Complex wave propagation in the Campi Flegrei Caldera, Italy,from Source and receiver-array analysis of sea-shot recordings

We investigate wave propagation in the complex shallow crust of Campi Flegrei Volcanic Complex, Italy, using array recordings of air-guns. We apply source- and receiver-array analysis to define the independent variation of horizontal slowness at both the source and receiver regions. This method allows the identification of asymmetric ray-paths associated with near-source and near-observer velocity heterogeneities. P-wave wave-vectors at both the source and receiver arrays depict discrepancies as large as 50° with respect to the values expected for the 3D velocity structure of the Gulf. At the source region, these discrepancies may be associated with either un-modelled complexities in the geometry of the buried caldera rim, or with velocity variations beneath the source-array. At the receiver array, the inferred anomalies may be attributed to velocity variations marking the Solfatara crater rim, or to a near-receiver, low-velocity body whose position would coincide with negative gravimetric anomalies and a low Vp/Vs ratio region inferred by independent geophysical and seismological studies

Detailed analysis of wave propagation beneath the Campi Flegrei Caldera, Italy

We investigate the complex propagation of seismic waves beneath the Campi Flegrei caldera, Italy, using multichannel recordings of artificial explosions. The sources consisted of air gun explosions shot in the Gulf of Pozzuoli at offsets ranging between 3 and 7 km. A multichannel recording device was deployed in the Solfatara crater and consisted of ten vertical-component and two three-component short-period seismometers with a maximum aperture of about 150 m. The zero-lag correlation (ZLC) technique was adopted to estimate horizontal slowness and backazimuth of coherent waves crossing the array. For sources located in the northern sector of the Gulf, with maximum offset 5 km, ray parameters and backazimuths are in agreement with those predicted for the 1D velocity model used for routine locations. For sources at offsets larger than approximately 5 km, the ZLC curves depict prominent maxima associated with a secondary phase propagating with a lower velocity than the first-arrival P wave. Using finite-difference synthetic seismograms generated for a 2D realistic velocity model, we explain these late arrivals in terms of a lateral velocity variation located at depths of about 1 km. Such discontinuity would correspond to a positive V (sub p) anomaly imaged by a recent 3D tomographic study, and interpreted as the submerged southern rim of Campi Flegrei caldera collapsed during the explosive eruption of 12 ky B.P. The small spacing among adjacent shot points allowed simultaneous wave-field decomposition at the source and receiver arrays. Using a modified version of the double-beam method, we retrieve the independent variation of horizontal slowness at both the source and receiver regions. For both cases, we found azimuthal deviations as large as 50 degrees with respect to the great circle path. At the source region, these discrepancies may be interpreted in terms of ray bending at the interface of the aforementioned positive anomaly. At the receiver array, the observed anomalies may be attributed to either velocity variations marking the Solfatara crater rim, or to a near-receiver, low-velocity body whose position would coincide with negative gravimetric anomalies and a high V (sub p) /V (sub s) ratio region inferred by independent geophysical and seismological studies

Shallow shear-wave velocity structure of Solfatara volcano (Campi Flegrei, Italy),from inversion of Rayleigh-wave dispersion curves

In this work, we infer the 1D shear-wave velocity model at Solfatara volcano using the dispersion properties of Rayleigh waves generated by artificial explosions. The groupvelocity dispersion curves are retrieved by applying the Multiple Filter Technique to single-station recordings of air-gun sea shots. Seismic signals are filtered in different frequency bands and the dispersion curves are obtained by evaluating the arrival times of the envelope maxima of the filtered signals. Fundamental and higher modes are carefully recognized and separated by using a Phase Matched Filter. The dispersion curves obtained indicate Rayleigh-wave fundamental-mode group velocities ranging from about 0.8 to 0.6 km/s over the 2-12 Hz frequency band. These group velocity dispersion curves are then inverted to infer a shallow shear-wave velocity model down to a depth of about 250 m. The shear-wave velocities thus obtained are compatible with those derived both from cross- and down-hole measurements in neighbouring wells and from laboratory experiments. These data are eventually interpreted in the light of the geological setting of the area. Using the velocity model obtained, we calculate the theoretical ground response to a vertically-incident S-wave getting two, main amplification peaks centered at frequencies of 2.2 and 5.4 Hz. The transfer function was compared to those obtained experimentally from the application of Nakamura’s technique to microtremor data, artificial explosions and local earthquakes. Agreement among the experimental and theoretical transfer functions is observed for the amplification peak of frequency 5.4 Hz

Seismic Noise by Wind Farms: A Case Study from the Virgo Gravitational Wave Observatory, Italy

We present analyses of the noise wave field in the vicinity of Virgo, the Italian–French gravitational wave observatory located close to Pisa, Italy, with special reference to the vibrations induced by a nearby wind farm. The spectral contribution of the wind turbines is investigated using (1) onsite measurements, (2) correlation of spectral amplitudes with wind speed, (3) directional properties determined via multichannel measurements, and (4) attenuation of signal amplitude with distance. Among the different spectral peaks thus discriminated, the one at frequency 1.7 Hz is associated with the greatest power, and under particular conditions it can be observed at distances as large as 11 km from the wind farm. The spatial decay of amplitudes exhibits a complicated pattern, which we interpret in terms of the combination of direct surface waves and body waves refracted at a deep (≈800 m) interface between the Plio-Pleistocenic marine, fluvial, and lacustrine sediments and the Miocene carbonate basement. We develop a model for wave attenuation that allows determining the amplitude of the radiation from individual turbines, which is estimated on the order of 300–400 μms_1/ √Hz for wind speeds over the 8–14 m=s range. On the basis of this model, we then develop a predictive relationship for assessing the possible impact of future wind farm projects

Wavelet decomposition and advanced denoising techniquesn for analysis and classification of seismic signals

This work describes an automatic classification procedure for seismic signals suitable for the analysis of complex, broad-band waveforms commonly associated with fluid-rock interaction in volcanic and hydrothermal systems. Based on Discrete Wavelet Transform, a set of significant seismic signal features that characterize the type of event is identified (e.g. noise, volcano tectonic, long period). These features are initially assessed for events whose category (class) can be previously determined by an expert analyst. A Bayesian Pattern Recognition supervised technique based on these features is adopted to classify a new ‘unlabelled pattern’, whose class is unknown. In this way values computed for known events are used to classify events of unknown identity ('supervised classification'). A test was performed on seismological data recorded at Campi Flegrei (Italy), which was divided into three classes. Automatic classification accuracy ranges from 82% to 100% over a broad range of datasets

Detailed Analysis of Wave Propagation Beneath the Campi Flegrei Caldera (Italy)

We investigate the complex propagation of seismic waves beneath the Campi Flegrei Caldera, Italy, using multichannel recordings of artificial explosions. The sources consisted of air gun explosions shot in the Gulf of Pozzuoli at offsets ranging between 3 and 7 km. The multichannel recording device was deployed in the Solfatara crater and consisted of 10 vertical-component and 2 three-component short-period seismometers with a maximum aperture of about 150m. The Zero-Lag-Correlation (ZLC) technique was adopted to estimate horizontal slowness and backazimuth of coherent waves crossing the array. For sources located in the northern sector of the Gulf, with maximum offset 5 km, ray parameters and backazimuths are in agreement with those predicted for the 1-D velocity model used for routine locations. For sources at offsets larger than ~5 km, the ZLC curves depict prominent maxima associated with a secondary phase propagating with a lower velocity than the first-arrival P-wave. Using finite-difference synthetic seismograms generated for a 2-D realistic velocity model, we explain these late arrivals in terms of a lateral velocity variation located at depths of about 1 km. Such discontinuity would correspond to a positive Vp anomaly imaged by a recent 3-D tomographic study, and interpreted as the submerged southern rim of Campi Flegrei caldera collapsed during the explosive eruption of 12 Ky b.p. The small spacing among adjacent shot points allowed simultaneous wavefield decomposition at the source and receiver arrays. Using a modified version of the double-beam method, we retrieve the independent variation of horizontal slowness at both the source and receiver regions. For both cases, we found azimuthal deviations as large as 50° with respect to the great circle path. At the source region, these discrepancies may be interpreted in terms of ray bending at the interface of the aforementioned positive anomaly. At the receiver array, the observed anomalies may be attributed to either velocity variations marking the Solfatara crater rim, or to a near-receiver, low-velocity body whose position would coincide with negative gravimetric anomalies and a high Vp/Vs ratio region inferred by independent geophysical and seismological studies

Source complexity of the May 20, 2012, MW 5.9, Ferrara (Italy) event

A Mw 3.9 foreshock on May 19, 2012, at 23:13 UTC, was followed at 02:03 on May 20, 2012, by a Mw 5.9 earthquake that hit a densely populated area in the Po Plain, west of the city of Ferrara, Italy (Figure 1). Over the subsequent 13 days, six Mw >5 events occurred; of these, the most energetic was a Mw 5.8 earthquake on May 29, 2012, 12 km WSW of the main shock. The tragic balance of this sequence was 17 casualties, hundreds of injured, and severe damage to the historical and cultural heritage of the area. From a seismological point of view, the 2012 earthquake was not an outstanding event in its regional context. The same area was hit in 1996 by a Mw 5.4 earthquake [Selvaggi et al. 2001], and previously in 1986 and in 1967 (DBMI11) [Locati et al. 2011]. The most destructive historical event was the 1570, Imax 8 event, which struck the town of Ferrara [Guidoboni et al. 2007, Rovida et al. 2011]. The 2012 seismic sequence lasted for several weeks and probably developed on a well-known buried thrust fault [Basili et al. 2008, Toscani et al. 2009, DISS Working Group 2010], at depths between 2 km and 10-12 km

Decrypting geophysical signals at Stromboli Volcano (Italy): Integration of seismic and Ground-Based InSAR displacement data

We present the integration of seismic and Ground‐Based Interferometric Synthetic Aperture Radar system (GBInSAR) displacement data at Stromboli Volcano. Ground deformation in the area of summit vents is positively correlated with both seismic tremor amplitude and cumulative amplitudes of very long period (VLP) signals associated with Strombolian explosions. Changes in VLP amplitudes precede by a few days the variations in ground deformation and seismic tremor. We propose a model where the arrival of fresh, gas‐rich magma from depth enhances gas slug formation, promoting convection and gas transfer throughout the conduit system. At the shallowest portion of the conduit, an increase in volatile content causes a density decrease, expansion of the magmatic column and augmented degassing activity, which respectively induce inflation of the conduit, and increased tremor amplitudes. The temporal delay between increase of VLP and tremor amplitudes/conduit inflation can be interpreted in terms of the different timescales characterizing bulk gas transfer versus slug formation and ascent

Temporal evolution of long-period seismicity at Etna Volcano, Italy, and its relationships with the 2004–2005 eruption

Between December 2004 and August 2005, more than 50,000 long-period events (LP) accompanied by very-long period pulses (VLP) were recorded at Mt. Etna, encompassing the effusive eruption which started in September 2004. The observed activity can be explained by the injection of a gas slug formed within the magmatic column into an overlying cavity filled by either magmatic or hydrothermal fluids, thus triggering cavity resonance. Although a large number of LP events exhibit similar waveforms before the eruption, they change significantly during and after the eruption. We study the temporal evolution of the LP-VLP activity in terms of the source movement, change of the waveforms, temporal evolution of the dominant resonance frequencies and the source Q factor and changes in the polarization of the signal. The LP source locations before and after the eruption, respectively, do not move significantly, while a slight movement of the VLP source is found. The intensity of the LP events increases after the eruption as well as their dominant frequency and Q factor, while the polarization of the signals changes from predominantly transversal to pure radial motion. Although in previous studies a link between the observed LP activity and the eruption was not found, these observations suggest that such a link was established at the latter end of the eruptive sequence, most likely as a consequence of a reestablishment of the pressure balance in the plumbing system, after it was undermined due to the discharge of large amounts of resident magma during the eruption. Based on the polarization properties of the signal and geological setting of the area, a fluid-filled crack is proposed as the most likely source geometry. The spectral analysis based on the autoregressive-models (SOMPI) is applied to the signals in order to analyse the resonance frequencies and the source Q-factors. The results suggest water and basalt at low gas volume fraction as the most likely fluids involved in the source process. Using theoretical relations for the “slow waves” radiated from the fluid-filled crack, we also estimate the crack size for both fluids, respectively

Seismic Noise by Wind Farms: A Case Study from the VIRGO Gravitational Wave Observatory, Italy.

We present analyses of the noise wavefield in the vicinity of VIRGO, the Italy-France gravitational wave observatory located close to Pisa, Italy, with special reference to the vibrations induced by a nearby wind park. The spectral contribution of the wind turbines is investigated using (i) on-site measurements, (ii) correlation of spectral amplitudes with wind speed, (iii) directional properties determined via multichannel measurements, and (iv) attenuation of signal amplitude with distance. Among the different spectral peaks thus discriminated, the one at frequency 1.7 Hz has associated the greatest power, and under particular conditions it can be observed at distances as large as 11 km from the wind park. The spatial decay of amplitudes exhibits a complicate pattern, that we interpret in terms of the combination of direct surface waves and body waves refracted at a deep (_ 800 m) interface between the plio-pleistocenic marine, fluvial and lacustrine sediments and the Miocene carbonate basement. We develop a model for wave attenuation which allows determining the amplitude of the radiation from individual turbines, which is estimated on the order of 300-400 μms−119 /pHz for wind speeds over the 8-14 m/s range. On the base of this model, we then develop a predictive relationship for assessing the possible impact of future, project wind farms