Spatial Distribution Of Seismicity Research Articles

Seismotectonic role of transversal structures in the Plio-Quaternary evolution of the external Marche Apennines (Italy)

We defined the seismotectonic setting of the external Marche Apennines (central Italy) by integrating geomorphological, structural and seismological data. The area is characterized by strong historical earthquakes, but the geometries and kinematics of the seismogenic sources have not previously been well defined. Plio-Quaternary Apennine compressional NW–SE structures are segmented by NE–SW-oriented transversal faults, the origin and role of which are still debated. We characterized the geometry, kinematics and activity of the four main transversal faults to better define their seismogenic potential. These high-angle, deeply rooted transversal fault systems have been mapped and separate the external Apennine sector into blocks and sub-blocks with different structural and evolutionary features. The integrated dataset showed that some inherited fault segments have recently been reactivated, displacing Quaternary deposits. The spatial distribution of seismicity indicates that some clusters of hypocentres are located within the basement. Stress field analysis using available focal mechanism solutions confirms the prevalence of left-lateral kinematics on roughly SW–NE-oriented structures. The transversal structures thus contribute to the longitudinal segmentation of the Apennine structures and, more generally, are involved in the northern part of the Adria plate kinematics towards the north and NNW. The assessment of seismic hazards and planning to mitigate risks in populated areas close to the Adriatic coast should consider these potentially active faults evidenced by instrumental seismicity and important historical earthquakes.

Deformation, seismicity, and monitoring response preceding and during the 2022 Fagradalsfjall eruption, Iceland

Following two periods of dike intrusion in 2021 at Fagradalsfjall, Iceland, one of which led to an eruption, a third dike intrusion commenced on 30 July 2022. A sudden increase in seismicity occurred within the diking area, with approximately 1700 automatically detected earthquakes > M1 within 24 h. Strong earthquakes were felt over several days within a wider area (largest MW 5.3). The timeline and spatial distribution of seismicity suggested it resulted from diking, together with triggered seismicity in nearby areas releasing stored tectonic stress. Geodetic observations revealed displacements consistent with a dike intrusion, and geodetic modeling on 2 August revealed a best-fit model with a shallow top depth of the dike (~1 km), and high magma inflow rate (~49 m3/s). Also considering a decline in seismicity, a warning was issued that the likelihood of a new eruption in the coming days was high. An effusive eruption started the next day (3 August) on a ~375-m-long fissure, with an initial extrusion rate of 32 m3/s. The projected surface location of the dike (from the optimal model) was within 49–110 m of the eruptive fissure. We present a timeline of the activity and monitoring response in the days both preceding and following the eruption onset. We compare the details of the activity that occurred prior to this diking and eruption to the previous events at Fagradalsfjall to improve understanding of unrest preceding eruptions.

Seismicity of the Western Sector of the Russian Arctic

The paper presents the results of many years of research on the compilation of a consolidated, refined, unified earthquake catalog for the western sector of the Russian Arctic for the entire instrumental observation period, in particular for the period from 1908 to 2020. The catalog includes data on recent seismicity in areas previously unavailable for detailed seismic monitoring. Based on the results of the study, we were able to more accurately determine the spatial distribution of seismicity in the region and compare the instrumental data with parameters of lineament-domain-focal (LDF, or, alternatively, lineament-domain-source (LDS)) models of the General Seismic Zoning maps (GSZ, from Russian term abbreviated as OSR)—OSR-97 and OSR-2016. In the western sector of the Russian Arctic, the highest seismicity is observed in the continent-ocean transition zone, in the Belyi Island, Novaya Zemlya and Severnaya Zemlya archipelagos. The very shelf of the Barents and Kara Seas is characterized by rare and scattered seismicity. The configuration, the maximum possible magnitudes, and the depths of the seismogenic layers in the LDF models of OSR-97 and OSR-2016 maps are not always consistent with the instrumental data and need a correction. The consolidated refined earthquake catalog can be used as a basis for the further research aimed at seismic hazard assessment of the territory, construction of geodynamic models, and study of the stress-strain state of the Earth’s crust.

Statistical power of spatial earthquake forecast tests

SUMMARYThe Collaboratory for the Study of Earthquake Predictability (CSEP) is an international effort to evaluate earthquake forecast models prospectively. In CSEP, one way to express earthquake forecasts is through a grid-based format: the expected number of earthquake occurrences within 0.1° × 0.1° spatial cells. The spatial distribution of seismicity is thereby evaluated using the Spatial test (S-test). The high-resolution grid combined with sparse and inhomogeneous earthquake distributions leads to a huge number of cells causing disparity in the number of cells, and the number of earthquakes to evaluate the forecasts, thereby affecting the statistical power of the S-test. In order to explore this issue, we conducted a global earthquake forecast experiment, in which we computed the power of the S-test to reject a spatially non-informative uniform forecast model. The S-test loses its power to reject the non-informative model when the spatial resolution is so high that every earthquake of the observed catalog tends to get a separate cell. Upon analysing the statistical power of the S-test, we found, as expected, that the statistical power of the S-test depends upon the number of earthquakes available for testing, e.g. with the conventional high-resolution grid for the global region, we would need more than 32 000 earthquakes in the observed catalog for powerful testing, which would require approximately 300 yr to record M ≥ 5.95. The other factor affecting the power is more interesting and new; it is related to the spatial grid representation of the forecast model. Aggregating forecasts on multi-resolution grids can significantly increase the statistical power of the S-test. Using the recently introduced Quadtree to generate data-based multi-resolution grids, we show that the S-test reaches its maximum power in this case already for as few as eight earthquakes in the test period. Thus, we recommend for future CSEP experiments the use of Quadtree-based multi-resolution grids, where available data determine the resolution.

Source process of two Mw 6.9 aftershocks of the 2015 Mw 8.3 Illapel earthquake

The rupture of the September 16, 2015, Mw 8.3 Illapel earthquake extended over 200 km length along strike, where the southern portion of the rupture area is controlled tectonically by the collision of the Juan Fernandez Ridge at ∼32.5°S. On November 11th, two large aftershocks occurred in less than an hour, at shallow depths and epicenters closely located. Both events occurred beyond, but nearby, the northern limit of the Illapel rupture. We inverted kinematic rupture models by joint inversion of strong-motion and teleseismic body waves datasets. The Akaike's Bayesian Information Criterion (ABIC) was used to estimate the relative weighting between datasets, as well as the weighting of spatial and temporal constraints. The rupture of the first event propagated updip from the hypocenter, and later in a bilateral mode, with large slip to the south. The slip solution yields a total seismic moment of 4.44 × 1019 N m (Mw 7.0), and peak-slip of 0.82 m. The epicenter of the second event relocates 15 km southwest with respect to the first one. The rupture of the second event propagated mainly southwards and updip to the trench, re-rupturing an area already broken by the first event. The estimated seismic moment is 3.11 × 1019 N m (Mw 6.9), with a maximum slip of 0.59 m. The source durations from their respective moment-rate functions are 35 s, in both models. The spatial slip distribution retrieved for the two events suggests that the upper portion of the megathrust slipped, a region that is usually thought to behave aseismically, controlled by velocity-strengthening friction. We also relocated the earthquake catalog using a double-difference algorithm. The spatial distribution of seismicity shows clusters that are analyzed in different groups, and the most remarkable feature being a strip running parallel - and close by - to the trench region with very low seismicity. The majority of regional moment tensor solutions are thrust faulting mechanisms, and given the centroid depths retrieved, these events are consistent with interplate seismicity.

Complex fault geometry of the 1976 Ms 7.8 Tangshan earthquake source region in North China

Investigating the fine structure of fault zones is essential for understanding the mechanism of large earthquake nucleation. In this paper we study the complex fault geometry of the Tangshan fault zone in North China using a new seismic data set from 2010 to 2020 that spans ∼10 years. Based on the well-located hypocenters and focal mechanism solutions, we obtained more detailed information on the fault zone structural features: (1) the fault zone is divided into five segments according to different dip angles of the fault plane; (2) the spatial distribution of seismicity reveals conjugate fault branches and a flower-shaped structure at the northern part of the fault zone; and (3) the fault zone is dominated by a strike-slip type, but their intersecting segments are characterized by a normal-fault type. We further obtained high-resolution seismic images of Vp, Vs and Vp/Vs around the Tangshan source region. Combining the fault geometry with the low-velocity anomalies below 10 km depth, we infer that the ascending of hot mantle material into the lower crust may have weakened the Tangshan fault zone and generate the 1976 Ms. 7.8 Tangshan earthquake.

3-D spatial cluster analysis of seismic sequences through density-based algorithms

SUMMARY With seismic catalogues becoming progressively larger, extracting information becomes challenging and calls upon using sophisticated statistical analysis. Data are typically clustered by machine learning algorithms to find patterns or identify regions of interest that require further exploration. Here, we investigate two density-based clustering algorithms, DBSCAN and OPTICS, for their capability to analyse the spatial distribution of seismicity and their effectiveness in discovering highly active seismic volumes of arbitrary shapes in large data sets. In particular, we study the influence of varying input parameters on the cluster solutions. By exploring the parameter space, we identify a crossover region with optimal solutions in between two phases with opposite behaviours (i.e. only clustered and only unclustered data points). Using a synthetic case with various geometric structures, we find that solutions in the crossover region consistently have the largest clusters and best represent the individual structures. For identifying strong anisotropic structures, we illustrate the usefulness of data rescaling. Applying the clustering algorithms to seismic catalogues of recent earthquake sequences (2016 Central Italy and 2016 Kumamoto) confirms that cluster solutions in the crossover region are the best candidates to identify 3-D features of tectonic structures that were activated in a seismic sequence. Finally, we propose a list of recipes that generalizes our analyses to obtain such solutions for other seismic sequences.

Nonlinear Statistical Features of the Seismicity in the Subduction Zone of Tehuantepec Isthmus, Southern México.

After the M8.2 main-shock occurred on 7 September 2017 at the Isthmus of Tehuantepec, Mexico, the spatial distribution of seismicity has showed a clear clusterization of earthquakes along the collision region of the Tehuantepec Transform/Ridge with the Middle America Trench off Chiapas. Furthermore, nowadays, the temporal rate of occurrence in the number of earthquakes has also showed a pronounced increase. On the basis of this behavior, we studied the sequence of magnitudes of the earthquakes which occurred within the Isthmus of Tehuantepec in southern Mexico from 2010 to 2020. Since big earthquakes are considered as a phase transition, after the M8.2 main-shock, one must expect changes in the Tehuantepec ridge dynamics, which can be observed considering that the b-value in the Gutenberg–Richter law, has also showed changes in time. The goal of this paper is to characterize the behavior of the seismic activity by using the Gutenberg–Richter law, multifractal detrended fluctuation analysis, visibility graph and nowcasting method. Those methods have showed important parameters in order to assess risk, the multifractality and connectivity. Our findings indicate, first that b-value shows a dependency on time, which is clearly described by our analyses based on nowcasting method, multifractality and visibility graph.

On the Depth of Earthquakes in the Delaware Basin: A Case Study along the Reeves–Pecos County Line

Abstract Increased rates of seismicity in the Delaware basin, Texas, accompanying unconventional petroleum development have created intensive interest in determining their cause. Detailed and accurate spatial distribution of seismicity and focal mechanisms are critical components for understanding the underlying industrial processes responsible for inducing seismicity. We focus on a highly seismically active area straddling the Reeves–Pecos County line where two TexNet stations sit atop the seismicity, which includes 21 ML 3+ events from 2017 to 2020 (Advanced National Seismic System Comprehensive Earthquake Catalog). Short epicentral distance enables us to reliably estimate the hypocentral depth using seismic phase picks and standard location methods. We use a deep-learning-based method to detect earthquakes and time the phase arrivals. Hypocentral locations computed in a velocity model constrained by local well data reveal that the seismicity concentrates between 1.5 and 2.5 km below ground in the Delaware Mountain Group, the primary wastewater disposal zone at this location. Waveform inversions for the moment tensor and focal depth independently confirm the shallow depths. The moment tensor solutions define critically stressed high-angle normal faults, suggesting a causal connection between injection and seismicity.

Poroelastic model in a vertically sealed gas storage: a case study from cyclic injection/production in a carbonate aquifer

SUMMARY Natural gas can be temporarily stored in a variety of underground facilities, such as depleted gas and oil fields, natural aquifers and caverns in salt rocks. Being extensively monitored during operations, these systems provide a favourable opportunity to investigate how pressure varies in time and space and possibly induces/triggers earthquakes on nearby faults. Elaborate and detailed numerical modelling techniques are often applied to study gas reservoirs. Here we show the possibilities and discuss the limitations of a flexible and easily formulated tool that can be straightforwardly applied to simulate temporal pore-pressure variations and study the relation with recorded microseismic events. We use the software POEL (POroELastic diffusion and deformation) which computes the poroelastic response to fluid injection/extraction in a horizontally layered poroelastic structure. We further develop its application to address the presence of vertical impermeable faults bounding the reservoir and of multiple injection/extraction sources. Exploiting available information on the reservoir geometry and physical parameters, and records of injection/extraction rates for a gas reservoir in southern Europe, we perform an extensive parametric study considering different model configurations. Comparing modelled spatiotemporal pore-pressure variations with in situ measurements, we show that the inclusion of vertical impermeable faults provides an improvement in reproducing the observations and results in pore-pressure accumulation near the faults and in a variation of the temporal pore-pressure diffusion pattern. To study the relation between gas storage activity and recorded local microseismicity, we applied different seismicity models based on the estimated pore-pressure distribution. This analysis helps to understand the spatial distribution of seismicity and its temporal modulation. The results show that the observed microseismicity could be partly linked to the storage activity, but the contribution of tectonic background seismicity cannot be excluded.

Large-scale seismic seafloor stability analysis in the South China Sea

Earthquakes are considered as the major trigger of submarine landslides. This study analyzes the large-scale seismic seafloor stability in the South China Sea (SCS), taking into consideration the earthquake probability and spatial distribution of seismicity and seafloor soil properties. For this cause, the geographical, seismic, and seafloor soil characteristics are first determined. The digital elevation model (DEM) of the study area is established using the geographic information system (GIS). Large-scale Peak Ground Acceleration (PGA) maps under different exceedance probabilities (EPs) are derived using the Chinese Probability Seismic Hazard Analysis (CPSHA) method. Three-dimension continuous models for seafloor soil properties are obtained using a proposed best-fit distribution-based Kriging method. With this information, the large-scale seafloor stability under different EPs is evaluated with an infinite slope model in the study area of the SCS. The proposed large-scale seismic seafloor stability is validated against recorded earthquake-induced submarine landslides. The analysis shows that the seafloor stability is found to be highly nonuniform, and the study area can be divided into three main parts based on their distinct seafloor stabilities. This study sheds new light on the large-scale seafloor stability, and can assist the development of ocean engineering projects in the SCS.

Seismic Hazards Assessment in the Eastern Himalayas Region

Spatial distribution of seismicity and seismic characteristics (fractal dimension, b-value, energy release, reoccurrence period) are assessed for the Himalayan Region (27° – 30°N and 85°– 97°E). The database consists of relocated earthquakes M ≥ 3.8 selected for the period 1964-2017 from the International Seismological Centre (ISC) catalogues (Engdahl, van der Hilst, and Buland (EHB) 2007). The <i>Gutenberg - Richter</i> frequency-magnitude relation (b-value) is calculated by the Maximum Likelihood Method (MLM) and by Least Square Method. The fractal dimension is estimated using the correlation integral method. The entire event set was also used for estimating radiated energy in the region. Four probabilistic models namely, <i>Weibul, Gamma, Lognormal and Exponential</i> have been used to estimate the probability of the occurrence of moderate earthquakes (M ≥ 5.5 and M ≥ 6.5) during a specified interval of time using the Maximum Likelihood Estimates (MLE) for estimating the model parameters. The highly stressed zones in the entire region are indicated by low b-values, low fractal dimension and low radiated energy. The vulnerable zones (Arunachal Himalayas, Mishami thrust zone) have been identified by these maps which are further corroborated with the probabilistic models to assess the seismic hazards in the Himalaya region. These areas are indicative of future probable earthquakes regions.

Fault network reconstruction using agglomerative clustering: applications to southern Californian seismicity

Abstract. In this paper we introduce a method for fault network reconstruction based on the 3D spatial distribution of seismicity. One of the major drawbacks of statistical earthquake models is their inability to account for the highly anisotropic distribution of seismicity. Fault reconstruction has been proposed as a pattern recognition method aiming to extract this structural information from seismicity catalogs. Current methods start from simple large-scale models and gradually increase the complexity trying to explain the small-scale features. In contrast the method introduced here uses a bottom-up approach that relies on initial sampling of the small-scale features and reduction of this complexity by optimal local merging of substructures. First, we describe the implementation of the method through illustrative synthetic examples. We then apply the method to the probabilistic absolute hypocenter catalog KaKiOS-16, which contains three decades of southern Californian seismicity. To reduce data size and increase computation efficiency, the new approach builds upon the previously introduced catalog condensation method that exploits the heterogeneity of the hypocenter uncertainties. We validate the obtained fault network through a pseudo prospective spatial forecast test and discuss possible improvements for future studies. The performance of the presented methodology attests to the importance of the non-linear techniques used to quantify location uncertainty information, which is a crucial input for the large-scale application of the method. We envision that the results of this study can be used to construct improved models for the spatiotemporal evolution of seismicity.

Influence of Lithosphere Rheology on Seismicity in an Intracontinental Rift: The Case of the Rhine Graben

The Upper Rhine Graben (URG), as a part of the wider European Cenozoic Rift System, is a tectonically active area that has been extensively investigated for its geothermal energy potential. In this study, we carry out a first investigation of the present-day thermo-mechanical stability of the area as based on a detailed 3D geological and thermal model. The overall goal is, therefore, to assess how the lithospheric strength varies within the URG in response to the natural tectonic setting as well as the internal thermal configuration, and how those variations can be related to the recorded seismicity. The results from the modelling indicate that there is a spatial correlation between the predictions for the graben-wide rheological configuration with both the deep thermal field and the configuration of the crystalline crust. We find that the regional characteristics of the long-term strength of the lithosphere match the spatial distribution of seismicity, indicating that the mechanical stability of the area is primarily controlled by resolved strength variations. By cross-plotting the modelled strength distribution with available seismicity catalogues, our results suggest that seismicity in the graben area is shallower and of lower intensity due to a hotter and weaker crust compared to its surrounding areas. In contrast, seismic energy release appears to occur at deeper levels and being of larger magnitudes east of the graben and in the adjacent Lower Rhine Graben to the north. These results demonstrate the relevance of a proper quantification of the lithospheric rheological configuration and its spatial variability in response to its tectonic inheritance as an asset to interpret the pattern and distribution of seismicity observed in the area.

One-off deep crustal earthquake swarm in a stable intracontinental region of the southwestern Korean Peninsula

A deep crustal strike-slip earthquake swarm that occurred in the intracontinental region of the southwestern Korean Peninsula is investigated. The earthquakes are detected using matched filter analysis. The hypocenters are then refined, showing more than 500 micro to small strike-slip earthquakes concentrated on a fault plane with dimensions of 500 m by 300 m at a depth of ~21 km. The low temperature in the source depth facilitates the seismicity. The earthquake swarm was transiently active for 15 days from 25 April 2020 to 12 May 2020, presenting a high seismicity rate with a Gutenberg-Richter b value of 2.01 (±0.05). The earthquake swarm began in the central area and expanded outward with time. The spatiotemporal seismicity distribution reflects bilateral rupture. The focal mechanism solutions suggest a left-lateral strike-slip fault with a strike of 103.6∘ ± 3.2∘ and a dip of 73.3∘ ± 11.7∘. The spatial distribution of seismicity generally agrees with the focal mechanism solutions. The orientation of the inverted stress field agrees with the ambient stress field, suggesting that the earthquake swarm occurred as a consequence of ambient lithostatic stress loading. Coulomb stress changes suggest that the stress increased in the directions of WNW-ESE and NNE-SSW from the fault. The transiently active earthquake swarm began to occur after the 2011 MW9.0 Tohoku-Oki megathrust earthquake, suggesting a rapid release of accumulated stress in response to changes in the properties of the medium caused by the megathrust earthquake.

The 3D thermal field across the Alpine orogen and its forelands and the relation to seismicity

Abstract Temperature exerts a first order control on rock strength, principally via thermally activated creep deformation and on the distribution at depth of the brittle-ductile transition zone. The latter can be regarded as the lower bound to the seismogenic zone, thereby controlling the spatial distribution of seismicity within a lithospheric plate. As such, models of the crustal thermal field are important to understand the localisation of seismicity. Here we relate results from 3D simulations of the steady state thermal field of the Alpine orogen and its forelands to the distribution of seismicity in this seismically active area of Central Europe. The model takes into account how the crustal heterogeneity of the region effects thermal properties and is validated with a dataset of wellbore temperatures. We find that the Adriatic crust appears more mafic, through its radiogenic heat values (1.30E-06 W/m3) and maximum temperature of seismicity (600 °C), than the European crust (1.3–2.6E-06 W/m3 and 450 °C). We also show that at depths of

Crustal Stress in the Northern Red Sea Region as Inferred from Seismic b-values, Seismic Moment Release, Focal Mechanisms, Gravity, Magnetic, and Heat Flow Data

This study explores the spatial distribution of seismicity, the pattern of b-values, gravity and magnetic anomalies, heat flow data, and focal mechanism solutions to more thoroughly understand the present-day stress distribution and the nature of the crust, which characterize the rifting process in the northern Red Sea region. The region shows alternating low and high b-values in congruence with low and high cumulative seismic moment releases, respectively, and with negative and positive gravity anomalies, respectively. In general, except for larger-sized earthquake locations that characterized by high stress accumulation, low b-values, low seismic moment release, and low-gravity anomalies dominate the zones of the Gulf of Aqaba and the Gulf of Suez, thereby implying low stress accumulation consistent with an old crust of no differential development. Conversely, the rest of the Northern Red Sea region exhibited heterogeneities in the spatial distribution of b-values, cumulative seismic moment releases, and gravity anomalies, thereby implying stress heterogeneities. The stress heterogeneities may impute to the differences in material properties of the upper crust in the region. Zones of a positive Bouguer anomaly and high stress are notably associated with a relatively weak crust characterized by high rate of seismicity, while zones of the negative Bouguer anomaly and low stress characterized an older and more stable crust. However, details on the nature of crust required additional geophysical and geological data of high resolution in future studies.

Effects of imparted Coulomb stress changes in the seismicity and cluster of the December 2017 Hojedk (SE Iran) triplet

The Hojedk area in the southeastern Iran experienced three earthquakes during 12 days in December 2017. These events occurred in proximity to a region that has experienced several moderate and large events in the past. In this paper, the role of Coulomb stress changes in occurrence of the Hojedk cluster was investigated, and also the produced stress changes due to these events on the surrounding and optimally oriented faults was calculated. Previous earthquakes (events in the Golbaf–Sirch region, the Shahdad slip, and the Dahuiyeh–Zarand earthquake) imparted positive stress changes on the fault planes of the Hojedk earthquakes, especially on the ruptured plane of the first main shock. The Hojedk first main shock triggered the second one by imparting maximum stress changes of about 11.7 MPa on the fault plane of this event, and these two events brought the third main shock to failure by producing more than 6.0 MPa stress changes on its ruptured plane. Among the major active faults in the surrounding area, the middle part of the Nayband fault, northern part of the Golbaf–Sirch fault, and southern part of the Kuhbanan fault received positive stress changes and southen parts of the Ravar and all parts of the Chatrud faults received negative stress changes due to the Hojedk earthquakes. Furthermore, our results for correlation between Coulomb stress changes and seismicity distribution showed that following the Dahuiyeh–Zarand earthquake, the majority of the seismicity located on the positive stress area and Coulomb stress changes have a controlling role on the spatial distribution of seismicity even after a decade. The occurrence of the Hojedk first main shock affects the spatial distribution of aftershocks until the occurrence of the next main hocks. By triggering the next one, the stress patterns change in the area, and seismicity follows from the stress patterns of the last event.

Seismic sources and Earth structure in the transition zone between Fore-Balkan unit and Moesian platform, NE Bulgaria

Investigations of hydraulically induced micro-seismicity are typically connected with magma injections, exploitation of geothermal fields and salt mines. Our study examines the seismicity and seismic sources in a peri-platform region with long salt production history. In addition, a very large site for extraction of aggregates and limestone is presented in the area. Increased seismicity in the region has been observed for several decades as during the study period of the last 12 years, more than 1000 events with magnitude $$M < 4.3$$ have been registered. To study the complex seismic sources concentrated in such small area spatial distribution of seismicity and fault plane solutions are analyzed together with available geological and geophysical data, in order to connect registered earthquakes swarms with geological structures. Results show that several faults are outlined from geological information and confirmed by interpretation of geophysical data. Very different focal mechanisms calculated for the 13 earthquakes in the near region of the salt production site cannot propose a dominant fault mechanism and confirm the complexity of the seismic sources in the area. Our results suppose that activation of some of these structures is more likely due to water injected in the salt dome as part of the production process, which changes the stress equilibrium in the Earth crust.

Volcanic structures investigation through SAR and seismic interferometric methods: The 2011–2013 Campi Flegrei unrest episode

Observations from satellites provide high-resolution images of ground deformation allowing to infer deformation sources by developing advanced modeling of magma ascent and intrusion processes. Nevertheless, such models can be strongly biased without a precise model of the internal structure of the volcano. In this study, we jointly exploited two interferometric techniques to interpret the 2011–2013 unrest at Campi Flegrei caldera (CFc). The first is the Interferometric Synthetic Aperture Radar (InSAR) technique, which provides highly-resolved spatial and temporal images of ground deformation. The second is the Ambient Noise Tomography (ANT), which images subsurface structures, providing the constraints necessary to infer the depth of the shallow source at CFc (between 0.8 and 1.2 km). We applied for the first time a tool to delineate the deformation source boundaries from the observed deformation maps: the Total Horizontal Derivative (THD) technique. The THD processes the vertical component of the ground deformation field detected through InSAR applied to COSMO-SkyMed data. The patterns retrieved by applying the THD technique show consistent spatial correlations with (1) the seismic group-velocity maps achieved through the ANT and (2) the distribution of the earthquakes nucleated during the unrest at ~1 km. High-velocity anomalies, the retrieved geometrical features of the deformation field, and the spatial distribution of seismicity coincide with extinct volcanic vents in the eastern part of the caldera (Solfatara/Pisciarelli and Astroni). Such a coincidence hints at a significant role of the extinct plumbing system in either constraining or channeling the eastward propagation of magmatic fluids. Here, we demonstrated that a joint analysis of the InSAR patterns, seismic structures, and seismicity allows us to model in space and time the characteristics and nature of the shallow deformation source at CFc. Using published literature, we show that the effects of structural heterogeneities at shallow depths may have a more significant early-stage impact on the evolution of the surface displacement signals than deeper magmatic sources: these secondary structural effects may produce local amplification in the deformation records which can be mistakenly interpreted as early signals of impending eruptions. The achieved results are particularly relevant for the understanding of the origin of deformation signal at volcanoes where magma propagation within sills is expected, as at CFc.