Friuli Earthquake Research Articles

A scenario-based approach for immediate post-earthquake rockfall impact assessment

Different approaches exist to describe the seismic triggering of rockfalls. Statistical approaches rely on the analysis of local terrain properties and their empirical correlation with observed rockfalls. Conversely, deterministic, or physically based approaches, rely on the modeling of individual trajectories of boulders set in motion by seismic shaking. They require different data and allow various interpretations and applications of their results. Here, we present a new method for earthquake-triggered rockfall scenario assessment adopting ground shaking estimates, produced in near real-time by a seismological monitoring network. Its key inputs are the locations of likely initiation points of rockfall trajectories, namely, rockfall sources, obtained by statistical analysis of digital topography. In the model, ground shaking maps corresponding to a specific earthquake suppress the probability of activation of sources at locations with low ground shaking while enhancing that in areas close to the epicenter. Rockfall trajectories are calculated from the probabilistic source map by three-dimensional kinematic modeling using the software STONE. We apply the method to the 1976 MI = 6.5 Friuli earthquake, for which an inventory of seismically-triggered rockfalls exists. We suggest that using peak ground acceleration as a modulating parameter to suppress/enhance rockfall source probability, the model reasonably reproduces observations. Results allow a preliminary impact evaluation before field observations become available. We suggest that the framework may be suitable for rapid rockfall impact assessment as soon as ground-shaking estimates (empirical or numerical models) are available after a seismic event.

Three large prehistoric earthquakes in the Eastern Alps evidenced by cave rupture and speleothem damage

Spectacular geomorphic evidence of recent tectonic activity and three potentially strong prehistoric earthquakes were discovered in Obir Caves in the Karawanken Mountains. This paper presents active fault structures, geomorphic features, and the age constraints attributed to particular seismotectonic events. The 1976 Mw 6.7 Friuli earthquake about 100 km from Obir Caves caused only local speleothem damage. The 100 km hypocentral distance enabled attenuation of the seismic waves so speleothem damage due to stalagmite resonance frequency was dampened. The documented dripstone column damage was rather caused by its sudden shortening due to passing elastic S-waves with estimated 3.5 (+8.2/−1.5) mm PGD amplitudes. At least three distinct seismotectonic events in the Late Pleistocene and Early to Middle Holocene were constrained by radiometric dating. The 40.6 cm sinistral fault slip occurred between 41.8 ± 1.3 ka and 18.7 ± 0.4 ka ago, the 2.6 cm dextral oblique reverse fault slip happened between 10.73 ± 0.23 and 8.61 ± 0.15 ka ago, and another speleothem damage event took place between 6.28 ± 0.24 ka and 5.7 ± 1.2 ka ago. These events were most probably accompanied by distinct destructive to very destructive paleoearthquakes with local intensities ranging from VIII to X on the ESI 2007 scale, significant rock weakening, and forming the large deep-seated gravitational slope deformation on the southeastern mountain slopes adjacent to Obir Caves.

Systematic Triggering of Large Earthquakes by Karst Water Recharge: Statistical Evidence in Northeastern Italy

It is known that surface water accumulation by natural or anthropic causes like precipitation and reservoir impoundment can trigger earthquakes. The phenomenon is amplified and sped up in karst areas, where fracture systems can store large quantities of water and facilitate its percolation to seismogenic depths, increasing both elastic stress and pore pressure on pre-existent faults. The present work explored the possibility that this mechanism had systematically triggered major earthquakes in northeastern Italy, where seismicity is concentrated along the pre-Alpine thrust belt, characterized by the alignment of a series of karst massifs. The time occurrence of damaging and destructive earthquakes (moment magnitude Mw between 4.8 and 6.4) since 1901 was compared with the evolution of the Palmer Drought Severity Index (PDSI), an index of soil moisture that summarizes precipitation and, through temperature, water evaporation. Statistical analysis based on the bivariate Ripley’s K-function shows a significant time correlation between earthquakes and the peaks of PDSI since 1934, with the two strongest earthquakes (1936 Alpago-Cansiglio earthquake and 1976 Friuli earthquake, Mw 6.1 and 6.4, respectively) placed by the two PDSI maxima. The analysis was extended back in time to the last millennium, showing a time correlation between the occurrence of destructive earthquakes (Mw ≥ 6.2) and the peaks of ice extension in the European Alps, assumed as a proxy for groundwater accumulation in the study area. This evidence presented herein coupled with geological characteristics of the area and recent observations on large crustal deformations induced by heavy precipitation suggests that, if PDSI is a valid ground water indicator, karst water recharge may play a role in triggering major earthquakes in northeastern Italy, also relating their occurrence to the large scale climate changes affecting precipitation and evaporation.

A focal mechanism catalogue of earthquakes that occurred in the southeastern Alps and surrounding areas from 1928–2019

Abstract. We present a focal mechanism catalogue of earthquakes that occurred in the southeastern Alps and surrounding areas from 1928 to 2019. The area involved in the process of convergence between the Adria microplate and Eurasia is one of the most seismically active regions in the Alpine Belt. The seismicity is minor, with the Ms= 6.5 Friuli earthquake being the strongest event recorded in the area, but the seismic risk is relevant because it is a highly populated region. For this reason, numerous studies have been carried out over time to investigate the stress field and the geodynamic characteristics of the region using focal mechanisms. To provide a comprehensive set of revised information, which is challenging to build quickly because the data are dispersed over many papers, we collected and revised the focal mechanisms that were previously published in the literature. Additionally, depending on the data quality and availability, we computed new focal mechanisms by first arrival polarity inversion or seismic moment tensor. Finally, we merged all the fault plane solutions to obtain a catalogue for a selection of 772 earthquakes with 1.8≤M≤6.5. For each earthquake, we reported all the available focal mechanisms obtained by different authors. We also suggested a preferred solution for users who need information provided expeditiously. The catalogue (https://doi.org/10.5281/zenodo.4660412; Saraò et al., 2021) is given as the Supplement of this paper and will be updated periodically (https://doi.org/10.5281/zenodo.4284970).

Decoding a complex record of anthropogenic and natural impacts in the Lake of Cavazzo sediments, NE Italy

The Lake of Cavazzo is a natural, lacustrine basin in the Friuli-Venezia Giulia region (NE Italy), which occupies a fluvio-glacial suspended valley of the Tagliamento river, at the southern front of the Alpine chain. The lake formed after the melting of major glacial tongues at the end of the Last Glacial Maximum, and has been affected in recent years by natural and anthropogenic events, with economic, cultural and environmental consequences. According to local witnesses, the lake environment has changed dramatically after the 1950s, when the basin was connected to a hydroelectric power plant collecting waters from a wider catchment area, as it served as its final discharge basin.In this work, based on a densely-spaced grid of high-resolution seismic reflection profiles complemented by sediment cores, we analysed the uppermost lake stratigraphy, highlighting major changes occurring at a decadal scale. Our main purpose is to verify whether and how the lake sediments record the transition from pristine/natural to artificial conditions, as well as the effects of multiple natural impacts including the 1976–77 Friuli earthquake sequence. The results of our analysis suggest varying environmental conditions of the lake after the 1950s indicated by changing sediment mineralogy, increased deposition of allochthonous clastic sediments, and recurrent episodes of anoxic conditions at the lake's floor, likely triggered by hyperpycnal flows derived from the hydroelectric power plant discharges. We also observe that the effects of the 1976–77 seismic sequence are recorded in the lake as resedimented levels, likely due to shaking, in situ deformation, landslides, and turbidity currents. Our results stress the importance of lacustrine environments as efficient recorders of anthropogenic and natural events.

Could a Decentralized Onsite Earthquake Early Warning System Help in Mitigating Seismic Risk in Northeastern Italy? The Case of the 1976 Ms 6.5 Friuli Earthquake

Abstract In May 1976, a devastating earthquake of magnitude Ms 6.5 occurred in Friuli, Italy, resulting in 976 deaths, 2000 injured, and 60,000 homeless. It is notable that, at the time of the earthquake, only one station was installed in the affected region. The resulting lack of information, combined with a dearth of mitigation planning for responding to such events, lead to a clear picture of the impact of the disaster being available only after a few days. This region is now covered by nearly 100 seismological and strong-motion stations operating in real time. Furthermore, 30 average-cost strong-motion stations have been recently added, with the goals of improving the density of real-time ground-motion observations and measuring the level of shaking recorded at selected buildings. The final goal is to allow rapid impact estimations to be made to improve the response of civil protection authorities. Today, considering the higher density seismological network, new efforts in terms of the implementation and testing of earthquake early warning systems as a possible tool for mitigating seismic risk are certainly worthwhile. In this article, we show the results obtained by analyzing in playback and using an algorithm for decentralized onsite earthquake early warning, broadband synthetic strong-motion data calculated at 18 of the stations installed in the region, while considering the magnitude and location of the 1976 Friuli earthquake. The analysis shows that the anisotropy of the lead times is related not only to the finite nature of the source but also to the slip distribution. A reduction of 10% of injured persons appears to be possible if appropriate mitigating actions are employed, such as the development of efficient automatic procedures that improve the safety of strategic industrial facilities.

Gravel Liquefaction Assessment Using the Dynamic Cone Penetration Test Based on Field Performance from the 1976 Friuli Earthquake

AbstractThe dynamic cone penetration test (DPT) developed in China has been correlated with liquefaction resistance of gravelly soils based on field performance data from the Mw7.9 Wenchuan earthqu...

Calibration of a mechanics-based method for large-scale vulnerability assessment

This paper presents an analytical method for large-scale vulnerability assessment used in order to simulate damage scenarios corresponding to those observed during past earthquakes in Italy. The method, already published in the technical literature with the acronym SP-BELA (Simplified Pushover-Based Earthquake Loss Assessment), has been adopted to calculate the vulnerability of the Italian building stock classified as reinforced concrete and masonry buildings. The current paper describes the calibration of the method through the comparison of numerically calculated damage scenarios and data on observed damage collected during earthquakes occurred in Italy starting from the 1976 Friuli earthquake. The use of observed damage data allowed to validate the method and to add reliability to the calculation of damage scenarios. The damage scenarios are useful to plan the emergency response immediately after an earthquake. Finally, a Web Geographical Interface System tool that implements SP-BELA to calculate seismic risk and real time damage scenario is presented.

Central Italy sequence: simulated damage scenario for the main 2016 shocks

Damage scenarios for the main events of the 2016 Central Italy earthquake sequence have been simulated. In order to describe the seismic performance of the as-built in the area, the mechanic based method simplified pushover-based earthquake loss assessment has been adopted. This methodology has been extensively validated by means of comparison between numerically calculated damage scenario and observed damage. These data have been collected during the surveys in areas hit by earthquakes in Italy in the last 40 years (starting form 1976 Friuli earthquake until 2012 Emilia earthquake). The calculation of damage scenario has been performed in a user friendly WebGIS platform developed for the Italian Department of Civil Protection. The paper gives an overview of methods and informatics tools prior to present the results of the damage scenario. The results of damage scenarios are summarized in tables, charts and maps. Further, improvements of the tools now available are also identified for future developments.

Cansiglio Karst Plateau: 10 Years of Geodetic–Hydrological Observations in Seismically Active Northeast Italy

Ten years’ geodetic observations (2006–2016) in a natural cave of the Cansiglio Plateau (Bus de la Genziana), a limestone karstic area in northeastern Italy, are discussed. The area is of medium–high seismic risk: a strong earthquake in 1936 below the plateau (Mm = 6.2) and the 1976 disastrous Friuli earthquake (Mm = 6.5) are recent events. At the foothills of the karstic massif, three springs emerge, with average flow from 5 to 10 m3/s, and which are the sources of a river. The tiltmeter station is set in a natural cavity that is part of a karstic system. From March 2013, a multiparametric logger (temperature, stage, electrical conductivity) was installed in the siphon at the bottom of the cave to discover the underground hydrodynamics. The tilt records include signals induced by hydrologic and tectonic effects. The tiltmeter signals have a clear correlation to the rainfall, the discharge series of the river and the data recorded by multiparametric loggers. Additionally, the data of a permanent GPS station located on the southern slopes of the Cansiglio Massif (CANV) show also a clear correspondence with the river level. The fast water infiltration into the epikarst, closely related to daily rainfall, is distinguished in the tilt records from the characteristic time evolution of the karstic springs, which have an impulsive level increase with successive exponential decay. It demonstrates the usefulness of geodetic measurements to reveal the hydrological response of the karst. One outcome of the work is that the tiltmeters can be used as proxies for the presence of flow channels and the pressure that builds up due to the water flow. With 10 years of data, a new multidisciplinary frontier was opened between the geodetic studies and the karstic hydrogeology to obtain a more complete geologic description of the karst plateau.

The 6 May 1976 Friuli earthquake: re-evaluating and consolidating transnational macroseismic data

Reinterpretace makroseismickeho pole silneho zemětřeseni Friuli 1976, Mw 6,4. Studie prezentuje rozsahlý soubor nově vyhodnocených makroseismických pozorovani ze 14 evropských zemi.

The phases of the Friuli earthquake

La vicenda del terremoto del Friuli puo essere letta suddividendo le conseguenze psicosociali del disastro in tre distinte fasi temporali: l’emergenza, la ricostruzione e la memoria. Mentre i comportamenti sociali nella prima fase sono abbastanza generalizzabili, specialmente nei contesti in cui e ravvisabile una subcultura da disastro, la ricostruzione e maggiormente legata a dimensioni ambientali, culturali e politiche specifiche. Le indagini effettuate a distanza di alcuni anni dall’evento hanno permesso l’identificazione di diverse modalita di risposta al trauma ambientale

Influence of soil deformability on the seismic response of a masonry tower

The manuscript deals with the dynamic interaction between the seismic response of a masonry bell-tower and the soil layer below its foundations. The study is based on a direct soil-structure interaction analysis, by means of a complete 3D FEM model in which, beside the non-linear behaviour of the masonry material, a large portion of the foundation soil is taken into account, assuming a condition of non-reflection of the elastic waves within the domain of interest by means of infinite elements. A fairly refined description of the tower geometry allowed a thorough examination of the interaction between the characteristics of the motion transmitted from the ground and the damage mechanisms of the tower, which may include crack patterns related to the second mode of vibration and the collapse of the belfry. The results of modal analyses highlighted the extent to which the soil deformability modifies the shape of second natural vibration modes of the tower. A relevant reduction of the high frequency content of the vertical component at the base of the tower was also observed, especially in the case of a more deformable soil. In the non-linear dynamic analyses, a constitutive relation with plasticity and damage for the walls material was assumed, while as forcing actions, the strong ground motions recorded in Mirandola during the shock of May 29, 2012, and in Gemona during the Friuli Earthquake on September 15, 1976 were assumed. These analyses allowed us to investigate two main issues: (1) the capability to take into account the higher vibration modes, which tend to have a relevant role in the damage of the upper part, especially the tower crown and belfry; (2) the specific effect of the vertical component of ground acceleration, usually considered quite relevant for this structural typology. The damage scenarios obtained in a condition of deformable foundation soil resulted different from those obtained in the hypothesis of a fixed base support. In this case, the resort to non-linear dynamical models, which also include a significant volume of deformable soil, is recommended in order to perform an accurate assessment of damage scenarios.

Trans-national earthquake early warning (EEW) in north-eastern Italy, Slovenia and Austria: first experience with PRESTo at the CE<sup>3</sup>RN network

Abstract. The region of central and eastern Europe is an area characterised by a relatively high seismic risk. Since 2001, to monitor the seismicity of this area, the OGS (Istituto Nazionale di Oceanografia e di Geofisica Sperimentale) in Italy, the Agencija Republike Slovenije za Okolje (ARSO) in Slovenia, the Zentralanstalt für Meteorologie und Geodynamik (ZAMG) in Austria, and the Università di Trieste (UniTS) have cooperated in real-time seismological data exchange. In 2014 OGS, ARSO, ZAMG and UniTS created a cooperative network named the Central and Eastern European Earthquake Research Network (CE3RN), and teamed up with the University of Naples Federico II, Italy, to implement an earthquake early warning system based on the existing networks. Since May 2014, the earthquake early warning system (EEWS) given by the integration of the PRESTo (PRobability and Evolutionary early warning SysTem) alert management platform and the CE3RN accelerometric stations has been under real-time testing in order to assess the system's performance. This work presents a preliminary analysis of the EEWS performance carried out by playing back real strong motion recordings for the 1976 Friuli earthquake (MW= 6.5). Then, the results of the first 6 months of real-time testing of the EEWS are presented and discussed.

OASIS: The OGS Archive System of Instrumental Seismology

Online Material: List of main seismological databases, OASIS database general schema, tables of networks and stations, and example station monography and extracted waveform time series. The availability of an extensive dataset of seismological data, together with the description of the recording site, can help improve studies on the causes and effects of earthquakes, such as studies on seismic hazard, site‐response analysis, soil–structure interaction, seismic source properties, shallow and deep earth structure, and so on. The number of seismological stations installed worldwide for either monitoring seismicity at regional or national scale or deployed for specific studies has dramatically increased in the last years. As a consequence, there is a growing need on one hand to archive the huge quantity of acquired data in a safe and organized way and, on the other hand, to provide suitable access and search‐capability tools to those data. Several research institutions from different countries currently collect their seismological data into databases, making them available on the Web. ![Graphic][1] A list of the main worldwide, national, and Italian seismological databases is provided in the electronic supplement to this article. In this article, we present the OGS Archive System of Instrumental Seismology (OASIS), the new information system developed at the National Institute of Oceanography and Experimental Geophysics (Istituto Nazionale di Oceanografia e di Geofisica Sperimentale [OGS]), settled in Italy, with the aim of organizing, archiving, and providing access to the whole set of its seismological instrumental data. OGS is a research institute located in Trieste, northern Italy, which has a well‐established seismological tradition and had a relevant growth after the 1976 M w 6.4 Friuli earthquake with the development of the northeast Italy seismometric network (rts.crs.inogs.it; last accessed February 2015). During the last decades, and especially after 1995 when OGS switched its regional network from analog to digital … [1]: /embed/inline-graphic-1.gif

Investigation of computational and accuracy issues in POD-based reduced order modeling of dynamic structural systems

In this paper, we investigate the performance of reduced order modeling of dynamic structural systems based on the proper orthogonal decomposition (POD) technique. Singular value decomposition of the so-called snapshot matrix is adopted to generate the reduced space, onto which the system equations of motion are projected to speedup the computations.To get insights into the achievable speedup and the capability of POD to provide an input-independent reduced model, we consider the 39-story Pirelli tower in Milan-Italy. First, we assume that a shear model of the building is excited by the May 18-1940, Mw 7.1, El Centro earthquake, and generate the data ensemble necessary to build the reduced model. Second, we assess the local and global accuracies of the same reduced model in tracking the dynamics of the building, if excited by the May 6-1976, Mw 6.4, Friuli earthquake and by the January 17-1995, Mw 6.8, Kobe earthquake, which differ from the El Centro one in terms of excited vibration frequencies. We show that POD allows to attain a speedup approaching 250, when the reduced order model is asked to feature a high accuracy; moreover, POD tends to outperform a standard modal analysis at increasing number of modes retained in the model.

Seismic Microzonation of Breginjski Kot (NW Slovenia) Based on Detailed Engineering Geological Mapping

Breginjski kot is among the most endangered seismic zones in Slovenia with the seismic hazard assessed to intensity IX MSK and the design ground acceleration of 0.250 g, both for 500-year return period. The most destructive was the 1976 Friuli Mw = 6.4 earthquake which had maximum intensity VIII-IX. Since the previous microzonation of the area was based solely on the basic geological map and did not include supplementary field research, we have performed a new soil classification of the area. First, a detailed engineering geological mapping in scale 1 : 5.000 was conducted. Mapped units were described in detail and some of them interpreted anew. Stiff sites are composed of hard to medium-hard rocks which were subjected to erosion mainly evoked by glacial and postglacial age. At that time a prominent topography was formed and different types of sediments were deposited in valleys by mass flows. A distinction between sediments and weathered rocks, their exact position, and thickness are of significant importance for microzonation. On the basis of geological mapping, a soil classification was carried out according to the Medvedev method (intensity increments) and the Eurocode 8 standard (soil factors) and two microzonation maps were prepared. The bulk of the studied area is covered by soft sediments and nine out of ten settlements are situated on them. The microzonation clearly points out the dependence of damage distribution in the case of 1976 Friuli earthquake to local site effects.

Simplified Mechanical Model for the Seismic Vulnerability Evaluation of Belfries

The systematic observation of damage caused by recent Italian earthquakes has highlighted the high seismic vulnerability of belfries, the inner part of bell towers. In this study, the authors propose a simplified methodology for safety checks of belfries that is usable for risk analysis at territorial level for the whole bell tower, according to the method proposed by the Italian Directive of October 12, 2007, Guidelines for the Evaluation and the Reduction of the Seismic Vulnerability of Cultural Heritage. The new mechanical method takes into account the possible activation of two different collapse mechanisms, connected to combined actions of compression and bending or shear action, supposing different static schemes in relation to the real belfry configuration. The seismic force at the base of the belfry is calculated also considering the influence of the second mode of vibration through a correction factor obtained by spectral analyses on several bell towers damaged by the 1976 Friuli earthquake. A validation of the proposed approach has been shown for two belfries damaged by the 2002 Molise earthquake and by the 2009 Abruzzi earthquakes, to highlight the reliability of the method in comparison to the simplified approach proposed in the literature.

Reassessment of the source of the 1976 Friuli, NE Italy, earthquake sequence from the joint inversion of high-precision levelling and triangulation data

SUMMARY In this study, we revisit the mechanism of the 1976 Friuli (NE Italy) earthquake sequence (main shocks Mw 6.4, 5.9 and 6.0). We present a new source model that simultaneously fits all the available geodetic measurements of the observed deformation. We integrate triangulation measurements, which have never been previously used in the source modelling of this sequence, with high-precision levelling that covers the epicentral area. We adopt a mixed linear/non-linear optimization scheme, in which we iteratively search for the best-fitting solution by performing several linear slip inversions while varying fault location using a grid search method. Our preferred solution consists of a shallow north-dipping fault plane with assumed azimuth of 282° and accommodating a reverse dextral slip of about 1 m. The estimated geodetic moment is 6.6 × 1018 Nm (Mw 6.5), in agreement with seismological estimates. Yet, our preferred model shows that the geodetic solution is consistent with the activation of a single fault system during the entire sequence, the surface expression of which could be associated with the Buia blind thrust, supporting the hypothesis that the main activity of the Eastern Alps occurs close to the relief margin, as observed in other mountain belts. The retrieved slip pattern consists of a main coseismic patch located 3–5 km depth, in good agreement with the distribution of the main shocks. Additional slip is required in the shallower portions of the fault to reproduce the local uplift observed in the region characterized by Quaternary active folding. We tentatively interpret this patch as postseismic deformation (afterslip) occurring at the edge of the main coseismic patch. Finally, our rupture plane spatially correlates with the area of the locked fault determined from interseismic measurements, supporting the hypothesis that interseismic slip on the creeping dislocation causes strain to accumulate on the shallow (above ∼10 km depth) locked section. Assuming that all the long-term accommodation between Adria and Eurasia is seismically released, a time span of 500–700 years of strain-accumulating plate motion would result in a 1976-like earthquake.

Acquiring, archiving, analyzing and exchanging seismic data in real time at the Seismological Research Center of the OGS in Italy

After the 1976 Friuli earthquake (Ms = 6.5) in north-eastern Italy that caused about 1,000 casualties and widespread destruction in the Friuli area, the Italian government established the Centro di Ricerche Sismologiche (CRS). This is now a department of the Istituto Nazionale di Oceanografia e di Geofisica Sperimentale (OGS), and it is specifically devoted to the monitoring of the seismicity of north-eastern Italy. Since its inception, the North-East Italy Seismic Network has grown enormously. Currently, it consists of 14 broad-band and 20 short-period seismic stations, all of which are telemetered to and acquired in real time at the OGS-CRS data center in Udine. Data exchange agreements in place with other Italian, Slovenian, Austrian and Swiss seismological institutes lead to a total number of 94 seismic stations acquired in real time, which confirms that the OGS is the reference institute for seismic monitoring of north-eastern Italy. Since 2002, CRS has been using the Antelope software suite as the main tool for collecting, analyzing, archiving and exchanging seismic data. SeisComP is also used as a real-time data exchange server tool. A customized webaccessible server is used to manually relocate earthquakes, and automatic procedures have been set-up for moment-tensor determination, shaking-map computation, web publishing of earthquake parametric data, waveform drumplots, state-of-health parameters, and quality checks of the station by spectra analysis. Scripts for email/SMS/fax alerting to public institutions have also been customized. Recently, a real-time seismology website was designed and set-up (http://rts.crs.inogs.it/).