Seismic Sequence Research Articles

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.

Observed damage and empirical predictive model of the school building heritage of the Marche region

The 2016 seismic sequence that struck Central Italy, with its three mainshocks and numerous aftershocks, caused widespread damages to building heritage. Among the buildings that suffered damages, schools are considered relevant structures from multiple points of view. Educational institutes, indeed, host children and young people during their training years in facilities that are often old and realized without recent safety Standards becoming, therefore, non-resilient, nor robust structures towards multiple natural hazards. In this paper, a database containing information regarding the geolocation of individual buildings, no. of floors, the floor surface, the construction period, the structural type and the potential presence of landscape or hydrogeological constraints, has been realized with reference to the schools of the Marche region, by merging different sources of information. In addition, the database also collects information regarding observed damages and recorded seismic intensities to schools during the 2016 Central Italy earthquake.Once built up the database, an empirical predictive model, known in literature and already used for the evaluation of potential post-earthquake scenarios to cultural heritage, is applied to school buildings located into the seismic crater of the Marche region, by estimating the parameters of the model through inference techniques. The probabilistic model applied, relates a measure of the ground motion intensity to an overall damage index, continuous in the range [0,1]. Moreover, if one integrates the partial information related to damaged buildings often provided by the post-earthquake surveys, with the data related to the undamaged and/or collapsed structures, the model can provide a full risk assessment procedure. Finally, fragility curves are also derived, to make it possible the use of the research outcomes in risk frameworks based on a discrete description of the consequences.

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

Small-magnitude earthquakes triggering fluid vents in a pressurised CO2 system, Caprese Michelangelo (Central Italy)

The CO2-(N2)-rich pressurised Caprese Reservoir (Northern Apennines, central Italy) is a structural system controlled by a subsurface fold anticline. Mud volcano-like structures and cold gas seeps lie along steep, ∼NE-trending faults, which are intimately connected to the deep fluid reservoir. In August 2010, five vents erupted mud flows after a local, small magnitude seismic sequence (MLmax 3.2). Although the exact dates of such paroxysmal episodes are unknown, they occurred within 55 days after the main shocks. These eruptions are evidence of triggered responses of mud volcano-like vents induced by earthquakes with small magnitude (ML3.0–3.2). Static stresses at the erupted vents are negligible, thereby dynamic strain is inferred to be the sole triggering mechanism. The main earthquakes produced a near-vent peak ground velocity (PGV) of ∼0.5 cm s−1 (ML3.2) and ∼ 0.2 cm s−1 (ML3.0). Although small, they are similar to other PGV values that triggered eruptive events in similar systems worldwide. A potential triggering mechanism can be searched in the enhanced permeability of fluid pathways produced by the passage of seismic waves. Fluid pressurization within the antiformal-shaped Caprese Reservoir by seismic wave focusing can also have played a significant role. This implies that seismic waves carrying even small perturbing stresses have the ability to stimulate hydrogeological systems to produce eruptions. Furthermore, our investigations allowed to assess the relationship between distance and earthquake magnitude for an M ∼ 3 event, and corroborate the validity of some existing empirical triggering thresholds of mud volcano-like systems even in the case of small earthquake magnitudes.

Effect of hemp bio composite strengthening on masonry barrel vaults damage

Masonry vaults are common structural elements of the Italian architectural heritage, significantly vulnerable to seismic loads. For this reason strengthening is often required. In the last decade, the use of traditional reinforcement techniques has been progressively replaced by Fiber Reinforced Polymers (FRP) bonded at the vault extrados/intrados. However, this approach has shown drawbacks mainly associated to the polymeric nature of matrix and physical compatibility between the matrix and the masonry support. Consequently, FRP strengthening evolved to innovative composite materials obtained by cement based matrix, i.e. Fiber Reinforced Cementitious Matrix (FRCM). Moreover, within the growing environmental sustainability issues, the scientific research is focusing on the development of biodegradable composite materials, based on vegetal fibers and natural matrices. This paper presents the results of experimental tests on two full-scale masonry vaults with the same geometry and mechanical properties: one is unreinforced and one is reinforced at the extrados by continuous bidirectional hemp ropes bonded in a layer of cocciopesto matrix (i.e. a mix of ground clay bricks and organic binder). The two models were built and tested in the 3D shaking table system of the Laboratory of Earthquake Engineering and Dynamic Analysis (LEDA) at Enna Kore University. Each vault was subjected to seismic loads and dynamic identification tests: the seismic load was given with increasing amplitude at the base of the two models while the dynamic identification tests were carried out after each seismic sequence in order to evaluate the variation of the dynamic properties, testifying structural damage. The strengthening effectiveness was assessed by comparing the damages detected on the unreinforced and reinforced masonry vaults. Results showed that the proposed bio composite strengthening material is effective for improving the seismic performance of masonry vaults.

Reaction of the carbonate Sibillini Mountains Basal aquifer (Central Italy) to the extensional 2016–2017 seismic sequence

Hydrogeological perturbations in response to earthquakes are widely described worldwide. In carbonate aquifers, a post-seismic discharge increase is often attributed to an increase of bulk permeability due to co-seismic fracturing and the attention on the role of faults to explain the diversion of groundwater is increasing. We focus on the reaction of carbonate hydrogeological basins to extensional seismicity, taking as an example the effects of the Central Italy 2016–2017 seismic sequence, on the Basal aquifer of the Sibillini Mountains area. Geo-structural, seismological and ground deformation data were collected and merged with artificial tracer tests results and with a 4-years discharge and geochemical monitoring campaign. The main NNW-directed groundwater flow was diverted to the west and a discharge deficit was observed at the foot-wall of the activated fault system with a relevant discharge increase, accompanied by geochemical variations, at the fault system hanging-wall. The observed variations are consistent with the combined action of a permeability increase along the activated fault systems, which modified the predominant pre-seismic along-strike regional flow, and with hydraulic conductivity increase due to fracturing, determining a fast aquifers emptying. We show that the prevailing mechanism depends on the aquifer systems position with respect to the activated faults.

A stochastic approach for upgrading Cultural Heritage framed concrete buildings by cable-ties to prevent progressive collapse under seismic sequences

Old industrial reinforced concrete (RC) structures included to built Cultural Heritage are subjected sometimes to obligatory removal of some structural element-members, e.g. columns. In such cases, a modification of the structural response and a redistribution of internal actions can result to a requirement for upgrading the remaining structure after the removal of the degraded elements in order to avoid a progressive collapse. The present study deals with such a case, which concerns the stochastic computational analysis of historic framed RC structures under the removal of some columns and the so-induced requirement of a strengthening by ties (tension only elements). The unilateral behaviour of these cable-ties, which can undertake only tension, is strictly considered. The response under seismic sequences of the remaining historic RC structure strengthened by ties is computed considering uncertain-but-bounded input parameters.

An Overview of the Historical Retrofitting Interventions on Churches in Central Italy

The seismic sequence occurring in Central Italy in 2016 represents a new test benchmark for historical masonry churches and a chance for a better comprehension of their structural behavior under earthquake actions. The many earthquakes that took place in the past have led to stratifications of repair and retrofitting interventions that sometimes worsened the structural behavior, especially when resulting in the introduction of elements not compatible with the churches’ original layout. Within this framework, the present paper intends to provide a critical review of the main interventions carried out in the churches of Central Italy for mitigating their seismic vulnerability and to evaluate their effectiveness in light of the damage surveyed on a representative sample of masonry churches after the 2016–2017 seismic sequence. The work is organized into three parts: (1) historical analysis of the territory; (2) review of the featured interventions; (3) critical appraisal of the interventions in relation to the surveyed data and assessment of their effectiveness. The goal of the work is to shed light on the correct design of retrofitting interventions in ancient masonry structures in order to enhance the structural safety of such artefacts without compromising their historical and cultural value.

Non-linear Dynamic Analysis of RC Structures Under Earthquake Sequences

Earthquakes in active seismic regions usually occur in a series of medium to strong intensity ground motions at small intervals of times. A large intensity mainshock (MS) is often followed by a series of aftershocks (AS) or even preceded by smaller foreshocks. This sequence type of mainshock-aftershock (MS-AS) ground motions with varying intensity pose major seismic hazard as there is limited scope of repair and retrofitting between seismic events. Due to continuous and repeated seismic ground motions over a brief period of time, the damages in the structure gets accumulated and structure collapse. This study navigates the behaviour of reinforced concrete structures under such seismic sequences. For this purpose, the nonlinear response of three 12 storey reinforced concrete buildings (regular plan, mass irregularity and diaphragm irregularity) is evaluated. The buildings are subjected to five real seismic sequences from previous earthquakes. Nonlinear dynamic analyses are carried out to study the response of buildings under MS and MS-AS sequences considering: a) material and geometric non linearities and b) irregularities. A single highest aftershock is considered in the present study. The results in this study indicates that MS-AS seismic sequence considering both material and geometric nonlinearities has significant effect on the response of structure. It also showed that seismic sequences significantly alter the response of irregular structure.

A Review on the Selection of Real and Artificial Seismic Sequences for Analysis

A huge mainshock activates several aftershocks, divulging the public to serious risk and impeding building repair, rehabilitation, and restoration works. The performance of a structure during multiple earthquakes depends on structural properties and the characteristics of ground motion. Hence, the selection of seismic sequence plays a vital role in the analysis. Presumption of spatial and temporal characteristics of aftershocks is needed for the identification of these repeated motions. This paper further looks into statistical variability connected with aftershock sequences. A critical review of different real and artificial seismic sequences taken for the analysis is carried out. A few studies reveal that a strong motion database for procuring actual mainshock aftershock sequences could underrate the aftershock effects as the database is inadequate and imperfect. It is also observed that artificial sequences can take the place of real sequences especially when an ample data set of real mainshock aftershock sequences are not accessible.

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)

Deep learning for earthquake hydrology? Insights from the karst Gran Sasso aquifer in central Italy

By leveraging monitoring data for the Gran Sasso carbonate aquifer during two significant seismic sequences that hit central Italy in recent years, this study investigates the possibility of using memory-enabled deep learning algorithms as meaningful tools for an enhanced modelling of the hydrological response of karst aquifers subject to earthquake phenomena. Meteorological, hydrological and seismic data are used to train and validate long short-term memory networks (LSTM) in one- and multiple-day ahead flow forecasting exercises, aimed at assessing model sensitivities to input variables and modelling choices (training data and parameters of the models). Results indicate that the models fairly reproduce the flow patterns for the considered spring in the Gran Sasso aquifer, thus supporting the potential use of these models for hydrological applications in similar areas, provided that sufficient data are available for the training of the network.

Estimating Coseismic Deformation of Southwestern Puerto Rico from the 7 January 2020 Mw 6.4 Earthquake: Constraints from Campaign and Continuous GPS

ABSTRACT The Puerto Rico–Virgin Islands (PRVI) block lies within the Northern Caribbean Plate Boundary Zone—a zone accommodating stresses between the larger North America and Caribbean plates. Data from Global Positioning System (GPS) sites throughout the PRVI block have been used to confirm the existence of a distinct microblock in the southwest. It is no coincidence that this portion of the PRVI block is the epicentral region of the 7 January 2020 Mw 6.4 earthquake and the ensuing seismic sequence. Prior to the mainshock, the southwestern Puerto Rico (SWPR) region exhibited most of the onland seismic activity. The 2020–2021 SWPR earthquake seismic sequence has been characterized by having an atypical aftershock decay distribution occurring along multiple faults. As a result, fault parameters of the 7 January 2020 mainshock have been poorly defined by conventional seismic methods. Here, we present results from campaign and continuous GPS sites in SWPR, and compare GPS-derived displacements to those computed from the U.S. Geological Survey National Earthquake Information Center (NEIC) focal mechanism. We conclude that irrespective of which nodal plane is used, the observed coseismic displacements from GPS differ from those predicted using a simple elastic model and the NEIC focal mechanism. We infer based on these observations that the complex mainshock rupture resulted in a suboptimal double-couple solution.

Return of the Atacama deep Slow Slip Event: The 5-year recurrence confirmed by continuous GPS

Four years ago, using survey GPS measurements, the first deep slow slip event (SSE) was detected in Chile (near Copiapó, Atacama region), unrelated to any major earthquake. It was located between 40 and 60 km depth on the subduction interface, lasted approximately 18 months (2014–2016.5) and reached an equivalent magnitude of Mw 6.9. The single permanent station operating in the region between 2002 and 2015 revealed that similar events had occurred at least twice before around 2006 and 2010, suggesting a 5-year repeat time. In anticipation of the next event expected for 2020, we densified the existing continuous GNSS network in the region with 5 new stations in early 2019. Here we show that the SSE occurred in 2020 as expected with the 5 year recurrence time. The event started around March 2020 and developed during 6 months, before it was perturbed by the 2020 Atacama seismic sequence that occurred nearby. During those initial 6 months, the 2020 event had the same characteristics as the 2014 SSE. It occurred in the same area and at the same depth, repeating a similar pattern of surface deformation. Before the occurrence of the nearby seismic sequence of September 2020, it had reached a third of the total amplitude of the 2014 SSE which had lasted three times longer. Whether the 2020 SSE was aborted when the nearby seismic sequence occurred or continued in the background is unknown but this will be resolved with longer times series.

Comparing and integrating artificial intelligence and similarity search detection techniques: application to seismic sequences in Southern Italy

SUMMARYUnderstanding mechanical processes occurring on faults requires detailed information on the microseismicity that can be enhanced today by advanced techniques for earthquake detection. This problem is challenging when the seismicity rate is low and most of the earthquakes occur at depth. In this study, we compare three detection techniques, the autocorrelation FAST, the machine learning EQTransformer, and the template matching EQCorrScan, to assess their ability to improve catalogues associated with seismic sequences in the normal fault system of Southern Apennines (Italy) using data from the Irpinia Near Fault Observatory (INFO). We found that the integration of the machine learning and template matching detectors, the former providing templates for the cross-correlation, largely outperforms techniques based on autocorrelation and machine learning alone, featuring an enrichment of the automatic and manual catalogues of factors 21 and 7, respectively. Since output catalogues can be polluted by many false positives, we applied refined event selection based on the cumulative distribution of their similarity level. We can thus clean up the detection lists and analyse final subsets dominated by real events. The magnitude of completeness decreases by more than one unit compared to the reference value for the network. We report b-values associated with sequences smaller than the average, likely corresponding to larger differential stresses than for the background seismicity of the area. For all the analysed sequences, we found that main events are anticipated by foreshocks, indicating a possible preparation process for main shocks at subkilometric scales.

Improvements to seismicity forecasting based on a Bayesian spatio-temporal ETAS model

The epidemic-type aftershock sequence (ETAS) model provides an effective tool for predicting the spatio-temporal evolution of aftershock clustering in short-term. Based on this model, a fully probabilistic procedure was previously proposed by the first two authors for providing spatio-temporal predictions of aftershock occurrence in a prescribed forecasting time interval. This procedure exploited the versatility of the Bayesian inference to adaptively update the forecasts based on the incoming information provided by the ongoing seismic sequence. In this work, this Bayesian procedure is improved: (1) the likelihood function for the sequence has been modified to properly consider the piecewise stationary integration of the seismicity rate; (2) the spatial integral of seismicity rate over the whole aftershock zone is calculated analytically; (3) background seismicity is explicitly considered within the forecasting procedure; (4) an adaptive Markov Chain Monte Carlo simulation procedure is adopted; (5) leveraging the stochastic sequences generated by the procedure in the forecasting interval, the N-test and the S-test are adopted to verify the forecasts. This framework is demonstrated and verified through retrospective early forecasting of seismicity associated with the 2017–2019 Kermanshah seismic sequence activities in western Iran in two distinct phases following the main events with Mw7.3 and Mw6.3, respectively.

The Adriatic Thrust Fault of the 2021 Seismic Sequence Estimated from Accurate Earthquake Locations Using sP Depth Phases

ABSTRACT An earthquake sequence occurred in the Central Adriatic region during March–June 2021. This sequence started on 27 March with a mainshock of moment magnitude (Mw) 5.2 occurring at 13:47 coordinated universal time (UTC). No foreshock was observed before this mainshock. The sequence lasted approximately three months, until the end of June 2021. Approximately 200 seismic events were recorded by the regional seismic network during this time, including four M ≥ 4.0 earthquakes. The 27 March 2021 earthquake was one of the strongest instrumentally recorded events in the area bounded approximately by the Ancona–Zadar line to the north and the Gargano–Dubrovnik line to the south. The mainshock originated at a focal depth of 9.9 km. The seismicity spread from the mainshock up-dip and down-dip along a northeast-dipping plane. Here, we investigate the geometry of the fault activated by this seismic sequence by using sP depth phases. We aim to significantly reduce the large uncertainties associated with the hypocentral locations of offshore earthquakes beneath the Adriatic Sea—an area that plays a fundamental role in the geodynamics of the Mediterranean. These refined earthquake locations also allow us to make inferences with regards to the seismotectonic context responsible for the analyzed seismicity, thus identifying a structure (here referred to as the Mid-Adriatic fault) consisting of a northwest–southeast-striking thrust fault with a ∼35° northeast-dipping plane. The use of depth-phase arrival times to constrain off-network event locations is of particular interest in Italy due to both the peculiar shape of the peninsula and the extreme scarcity of seafloor stations, the cost and management of which are very expensive and complex. Here, we present the first attempt to apply this off-network locating technique to the Italian offshore seismicity research with the aim of improving hazard estimations in these hard-to-monitor regions.

Seismic deformation in the Adriatic Sea region

We present an overall analysis of the recent seismic activity occurred in the Adriatic Sea region, a strongly debated sector of the Mediterranean area, where several authors have proposed different models of plate configuration and kinematics. In the past, seismic investigations of this marine area have been strongly hampered by non-optimal network geometries, but data quality increase and recent methodological improvements lay the groundwork to attempt more accurate analyses including proper evaluations of result reliability. On these grounds, we investigated the seismic activity of the last decades by means of new hypocenter locations, waveform inversion focal mechanisms and seismogenic stress fields. We used the Bayloc non-linear probabilistic algorithm to compute hypocenter locations for the most relevant seismic sequences by carefully evaluating location quality and seismolineaments reliability. We also provided an updated database of waveform inversion focal mechanisms including original solutions estimated by applying the waveform inversion method Cut And Paste and data available from official catalogs. Then, focal mechanism solutions have been used to estimate seismogenic stress fields through different inversion algorithms. Seismic results indicate a relevant degree of fragmentation and different patterns of deformation in the Central Adriatic region. In particular, our analyses depicted two NW-SE oriented, adjacent volumes: (i) a pure compressive domain with NNE-trending axis of maximum compression characterizes the northeastern volume where the seismic activity occurs on W-to-NW oriented seismic sources; (ii) a transpressive domain with NW-trending axis of maximum compression characterizes the southwestern sector where thrust faulting preferentially occurs on ENE-to-NE oriented planes and strike-slip faulting on E-W ones. Joint evaluation of seismic findings of the present study and kinematic models proposed in the literature indicates just in the Central Adriatic region the presence of a broad deformation zone, accommodating a still evolving fragmentation of the Adriatic domain in two blocks rotating in opposite directions. On these grounds, the obtained results not only furnish new seismological evidence supporting the "two-blocks model" proposed by previous authors, but they also provide additional constraints, useful for better understanding and modeling the seismotectonic processes occurring in the Adriatic region. Data availabilityData used in the present study were collected from catalogs and bibliographic sources indicated in detail in the article. Waveform inversions performed in this study used data available in the database EIDA, http://orfeus-eu.org/webdc3/ (accessed February 2022)