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To determine the crustal rock rheological properties and model wave propagation in an anelastic attenuating medium, it is necessary to determine the quality factor Q, which expresses the fraction of friction-dissipated energy to total seismic energy. Measuring time-broadening of the first P- and S-wave pulses, we propose a time-domain method to estimate the frequency-independent Q parameter of body waved from microearthquake records. We assume a uniform velocity, circular rupture model as represented by a triangular moment rate/displacement function, whose attenuated velocity pulse widths are analyzed in the near-source distance range. The attenuated velocity pulse width data allow the calculation of the source parameters, including rupture duration/radius and stress drop values, as well as the attenuation factor t* (travel distance/quality factor), used to determine the attenuation structure in the study area. It is noted that the constant coefficient of the pulse-width vs t* relationship, required for calculating the t* catalog, have been calibrated for a triangular displacement waveform through simulation analysis. An evaluation of the methodology was carried out on 126 micro-events with Mw ranging from 1 to 3 located around the PRATI-9 and PRATI-29 injection wells at the Geyser geothermal field, California. The analysis of the P- and S-waves indicates a Qp range of 55 to 100 and a Qs range of 89 to 189. To validate the the t* data, we have inverted them to obtain a 1D QP model that matches consistently with the profiles derived from existing tomographic QP models in the area.

The rupture process of the recent moderate-to-large earthquakes in the longest seismic sector in Iran's plateau, the Zagros area, has been modeled using the strong motion data provided by the Iranian Building and Housing Research Center (BHRC). The selected dataset includes the largest and deadliest seismic event, the 2017 Mw 7.3, Sarpol-e Zahab earthquake. The earthquake source parameters (moment magnitude, source duration, rupture dimension, and average stress drop) are determined by implementing a parametric modeling technique in the time domain based on the time evolution of the P-wave displacement signals. The seismic source parameters are calculated from simulated trapezoidal and triangular moment-rate functions assuming the unilateral rectangle and circular crack models, respectively, where the rupture propagates at a constant velocity as a fraction (90%) of the average shear-wave velocity in the medium. The anelastic attenuation effect assuming the independent frequency-Q parameter ranging from 50 to 200 is accounted for by a post-processing procedure that retrieved the observed moment-rate triangular shape. Hence, the average stress drop with different varies between =1.50 (1.14−1.95) and =0.90 (0.71−1.14) MPa. Assuming a circular rupture model for Sarpol-e-Zahab, we estimate a moment magnitude of 7, rupture duration of 7 seconds, source radius of 16 km, and statistical stress drop of about 3.5 MPa. Alternatively, a unilateral rupture model calculates the fault length and width at 45 and 16 km, with a lower stress drop of 2 MPa.

Non-linear response of the elastic properties of the crust has been studied using the analysis of seismic velocity variations induced by various natural forcing agents (earthquakes, tides, volcanic processes, and others). Here we study 1) the variations of seismic velocities in response non-tectonic deformations associated to phases of groundwater recharge/discharge in large karstic aquifers in the Southern Apennines of Italy and discuss 2) the implications in terms of non-elastic behavior of the crust. Karst systems are complex aquifers common within the carbonate rocks of the Apennines. They store large amount of groundwater producing significant horizontal dilatational strains that modulate the secular, tectonic deformation (~3 mm/yr extension across the Apennines) and background seismicity (Silverii et al., 2019; D’Agostino et al., 2018) with seasonal and multi-seasonal signatures. The availability of accurate and temporally-long hydrological measurements (rainfall and karst spring discharge) in addition to dense seismic and geodetic networks provide the opportunity to assess the elastic response of the crust to strain forcing at various periods and the sensitivity of relative velocity variations to non-tectonic, hydrological strain variations. We performed velocity variation measurements on seismic noise autocorrelation signals recorded at seismic stations for different coda waves time lapse and compared them with strain measured by the GPS network. We observe that seismic velocities decrease during dilatation episodes (high hydraulic head) and increase during contraction (low hydraulic head). We find that the retrieved strain sensitivity of seismic velocity changes is of the order of ~103 and discuss such sensitivity with previous natural and laboratory results.

Water injection in geothermal areas is the preferential strategy to sustain the natural production of geothermal resources. In this context, monitoring microearthquakes is a fundamental tool to track changes in the reservoirs in terms of soil composition, response to injections, and resource exploitation in space and time. Therefore, the refined source characterization is crucial to better estimate the size, source mechanism, and rupture process of microearthquakes, as possibly related to industrial activities and to identify any potential variation of the background seismicity. Standard approaches for source parameters estimation are ordinarily based on the modelling of Fourier displacement spectra and its characteristic parameters, the low-frequency spectral level and corner frequency. Here we apply a time-domain innovative technique that uses the curves of P-wave amplitude vs time along the seismogram. The methodology allows estimating seismic moment, source radius, and static stress drop from the plateau level and the corner-time and of the average logarithm of P-wave displacement versus time with the assumption of a triangular moment rate function, uniform rupture speed, and constant/frequency-independent Q-factor. In the current paper, this time-domain methodology is implemented to a selected catalog of micro-earthquakes consists of 83 events with moment magnitude ranging between 1.0 and 1.5, occurred during 7 years (2007-2014) of fluid extraction/injection around Prati-9 and Prati-29 wells at The Geysers Geothermal field.

With the aim to investigate the rupture complexity and the radiated wave field of 2017, Mw 3.9, Ischia earthquake, south-west of Naples (Italy), we used finite-fault modeling to invert the near-source (and searched for the best-fit kinematic rupture parameters. This analysis showed that the rupture nucleated at about 600 m west of IOCA and 1.1-km depth, along a 1 km, NW-SE striking fault (i.e., thrust with right-lateral component), with a rupture velocity of about 0.7 km/s. The retrieved rupture model coupled with multipath reverberations effects related to a thin, low-velocity near-surface volcanic sedimentary layer, well explains the observed long ground motion duration and the large amplitudes recorded all over the island. Finally, the apparent source time function (STF), obtained from inverse modeling using a theoretical Green’ function (GF), is validated by implementing an empirical GF (EGF) analysis.

Fault zones are major sources of hazard for many populated regions around the world. Earthquakes still occur unanticipated, and research has started to observe fault properties with increasing spatial and temporal resolution, having the goal of detecting signs of stress accumulation and strength weakening that may anticipate the rupture. The common practice is monitoring source parameters retrieved from measurements; however, model dependence and strong uncertainty propagation hamper their usage for small and microearthquakes. Here, we decipher the ground motion (i.e., ground shaking) variability associated with microseismicity detected by dense seismic networks at a near-fault observatory in Irpinia, Southern Italy, and obtain an unprecedentedly sharp picture of the fault properties evolution both in time and space. We discuss the link between the ground-motion intensity and the source parameters of the considered microseismicity, showing a coherent spatial distribution of the ground-motion intensity with that of corner frequency, stress drop, and radiation efficiency. Our analysis reveals that the ground-motion intensity presents an annual cycle in agreement with independent geodetic displacement observations from two Global Navigation Satellite System stations in the area. The temporal and spatial analyses also reveal a heterogeneous behavior of adjacent fault segments in a high seismic risk Italian area. Concerning the temporal evolution of fault properties, we highlight that the fault segment where the 1980 Ms 6.9 Irpinia earthquake nucleated shows changes in the event-specific signature of ground-motion signals since 2013, suggesting changes in their frictional properties. This evidence, combined with complementary information on the earthquake frequency–magnitude distribution, reveals differences in fault segment response to tectonic loading, suggesting rupture scenarios of future moderate and large earthquakes for seismic hazard assessment.

The West Delta Deep Marine (WDDM) concession is one of the abundant natural gas resources in the world characterised by the presence of several active gas chimneys conduit feeding pockmarks. The detection of shallow gas accumulations has been gaining importance in hazard assessments before and during offshore drilling operations, whereas there is no way to estimate the exact pressure of the gas content in sediment to expect the potential gas hazards before and during the offshore installations, operations, and drilling. Monitoring of the gas chimneys and pockmarks plays an important role as an early warning for the oil industry to more focus on this kind of activities which may represent a catastrophic event in the future for the offshore installations at the WDDM region. Moreover, the “‘return period” of the pockmark activities is a region-dependence parameter according to the continuity of the gas supply and stability of the seepage pathway. However, in this study, the identification of the direct and indirect evidence of the shallow gas presence and migration pathways to the seafloor is achieved by extracting various-chosen seismic attributes, such as Root Mean Square, envelope, energy, the cosine of the instantaneous phase, variance, and chaos attributes. These attributes improve the imaging of several seismic evidence such as bright anomalies, enhanced reflectors, gas chimneys, and associated seabed features formed by the migration of the fluids from deep reservoirs through gas chimneys up to the seafloor. The presented profiles clearly show that gas chimneys and pockmarks combined with other associated features represent common features of the WDDM concession that can severely impact during and after offshore drilling operations in the aspects of safety, environment, and cost.

Seismic attributes supported by composite logs are the best way that can enable the interpreter to understand seismic data very well and generate a new view of the output results. Detection of the reservoir zone can be enhanced by analyzing wells log data based on Gamma-ray, Resistivity, and Vp sonic logs respectively. Composite logs of Scarab-1, Scarab-De, Scarab-Da, Scarab-Dd, and Scarab-2 wells indicate the lateral and vertical variation of the gas reservoir in El Wastani Formation. However, there are several seismic attributes that can be used to support reservoirs identification. For enhancement the detection of the hydrocarbon reservoirs, it is important to carefully analyze the 2D seismic data, which in this study will be primarily prepared to enhance seismic attributes results for the identification of gas chimneys, gas zones as channels, enhance stratigraphic and structural interpretations. In this article, we have performed data conditioning, quality control and seismic well ties including the preliminary wavelet extractions to get accurate output. Then, we have extracted of several classes of physical, geometrical and complex attributes as a direct hydrocarbon indicator to identify the gas zones, channels and chimneys and to identify the faults and discontinuities. The main contribution of this work is to provide a more detailed seismic reflection image supported by several seismic attributes classes and well logs to show a visual and quantitative evidence to identify the gas channels and gas chimneys with improving the detection of the faults and discontinuities. Keywords: Seismic attributes, Offshore nile delta, Gas channels, Gas bearing sand zones, Gas chimney, Well logging

Repeated tomographic inversions in time (the so called 4D tomography) track physical properties and stress changes in the medium hosting fault systems by measuring changes in P and S seismic velocities. These changes may provide insights on fault system dynamics and earthquake triggering mechanisms. We applied 4D tomography to the volume embedding the Irpinia Fault System (IFS, southern Italy) using more than ten years of continuous seismicity monitoring. The IFS is one of the Italian most hazardous fault systems, being able to generate the 1980 Ms 6.9 earthquake, characterized by a multi-segmented rupture. Seismicity was divided into uneven epochs having almost the same spatial resolution of the volume hosting the IFS.The resulting images show time-invariant features, clearly related to crustal lithology, and time-changing (up to 20%) velocity anomalies in the central region. Vp, Vs and Vp/Vs anomalies are referred to the tomographic model obtained using all the data set, and occur at depths ranging between 1 and 5 km, and between 8 and 12 km. These anomalies are temporally well-correlated with groundwater recharge/discharge series and geodetic displacements during the same time intervals. This correlation provides evidence for the existence of pulsating pore pressure changes in a fractured crustal volume at depth of 8-12 km, saturated with a predominant gas phase (likely CO2) and correlated with groundwater recharge processes, We suggest that tomographic measurements of the Vp-to-Vs spatiotemporal changes are a suitable proxy to track the pore pressure evolution at depth in highly sensitive regions of fault systems.

The estimation of spatial and temporal changes in the host medium physical properties is a quest to improve risk evaluation and hazard forecasting application. The space-time evolution of the seismicity gives information about stress variations, fluid content, and pore-pressure changes inside the medium. Thus, the accuracy of arrival-time measurements is crucial for travel-time-based seismological applications, such as earthquake location and travel(delay)-time tomography, especially when double-difference times are used. Standard monitoring networks and tools implements single-station, STA/LTA-based, automatic event detection/location procedures, which may produce inconsistent arrival-times of the same phase among stations. To overcome this problem, refined cross-correlation (CC) techniques for time picking have been recently developed. Their basic approach is to use CC to refine picks of event pairs with high waveform similarity. Similar events are grouped in families, considering the max CC values, the inter-distance and/or the focal mechanism similarity. Two drawbacks of this common approach are (1) the impact of noise from individual receiver levels on the quality of reference trace (RT) and (2) the inability to adjust the systematic shift of automatic picks.Here we propose a new, fully automatic approach to refine the phase time picks. The CC is used to identify family members with a hierarchical clustering procedure. In each family, after the trace alignment, we build the RT by stacking the events weighted by the signal-to-noise ratio and the polarity. We applied this technique to a catalog of 3574 events of the 2014 MJMA 6.7 sequence occurred at the Northern Nagano prefecture. The results indicate that we can improve the precision of phase picks of similar events and to adjust the systematic shift introduced by the automatic picker with mean differences between refined and automatic picks up to 0,5-1 s.The high consistency of the phase picks allows to increase the accuracy of absolute location by reducing the mean location error from 0,6 km to 0,1 km and the root-mean-square from 0,15 to 0,075. Consequently, we observe an alignment of the seismicity respect to the main fault plane with an 30°-45° east-dipping angle for the shallow part while the deeper part dips at 50°-65°. Then, the double difference location provides highly resolved hypocenter locations and medium parameters by considering events of the same family as events pair. This improvement allows to use fast-tracking methods, as the Vp/Vs in time and the Coda-Wave Interferometry, to get information about the velocity variations before and during the sequence. By using those methods, we expect to get accurate information of the physical properties evolution and especially about the role of fluid in the triggering of the sequence.

The imaging of volcanic structures by means of seismic techniques is aimed at the structural characterization and monitoring purposes. The quiescent volcano of the Solfatara belong to the caldera of the Campi Flegrei Italy, a resurgent nested caldera that has been extensively investigated through active seismic investigation.The fumaroles of Bocca Grande and Bocca Nuova at the Solfatara volcano, represent some of the main markers of deep magmatic shallow hydrothermal activity. In this article we identify the gas accumulation zone using the attributes and scaled Poisson ratio extracted from multi-2D seismic profiles. The 400 m long profiles, have been acquired during the active experiment RICEN (Repeated Induced Earthquake and Noise) performed in the context of the EU project MEDSUV between May and November 2014. The seismic arrays were deployed along the NE-SW and NW-SE directions within the crater across the zones of the fumaroles and the “fangaia”.The time- and depth-sections are reconstructed after applying residual statics, DMO corrections, CMP gathering, and the post-stack Kirchhoff migration technique. The energy, root mean square, envelope, and sweetness attributes have been computed and extracted for determining the maximum and minimum values of amplitude zones on the migrated, post-stack seismic sections. Furthermore, we have investigated the time-gain attribute, which is used to interpret deep reflectors, and the variance attribute, that is a geometrical attribute providing information on location of faults, discontinuities, and chaotic zones. To better detail the reflectivity of shallow events, enhanced by the post stack attributes, the Amplitude Versus Offset (AVO) technique has also been used to discriminate and identify shallow gas pockets. The seismic profile, seismic attributes, and near-surface structural interpretation of the Solfatara volcano have been combined into a final structural image of the Solfatara subsoil. This show a clear evidence of the fluids trapping zones at 10-50 m depth beneath the crater's surface, as well as their migration paths down to 150 meters depth.

We developed a Bayesian technique to infer the double-couple, focal mechanism parameters (strike, dip and slip angles) of an earthquake source. The method uses 3 independent datasets: P-wave peak amplitude and polarity and S-to-P amplitude ratio wherever it is available.The Bayesian technique works even in absence of one dataset and easily integrates any prior information about the region of study. The parameter space is explored thanks to an octree strategy. The method estimates the Posterior pdf, where the maximum likelihood parameter values (MAP model) both for the principal and auxiliary plane are chosen as the final fault mechanism solution. Furtherly, the uncertainties as the projections of the semi-axis of the 68% confidence ellipsoid centred on the MAP model are provided.The joint use of the three datasets allows to determine a solution even in the case of a limited number of stations that have recorded the event, which is the case for example for small magnitude earthquakes (MWe applied and tested the methodology to a microearthquake sequence (ML 0.4-3.0) occurred in the Irpinia region, South Italy, using an uninformative prior distribution for the parameters. In this area, the background seismicity occurs in a volume delimited by the faults activated during the 1980 Irpinia M 6.9 earthquake. This faults system is complex and composed of northwest–southeast striking normal faults along the Apennines chain. A network of 3-component accelerometers and velocimeters is currently monitoring the area (Irpinia Seismic NETwork).We inferred the focal mechanism of the earthquakes of the sequence. Our results show fault mechanism solutions which are consistent with previous studies, well reflecting the regional stress field. The focus on micro-seismicity can reveal characteristics useful to highlight behaviours of larger scale seismicity.

We present a comprehensive review of seismic and gravity observations and tomographic models produced over the past four decades in order to understand the structure of the crust beneath the Campi Flegrei caldera. We describe the main lithological and structural discontinuities defined through these observations, illustrate their geophysical responses, and discuss the constraints they give to the understanding of magmatic and volcanic processes. Micro-seismic crises related to caldera unrest, and ambient seismic noise measurements provide comprehensive seismic data to local earthquake and ambient noise tomography. In combination with reflection data from onshore and offshore active seismic experiments, velocity tomography reconstructs the elastic properties of the caldera between surface and ~4 km depth. Active experiments also define the depth of lithological interfaces and deep (~7.5 km) partially molten bodies. Seismic attenuation tomography provides information complementary to velocity tomography, defining lateral lithological changes and the geometry of onshore and offshore fluid and magma bodies down to 4 km depth. Once compared with seismic analyses, gravity data highlight lateral changes in the offshore caldera structures. During the deformation and seismo-geochemical unrest (1982–1984), they permitted to reconstruct a minor (

Earthquake early warning systems are able to issue an alert just after the start of the seismic rupture on the fault plane, but before the ground shaking reaches the sites to be protected. The available lead-time for security actions is very short (seconds to several dozens of seconds), requiring fast and robust methods to make these systems operational. Here, general principles about the two paradigms of early warning systems are presented: the regional and the on-site systems. When these methods are applied to data recorded by very dense networks, such as in near-fault observatories, the blind zone can be shortened to 15–25km. Next generation of early warning systems should reduce the epistemic variability by implementing modeling of finite-source and site effects. Finally, the combination of on-site and regional systems may lead to a real-time estimation of the ground-shaking maps, overcoming the problem of computing the event location, and magnitude.

The reliable determination of earthquake source parameters is a relevant task of seismological investigations which ground nowadays on high-quality seismic waveforms collected by near-source dense arrays of ground motion sensors. Here, we propose a parametric modeling technique which analyzes the time-domain P-wave signal recorded in the near-source range of small-to-large-size earthquakes. Assuming a triangular moment rate function, a uniform speed, and circular rupture model, we develop the equations to estimate the seismic moment, rupture radius, and stress drop from the corner time and plateau level of the average logarithm of the P-wave displacement versus time curves (LPDT). The constant- Q , anelastic attenuation effect, is accounted by a postprocessing procedure that evaluates the Q -unperturbed moment rate triangular shape. The methodology has been validated through application to the acceleration records of the 2016-2017 Central Italy and 2007-2019 Japan earthquake sequences covering a wide moment magnitude range (Mw 2.5-6.5) and recording distance 5.5 events in Central Italy relative to previous estimates using spectral methods.

In this study we implemented and tested the Earthquake Early Warning system PRESTo (PRobabilistic and Evolutionary early warning System, Satriano et al., 2011) on the Greek Ionian islands of Lefkada, Zakynthos and Kefalonia. PRESTo is a free and open source platform for regional Earthquake Early Warning developed at the University of Naples Federico II, which is currently under experimentation in Southern Italy, in the area covered by the Irpinia Seismic Network. The three Ionian islands selected for this study are located on the North-Western part of the Hellenic trench. Here the seismicity rate and the seismic hazard, coupled with the vulnerability of existing critical infrastructures, make this region among the highest seismic risk areas in Europe, where the application of Earthquake Early Warning systems may become a useful strategy to mitigate the potential damage caused by earthquakes. Here we studied the feasibility of implementing an Earthquake Early Warning system on an existing seismic network, which was not specifically made for earthquake early warning purposes, and evaluated the performance of the system, using a data set of real-earthquake recordings. We first describe the technical details of the implementation of PRESTo in the area of interest, including the preliminary parameter configuration and the empirical scaling relationship calibration. Then we evaluated the performance of the system through the off-line analysis of a database of real earthquake records belonging to the most recent M > 4.0 earthquakes occurred in the area. We evaluated the performance in terms of source parameter estimation (location, magnitude), accuracy of ground shaking prediction and lead-time analysis. Finally, we show the preliminary results of the real-time application of PRESTo, performed during the period 01–31 July 2019.

In the West Delta Deep Marine (WDDM) concession many important gas reservoirs have been recently discovered. To improve the possibility of discovering new natural gas resources, an integrated predi...

The continuous monitoring of the space-time-magnitude evolution of seismicity is a crucial task to assess the geo-mechanical conditions of hydrocarbon reservoirs. It is fundamental to design optimized high-sensitivity and cost-effective seismic networks able to detect and locate low magnitude events with high accuracy. In Italy, after the concern for the 2012 Emilia earthquake, the government released the guidelines for monitoring the seismicity in areas surrounding subsurface industrial activities. In this work, we propose an optimization study for seismic monitoring networks based on the guideline requirements. We evaluated the performance, in terms of detection/location thresholds and events location errors, of different network layouts constructed by varying the station density and geometry and considering also the integration of seismic arrays. We simulated sets of seismic sources at different depths by varying magnitude and stress drop values. We considered two different values of noise level at the stations. The results show that the station density and the noise level represent the crucial parameters for the seismic network performances. The performances of a standard network comply the guidelines requirements in terms of detection/location thresholds. Only the integration of seismic networks with arrays allows to decrease the location errors of several hundreds of meters and to reach the location accuracy required by the Italian guidelines. We foresee the integration of standard networks with seismic arrays to achieve high performance in terms of detection capability and location accuracy.

The reliable determination of earthquake source parameters is a relevant task of seismological investigations which ground nowadays on high quality seismic waveforms collected by near-source dense arrays of ground motion sensors. Here we propose a parametric modelling technique which analyzes the time-domain P-wave signal recorded in the near-source range of small-to-large size earthquakes. Assuming a triangular moment-rate function and a uniform speed, circular rupture model, we develop the equations to estimate the seismic moment, rupture radius and stress-drop from the corner-time and plateau level of the average logarithm of the P-wave displacement vs time curves (LPDT). The constant-Q, anelastic attenuation effect is accounted by a post-processing procedure that evaluates the Q-unperturbed moment-rate triangular shape.The methodology has been validated through the application to the acceleration records of the 2016-2017 Central Italy and 2007-2019 Japan earthquake sequences covering a wide moment magnitude range (Mw 2.5 - 6.5) and recording distance < 100 km. After correcting for the anelastic attenuation function, the estimated average stress-drop and the confidence interval (〈∆σ〉=0.60 (0.42-0.87) MPa and 〈∆σ〉=1.53 (1.01-2.31) for crustal and subcrustal events of Japan and 〈∆σ〉=0.36(0.30-0.44) MPa for Central Italy) show, for both regions, a self-similar, constant stress-drop scaling of the rupture duration/radius with seismic moment. The smaller sensitivity of the spatially averaged, time-varying peak displacement amplitude to the radiation from localized high slip patch on the fracture surface, could explain the retrieved smaller average stress-drops for sub-crustal earthquakes in Japan and M>5.5 events in Central Italy relative to previous estimates using spectral methods.

A primary task of a network-based, earthquake early warning system is the prompt event detection and location, needed to assess the magnitude of the event and its potential damage through the predicted peak ground shaking amplitude using empirical attenuation relationships. Most of real-time, automatic earthquake location methods ground on the progressive measurement of the first P-wave arrival time at stations located at increasing distances from the source but recent approaches showed the feasibility to improve the accuracy and rapidity of the earthquake location by using the additional information carried by the P-wave polarization or amplitude, especially with unfavorable seismic network lay-outs. Here, we propose an evolutionary, Bayesian method for the real-time earthquake location which combines the information derived from the differential P-wave arrival times, amplitude ratios and back-azimuths measured at a minimum of two stations. As more distant stations record the P-wave, the posterior pdf is updated and new earthquake location parameters are determined along with their uncertainty. To validate the location method, we performed a retrospective analysis of mainshocks (M > 4.5) occurred during the 2016–2017 Central Italy earthquake sequence by simulating the typical acquisition layouts of in-land, coastal and linear array of stations. Results show that with the combined use of the three parameters, 2–4 s after the first P-wave detection, the method converges to stable and accurate determinations of epicentral coordinates and depth even with a nonoptimal coverage of stations. The proposed methodology can be generalized and adapted to the off-line analysis of seismic records collected by standard local networks.

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

Seismic arrays are instruments capable to lower the magnitude threshold of earthquake detection by improving the signal-to-noise ratio of recordings. Seismic arrays have been used since 1960s, to investigate global earthquakes or small-scale structure of the Earth’s interior. We designed a cost-effective seismic monitoring array-system made by autonomous stations specifically designed for studying local micro-seismicity. The data collected by this system allow to develop and apply a new method for earthquake detection and location of micro-seismicity based on a frequency-wavenumber (f-k) domain analysis of continuous data recorded by small seismic arrays, in order to separate the coherent signal of low magnitude events from the surrounding noise.Field surveys have been carried out in two Italian regions, which are seismically active and already monitored by high quality, standard seismic networks, so to allow us to test the performance of the proposed array configuration and processing algorithm.The first survey was carried out in the Irpinia region (Southern Italy), near the main fault segment activated during the Ms 6.9 Irpinia earthquake occurred in 1980. The natural seismicity of the area features occasional small seismic sequences, with magnitude (ML) less than 3, that are recorded by the local seismic network that monitors the Irpinia fault-system (ISNet - Irpinia Seismic Network). Three small aperture seismic arrays (few hundred meters wide) were deployed at distance of few tens of kilometers each other for three months. Each array was made up of seven 3-component stations, arranged in irregular geometry.The second experiment was carried out in the Montello-Collalto area (Veneto region, North-East Italy), where an underground gas storage concession, known as “Collalto Stoccaggio”, exists. The gas storage activity is monitored by a local seismic network named “Collalto Seismic Network” (Rete Sismica di Collalto, or RSC). This dense network was designed and is managed by the National Institute of Oceanography and Applied Geophysics (OGS) on behalf of Edison Stoccaggio S.p.A., the storage concession holder. A seismic array composed of eight 3-component stations with 2 km of maximum aperture was deployed for a couple of months to monitor the micro-seismicity occurring nearby.Our preliminary results suggest that the f-k earthquake detection algorithm of micro-seismicity using seismic arrays can become a valid tool to complement standard seismic networks in monitoring and studying natural and induced seismicity. In the Irpinia region, for instance, we have detected and located about six times the earthquakes recorded by the local network lowering the magnitude down to 0.1, smaller than the catalogue minimum magnitude that is equal to 1.2.