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We present a 1:25,000 scale map of the coseismic surface ruptures following the 30 October 2016 M w 6.5 Norcia normal-faulting earthquake, central Italy. Detailed rupture mapping is based on almost 11,000 oblique photographs taken from helicopter flights, that has been verified and integrated with field data (>7000 measurements). Thanks to the common efforts of the Open EMERGEO Working Group (130 people, 25 research institutions and universities from Europe), we were able to document a complex surface faulting pattern with a dominant strike of N135°-160° (SW-dipping) and a subordinate strike of N320°-345° (NE-dipping) along about 28 km of the active Mt. Vettore-Mt. Bove fault system. Geometric and kinematic characteristics of the rupture were observed and recorded along closely spaced, parallel or subparallel, overlapping or step-like synthetic and antithetic fault splays of the activated fault systems, comprising a total surface rupture length of approximately 46 km when all ruptures were considered.

In this work, we propose a geodetic model for the seismic sequence, with doublet earthquakes, that occurred in Bandar Abbas, Iran, in November 2021. A dataset of Sentinel-1 images, processed using the InSAR (Interferometric Synthetic Aperture Radar) technique, was employed to identify the surface deformation caused by the major events of the sequence and to constrain their geometry and kinematics using seismological constraints. A Coulomb stress transfer analysis was also applied to investigate the sequence’s structural evolution in space and time. A linear inversion of the InSAR data provided a non-uniform distribution of slip over the fault planes. We also performed an accurate relocation of foreshocks and aftershocks recorded by locally established seismographs, thereby allowing us to determine the compressional tectonic stress regime affecting the crustal volume. Despite the very short time span of the sequence, our results clearly suggest that distinct blind structures that were previously unknown or only suspected were the causative faults. The first Mw 6.0 earthquake occurred on an NNE-dipping, intermediate-angle, reverse-oblique plane, while the Mw 6.4 earthquake occurred on almost horizontal or very low-angle (SSE-dipping) reverse segments with top-to-the-south kinematics. The former, which cut through and displaced the Pan-African pre-Palaeozoic basement, indicates a thick-skinned tectonic style, while the latter rupture(s), which occurred within the Palaeozoic–Cenozoic sedimentary succession and likely exploited the stratigraphic mechanical discontinuities, clearly depicts a thin-skinned style.

By referring to the two strongest earthquakes of the 2016–2017 Central Italy seismic sequence, this paper presents a procedure to make shaking maps through empirical relationships between macroseismic intensity and ground-motion parameters. Hundreds of waveforms were processed to obtain instrumental ground-motion features which could be correlated with the potential damage intensities. To take into account peak value, frequency, duration, and energy content, which all contribute to damage, cumulative absolute velocity and Arias intensity were used to quantify the features of the ground motion. Once these parameters had been calculated at the recording sites, they were interpolated through geostatistical techniques on the whole struck area. Finally, empirical relationships were used for mapping intensities, i.e., potential effects on the built environment. The results referred to both earthquake scenarios that were analyzed and were also used for assessing the influence of the spatial coverage of the instrumental network. In fact, after the first events, the Italian seismic network was subjected to the addition and thickening of sensors in the epicentral area, especially. The results obtained by models only dependent on ground-motion parameters or even on the epicentral distance were compared with the official ShakeMaps and the observed intensities for assessing their reliability. Finally, some suggestions are proposed to improve the procedure that could be used for rapidly assessing ground shaking and mapping damage potential producing useful information for non-expert audience.

Immediately after a large earthquake, the main question asked by the public and decision-makers is whether it was the mainshock or a foreshock to an even stronger event yet to come. So far, scientists can only offer empirical evidence from statistical compilations of past sequences, arguing that normally the aftershock sequence will decay gradually whereas the occurrence of a forthcoming larger event has a probability of a few per cent. Here we analyse the average size distribution of aftershocks of the recent Amatrice–Norcia and Kumamoto earthquake sequences, and we suggest that in many cases it may be possible to discriminate whether an ongoing sequence represents a decaying aftershock sequence or foreshocks to an upcoming large event. We propose a simple traffic light classification to assess in real time the level of concern about a subsequent larger event and test it against 58 sequences, achieving a classification accuracy of 95 per cent. Changes in the average size distribution of earthquakes are used to discriminate between foreshocks and aftershocks, and a traffic light classification is proposed for the real-time assessment of the probability of a subsequent larger event.

Deep 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. Plain Language Summary There is widespread evidence that the Amatrice‐Visso‐Norcia seismic sequence (2016–2017, Central Italy) was triggered by fluid circulation across the Apennine Chain. However, how, and why fluids migrated across the fault network is still under debate. Seismic attenuation describes how seismic waves lose energy during their propagation. When used as an imaging attribute, it has demonstrated the potential to recover the spatial extension and mechanisms of fracturing and fluid movement across volcanoes and faults. Here, we map scattering attenuation through the peak delay measurements in 3D before (2013–2016) and during the 2016–2017 sequence. Scattering attenuation separated fractured zones from regions of compaction, controlled, before and during the sequence by thrusts and lithological differences. High scattering (strong fracturing) increases through time due to intense fracturing, while low scattering (higher compaction of the rocks) marks areas where earthquakes will occur. Our results highlight the importance of the main thrusts, as they separate compartments of the shallow crust characterized by different scattering attenuation anomalies, the Triassic deposits in fluid accumulation, and subsequent triggering of normal faults. Key Points Scattering attenuation detects the control of thrusts and lithology on post‐seismic fracturing and fluid migration during the AVN sequence Overpressurized fluids compact low‐scattering rocks at thrusts' roots before earthquakes Detecting fluid overpressure and fracturing suggests an unexploited monitoring potential of scattering attenuation

On 26th December 2018, an earthquake (Mw = 4.9) hits the eastern flank of the Etna volcano (Italy). It was the strongest seismic event among seventy with Mw > 2.5 occurring from 23rd December 2018. After the 2018 seismic sequence, seismic microzonation activities have been conducted for several municipalities located in the Etna area. The paper presents the results of seismic microzonation studies for the municipality of Trecastagni damaged by the 2018 seismic sequence. One important aspect in seismic microzonation studies is the definition of subsoil models derived from geological and geotechnical investigations. In this work, an intense investigation campaign has been carried out to define the subsoil models to be used in local seismic response analyses. A peculiarity of the area is constituted by the presence of shear wave velocity inversions typical of volcanic contexts. Another key aspect in seismic microzonation studies is the selection of the most suitable input motions matching on average a reference spectrum in a defined range of periods. In this study, the acceleration response spectrum prescribed by the Italian seismic code for outcropping rock condition and 475 years return period has been assumed as reference. The results of the site response numerical analyses are presented in terms of elastic acceleration response spectra, amplification functions and amplification factors of spectral acceleration defined according to national guidelines and standards for microzonation studies. Findings have been synthetized into three maps representing the amplification factors computed within three ranges of periods. In order to establish an absolute ranking of seismic hazard, a new methodology has been applied by a synthetic damage-constrained parameter (called HSM) whose classification is based on the correlation with the European Macroseismic intensity Scale. Finally, a damage grade map has been developed for a seismically homogeneous microzone based on the macroseismic intensities and the vulnerability classes of the buildings encountered in the area. This study highlights the importance of the HSM parameter in developing emergency planning and mitigation policies.

Seismic microzonation represents a basic tool for local administrations in the developing of cost-effective risk reduction strategies and emergency planning. In recent years, the Italian scientific community has been deeply involved in implementing best-practices and tools to make seismic microzonation studies affordable by allowing its widespread application. Specific guidelines were provided that are representative of the international state of the art in this field. Moreover, the national Center for Seismic Microzonation and its applications (CentroMS) was established, which includes the most important Italian scientific institutions involved in seismic microzonation studies during last years. One of the purposes of this Center is supporting local Authorities and professionals in the field practice. Effectiveness of this organization has been tested during most recent seismic sequences, where the Center was charged to support the development of reconstruction plans. In this review paper the main features of these activities are addressed by focusing on experiences gained in the seismic microzonation studies carried out at 138 Municipalities in the area of Central Italy damaged by the 2016–2017 seismic sequence.

Four regions of central Italy were struck by the seismic sequence of the 2016 earthquake in the country: Lazio, Abruzzo, Umbria and Marche. This highlighted the different behaviour of masonry constructions depending on the prevention actions carried out after previous earthquakes. In particular, although damaged, the masonry buildings in the historical centre of Norcia (Umbria region) behaved significantly better than those in other regions. Indeed, the strengthening interventions carried out after the earthquakes of 1971, 1979 and 1997 greatly affected the seismic behaviour of masonry aggregates (contiguous masonry structural units, MSUs) in the historical centre, which sustained limited damage and a low number of collapses. This paper discusses the empirical data on damage collected with respect to 670 MSUs by means of the first level survey form concerning post-earthquake damage, and usability assessments (AeDES). The forms completed for the survey relate to MSUs in the historical centre of Norcia and were produced by the technicians of the Umbria Seismic Risk Office. The analysis shows the correlation between the MSU characteristics of: age of construction and renovation work; type of vertical and horizontal structures; roof types and usability rating; and the damage level and extent thereof detected in vertical structures. The effectiveness of previous strengthening interventions and the analyses of the types of strengthening solution are also discussed. A case study aggregate is analyzed in detail in order to illustrate the importance of strengthening interventions on vertical bearing elements. The strengthening interventions resulted in a sound strategy to strongly reduce losses, even in a very vulnerable centre comprised of old residential masonry aggregates.

Every gigantic earthquake begins as a tiny rock failure at almost a point, followed by successive slip of the complex fault system, before radiating strong shaking from a vast rupture area extending over hundreds of kilometres. Whether the growth process of the rupture of a large earthquake is predictable and whether it produces observable signatures different from that of smaller events 1 – 5 are fundamental questions related to the potential for earthquake early warning and probabilistic forecasting. Inspired by a recent discovery that large earthquakes might have seismic waves, and probably rupture processes, that are almost identical to those of smaller events 6 – 8 , we show that such similarity characterized by large cross-correlation is a common feature of earthquakes in the Tohoku–Hokkaido subduction zone, Japan. A systematic comparison of 15 years of high-sensitivity seismograph records for approximately 100,000 events reveals 80 extremely similar and 390 very similar pairs of large (moment magnitude M  > 4.5) and small ( M  < 4.0) earthquakes, co-located within about 100 metres. An extremely high similarity is observed for pairs of subduction-type earthquakes (170 of 899 large events) separated by a long period of up to 15 years, whereas for pairs of other types of large earthquakes only the foreshocks and aftershocks are similar. This frequently occurring similarity between different-sized subduction-type earthquakes suggests repeated cascading rupture processes in a widespread hierarchical structure 9 – 12 along the plate interface and indicates a specific but probabilistically limited predictability of the final size of the earthquake (that is, the location and a set of possible sizes of an earthquake are well predicted, but its final size is not at all well constrained). Analysis of a dataset of high-sensitivity Tohoku–Hokkaido seismograph records shows that pairs of subduction-type earthquakes of different sizes have very similar initial characteristics, implying that the final size of an earthquake cannot be reliably predicted from these.

Successive locations of individual large earthquakes ( M w  > 5.5) over years to centuries can be difficult to explain with simple Coulomb stress transfer (CST) because it is common for seismicity to circumvent nearest-neighbour along-strike faults where coseismic CST is greatest. We demonstrate that Coulomb pre-stress (the cumulative CST from multiple earthquakes and interseismic loading on non-planar faults) may explain this, evidenced by study of a 667-year historical record of earthquakes in central Italy. Heterogeneity in Coulomb pre-stresses across the fault system is >±50 bars, whereas coseismic CST is <±2 bars, so the latter will rarely overwhelm the former, explaining why historical earthquakes rarely rupture nearest neighbor faults. However, earthquakes do tend to occur where the cumulative coseismic and interseismic CST is positive, although there are notable examples where earthquake propagate across negatively stressed portions of faults. Hence Coulomb pre-stress calculated for non-planar faults is an ignored yet vital factor for earthquake triggering. Scattered earthquake locations in the same region cannot be explained solely by coseismic Coulomb stress on planar faults. Instead, the authors suggest Coulomb pre-stress to influence earthquake locations. Pre-stress was modelled on strike-variable faults and consists of coseismic and interseismic Coulomb stress.