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The study of the incidence of coseismic phenomena is becoming an increasingly demanding and fundamental need in terms of civil protection agencies. Especially landslides triggered by earthquakes can cause significant impacts and losses across wide areas affected by earthquake shaking.In this context, we present a new database of historical earthquake-induced landslides (HEILs) created within the project “Multi-scale, integrated approach for the definition of earthquake-induced landslide hazard in Italy”, funded by the Italian Ministry for the Environment. The goal of this project was to develop a multidisciplinary approach for assessing the earthquake-induced landslide hazard at national, regional and local scales, and integrating existing databases with the results from previous projects and research activities.The Catalogue of Strong Earthquakes in Italy (CFTI) database holds a central role in this research. It collects the results of over three decades of research on historical seismicity in Italy. What makes CFTI different from other earthquake catalogues is that its database does contain not only parametric data and macroseismic intensities assigned to individual localities but also synthetic descriptions of the seismic scenario for each investigated earthquake sequence. It provides a complete account of the effects on the built and natural environment. In addition, for every investigated earthquake sequence, CFTI supplies the relevant bibliography in an organized form, allowing to navigate upstream from the parameters of a specific earthquake to the original sources used to investigate that event.​​CFTI also provides descriptions of the effects induced by earthquakes on the natural environment, such as ground cracks, chasms, landslides, rockfalls, changes in the discharge rate of rivers and springs, tsunami effects, overflowing of lakes, etc. Specifically, its latest version, CFTI5Med, documents about 600 landslides associated with strong historical earthquakes.We thus reviewed and integrated data relating to HEILs, already included in the CFTI database, by identifying new landslides. We focused on the review of historical sources, newly found or already archived in the CFTI database, the analysis of recent scientific articles and technical reports. Moreover, we carried out a comparison with other digital archives such as the CEDIT (https://doi.org/10.4408/IJEGE.2012-02.O-05) and the EEE catalogue (http://eeecatalogue.isprambiente.it/). The goal was reaching a more accurate localization and definition of the slope movement types of the HEILs, when the descriptions of the historical sources allowed it, through the geographical comparison with data of different origins, such as aerial photographs, geomorphological and instability maps. These effects were associated, where possible, with the individual landslides registered in the IFFI database (https://www.progettoiffi.isprambiente.it/).The final result is a dataset with about 1,000 landslides divided into classes of location accuracy. The dataset is addressed to a large audience of potential users: researchers and scholars, administrators and technicians of local institutions, and civil protection authorities.The results are collected in a new independent database, CFTI Landslides, connected to the CFTI5Med, which is publicly accessible online through a dedicated open-source geographic interface, designed to be interoperable with both INGV and external databases.

Late Quaternary paleovalley systems are sedimentary bodies, tens of m thick and a few km wide, that are typically buried beneath modern deltas and coastal plains and that have no obvious geomorphological expression. Paleovalley systems are increasingly studied worldwide as they are considered possible amplifiers of earthquake damage due to the sharp contrast between their soft and unconsolidated sediment fill and the adjacent substrate. In this study, using previous high-resolution stratigraphic reconstructions of the Pescara and Manfredonia paleovalleys in the Adriatic coastal plain (Italy), we investigate the potential of the microtremor-based horizontal-to-vertical spectral ratio technique (mHVSR) to identify these sediment bodies in the subsurface. We acquired 23 microtremor measurements in the Pescara area and 54 at Manfredonia along two transects transversal to the paleovalley axes. At both sites, we were able to detect resonance peaks that we correlated with stratigraphic data. In the Pescara paleovalley system, we identified low-amplitude resonance peaks at frequencies varying between 0.9 and 4 Hz, clearly denoting a U-shaped feature with lower frequencies in the central part. At Manfredonia, the resonance peaks are more prominent (with mHVSR amplitude up to 7), and the paleovalley system is denoted by resonance frequencies between 0.9 and 2.5 Hz, with a more complex geometry shaped by the interactions of the Candelaro, Cervaro, and Carapelle rivers. Using the well-known facies architecture as a guide, we constrained the mHVSR resonance peaks to create a Frequency-Depth model and transformed the mHVSR curves from the frequency to the spatial domain to reconstruct paleovalley geometries and infer Vs models. We thus obtained the 2D models of the paleovalleys profiles. The Pescara mHVSR model shows a sedimentary cover thickness varying from 10 m (on the interfluves) to 40 m (in the depocentre). At Manfredonia, the sedimentary cover has similar thickness, in the range of 10-45 m, with variations that reflect its complex internal geometry. At both sites, paleovalley fills are characterized by low Vs velocities: comparable Vs values were obtained from the two depocentres (about 180 m/s at Pescara and 140 m/s at Manfredonia), which places paleovalley fills into the ground type D of the Standard Eurocode 8. The fundamental resonance frequencies show considerable variability along the investigated transects on very short distances (few hundred meters), in a range of frequencies that can interact with the most common building types. We mapped this variability and observed excellent correlation with the geologic cross-sections, proving the mHVSR to be an effective tool for mapping these particular sediment bodies. The 2D models obtained will serve as a basis for future seismic response simulations.

Katla is one of the most active volcanoes in Iceland and is characterised by persistent seismicity. It is partly covered by the Mýrdalsjökull glacier and its historic activity is dominated by phreatomagmatic eruptions within the caldera associated with catastrophic glacial floods. In July 2011 a sudden jökulhlaup was released from the glacier, associated with tremor, elevated seismicity inside the caldera and a new cluster of seismicity on the south flank. This was likely caused by a hydrothermal or magmatic event, possibly a small subglacial eruption. Similar unrests occurred in 1955 and 1999. We have identified changes of the seismicity pattern coinciding with the 2011 unrest, suggesting a modification in the volcanic system. It may be speculated that if the persistent seismicity at Katla is an indication of a pressurized magma system ready to erupt, small events like those of 1955, 1999 and 2011 may trigger larger eruptions in the future. We have also conducted a pilot study of the geology of the southern flank, where the new seismicity is recorded, and identified sources for flank eruptions in the recent eruptive history of Katla. These include rhyolitic domes and surtseyan craters. Therefore, a wide range of volcanic processes have to be taken into account as possible source for the new seismicity and volcanic hazard.

Ground motion prediction is one of the main goals in seismic hazard assessment. Empirical ground motion prediction equations may fail to reproduce the complexity of ground shaking in complex 3D media and therefore the use of full waveform modelling is increasingly adopted to model ground shaking. The knowledge of the 3D crustal structure in terms of geometries of the main discontinuities and velocities is fundamental to model wave propagation. However, we often lack detailed geological and geophysical information to build reliable models.We exploit here a large set composed of high-resolution 2D and 3D seismic data and of about 40 wells with stratigraphic and velocity information, both onshore and offshore, to constrain a 3D crustal velocity model in a sector of the Calabrian accretionary prism (southern Italy). We interpret the main reflection discontinuities and constrain their depth at all available wells in the study area and we use well’s check-shots and velocity data to estimate interval-velocities of the main stratigraphic units. We then combine all depth and velocity information into a regional 3D crustal velocity model of the first 8-10 km. This is subsequently extended to a depth of ~50 km using available regional crustal models to obtain the final model used for ground motion simulation.We implement our crustal model in the spectral-element code SPECFEM3D_Cartesian to simulate wave propagation in the 3D velocity model honoring surface topography. This allows reconstructing the low-frequency part of the waveforms (up to ~1 Hz), which is then combined with high-frequency seismograms obtained with a stochastic method following the hybrid broadband simulation approach by Graves and Pitarka (2010).We evaluate the goodness of our model by simulating real earthquakes and comparing simulated and recorded waveforms at the available seismic stations in the area. We compare the results from our 3D model with the ones obtained using a local tomography model and the European crust model EPcrust. The maps of ground motion obtained from the simulated broadband waveforms are then compared with empirical ShakeMaps. These results will also be useful for earthquake scenario calculations, by simulating potential seismic sources identified from structural analysis of geological and seismic data.

The Calabrian accretionary prism is the result of a complex interaction between subduction-related tectonics and sedimentation, active since the Eocene. The limited seismicity recorded in recent years in the area appears mostly associated to the subduction interface and could reflect either a weak subduction coupling or a slow subduction rate. Nevertheless, recent intense deformation and uplift of the seafloor has been observed within the accretionary prism.The analysis of multichannel 2D and high-quality 3D seismic data, morphobathymetric data and instrumental seismicity, allows defining and characterizing both the deeper and shallower tectonic deformation affecting the northeastern sector of the Calabrian accretionary prism. Besides the uppermost thrust fault of the Calabrian accretionary prism, that outlines the Crotone promontory, the shallow tectonic pattern of the prism is characterized by a belt of broad flat-topped anticlines, and a set of minor narrow structures, mainly NNW-SSE to N-S oriented, that present a variable relationship with the underlying main thrust faults. The uppermost sedimentary strata within the anticlines are affected by numerous small-scale extensional faults, not rooted at depth, likely due to outer-arc extension above uplifted depocenters. In places, the inversion of basin-bounding faults is also visible. More regularly spaced and cylindrical NW-SE anticlines are also observed in the Gulf of Taranto, in the outer sector of the accretionary prism, where a thrust/back-thrust tectonic style is present. The origin of the anticlines varies within the overall set and reflects the long-term tectonic evolution of the accretionary prism, with the oblique docking of the Calabrian accretionary prism onto the Apulian Escarpment as a key feature.

Double differences (DD) seismic data are widely used to define elasticity distribution in the Earth's interior, and its variation in time. DD data are often pre-processed from earthquakes recordings through expert-opinion, where couples of earthquakes are selected based on some user-defined criteria, and DD data are computed from the selected couples. We develop a novel methodology for preparing DD seismic data based on a trans-dimensional algorithm, without imposing pre-defined criteria on the selection of couples of events. We apply it to a seismic database recorded on the flank of Katla volcano (Iceland), where elasticity variations in time has been indicated. Our approach quantitatively defines the presence of changepoints that separate the seismic events in time-windows. Within each time-window, the DD data are consistent with the hypothesis of time-invariant elasticity in the subsurface, and DD data can be safely used in subsequent analysis. Due to the parsimonious behavior of the trans-dimensional algorithm, only changepoints supported by the data are retrieved. Our results indicate that: (a) retrieved changepoints are consistent with first-order variations in the data (i.e. most striking changes in the DD data are correctly reproduced in the changepoint distribution in time); (b) changepoint locations in time do correlate neither with changes in seismicity rate, nor with changes in waveforms similarity (measured through the cross-correlation coefficients); and (c) noteworthy, the changepoint distribution in time seems to be insensitive to variations in the seismic network geometry during the experiment. Our results proofs that trans-dimensional algorithms can be positively applied to pre-processing of geophysical data before the application of standard routines (i.e. before using them to solve standard geophysical inverse problems) in the so called exploration of the data space.

Measuring ground resonances is of great importance for seismic site amplification studies. The task is usually addressed with the common H/V (horizontal to vertical spectral ratio) approach, which is widely used for both microzonation studies and stratigraphic imaging. Peaks on the H/V function are used to identify ground resonance frequencies, usually assuming 1D site conditions, i.e. with plane-parallel stratigraphy. In the simple case of a horizontal soft layer overlying a bedrock, 1D resonance is linked to the local bedrock depth (as a function of the shear wave velocity of the sediment layer). Therefore, when the 1D approximation holds, spatial variations of the resonance frequency reflect changes of bedrock depth (when lateral homogeneity of the sediment cover can be assumed). However, at sites with non-plane subsurface geometries, more complex resonance patterns may develop, such as 2D resonance patterns that typically occur within sediment-filled valleys. In this case, 2D resonance involves simultaneous vibration of the whole sedimentary infill at the same frequency, which may lead to large seismic amplification. 2D ground resonances can no longer be linked to the local depth-to-bedrock directly below the measurement site, but depend on the whole valley geometry and mechanic properties. Distinguishing between the 1D and 2D nature of a site is mandatory to avoid wrong stratigraphic and dynamic interpretations, which is in turn extremely relevant for seismic site response assessment.We investigated the problem in the Bolzano sedimentary basin (Northern Italy), which lies at the intersection between three valleys, using a single-station microtremor approach, the same usually applied for H/V surveys. We observed that the footprints of 1D and 2D resonances reside in different behaviors along the three components of motion. This is because, while the dynamic behavior of a 1D-site is the same along all horizontal directions, 2D resonances differ along the longitudinal and transversal directions of the resonating body, e.g. parallel and perpendicular to the valley axis. In addition, 2D resonance modes involve also a vertical component. This implies that the H/V method, by mixing the information along the three components, is not suitable to detect 2D resonances, that can be acknowledged only by looking at the individual spectral components and not at the H/V curves alone.By analyzing several hundred single-station microtremor measurements, we identified a list of frequency and amplitude features that characterize 1D and 2D resonances on individual spectral components of motion and on H/V ratios, on a single measurement and on several measurements acquired along profiles across the investigated valleys. We identified valleys characterized by 1D-only, 1D+2D and 2D-only resonance patterns and we propose a workflow scheme to conduct experimental measurements and data analysis in order to directly assess the 1D or 2D resonance nature of a site with a single-station approach, rather than evaluating this indirectly with numerical modelling.

SUMMARYThe vibration modes of the ground have been described both in the 1-D and 2-D case. The 1-D resonance is found on geological structures whose aspect ratio is low, that is on layers with a lateral width much larger than their thickness. A typical example is that of a horizontal soft sediment layer overlying hard bedrock. In this case, the 1-D resonance frequency, traditionally detected by means of the microtremor H/V (horizontal to vertical spectral ratio) technique, depends on the bedrock depth and on the shear wave velocity of the resonating cover layer. The H/V technique is thus used both to map the resonance frequencies in seismic microzonation studies and for stratigraphic imaging. When 2-D resonance occurs, generally on deep and narrow valleys, the whole sedimentary infill vibrates at the same frequency and stratigraphic imaging can no longer be performed by means of the 1-D resonance equation. Understanding the 1-D or 2-D resonance nature of a site is therefore mandatory to avoid wrong stratigraphic and dynamic interpretations, which is in turn extremely relevant for seismic site response assessment. In this paper, we suggest a procedure to address this issue using single-station approaches, which are much more common compared to the multistation synchronized approach presented by research teams in earlier descriptions of the 2-D resonances. We apply the procedure to the Bolzano sedimentary basin in Northern Italy, which lies at the junction of three valleys, for which we observed respectively 1-D-only, 1-D and 2-D, and 2-D-only resonances. We conclude by proposing a workflow scheme to conduct experimental measurements and data analysis in order to assess the 1-D or 2-D resonance nature of a site using a single-station approach.

Any stratigraphic reconstruction by means of surface geophysical methods is affected by the nonuniqueness of data inversion and by the resolution-depth trade-off. The combination of different geophysical techniques can reduce the number of degrees of freedom of the problem. We have focused on two low-impact single-station geophysical techniques: microtremor and gravity. These have been used by previous authors for stratigraphic mapping only by comparing the results independently. We suggest a procedure to combine microtremor and gravity data into a unique subsoil model and explore to what extent their combined use can overcome their individual weaknesses and constrain the final result. We apply the procedure to the Bolzano sedimentary basin, Northern Italy, to derive a 3D bedrock model of the basin. We use microtremor data to map the ground resonance frequencies and derive an initial 3D bedrock depth model by assuming a [Formula: see text] profile for the sediment fill. Then, we define a density model for rock and sediments and perform 3D gravity forward modeling. We then perturb the [Formula: see text] and density models and find the parameters that best fit the observed gravity anomalies. Data uncertainties are examined to explore the significance of the results. Joint use of the two techniques successfully helps interpret the stratigraphic model: Ground resonance frequencies guarantee the spatial resolution of the bedrock geometry model, whereas gravity data help constrain the frequency to depth conversion.

A peculiar cluster of seismicity near the tip of Sandfellsjökull on the eastern flank of Katla volcano in southern Iceland has been analyzed in detail using data from a temporary seismic network. A total of 300 events were detected between July 2011 and August 2013, most of them from a swarm between December 4th and 12th, 2011. The sparser permanent network detected a small fraction of these events, but also a larger swarm in November 2010. When seismic activity started in this area is uncertain because of changes in the detection capability of the network over time. The events are of low magnitude (–0.5 < $M_L$ < 0.5) and the b-value of their magnitude distribution is high (1.6±0.1). Based on their frequency content (4–25 Hz) and clear P and S arrivals, the events are classified as volcano–tectonic. Two multiplets probably with different source mechanism are identified in their population. The events locate at approximately 3.5 km depth. Most of them are tightly clustered according to double difference relative locations in a volume that is only about 400 m in diameter in all directions. Several events are scattered up to 800 m beneath this volume. There is some suggestion of elongate structure in the cluster with a NNE/SSW strike and a dip of 60 degrees. We argue that these events cannot be due to a glacial or a broad tectonic process. Possibly, a localized source of fluid pressure, e.g., a small magma body at depth may be the source of these events.

A 23-hour tremor burst was recorded on July 8–9th 2011 at the Katla subglacial volcano, one of the most active and hazardous volcanoes in Iceland. This was associated with deepening of cauldrons on the ice cap and a glacial flood that caused damage to infrastructure. Increased earthquake activity within the caldera started a few days before and lasted for months afterwards and new seismic activity started on the southern flank. No visible eruption broke the ice and the question arose as to whether this episode relates to a minor subglacial eruption with the tremor being generated by volcanic processes, or by the flood. The tremor signal consisted of bursts with varying amplitude and duration. We have identified and described three different tremor phases, based on amplitude and frequency features. A tremor phase associated with the flood was recorded only at stations closest to the river that flooded, correlating in time with rising water level observed at gauging stations. Using back-projection of double cross-correlations, two other phases have been located near the active ice cauldrons and are interpreted to be caused by volcanic or hydrothermal processes. The greatly increased seismicity and evidence of rapid melting of the glacier may be explained by a minor sub-glacial eruption. A less plausible interpretation is that the tremor was generated by hydrothermal boiling and/or explosions with no magma involved. This may have been induced by pressure drop triggered by the release of water when the glacial flood started. All interpretations require an increase of heat released by the volcano.

We introduce a back-projection method to locate tremor sources using products of cross-correlation envelopes of time series between seismic stations. For a given subset of n stations, we calculate ...

A key element for assessing seismic hazard and risk is the availability of a comprehensive dataset on past earthquakes. Here we present the rationale, structure and contents of CFTI5Med (10.6092/ingv.it-cfti5), the 2018 version of the Catalogue of Strong Earthquakes in Italy: a large multidisciplinary effort including historians, seismologists and geologists. It was conceived in 1989, following the inception of GIS technology, and first published in 1995 to offer a full account of Italy’s strongest earthquakes, of their territorial impact and associated social and economic upheaval. Subsequent versions (1997, 2000, 2007) entailed a fine tuning of research methodologies, included additional research on Italian earthquakes, and were extended to large earthquakes of the Mediterranean area. CFTI5Med comprised an opportunity to streamline the structure of the Catalogue database and propose a renovated user interface. The new front-end (1) grants an easier, intuitive access to the data, including earthquake effects on the environment, and (2) allows all data to be displayed jointly with relevant topographic, geological and seismological overlays published as web services., Design Type(s)data integration objective • source-based data analysis objectiveMeasurement Type(s)earthquakeTechnology Type(s)digital curationFactor Type(s)geographic location • temporal_intervalSample Characteristic(s)Mediterranean Region Machine-accessible metadata file describing the reported data (ISA-Tab format)

A double-correlation method is introduced to locate tremor sources based on stacks of complex, doubly-correlated tremor records of multiple triplets of seismographs back projected to hypothetical source locations in a geographic grid. Peaks in the resulting stack of moduli are inferred source locations. The stack of the moduli is a robust measure of energy radiated from a point source or point sources even when the velocity information is imprecise. Application to real data shows how double correlation focuses the source mapping compared to the common single correlation approach. Synthetic tests demonstrate the robustness of the method and its resolution limitations which are controlled by the station geometry, the finite frequency of the signal, the quality of the used velocity information and noise level. Both random noise and signal or noise correlated at time shifts that are inconsistent with the assumed velocity structure can be effectively suppressed. Assuming a surface-wave velocity, we can constrain the source location even if the surface-wave component does not dominate. The method can also in principle be used with body waves in three dimensions, although this requires more data and seismographs placed near the source for depth resolution.

Relative locationmethods are commonly used to precisely locate earthquake clusters consisting of similar waveforms. Repeating waveforms are often recorded at volcanoes, where, however, the crust structure is expected to contain strong heterogeneities and therefore the 1-D velocity model assumption that is made in most location strategies is not likely to describe reality. A peculiar cluster of repeating low-frequency seismic events was recorded on the south flank of Katla volcano (Iceland) from 2011. As the hypocentres are located at the rim of the glacier, the seismicity may be due to volcanic or glacial processes. Information on the size and shape of the cluster may help constraining the source process. The extreme similarity of waveforms points to a very small spatial distribution of hypocentres. In order to extract meaningful information about size and shape of the cluster, we minimize uncertainty by optimizing the cross-correlation measurements and relative-location process.With a synthetic test we determine the best parameters for differential-time measurements and estimate their uncertainties, specifically for each waveform. We design a location strategy to work without a pre-defined velocity model, by formulating and inverting the problem to seek changes in both location and slowness, thus accounting for azimuth, take-off angles and velocity deviations from a 1-D model. We solve the inversion explicitly in order to propagate data errors through the calculation. With this approach we are able to resolve a source volume few tens of metres wide in horizontal directions and around 100 metres in depth. There is no suggestion that the hypocentres lie on a single fault plane and the depth distribution indicates that their source is unlikely to be related to glacial processes as the ice thickness is not expected to exceed few tens of metres in the source area. Our method is designed for a very small source region, allowing us to assume a constant slowness for the whole cluster and to include the effects of 3-D heterogeneity such as refraction. Similar circumstances may arise in other volcanic regions with a high level of heterogeneity and where densely clustered earthquakes are often recorded.

Katla is a threatening volcano in Iceland, partly covered by the M\'yrdalsj\"okull ice cap. The volcano has a large caldera with several active geothermal areas. A peculiar cluster of long-period seismic events started on Katla's south flank in July 2011, during an unrest episode in the caldera that culminated in a glacier outburst. The seismic events were tightly clustered at shallow depth in the Gvendarfell area, 4 km south of the caldera, under a small glacier stream on the southern margin of M\'yrdalsj\"okull. No seismic events were known to have occurred in this area before. The most striking feature of this seismic cluster is its temporal pattern, characterized by regular intervals between repeating seismic events, modulated by a seasonal variation. Remarkable is also the stability of both the time and waveform features over a long time period, around 3.5 years. No comparable examples have been found in the literature. Both volcanic and glacial processes can produce similar waveforms and therefore have to be considered as potential seismic sources. Discerning between these two causes is critical for monitoring glacier-clad volcanoes and has been controversial at Katla. For this new seismic cluster on the south flank we regard volcano-related processes as more likely than glacial ones for the following reasons: 1) the seismic activity started during an unrest episode involving sudden melting of the glacier and a j\"okulhlaup; 2) the glacier stream is small and stagnant; 3) the seismicity remains regular and stable for years; 4) there is no apparent correlation with short-term weather changes, such as rain storms. We suggest that a small, shallow hydrothermal system was activated on Katla's south flank in 2011, either by a minor magmatic injection or by changes of permeability in a local crack system.