of buildings with homogeneous vulnerability and to carry out risk mitigation strategies. The Department of Italian Civil Protection has developed, for each type of building (residential buildings, long-span buildings, churches, historic buildings), survey forms to collect the damage reported by the structures in a standardized way.This paper is focused on the form used for churches (A-DC2006 form), that analyzed the seismic behavior of a church by considering each macro-element that composes it independently (i.e., façade, nave, apse, transept ...). The form proposes for each macro-element a variable number of collapse mechanisms, for a total of 28.The analyses presented here concern the data of 225 churches inspected during the 2016-2017 Central Italy seismic sequence. All the churches were investigated twice by different technicians due to the occurrence of a second seismic event that required a further inspection. This chance has allowed to compare the data of two inspections relating to the same church and it highlighted the presence of frequent interpretative differences: often the collapse mechanisms identified by one technician did not coincide with those identified by the other technician. Each of the 225 churches was therefore carefully analyzed to identify the recurring situations that caused these critical issues.

Preliminary results in the automated detection of operational modal properties of the Portico Varano in the Camerino Ducal Palace

Portico Varano in the Ducal Palace of Camerino (Italy) is a renaissance monumental quadriporticus that was damaged by the 2016 Central Italy seismic sequence. Within the field activities for saving cultural heritage foreseen within a recent European research project named ARCH, a long-term monitoring system, comprising different types of sensors, such as accelerometers, displacement transducers, environmental sensors, and a weather station, was set up to achieve comprehensive measures of its operational behaviour and the evolution of the damage. The monitoring system, installed in October 2020, is currently operating, providing valuable information on the experimentally observed dynamic behaviour, also considering changes in the environmental conditions.Starting from the results of the dynamic characterization of the structure and after the optimization of the position of the sensors, this paper shows and discusses the efforts made to track over time the modal characteristics of the Portico Varano in order to detect changes in its conditions. In addition, a procedure has been proposed and implemented combining information available from Italian National Institute of Geophysics and Volcanology (INGV) to identify recorded data related to seismic events relevant to operational conditions.

Exceedance of design seismic actions during the 2016-2017 central Italy seismic sequence: sensitivity to two seismic hazard models

In Italy, seismic ground motion intensity for structural design relies on probabilistic seismic hazard analysis (PSHA). Research has shown that, even in the case of a moderate magnitude earthquake, it is expected that ground motion intensities larger than the design counterparts are observed at some sites in the epicentral region. The area where such an exceedance has possibly occurred can be assessed via ShakeMap. In the case of seismic sequences, ShakeMap envelopes allow to identify the fraction of the region of interest subjected to at least one exceedance of design seismic actions. For the 2016-2017 central Italy sequence, ShakeMap envelopes reveal that exceedance of the design intensity enforced by the current building code occurred in an area covering thousands of square kilometers, depending on the spectral and exceedance return period of the design intensity. This study quantifies the exceedance area of ground motion intensity according to the PSHA results based on two alternative PSHA models for Italy. One is that at the basis of the official PSHA for Italy adopted by the current building code, known as MPS04; the other relies on a grid-seismicity source model deriving from a set of source models recently developed for a new PSHA study for the country (MPS19). Looking at ShakeMap envelopes, it is found that the estimated exceedance area is of the same order of magnitude for the two hazard models, at least for the spectral and exceedance return periods considered herein.

Empirical fragility curves for macro-elements and single mechanisms of churches damaged during the 2016-2017 Central Italy seismic sequence

The churches represent the most widespread building typology within the monumental masonry heritage present in the Italian territory. A special attention needs to be paid to the assessment of the vulnerability of these structures because of the particular architectural features (slender walls with large openings, wide halls and huge vaults, large façades, etc…). This paper presents several statistical analyses carried out on a wide database that collects damage information and structural characteristics of a large sample of masonry churches (2884) inspected after the 2016-2017 Central Italy seismic sequence. The collected information is used for developing empirical fragility curves starting from the damage probability matrices (DPMs) for discrete intervals of PGA (peak ground acceleration), with reference to the following three damage estimates: i) the global damage referred to the whole church, ii) the damage referred to single macro-elements, and iii) the damage referred to single mechanisms. The fragility functions are determined via the maximum likelihood estimation method and adopt a lognormal cumulative function to define the exceedance probabilities for the five damage levels provided by the European macro-seismic (EMS) scale. The fragility curves related to the three damage estimates are compared to each other to identify the weight of specific macro-elements and mechanisms in determining the global damage level and eventually to point out proper and specific interventions to reduce the vulnerability of the whole church.

Damage assessment of Apennine masonry churches severely damaged during the 2016 Central Italy seismic sequence

Damage assessment of Apennine masonry churches severely damaged during the 2016 Central Italy seismic sequence

Multivariate statistical analysis of site effect indicators for the Montereale and Capitignano area (AQ) following the seismic sequence in central Italy (2016–2017)

Multivariate statistical analysis of site effect indicators for the Montereale and Capitignano area (AQ) following the seismic sequence in central Italy (2016–2017)

A comprehensive suite of earthquake catalogues for the 2016-2017 Central Italy seismic sequence

The protracted nature of the 2016-2017 central Italy seismic sequence, with multiple damaging earthquakes spaced over months, presented serious challenges for the duty seismologists and emergency managers as they assimilated the growing sequence to advise the local population. Uncertainty concerning where and when it was safe to occupy vulnerable structures highlighted the need for timely delivery of scientifically based understanding of the evolving hazard and risk. Seismic hazard assessment during complex sequences depends critically on up-to-date earthquake catalogues—i.e., data on locations, magnitudes, and activity of earthquakes—to characterize the ongoing seismicity and fuel earthquake forecasting models. Here we document six earthquake catalogues of this sequence that were developed using a variety of methods. The catalogues possess different levels of resolution and completeness resulting from progressive enhancements in the data availability, detection sensitivity, and hypocentral location accuracy. The catalogues range from real-time to advanced machine-learning procedures and highlight both the promises as well as the challenges of implementing advanced workflows in an operational environment.

Detection of Spatial and Temporal Stress Changes During the 2016 Central Italy Seismic Sequence by Monitoring the Evolution of the Energy Index

AbstractWe consider approximately 23,000 microearthquakes that occurred between 2005 and 2016 in central Italy to investigate the crustal strength before and after the three largest earthquakes of the 2016 seismic sequence (i.e., the Mw 6.2, 24 August 2016 Amatrice, the Mw 6.1, 26 October 2016 Visso, and the Mw 6.5, 30 October 2016 Norcia earthquakes). We monitor the spatiotemporal deviations of observed radiated energy, ES, with respect to theoretical values, ESt, derived from a scaling model between ES and M0 calibrated for background seismicity in central Italy. These deviations, defined here as Energy Index (EI), allow us to identify in the years following the Mw 6.1, 2009 L’Aquila earthquake a progressive evolution of the dynamic properties of microearthquakes and the existence of high EI patches close to the Amatrice earthquake hypocenter. We show the existence of a crustal volume with high EI even before the Mw 6.5 Norcia earthquake. Our results agree with the previously suggested hypothesis that the Norcia earthquake nucleated at the boundary of a large patch, highly stressed by the two previous mainshocks of the sequence. We highlight the mainshocks interaction both in terms of EI and of the mean loading shear stress associated to microearthquakes occurring within the crustal volumes comprising the mainshock hypocenters. Our study shows that the dynamic characteristics of microearthquakes can be exploited as beacons of stress change in the crust and thus be exploited to monitor the seismic hazard of a region and help to intercept the preparation phase of large earthquakes.

Applying the damage assessment for rapid response approach to the august 24 M6 event of the seismic sequence in central Italy (2016)

Seismic monitoring networks are increasingly being used in urban areas to record and locate earthquakes. Recordings in the proximity of buildings also allow assessing, as a first approximation, the expected building damage. The DARR (Damage Assessment for Rapid Response) method provides local-scale information on expected damage patterns. The potential of this approach is discussed here for the August 24 M6 event of the Central Italy seismic sequence (2016–2017). We focus only on the first event of the sequence because cumulative damage is outside the scope of this study. The earthquake recordings are available from two Italian monitoring networks: the Italian Accelerometric Archive (ITACA) and the OSS (Osservatorio Sismico delle Strutture), which collects data from monitored buildings and bridges in Italy. We selected four target areas (Amatrice, Norcia, Visso and Sulmona) characterized by different epicentral distances and building typologies, that suffered different levels of damage during the M6 event on 24 August 2016. Using recordings either in the free field or in the basement of buildings, the expected relative displacement of building typologies common in the studied areas is calculated with the DARR method. Using predefined damage thresholds from literature, the obtained results allow quantifying the expected damage for dominant building typologies in the surroundings of the recording sites. We investigate and discuss the potential use and applicability of the DARR method in different areas depending on the epicentral distance and building characteristics. The results indicate that the DARR approach is useful for supporting and improving rapid response activities after a seismic event.