Deformation sources in volcanic areas are generally modeled in terms of pressurized tri-axial ellipsoids or pressurized cracks with simple geometrical shapes, embedded in a homogeneous half-space. ...However, the assumption of a particular source mechanism and the neglect of medium heterogeneities bias significantly the estimate of source parameters. A more general approach describes the deformation source in terms of a suitable moment tensor. Ratios between moment tensor eigenvalues are shown to provide a strong diagnostic tool for the physical interpretation of the deformation source and medium heterogeneities may be accounted for through 3D finite element computations. Leveling and EDM data, collected during the 1982–84 unrest episode at Campi Flegrei (Italy), are employed to retrieve the complete moment tensor according to a Bayesian inversion procedure, considering the heterogeneous elastic structure of the volcanic area. Best fitting moment tensors are found to be incompatible with any pressurized ellipsoid or crack. Taking into account the deflation of a deeper magma reservoir, which accompanies the inflation of a shallower source, data fit improves considerably but the retrieved moment tensor of the shallow source is found to be incompatible with pressurized ellipsoids, still. Looking for alternative physical models of the dislocation source, we find that the best fit moment tensor can be best interpreted in terms of a mixed mode (shear and tensile) dislocation at 5.5
km depth, striking EW and dipping by ~
25°–30° to the North. Gravity changes are found to be compatible with the intrusion of ~
60–70
·
10
6
m
3 of volatile rich magma with density ~
2400
kg/m
3.
Display omitted
► We model the 1982–84 uplift at Campi Flegrei by a general deformation source. ► The best model is a mixed mode crack at 5.5
km depth, striking EW, dipping 27°N. ► The numerical models include the elastic structure and a deflating sill at 7.5
km depth. ► The estimated intrusion density is ~
2400
kg/m
3 compatible with volatile rich magma.
We present a detailed analysis of the small magnitude (ML < 3) Reservoir Induced Seismicity associated with the Pertusillo water reservoir located in the high seismic hazard zone of Val d'Agri ...(Southern Italy). We apply template‐matching detection to a 13‐month‐long dense passive survey, obtaining a final high‐precision double‐difference catalog of 5,070 earthquakes (−0.7 < ML < 2.6, MC = 0.2). The new catalog allows precisely tracking the spatiotemporal distribution of the swarm‐seismicity and to map the b‐value of the Gutenberg‐Richter law. We combine seismicity data with available subsurface geophysical data, fostering an improved interpretation of the induced seismicity. We identify four seismicity‐clusters showing rapid changes in seismic rate which correlate to severe seasonal oscillations. Seismicity unravels new km‐scale faults or better define faults partially‐illuminated by template earthquakes. b‐value shows a significant spatial variability, with very‐high b‐value (up to 2) within areas of distributed seismicity and lower (∼1.3) b‐value for on‐fault seismicity featuring larger magnitude events. Seismicity is confined within a brine‐saturated fractured carbonate reservoir, while earthquake distribution and rate are controlled by the fault architecture and rock properties (e.g., inherited fluid pathways, rock fracturing, pore fluid pressure). In particular, most earthquakes reactivate, with extensional kinematics, pre‐existing reverse/transpressional faults favorably oriented in the present‐day extension. All observations suggest that a poroelastic stress transmission mechanism, due to the seasonal water level oscillation, can explain the Pertusillo lake seismicity. This study confirms the importance of investigating the complex interaction among stress changes caused by human activities, pre‐existing faults and local stress field to correctly assess the hazard posed by induced seismicity.
Plain Language Summary
The Val d'Agri basin in Southern Italy is characterized by a high seismic hazard as evidenced by the strong 1857 M7.1 earthquake, whose causative fault is still debated. The area is also a natural laboratory for studying anthropogenic seismicity since it hosts two well‐documented cases of seismicity induced by: (a) the exploitation of a giant oilfield with a clear case of wastewater injection‐induced‐seismicity and (b) the seasonal water level oscillation in the medium‐size Pertusillo water reservoir (PWR). We focus on the reservoir induced seismicity to better understand the physical mechanism behind its genesis and time‐space evolution. We applied a technique that allows us to extract smaller earthquakes from known ones (templates), increasing 10‐times the original catalog and helping to better characterize the spatiotemporal seismicity behavior. Seismicity is organized in four distinct clusters activated in response to the seasonal loading/unloading phases of the PWR. It delineates km‐scale faults and is confined within highly fractured aqueous carbonates. Our results furnish new data to understand how the stress induced by the lake level changes influences seismicity nucleation in this hazardous area, spurring for a more accurate estimation of the seismic hazard of the area.
Key Points
We use template‐matching to build an enhanced catalog of 5k earthquakes describing the Pertusillo reservoir induced seismicity
Seismicity is organized in four clusters featuring different b‐value and confined in an aqueous carbonate reservoir
Seismicity reactivates, in extension, inherited reverse faults and it is controlled by the physical properties of the fractured carbonates
SUMMARY
Einstein Telescope (ET) is a proposed underground infrastructure in Europe to host future generations of gravitational-wave (GW) detectors. One of its design goals is to extend the ...observation band of terrestrial GW detectors from currently about 20 Hz down to 3 Hz. The coupling of a detector to its environment becomes stronger at lower frequencies, which makes it important to carefully analyse environmental disturbances at ET candidate sites. Seismic disturbances pose the greatest challenge since there are several important mechanisms for seismic vibrations to produce noise in ET, for example, through gravitational coupling, stray light, or through harmful constraints on the design of ET’s control system. In this paper, we present an analysis of the time-variant properties of the seismic field at the Sardinia candidate site of ET connected to anthropogenic as well as natural phenomena. We find that temporal variations of source distributions and of the noise spectra generally follow predictable trends in the form of diurnal, weekly, or seasonal cycles. Specific seismic sources were identified such as road bridges, which produce observable disturbances underground. This information can be used to adapt a detector’s seismic isolation and control system.
Summary
The overall picture of Mount Etna deformation emerging since a couple of decades of geodetic surveys shows effects of magma accumulation, characterized by inflation/deflation cycle, ...accompanied by a sliding instability of the southeast flank, whose manifestation is an increase in the horizontal deformation away from the volcano summit. This is a very interesting case to test whether advanced models, taking into account topography, internal structure and frictional rheology, may contribute to a better understanding of the complex interplay among mechanical response, magmatic activity and gravitational load occurring in a volcanic system. Using finite element numerical models we make predictions of surface displacements associated with a simple expansion source and with a dike-like vertical discontinuity. A new methodology is developed to initialize the lithostatic stress field according to the material and geometrical complexities of the models considered. Our results show that, while an amplification of the horizontal displacement can be easily obtained up to a maximum distance of 10 km from the source, we have not been able to find any configuration to extend further this signal. For the case of Mount Etna this suggests that the large horizontal displacements observed in the east flank along the coast cannot be directly related to magma accumulation below the volcano's summit.
The increase of available seismic data prompts the need for automatic processing procedures to fully exploit them. A good example is aftershock sequences recorded by temporary seismic networks, whose ...thorough analysis is challenging because of the high seismicity rate and station density. Here, we test the performance of two recent Deep Learning algorithms, the Generalized Phase Detection and Earthquake Transformer, for automatic seismic phases identification. We use data from the December 2019 Mugello basin (Northern Apennines, Italy) swarm, recorded on 13 permanent and nine temporary stations, applying these automatic procedures under different network configurations. As a benchmark, we use a catalog of 279 manually repicked earthquakes reported by the Italian National Seismic Network. Due to the ability of deep learning techniques to identify earthquakes under poor signal‐to‐noise‐ratio (SNR) conditions, we obtain: (a) a factor 3 increase in the number of locations with respect to INGV bulletin and (b) a factor 4 increase when stations from the temporary network are added. Comparison between deep learning and manually picked arrival times shows a mean difference of 0.02–0.04 s and a variance in the range 0.02–0.07 s. The improvement in magnitude completeness is ∼0.5 units. The deep learning algorithms were originally trained using data sets from different regions of the world: our results indicate that these can be successfully applied in our case, without any significant modification. Deep learning algorithms are efficient and accurate tools for data reprocessing in order to better understand the space‐time evolution of earthquake sequences.
Plain Language Summary
We focus on the testing and comparison of recently published frameworks for seismic events and seismic waves detection based on deep learning. As an example we use data from an earthquake swarm that occurred in December 2019 in the Mugello basin (N of Florence, Italy). Earthquake swarms analysis is particularly challenging because of the high seismic rates, with events closely occurring in time and space. The results of the automatic analysis are compared with results reviewed by human analysts. The deep learning algorithms used in this application were originally trained on data collected from different regions of the world: Our results indicate their capability to apply the knowledge gained in different contexts to totally unseen data. However, some differences, due to the different design of the algorithms and to training data sets, arise and suggest that careful tuning and benchmarking are required to achieve stable and reliable results in seismic data analysis workflows.
Key Points
We analyze the Mw 4.5 Mugello (Italy) earthquakes sequence, December 2019, using deep learning algorithms, and compare with bulletin data
Deep learning performances on detections and phase picking works very well even without ad‐hoc retraining
Deep learning based earthquake catalogs show differences due to different algorithm designs
In this work we report the ongoing characterization of the Sos Enattos former mine (Sardinia, Italy), one of the two candidate sites for the Einstein Telescope (ET), the European third-generation ...underground interferometric detector of Gravitational Waves. The Sos Enattos site lies on a crystalline basement, made of rocks with good geomechanical properties, characterized by negligible groundwater. In addition, the site has a very low seismic background noise due to the absence of active tectonics involving Sardinia. Finally, the area has a low population density, resulting in a reduced anthropic noise even at the ground level. This location was already studied in 2012-2014 as a promising site for an underground detector. More recently, in March 2019, we deployed a new network of surface and underground seismometers at the site, that is currently monitoring the local seismic noise. Most of the energy carried by the seismic waves is due to the microseisms below 1 Hz, showing a significant correlation with the waves of the west Mediterranean sea. Above 1 Hz the seismic noise in the underground levels of the mine approaches the Peterson's low noise model. Exploiting mine blasting works into the former mine, we were also able to perform active seismic measurements to evaluate the seismic waves propagation across the area. In conclusion we also give a first assessment about the acoustic and magnetic noise in this underground site.
We investigate the 2004–2006 uplift phase of Campi Flegrei caldera (Italy) by exploiting the archive of ascending and descending ENVISAT SAR data acquired from November 2002 to November 2006. The ...SBAS‐DInSAR technique is applied to generate displacement mean velocity maps and time series. An appropriate post‐processing step is subsequently applied to map the areas whose temporal deformation behavior is correlated with that of the maximum uplift zone. Our results show that the deformation also extends outside the volcanological limits of the Neapolitan Yellow Tuff caldera, without significant discontinuities. The DInSAR data are inverted by considering a finite spheroid and an isotropic point‐source. The inversion results suggest that the new uplift is characterized by a source location similar to the previous small uplift event of 2000 and to the long term subsidence of the 1990's. In particular, the source is located at a depth of about 3.2 km and very close to the city of Pozzuoli (about 800 m offshore, to the SW); the associated volume variation is about 1.1 106 m3/year.
The 1982–1984 unrest episode at Campi Flegrei (CF) caldera, Italy, was characterized by huge deformation (more than 1.5 m uplift) concentrated inside the caldera. According to point source isotropic ...models in homogeneous elastic and visco-elastic half-spaces, the source depth is very shallow (∼3 km). If the source radius is ∼1 km, this implies that magma is at a depth of ∼2 km depth. However, several independent observations show that the top of the magma chamber at CF must be deeper than ∼4 km. This paper investigates how the inferred source depth increases when rigidity heterogeneities (obtained through seismic tomography at CF) are considered and when the long-term deformation takes place under drained conditions. Finite element models indicate that overpressure needed at the source to reproduce the 1.5 m maximum uplift is however beyond typical rock strength values. This evidence, together with the high thermal anomalies, the presence of fluids and the low cohesion of tuffs filling the caldera, suggests the use of elasto-plastic constitutive laws. For elasto-plastic behavior, the same deformation is obtained using a deeper source (with center at ∼5 km depth) and a lower overpressure (than required by elastic models). The plastic deformation concentrates both at the source boundaries and above the source, where seismic activity has been recorded. These results indicate that the rheological properties of the shallow crust of CF have important implications for hazard estimate during unrest episodes.
Third-generation gravitational wave observatories will extend the lower frequency limit of the observation band toward 2 Hz, where new sources of gravitational waves, in particular intermediate-mass ...black holes (IMBH), will be detected. In this frequency region, seismic noise will play an important role, mainly through the so-called Newtonian noise, i.e., the gravity-mediated coupling between ground motion and test mass displacements. The signal lifetime of such sources in the detector is of the order of tens of seconds. In order to determine whether a candidate site to host the Einstein Telescope observatory is particularly suitable to observe such sources, it is necessary to estimate the probability distributions that, in the characteristic time scale of the signal, the sensitivity of the detector is not perturbed by Newtonian noise. In this paper, a first analysis is presented, focused on the Sos Enattos site (Sardinia, Italy), a candidate to host the Einstein Telescope. Starting from a long data set of seismic noise, this distribution is evaluated considering both the presently designed triangular ET configuration and also the classical ”L” configuration.
The response of the Earth to the melting of the Late Pleistocene ice sheets is commonly studied by spherically layered models, based on well-established analytical methods. In parallel, a few models ...have been recently proposed to circumvent the limitations imposed by spherical symmetry, and to reproduce the actual structure of the lithosphere and of the upper mantle. Their main outcome is that laterally varying rheological structures may significantly affect various geophysical quantities related to glacial isostatic adjustment (GIA), and particularly post-glacial relative sea-level (RSL) variations and 3-D crustal velocities in formerly ice-covered regions. In this paper, we contribute to the ongoing debate about the role of lithospheric and mantle heterogeneities by new 3-D spherical Newtonian finite elements models and we directly compare their outcomes with publicly available global RSL data. This differs from previous investigations, in that have mainly focused on extensive sensitivity analyses or have considered a limited number of RSL observations from formerly glaciated regions and their periphery. In our study the lithospheric thickness mimics the global structure of the cratons based on geological evidence, and the upper mantle includes a low-viscosity zone beneath the oceanic lithosphere. We use two distinct global surface loads, based upon the ICE1 and ICE3G deglaciation chronologies, respectively. Our main finding is that using all of the available RSL observations in the last 6000 years it is not possible to discern between homogeneous and heterogeneous GIA models. This result, which holds for both ICE1 and ICE3G, suggests that the cumulative effects of laterally varying structures on the synthetic RSL curves cancel out globally, yielding signals that do not significantly differ from those based on the 1-D models. We have also considered specific subsets of the global RSL database, sharing similar geographical settings and distances from the main centres of deglaciation. When we consider the data from the margins of the Baltic region, a laterally varying lithospheric thickness improves significantly the agreement with the observations. This is not observed in other relevant situations, including the Hudson bay region. In the regions where the disagreement between predictions and observations is particularly evident, further investigations are needed to improve the geometry of the heterogeneous structures and of the surface ice-sheets distribution.
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