Festa, Gaetano

Pump-probe experiments investigate the strain sensitivity of crustal elastic properties, showing nonlinear variations during the strain cycle. In the laboratory, pre-seismic reductions in seismic velocity indicate that asperity contacts within the fault zone begin to fail before the macroscopic frictional sliding. The recognition of such effects in natural seismic-cycles has been challenging. Here we exploit seasonal hydrological strains, performing a natural analogue to a quasi-static laboratory pump-probe experiment to investigate the nonlinear strain sensitivity of crustal rocks and its role in seismic failure along the tectonically-active Irpinia Fault System (Southern Italy). By comparing 14-years-long series of spring discharge, strain, seismic velocity variations and earthquakes rate, we find that seismicity peaks during maximum hydrological forcing and minimum seismic velocity. Seasonal strains of ~10-6 are required for both earthquake triggering and significant nonlinearity effects arising from modulus reduction. We suggest that, for faults in a critical state, cyclical softening may lead to failure and seasonal seismicity.

Seismic monitoring in areas where induced earthquakes could occur is a challenging topic for seismologists due to the generally very low signal to noise ratio. Therefore, the seismological community is devoting several efforts to the development of high-quality networks around the areas where fluid injection and storage and geothermal activities take place, also following the national induced seismicity monitoring guidelines. The use of advanced data mining strategies, such as template matching filters, auto-similarity search, and deep-learning approaches, has recently further fostered such monitoring, enhancing the seismic catalogs and lowering the magnitude of completeness of these areas. In this framework, we carried out an experiment where a small-aperture seismic array was installed within the dense seismic network used for monitoring the gas reservoir of Collalto, in North Italy. The continuous velocimetric data, acquired for 25 days, were analysed through the application of the optimized auto-similarity search technique FAST. The array was conceived as a cost-effective network, aimed at integrating, right above the gas storage site, the permanent high-resolution Collalto Seismic Network. The analysis allowed to detect micro-events down to magnitude Ml = −0.4 within a distance of ~15 km from the array. Our results confirmed that the system based on the array installation and the FAST data analysis might contribute to lowering the magnitude of completeness around the site of about 0.7 units.

Transnational access (TNA) allows cross-border, short-term and frequently free-of-charge access to world-class research facilities, to foster collaborations and exchanges of experience. Specifically, TNA aims to encourage open science and innovation and to increase the efficient and effective use of scientific infrastructure. Within EPOS, the European Plate Observing System, the Volcano Observatories and Multi-scale Laboratories communities have offered TNA to their high-quality research facilities through national and European funding. This experience has allowed the definition, design, and testing of procedures and activities needed to provide transnational access inn the EPOS context. In this paper, the EPOS community describes the main objectives for the provision of transnational access in the EPOS framework, based on previous experiences. It includes practical procedures for managing transnational access from a legal, governance, and financial perspective, and proposes logistical and technical solutions to effectively execute transnational access activities. In addition, it provides an outlook on the inclusion of new thematic communities within the TNA framework, and addresses the challenges of providing market-driven access to industry.

The Near Fault Observatories (NFOs) community is one of the European Plate Observing System (EPOS, http://www.epos-eu.org) Thematic Communities, today consisting of six research infrastructures that operate in regions characterised by high seismic hazard originating from different tectonic regimes. Earthquakes respond to complex natural systems whose mechanical properties evolve over time. Thus, in order to understand the multi-scale, physical/chemical processes responsible for the faulting that earthquakes occur on, it is required to consider phenomena that intersect different research fields, i.e., to put in place multidisciplinary monitoring. Hence, NFOs are grounded on modern and multidisciplinary infrastructures, collecting near fault high resolution raw data that allows generation of innovative scientific products. The NFOs usually complement regional backbone networks with a higher density distribution of seismic, geodetic, geochemical and other geophysical sensors, at surface and sometimes below grade. These dense and modern networks of multi-parametric sensors are sited at and around active faults, where moderate to large earthquakes have occurred in the past and are expected in the future. They continuously monitor the underlying Earth instability processes over a broad time interval. Data collected at each NFO results in an exceptionally high degree of knowledge of the geometry and parameters characterizing the local geological faults and their deformation pattern. The novel data produced by the NFO community is aggregated in EPOS and is made available to a diverse set of stakeholders through the NFO Federated Specific Data Gateway (FRIDGE). In the broader domain of the Solid Earth sciences, NFOs meet the growing expectations of the learning and communication sectors by hosting a large variety of scientific information about earthquakes as a natural phenomenon and a societal issue. It represents the EPOS concept and objective of aggregating and harmonising the European research infrastructures capabilities to facilitate broader scientific opportunity. The NFOs are at the cutting edge of network monitoring. They conduct multidisciplinary experiments for testing multi-sensor stations, as well as realise robust and ultra-low latency, transmission systems that can routinely accommodate temporary monitoring densification. The effort to continuously upgrade the technological efficiency of monitoring systems positions the NFO at the centre of marketing opportunities for the European enterprises devoted to new sensor technology. The NFOs constitute ideal test beds for generating expertise on data integration, creating tools for the next generation of multidisciplinary research, routine data analysis and data visualization. In particular focus is often on near-real time tools and triggering alarms at different levels are tested and implemented, strengthening the cooperation with the Agencies for risk management. NFOs have developed innovative operational actions such as the Testing Centre for Earthquake Early Warning and Source Characterisation (CREW) and detailed fast ground shaking and damage characterization. Complementing the recent growth of modern laboratory and computational models, the NFOs can provide interdisciplinary observations of comparable high resolution to describe the behaviour of fault slip over a vast range of spatial and temporal scales and aiding to provide more accurate earthquake hazard characterizations.

Tsunamis constitute a significant hazard for European coastal populations, and the impact of tsunami events worldwide can extend well beyond the coastal regions directly affected. Understanding the complex mechanisms of tsunami generation, propagation, and inundation, as well as managing the tsunami risk, requires multidisciplinary research and infrastructures that cross national boundaries. Recent decades have seen both great advances in tsunami science and consolidation of the European tsunami research community. A recurring theme has been the need for a sustainable platform for coordinated tsunami community activities and a hub for tsunami services. Following about three years of preparation, in July 2021, the European tsunami community attained the status of Candidate Thematic Core Service (cTCS) within the European Plate Observing System (EPOS) Research Infrastructure. Within a transition period of three years, the Tsunami candidate TCS is anticipated to develop into a fully operational EPOS TCS. We here outline the path taken to reach this point, and the envisaged form of the future EPOS TCS Tsunami. Our cTCS is planned to be organised within four thematic pillars: (1) Support to Tsunami Service Providers, (2) Tsunami Data, (3) Numerical Models, and (4) Hazard and Risk Products. We outline how identified needs in tsunami science and tsunami risk mitigation will be addressed within this structure and how participation within EPOS will become an integration point for community development.

Seismic sequences are a powerful tool to locally infer geometrical and mechanical properties of faults and fault systems. In this study, we provided detailed location and characterization of events of the 3–7 July 2020 Irpinia sequence (southern Italy) that occurred at the northern tip of the main segment that ruptured during the 1980 Irpinia earthquake. Using an autocorrelation technique, we detected more than 340 events within the sequence, with local magnitude ranging between −0.5 and 3.0. We thus provided double difference locations, source parameter estimation, and focal mechanisms determination for the largest quality events. We found that the sequence ruptured an asperity with a size of about 800 m, along a fault structure having a strike compatible with the one of the main segments of the 1980 Irpinia earthquake, and a dip of 50–55° at depth of 10.5–12 km and 60–65° at shallower depths (7.5–9 km). Low stress drop release (average of 0.64 MPa) indicates a fluid-driven initiation mechanism of the sequence. We also evaluated the performance of the earthquake early warning systems running in real-time during the sequence, retrieving a minimum size for the blind zone in the area of about 15 km.

The complexity of coseismic slip distributions influences the tsunami hazard posed by local and, to a certain extent, distant tsunami sources. Large slip concentrated in shallow patches was observed in recent tsunamigenic earthquakes, possibly due to dynamic amplification near the free surface, variable frictional conditions or other factors. We propose a method for incorporating enhanced shallow slip for subduction earthquakes while preventing systematic slip excess at shallow depths over one or more seismic cycles. The method uses the classic k−2 stochastic slip distributions, augmented by shallow slip amplification. It is necessary for deep events with lower slip to occur more often than shallow ones with amplified slip to balance the long-term cumulative slip. We evaluate the impact of this approach on tsunami hazard in the central and eastern Mediterranean Sea adopting a realistic 3D geometry for three subduction zones, by using it to model ~ 150,000 earthquakes with 𝑀𝑤 from 6.0 to 9.0. We combine earthquake rates, depth-dependent slip distributions, tsunami modeling, and epistemic uncertainty through an ensemble modeling technique. We found that the mean hazard curves obtained with our method show enhanced probabilities for larger inundation heights as compared to the curves derived from depth-independent slip distributions. Our approach is completely general and can be applied to any subduction zone in the world.

Several great earthquakes occur on thrust faults along both subduction and continental collision zones. These events often feature a large shallow slip patch, an asymmetric pattern for the ground motion, and the static deformation between the hanging wall and footwall of the fault. From a mechanical point of view, this asymmetry can be partially explained taking into account the interaction between the fault and the seismic radiation emitted during rupture propagation and stored in the hanging wall in the vicinity of the free surface. We numerically investigate the rupture dynamics along a thrust dipping fault impacting onto the free surface at a dip angle of δ = 20°, in a 2‐D elastic model. We show how the wave interaction of the rupture with the free surface leads to a breaking of the reflection symmetry. Compared to a rupture propagating in an infinite medium, this interaction enhances the slip rate in the updip direction as an effect of the coupling between slip and normal traction around the crack front. The breaking of symmetry leads to sizeable acceleration of the rupture toward asymptotic speed with inertia acquisition, and dependence of the rupture dynamics on the level of friction along the interface might produce an interface opening over a finite length in the vicinity of the surface. We finally explore how the wave interaction drives amplification and asymmetry of the shallow slip and the vertical displacement at the surface. The described effects should be considered in various numerical approaches and in interpretation of geophysical observations.

Near-fault ground motion records often present impulsive signals, characterized by a largeamplitude in the velocity wavefield and by the energy concentrated in a short time window as comparedto the total earthquake duration. Thispulse-likebehavior is ascribed to the directivity of the seismic rupture,and it requires a stronger demand to the buildings not predicted by the classical design spectra. In this workwe investigate the pulse occurrence and duration in near-fault synthetic seismograms generated from anensemble ofk 2source models. We exploited the fault geometry of theMw= 6.3, 2009 L’Aquila earthquake,which represents a typical example of normal-fault earthquake for which several records in the fault vicinityare available for comparison with synthetics. We show that impulsive records are sensitive to the rupturevelocity, to the hypocenter depth, and to the station location, whether it is on the hanging wall or on thefootwall. The pulse duration was also shown to be proportional to the risetime, and it scales with thesource-receiver distance and inversely with the rupture velocity. We model these results as an effectof the coupled along-strike and updip directivity

In this paper we present innovative methodologies for seismic monitoring of volcanic structures in space and time (4D) which can possibly evolve toward an unrest stage. They are based on repeated phase and amplitude measurements done on active and/or passive seismic data including shots, vibrations, earthquakes and ambient noise in order to characterize the structure of the volcano and track its evolution through time. The characterization of the medium properties is performed through the reconstruction of an image of the elastic and anelastic properties of the propagation mediumcrossed by seismicwaves. This study focuses on the application of specific tomographic inversion methods to obtain high quality tomographic images. The resolution of the tomographic models is influenced by the number and spatial distribution of data. The expected resolution thus guides the setup of, for example, active seismic surveys. To recognize andmonitor changes in the properties of the propagation medium without performing an active survey we identify a fast proxy based on the time evolution of the Vp/Vs ratio. The advantages and limitations of the methods are discussed through synthetic tests, resolution analysis and case studies in volcanic areas such as the Campi Flegrei (southern Italy) and The Geysers geothermal area (California).