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Moment tensor inversions of broadband velocity data are usually managed by adopting Green's functions for 1D layered seismic wave speed models. This assumption can impact on source parameter estimates in regions with complex 3D heterogeneous structures and discontinuities in rock properties. In this work, we present a new centroid moment tensor (CMT) catalog for the Amatrice‐Visso‐Norcia (AVN) seismic sequence based on a recently generated 3D wave speed model for the Italian lithosphere. Forward synthetic seismograms and Fréchet derivatives for CMT‐3D inversions of 159 earthquakes with Mw ≥ 3.0 are simulated using a spectral‐element method (SEM) code. By comparing the retrieved solutions with those from time domain moment tensor (TDMT) catalog, obtained with a 1D wave speed model calibrated for Central Apennines (Italy), we observe a remarkable degree of consistency in terms of source geometry, kinematics, and magnitude. Significant differences are found in centroid depths, which are more accurately estimated using the 3D model. Finally, we present a newly designed parameter, τ, to better quantify and compare a‐posteriori the reliability of the obtained MT solutions. τ measures the goodness of fit between observed and synthetic seismograms accounting for differences in amplitude, arrival time, percentage of fitted seconds, and the usual L2‐norm estimate. The CMT‐3D solutions represent the first Italian CMT catalog based on a full‐waveform 3D wave speed model. They provide reliable source parameters with potential implications for the structures activated during the sequence. The developed approach can be readily applied to more complex Italian regions where 1D models are underperforming and not representative of the area. Plain Language Summary: The moment tensor (MT) is a mathematical representation of the movement on a fault during an earthquake, and of the size, or magnitude, of the event. Such tensor is often described through the beachballs, a graphic symbol that indicates the fault orientation and the type of slip that occurs during an earthquake. Usually, seismologists use 1D wave speed models (i.e., describing only the vertical velocity of seismic waves in the Earth interior) in order to compute MTs. In recent years, due to the incredible progresses of computer sciences, also 3D models, which are able to describe lateral velocity variations, have been successfully adopted to compute MTs. In this work, we use the recently developed 3D Italian wave speed model "IMAGINE_IT" ("Im25" for brevity) to compute the MT solutions for the Amatrice‐Visso‐Norcia (AVN) 2016–2018 earthquakes with magnitude larger than 3. This seismic sequence ruptured almost 80 km of the Apennines normal faults and resulted in 299 casualties and >20,000 homeless. Our newly developed MT catalog allows us to better understand the characteristics of the faults activated during the seismic sequence and to provide more reliable source parameters as magnitude and depth. Key Points: A new centroid moment tensor catalog for Central Italy earthquakes has been built based on a 3D wave speed model for the Italian lithosphereMoment magnitude, kinematic source parameters, and centroid depths inferred with a 3D wave speed model are accurately estimatedThe goodness of moment tensor solutions is tested via a newly designed quality factor [ABSTRACT FROM AUTHOR]

We performed measurements of velocity variations for variable coda waves time lapse using empirical Green's functions reconstructed by autocorrelation of seismic noise recorded during a period of 17 years in the region of L'Aquila, Italy. The time lapse approach permitted us to evaluate the spatial (depth) dependence of velocity variation (dv/v). By quantitatively comparing the 17 years of dv/v time series with independent data (e.g., strain induced by earthquakes and hydrological loading), we unravel a group of physical processes inducing velocity variations in the crust over multiple time and spatial scales. We find that rapid shaking due to three magnitude 6+ earthquakes mainly induced near surface velocity variations. On the other hand, slow strain perturbation (period 5 years, in the preseismic period) associated with hydrological cycles, induced velocity changes primarily in the middle crust. The observed behavior suggests the existence of a large volume of fluid‐filled cracks exist deep in the crust. Our study highlights the possibility of using seasonal and multiyear perturbations to probe the physical properties of seismogenic fault volumes and shed new light into the depth‐dependent rheology of crustal rocks in the region or L'Aquila. Key Points: We characterise long and short term velocity variation during 17 years around L'Aquila region (Italy)We resolved the source of seismic velocity variation and exploit them to obtain the depth dependent cause of velocity variation in the crustWe highlighted a strong sensitivity to strain perturbation isolated at depth [ABSTRACT FROM AUTHOR]

Rapid assessment of the volume and the rate at which gas and pyroclasts are injected into the atmosphere during volcanic explosions is key to effective eruption hazard mitigation. Here, we use data from a dense infrasound network deployed in 2017 on Mt. Etna, Italy, to estimate eruptive volume flow rates (VFRs) during small gas‐and‐ash explosions. We use a finite‐difference time‐domain approximation to compute the acoustic Green's functions and perform a full waveform inversion for a multipole source, combining monopole and horizontal dipole terms. The inversion produces realistic estimates of VFR, on the order of 4 × 104 m3/s and well‐defined patterns of source directivity. This is the first application of acoustic waveform inversion at Mt. Etna. Our results demonstrate that acoustic waveform inversion is a mature and robust tool for assessment of source parameters and holds potential as a tool to provide rapid estimates of VFR in near real time. Plain Language Summary: Rapid and realistic assessment of the volume of erupted material, and the rate at which gas and pyroclasts are injected into the atmosphere during volcanic explosions, is crucial for effective monitoring and hazard mitigation. These parameters, for instance, are key inputs into models of atmospheric rise and transport of volcanic plumes. Volcanic explosions, among many other phenomena, generate atmospheric pressure waves known as infrasound. These sound waves that propagate at frequencies below 20 Hz, represent a powerful tool to investigate the dynamics and source mechanisms of volcanic explosions. Here, we demonstrate how recordings of acoustic infrasound generated by explosions at Mt. Etna can be used to assess the volume flow history of these events. We introduce and apply a data modeling workflow that could be implemented in near real time at Mt. Etna and other volcanoes worldwide. Key Points: This is the first application of acoustic waveform inversion for small explosions at Mt. Etna volcano, ItalyWe demonstrate the feasibility of applying this inversion method for large volcanoes using up to 14 stationsAcoustic waveform inversion holds potential for volume flow estimation in near real time at Mt. Etna and other volcanoes [ABSTRACT FROM AUTHOR]

In the last decade the use of passive methods has become appealing in reconstructing the properties of the propagation medium by seismic ambient noise data, without the use of localized natural or artificial sources. A temporary seismic network was installed in the urban area of Benevento (southern Italy) in order to characterize the shallow structure of the city using stable methods for the analysis of the seismic noise continuously acquired by stations. The city of Benevento is one of the italian areas with highest seismic hazard, and at present the region is affected by low energy swarms and sparse events (Ml ≤ 4.1). It has been struck by several destructive historical earthquakes, the strongest of which occurred in 1456, 1688, 1805 with associated MCS intensity up to X–XI. We used the sixteen seismic stations installed in Benevento to record ambient noise for about 1 month. The stations were equipped with different seismic instruments: (i) digitizers Quanterra Q330 connected to Le3d-5 s short-period sensors; (ii) Nanometrics Centaur digitizers coupled with Trillium Compact 120s broad-band velocimeters; (iii) one station with Episensor force balance accelerometer connected to a D6BB-DIN Staneo digitizer. Interstations Green's functions were reconstructed by the cross-correlation of continuous ambient noise data, and surface waves signals were extracted from Green's Functions (GFs) for investigating the elastic properties of the subsurface structure. In this regard, we performed the beamforming analysis to test the hypothesis of isotropy distribution of noise sources on which the cross-correlation method is based, and the particle motion analysis to confirm the presence of surface Rayleigh waves in the GFs. We analysed the temporal stability of the cross-correlated signals and the results show that 2 weeks of continuous measurements are sufficient to stabilize the surface waves signal extracted from the GFs. The phase velocity dispersion curves are computed for 115 station pairs through the use of a far-field representation of the surface-wave GFs and an image transformation technique. Our strategy based on cross-correlation analysis provides robust phase-velocity dispersion curves that vary approximately from 1.4 km s–1 at 0.7 Hz to 0.6 km s–1 at 5 Hz. Different pairs were selected for the inversion of phase-velocity dispersion curves aimed to derive 1-D shear-wave velocity (Vs) profiles (up to a maximum depth of about 500 m) representative of some areas of the city characterized by different soil deposits. [ABSTRACT FROM AUTHOR]

Since a few decades volcanic long period (LP) events have been recorded on many active volcanoes and their study has been recognized as an important tool to characterize volcanic activity. LP event analyses through moment tensor (MT) inversions have led to kinematic descriptions of various source mechanisms. The main challenge in these inversions is to 'strip out' the propagation effect in order to isolate the source; hence the velocity model used controls the accuracy of the retrieved source mechanism. We first carry out several synthetic tests of inversions on Mt Etna volcano (Italy). Four geological models with topography are considered with increasing complexity: the most complex model is used to generate synthetic data, while the other three models are used to calculate the Green's functions for inversions. The retrieved solutions from the three velocity models are similar. The MT solutions for a deeper source are well retrieved, while a shallower source test suffers from high uncertainties and strong misinterpretation of the source orientation. The homogeneous model gives the lowest misfit value, but source location and mechanism decomposition are inaccurate. When a complex model different from the true one is used, a high misfit value and a wrong solution is obtained. We then incorporate our findings into the MT inversion of an LP event recorded on Mt Etna in 2008. We obtain very different solutions among the three models in terms of source location and mechanism decomposition. The overall shape of the retrieved source time functions are similar, but some amplitude differences arise, especially for the homogeneous model. Our work highlights the importance of including the unconsolidated surface materials in the computation of Green's functions especially when dealing with shallow sources. [ABSTRACT FROM AUTHOR]

We perform a systematic investigation of along-strike rupture directivity of 70 earthquakes (3.0 ≤ Mw ≤ 6.1) of the 2009 L'Aquila seismic sequence by analysing azimuthal variations of broad-band seismograms recorded in the distance range 60 km < R < 230 km. We use reference spectra of events with little directivity (similar to the empirical Green's function method) to deconvolve propagation-site effects and focus on source properties. A directivity index (0 ≤ IDIR ≤ 1) calculated for each earthquake quantifies the spectral separation above the corner frequency of the target event at opposite along-strike directions. A large number (73 per cent) of events including the Mw 6.1 main shock showhigh (>0.7) IDIR values indicating predominantly unilateral rupture propagation. The preferred rupture propagation direction is generally to the southeast with no dependence on the earthquake magnitude or occurrence time. Events on two main faults (L'Aquila and Campotosto) show somewhat different behaviour. Almost all earthquakes on the L'Aquila fault have strong unilateral directivity to the southeast, whereas earthquakes on the Campotosto fault show more diverse behaviour. However, there is a predominance of unilateral ruptures (14 out of 22) also on the Campotosto fault, and the few (five) earthquakes with ruptures to the northwest are limited to the most northwestern segment of the fault. The spectral results are consistent with time-domain analysis when the latter samples adequately the frequency band above corner frequency. The preferred rupture direction may be produced at least in part by a velocity contrast across the fault. The results provide important input for estimates of seismic motion and physics of earthquake ruptures. [ABSTRACT FROM AUTHOR]

The empirical Green's function (EGF) technique is applied in the frequency domain to 962 broad-band seismograms (3.3 ≤ MW ≤ 6.1) to determine stress drop and source scaling of the 2009 April L’Aquila earthquakes. The station distance varies in the range 100–250 km from the source. Ground motions of several L’Aquila earthquakes are characterized by large azimuthal variations due to source directivity, even at low magnitudes. Thus, the individual-station stress-drop estimates are significantly biased when source directivity is not taken into account properly. To reduce the bias, we use single-station spectral ratios with pairs of earthquakes showing a similar degree of source directivity. The superiority of constant versus varying stress-drop models is assessed through minimization of misfit in a least-mean-square sense. For this analysis, seismograms of 26 earthquakes occurring within 10 km from the hypocentres of the three strongest shocks are used. We find that a source model where stress drop increases with the earthquake size has the minimum misfit: as compared to the best constant stress-drop model the improvement in the fit is of the order of 40 per cent. We also estimate the stress-drop scaling on a larger data set of 64 earthquakes, all of them having an independent estimate of seismic moment and consistent focal mechanism. An earthquake which shows no directivity is chosen as EGF event. This analysis confirms the former trend and yields individual-event stress drops very close to 10 MPa at magnitudes MW > 4.5 that decrease to 1 MPa, on the average, at the smallest magnitudes. A varying stress-drop scaling of L’Aquila earthquakes is consistent with results from other studies using EGF techniques but contrasts with results of authors that used inversion techniques to separate source from site and propagation effects. We find that there is a systematic difference for small events between the results of the two methods, with lower and less scattered values of stress drop resulting from the EGF approach. [ABSTRACT FROM PUBLISHER]

SUMMARY The 2009 L'Aquila, Italy, earthquake ( Mw 6.3) produced an unprecedented number of seismological records for a normal faulting event, and hence represents an important case study of damaging moderate magnitude earthquakes occurring in densely inhabited areas. We have investigated the source process of the 2009 L'Aquila earthquake in both the low- and broad-band frequency ranges by analysing the teleseismic and strong motion data. A low-frequency (<0.5 Hz) source model has been determined by waveform inversion of teleseismic and near-field strong motion data, yielding slip distributions with a major asperity located about 8 km southeast of the hypocentre. The broad-band (0.2-10 Hz) source characteristics were estimated using the empirical Green's function method providing the strong motion generation area of about 42 km2 located southeast of the hypocentre below the village of Onna. This coincides with the location of the asperity from the inversion analysis, indicating that the low-frequency and broad-band ground motions produced during the earthquake were generated at similar locations on the fault plane. The position of the strong motion generation area also correlates well with the distribution of observed macroseismic (MCS) intensities reported by INGV. The maximum macroseismic intensities of IX-X were reported southeast of the epicentre, with an intensity of X measured in Onna. The source properties of the 2009 L'Aquila mainshock reveal the estimated size of the strong motion generation area to be in agreement with predictions made using empirical source scaling relation for inland crustal earthquakes, implying a stress drop of approximately 10 MPa. This further indicates that the 2009 L'Aquila earthquake involved a stress drop similar to that of previously recorded earthquakes in the Central and Southern Apennines. [ABSTRACT FROM AUTHOR]

In this paper, we adopt three ground-motion simulation techniques (the stochastic finite-fault simulation code from Motazedian and Atkinson, 2005; the hybrid deterministic-stochastic approach with approximated Green's functions from Pacor et al, 2005; and the broadband hybrid integral-composite technique with full-wavefield Green's functions from Gallovič and Brokešová, 2007), with the aim of investigating the different performances in near-fault strong-motion modeling and prediction from past and future events. The test case is the 1980 M 6.9 Irpinia earthquake, the strongest event recorded in Italy in the last 30 years. First, we simulate the recorded strong-motion data and validate the model parameters by computing spectral acceleration and peak amplitude residual distributions. The validated model is then used to investigate the influence of site effects and to compute synthetic ground motions around the fault. Afterward, we simulate the expected ground motions from scenario events on the Irpinia fault, varying the hypocenters, the rupture velocities, and the slip distributions. We compare the median ground motions and related standard deviations from all scenario events with empirical ground-motion prediction equations (GMPEs), The synthetic median values are included in the median ± 1 standard deviation of the considered GMPEs, Synthetic peak ground accelerations show median values smaller and with a faster decay with distance than the empirical ones. The synthetics total standard deviation is of the same order or smaller than the empirical one, and it shows considerable differences from one simulation technique to another. We decomposed the total standard deviation into its between-scenario and within-scenario components. The larger contribution to the total sigma comes from the latter, while the former is found to be smaller and in good agreement with empirical interevent variability. [ABSTRACT FROM AUTHOR]

Abstract: Almost 50,000 long period (LP) events were recorded at Mt. Etna from November 2003 to May 2006. We analysed these events, as well as very long period (VLP) events which were associated with some of them. During some intervals the spectral and wavefield features of LP events remained steady, with significant changes occurring between the intervals. Based on the times of the changes, we distinguish five different sub-periods. In particular, during sub-period III (June–November 2005) the wavefield at the stations nearest to the summit area was composed of P waves. Locations for 150 LP events occurring in sub-period III, determined using radial semblance, changed, at the same time as the events'' spectral features changed. It was during this sub-period that many of the LP events were associated with VLP events. Based on similarity of the waveforms, we distinguished two families of VLP events, with gradually evolving waveforms. The two families are located in slightly different places, but near the sources of the LP events. The change between the families occurred at the same time as the spectra of the LP events changed. Finally the source of the VLP events was investigated by performing complete waveform moment tensor inversion of stacks of the two families. Synthetic Green''s functions for the full moment tensor were calculated for a homogeneous half-space. For both families, the source region with the highest variance reduction lies approximately beneath the active craters, at 500m below the altitudes of the stations. The solutions for both families are very similar with sources that are between 60 and 70% isotropic. Attempts to determine deviatoric moment tensors produced consistently poorer fits. The remaining energy is poorly constrained and is likely to be noise. In conclusion, these results highlight changes in the LP and VLP events at Mt. Etna over time, and the causal relationship between them. [Copyright &y& Elsevier]

Abstract: We test the physically-based ground motion hazard prediction methodology of Hutchings et al. [Hutchings, L., Ioannidou, E., Kalogeras, I., Voulgaris, N., Savy, J., Foxall, W., Scognamiglio, L., and Stavrakakis, G., (2007). A physically-based strong ground motion prediction methodology; Application to PSHA and the 1999 M =6.0 Athens Earthquake. Geophys. J. Int. 168, 569–680.] through an a posteriori prediction of the 26 September 1997, M w 6.0 Colfiorito (Umbria–Marche, Italy) earthquake at four stations. By “physically-based” we refer to ground motion synthesized with quasi-dynamic rupture models derived from physics and an understanding of the earthquake process. We test five hypotheses proposed by Hutchings et al. [Hutchings, L., Ioannidou, E., Kalogeras, I., Voulgaris, N., Savy, J., Foxall, W., Scognamiglio, L., and Stavrakakis, G., (2007). A physically-based strong ground motion prediction methodology; Application to PSHA and the 1999 M =6.0 Athens Earthquake. Geophys. J. Int. 168, 569–680.] that support application of the methodology to physically-based probabilistic seismic hazard or risk analysis. We use two methods to test the hypotheses. First, we test whether observed records fall within the 68% log-normal confidence interval for the distribution of absolute acceleration response (AAR), pseudo velocity response (PSV), and Fourier amplitude spectra (FFT) created by a suite of source models. We also used the godness of fit between synthesized seismograms to verify whether at least one of the source models in the suite generates seismograms that match the observed waveforms, and if good fits to seismograms are due to source models that are close to what is actually known about the Colfiorito earthquake. We tested the hypotheses with a range of source parameters proposed by Hutchings et al. [Hutchings, L., Ioannidou, E., Kalogeras, I., Voulgaris, N., Savy, J., Foxall, W., Scognamiglio, L., and Stavrakakis, G., (2007). A physically-based strong ground motion prediction methodology; Application to PSHA and the 1999 M =6.0 Athens Earthquake. Geophys. J. Int. 168, 569–680.]. We synthesized records from 100 rupture scenarios that were generated by a Monte Carlo selection of parameters within the range. This range was based upon having some prior knowledge of where the earthquake would occur. Observed values of AAR, PSV and FFT fit within the 68% confidence interval for all four stations, and one of the models generated seismograms that had a good fit compared to the observations. Moreover, a strict test for validating a physically-based ground motion hazard prediction methodology is that as more information is known about the source, the uncertainty of the prediction should narrow, but still include the actual ground motion. Then, we tightened the source parameters to be centered about the known parameters for the Colfiorito earthquake, and allowed for less uncertainty in their values. We found this to be true for this test. While the 68% confidence interval narrowed from a factor of ± about 4 to ± about 2 for the distributions, observed values of AAR, PSV and FFT still fit within the distributions for all four stations. Ultimately, we have calculated peak ground velocity (PGV) and peak ground acceleration (PGA) for all the synthetic seismograms obtained from the computed scenarios, and we have found that they are comparable with the actual and with those from the attenuation relation. We conclude that the methodology of Hutchings et al. [Hutchings, L., Ioannidou, E., Kalogeras, I., Voulgaris, N., Savy, J., Foxall, W., Scognamiglio, L., and Stavrakakis, G., (2007). A physically-based strong ground motion prediction methodology; Application to PSHA and the 1999 M =6.0 Athens Earthquake. Geophys. J. Int. 168, 569–680.] is promising in giving ground motion hazard prediction estimates. [Copyright &y& Elsevier]

The factorization method (FM) has been applied to measurement data from a multistatic ground-penetrating radar operating in close proximity to the ground, which was used in a measurement campaign on the Joint Research Centre mine test lane in Ispra, Italy. This paper is targeted toward a future hand-held demining system. The according space limits restrict an independent positioning of transmit and receive antennas. Hence, very small multistatic datasets are obtained, representing a difficult case for the reconstruction with the FM. [ABSTRACT FROM AUTHOR]

Basaltic volcanoes are characterized by low-level explosions and lava fountaining, but are capable of violent subplinian to plinian eruptions. Stromboli Volcano has been observed with seismometers and tiltmeters to deform prior to explosion [Genco and Ripepe, 2010]. Quantitatively linking this precursory deformation, often interpreted as inflation due to an influx of magma and gas, with the volume of subsequent erupted material (gas and tephra) will be helpful for understanding eruption dynamics and in hazard mitigation, as we can better constrain expected volatile output from monitored volume input. To these aims, we temporarily deployed 7 seismometers, 7 infrasonic microphones, an FTIR, FLIR, gravimeter, and MultiGAS at Stromboli Volcano, Italy in May 2018. We use these data in combination with data from permanently installed seismometers, infrasound sensors, tiltmeters, and a UV camera to examine the volatile budget of Stromboli by comparing shallow volume input derived from broadband, tilt-affected seismic and tilt data with volume output from infrasound and gas data. First, we characterize and locate the seismically-derived tilt source using a nonlinear moment tensor inversion method [Tape and Tape, 2012; Waite and Lanza, 2016] and use the result to quantify the volatile volume input. Initial inversion results place the tilt-affected source ~300 m below the active craters. We will attempt to characterize the input volume density using gravity data. Independently, we characterize and locate the infrasonic explosion source and quantify the output volume using an infrasonic waveform inversion technique that accounts for the influence of topography by computing numerical Green's functions by way of a 3-D finite difference time domain method [Kim et al., 2015]. The seismically estimated volume input and infrasonically estimated volume output will then be validated with volume output estimated from gas observations. This quantitative examination of tilt, infrasound and gas data aims to advance our ability to determine the size of eruption prior to its occurrence. [ABSTRACT FROM AUTHOR]