Stefano Solarino

Stress tensor orientation derived from fault plane solutions in the southwestern Alps

Gephart and Forsyths method has been applied to estimate stress orientations from earthquake fault plane solutions of the southwestern Alps, a region where the tectonic stress regime is known to be fairly complex. Fault plane solutions have been either taken from the literature or computed using data from local and regional networks. Data refer to seismic events of magnitude in the range 2.5–5.3 which occurred in the last decades at depths between 0 and 25 km. Two zones with a different stress orientation have been identified in the studied area (44.0°–45.5°N, 6.5°–8.5°E): the western zone, corresponding to the crest of the alpine belt, where a high-dip maximum compressive stress is found, and the eastern zone (Alps chain to Po Plain transition), characterized by an almost horizontal E-W σ1 and a nearly vertical σ3. Hypocenters of earthquakes used for stress inversion lie in the depth ranges 0–15 km and 5–25 km in the western and eastern zones, respectively. The transition between the two stress domains is very sharp, and this is also indicated by space distribution of earthquake individual misfits to the respective stress models. The findings of the present study are a good match for tectonic models which assume E-W compression derived from the Adria-Europe interaction and producing: (1) major thrusting processes in the eastern side of the chain and (2) secondary tensional effects at very shallow depth beneath the alpine belt crest (western zone of the area studied in this work).

First seismic evidence for continental subduction beneath the Western Alps

The first discovery of ultrahigh-pressure coesite in the European Alps 30 years ago led to the inference that a positively buoyant continental crust can be subducted to mantle depth; this had been considered impossible since the advent of the plate tectonics concepts. Although continental subduction is now widely accepted, there remains debate because there is little direct (geophysical) evidence of a link between exhumed coesite at the surface and subducted continental crust at depth. Here we provide the first seismic evidence for continental crust at 75 km depth that is clearly connected with the European crust exactly along the transect where coesite was found at the surface. Our data also provide evidence for a thick suture zone with downward-decreasing seismic velocities, demonstrating that the European lower crust underthrusts the Adriatic mantle. These findings, from one of the best-preserved and long-studied ultrahigh-pressure orogens worldwide, shed decisive new light on geodynamic processes along convergent continental margins.

Litho—asthenospheric structures of northern Italy as inferred from teleseismic P-wave tomography

Abstract Regional three-dimensional inversions of teleseismic P-wave travel time residuals recorded by high-frequency regional and local seismic networks operating along the Western Alps and surrounding regions were carried out and lithosphere and upper mantle P-wave velocity models down to 300 km were obtained. Residuals of more than 500 teleseismic events, recorded by 98 fixed and temporary seismic stations, have been inverted. The comparison between real residuals and the ones obtained from tomographic model indicates that the method is able to solve the feature of the regional heterogeneities. Where the resolution is good, coherent lithospheric and upper mantle structures are imaged. In the shallower layers, high- and low-velocity anomalies follow the structural behaviour of the Alpine-Apenninic chains showing the existence of very strong velocity contrasts. In the deepest layers, velocity contrast decreases however two deep-seated high-velocity structures are observed. The most extended in depth and approximately trending NE-SW has been interpreted as a wreck of the oldest subduction responsible of the Alpine orogenesis. The second one, connected to the northwestern sector of the Apenninic chain, appears to vanish at depths greater than 180 km and is probably due to still active Apenninic roots. Cross-sections depict the spatial trend of perturbations and in particular outline the sub-vertical character of the Alpine and Apenninic anomalies. Under the Ligurian Sea, the 3-D inversion confirms the uplift of the asthenosphere in agreement with the tectonic evolution of the basin.

Continuity of the Alpine slab unraveled by high-resolution P wave tomography

The question of lateral and/or vertical continuity of subducted slabs in active orogens is a hot topic partly due to poorly resolved tomographic data. The complex slab structure beneath the Alpine region is only partly resolved by available geophysical data, leaving many geological and geodynamical issues widely open. Based upon a finite-frequency kernel method, we present a new high-resolution tomography model using P wave data from 527 broadband seismic stations, both from permanent networks and temporary experiments. This model provides an improved image of the slab structure in the Alpine region and fundamental pinpoints for the analysis of Cenozoic magmatism, (U)HP metamorphism, and Alpine topography. Our results document the lateral continuity of the European slab from the Western Alps to the central Alps, and the downdip slab continuity beneath the central Alps, ruling out the hypothesis of slab break off to explain Cenozoic Alpine magmatism. A low-velocity anomaly is observed in the upper mantle beneath the core of the Western Alps, pointing to dynamic topography effects. A NE dipping Adriatic slab, consistent with Dinaric subduction, is possibly observed beneath the Eastern Alps, whereas the laterally continuous Adriatic slab of the Northern Apennines shows major gaps at the boundary with the Southern Apennines and becomes near vertical in the Alps-Apennines transition zone. Tear faults accommodating opposite-dipping subductions during Alpine convergence may represent reactivated lithospheric faults inherited from Tethyan extension. Our results suggest that the interpretations of previous tomography results that include successive slab break offs along the Alpine-Zagros-Himalaya orogenic belt might be proficiently reconsidered.

KnowRISK on Seismic Risk Communication: The Set-Up of a Participatory Strategy- Italy Case Study

KnowRISK (Know your city, Reduce seISmic risK through non-structural elements) is a European project that addresses prevention measures to reduce non-structural damage caused by earthquakes. It is built on risk communication and takes action on pilot areas of the three participating countries: Portugal, Iceland, and Italy. The setting up of risk communication strategies in the project stands on the understanding local communities fragility, on their direct engagement, and on a holistic approach to vulnerability. The level of relevance of seismic compared to other hazards, the understanding, the memory of past disasters are indicators that affect the way a risk is perceived and preventive measures are taken. Similarly, the level of education, wealth, exposure to other, social, risks are aggravation parameters in risk computation to be accounted for when we communicate risk. Strategies for risk communication in KnowRISK rely on schools and citizen’s engagement, citizen’s science activities, tools for raising awareness.

KnowRISK Practical Guide for Mitigation of Seismic Risk Due to Non-structural Components

Good performance of non-structural elements can be decisive in saving lives and costs when an earthquake strikes. The European project KnowRISK aims to educate and encourage households to take the necessary precautionary measures to protect people, houses, and contents. Preparedness and prevention act on community resilience. Within the KnowRISK project, the idea of a Practical Guide has been conceived suggesting seismic mitigation solutions for non-structural components to non-experts stakeholders. It is intended to guide people into the first steps of prevention in a straightforward manner, minimizing or avoiding injuries, damage, and long-term financial consequences. The novelty of the Guide belongs to his philosophy: a path through increasing challenges corresponds to a growing level of safety. The idea is that anyone can mitigate seismic risk in its own environment by adopting simple and low cost measures. The Practical Guide may contribute to increase risk awareness. This kind of initiatives if undertaken at larger scales may also enhance social resilience.