Papers by Francesca Bianco
Journal of Seismology, 2009
Shear wave splitting parameters represent a useful tool to detail the stress changes occurring in... more Shear wave splitting parameters represent a useful tool to detail the stress changes occurring in volcanic environments before impending eruptions. In the present paper, we display the parameter estimates obtained through implementation of a semiautomatic algorithm applied to all useful datasets of the following Italian active volcanic areas: Mt. Vesuvius, Campi Flegrei, and Mt. Etna. Most of these datasets have been the object of several studies (Bianco et al., Annali di Geofisica, XXXXIX 2:429–443, 1996, J Volcanol Geotherm Res 82:199–218, 1998a, Geophys Res Lett 25(10):1545–1548, 1998b, Phys Chem Earth 24:977–983, 1999, J Volcanol Geotherm Res 133:229–246, 2004, Geophys J Int 167(2):959–967, 2006; Del Pezzo et al., Bull Seismol Soc Am 94(2):439–452, 2004). Applying the semiautomatic algorithm, we confirmed the results obtained in previous studies, so we do not discuss in much detail each of our findings but give a general overview of the anisotropic features of the investigated Italian volcanoes. In order to make a comparison among the different volcanic areas, we present our results in terms of the main direction of the fast polarization (φ) and percentage of shear wave anisotropy (ξ).
Geophysical Research Letters, 2009
1] In the present work we separated intrinsic from scattering attenuation coefficients both for t... more 1] In the present work we separated intrinsic from scattering attenuation coefficients both for the crust and the upper mantle in the tectonically highly active areas of the Southern-Central Andes -Nuevo Cuyo region, analyzing two groups of earthquakes, well separated in depth. This region is characterized by the presence of flat subduction. We apply MLTWA (Multiple Lapse Time Window Analysis), coda normalization and Q-coda techniques to measure the scattering and intrinsic attenuation coefficient and the total Q for S waves. We find that intrinsic attenuation does not decrease with depth whereas scattering attenuation is higher in the crust than in the upper mantle, and that intrinsic attenuation predominates over scattering attenuation. We interpret this observation in terms of the release of water and other fluids into the overlying lithosphere due to the dynamics of the subduction process, in agreement with most of the prevalent geodynamic models. Citation: Badi, G., E. Del Pezzo, J. M.
Physics of The Earth and Planetary Interiors, 2006
We present a high resolution 3D model of S-wave attenuation (Q −1 S ) for the volcanic structure ... more We present a high resolution 3D model of S-wave attenuation (Q −1 S ) for the volcanic structure of Mt. Vesuvius. Data from 959 waveforms relative to 332 volcano-tectonic earthquakes located close to the crater axis in a depth range between 1 and 4 km (below the sea level) recorded at 6 three-component seismic stations were used for the inversion. We obtained the estimate of Q −1 S for each source-station pair using a single-station method based on the normalization of the S-wave spectrum for the coda spectrum at 12 s lapse time. This is a modification of the well known coda-normalization method to estimate the average Q −1 S for a given area. We adopt a parabolic ray-tracing in the high resolution 3D velocity model which was previously estimated using almost the same data set; then we solve a linear inversion scheme using the L-squared norm with positive constraints in 900 m-side cubic blocks, obtaining the estimate of Q −1 S for each block. Robusteness and stability of the results are tested changing in turn the input data set and the inversion technique. Resolution is tested with both checkerboard and spike tests. Results show that attenuation structure resembles the velocity structure, well reproducing the interface between the carbonates and the overlying volcanick rocks which form the volcano. Analysis is well resolved till to a depth of 4-5 km. Higher Q contrast is found for the block overlying the carbonate basement and close to the crater axis, almost cohincident with a positive P-wave velocity contrast located in the same volume and previously interpreted as the residual high density body related to the last eruptions of Mt. Vesuvius. We interpret this high-Q zone as the upper part of carbonate basement in which most of the high energy seismicity take place. The low-Q values found at shallow depth are interpreted as due to the high heterogeneity mainly caused by the mixing of lava layers and pyroclastic materials extruded during the last eruptions.
Journal of Volcanology and Geothermal Research, 2003
The occurrence of a seismic series in Deception Island volcano (Antarctica), composed of hundreds... more The occurrence of a seismic series in Deception Island volcano (Antarctica), composed of hundreds of local volcano^tectonic earthquakes, has permitted us to study the seismic attenuation of such a volcanic environment in the short-distance and high-frequency range. This study has been performed using P-waves, S-waves and coda-waves and applying different, frequency dependent and independent, techniques. The methods used for this analysis have been: spectral and broadening-of-the-pulse, for direct P-and S-waves, coda normalization for S-waves, and single backscattering model for coda-waves. The results show that, in general, Q values are significantly smaller for the entire frequency range used (6^30 Hz) than those found in other volcanic and tectonic areas. The attenuation for P-waves is greater than for S-waves in the frequency independent methods, with a Q L /Q P ratio that ranges between 1.9 and 3.2. Comparing the Q-factor obtained for S-waves we have observed clear differences as a function of the method used; the coda normalization method has supplied significantly higher Q values (Q d ) than the other two methods (Q L ). We have interpreted this discrepancy as an effect of the methods: coda normalization and single back-scattering methods eliminate the contribution of the near-surface attenuation in their Q values. Comparing both Q L and Q d we have estimated the near-surface attenuation under the recording site, named Q U . On the other hand, we have observed that Q d has anomalous frequency dependence, with a minimum value at 21 Hz. This pattern is interpreted as an effect of strong scattering of the seismic waves in the source area of the earthquakes. Q c values depend clearly with frequency and lapse time and the lapse time dependence could be interpreted as a depth dependence of the seismic attenuation in Deception Island volcano, Antarctica. The obtained Q values have allowed us to separate the contribution of intrinsic and scattering attenuation, deriving that the scattering attenuation is predominant over the intrinsic effects. Finally, in order to investigate how the heterogeneous medium of the volcanic island could produce other effects, we have checked whether it produces polarization of the shear-waves. The preliminary results of the polarization direction indicate a main E^W strain direction. All these evidences reveal the strongly heterogeneous structure of Deception Island volcano. ß (C. Mart| ¤nez-Are ¤valo), [email protected] (F. Bianco), [email protected] (J.M. Iba ¤n ‹ ez), [email protected] (E. Del Pezzo).
Tectonophysics, 2006
The attenuation of seismic waves is one of the basic physical parameters used in seismological st... more The attenuation of seismic waves is one of the basic physical parameters used in seismological studies and earthquake engineering, which is closely related to the seismicity and regional tectonic activity of a particular area. In this work, the seismic attenuation in the Straits of Messina, affected by several and strong historical earthquakes, was studied using waveforms recorded by a local seismic network composed of seven stations. We measured: the coda quality factor (Q c ) in the Single Scattering model hypothesis; the direct quality factor (Q d ) applying the Coda Normalization method for S-waves; and the intrinsic and scattering quality factor (Q i and Q s ) by the Multiple Lapse Time Window Analysis (MLTWA) method. Coda Q values were obtained using different lapse times (40, 60 and 80 s) for the frequency bands centred at 1.5, 3, 6 and 12 Hz. Our findings indicate that Q c increases with increasing lapse time and that Q c is frequency dependent. This behaviour is usually correlated to the degree of tectonic complexity and to the presence of heterogeneities at several scales. In fact, by using the Coda Normalization method we obtained low Q d values, as expected for a heterogeneous and active zone.
Geophysical Research Letters, 2001
The energy transfer theory in the hypothesis of 2-D isotropic multiple scattering is applied to t... more The energy transfer theory in the hypothesis of 2-D isotropic multiple scattering is applied to the propagation of volcanic tremor. Under the assumption of a time stationary energy source, we calculate the pattern of energy vs distance as a function of intrinsic and scattering Q. Experimental values of attenuation coefficient ?-• measured at Etna and Masaya Volcanoes are fitted to the theory in order to contemporaneously estimate the loss of energy associated to scattering processes and that produced by intrinsic dissipation. Despite the high degree of trade off between the estimate of intrinsic and scattering attenuation deduced by the data analysis, results indicate that the tremor energy decay with distance is more strongly controlled by intrinsic dissipation than by scattering. ---3 25 31 9 25-65 180 •100
Bulletin of The Seismological Society of America, 2004
We study the time changes of (1) the b-value of the Gutenberg-Richter distribution, (2) the inver... more We study the time changes of (1) the b-value of the Gutenberg-Richter distribution, (2) the inverse coda Q( ), and (3) the shear-wave splitting parameters 1מ Q C (i.e., the time delay T d between qS 1 and qS 2 phases and the polarization direction of the qS 1 wave) for small-magnitude volcano-tectonic earthquakes of Mt. Vesuvius, Italy. We used for (1) the seismic catalog of Mt. Vesuvius seismicity starting from January 1994, for (2) a selected (on the basis of the best signal-to-noise ratio) set of data with hypocentral distances smaller than 4 km recorded at station BKE (analogical) with a 1-Hz vertical seismometer during the period from January 1994 until the present, and for (3) a set of data recorded at two digital, high dynamical range, portable short-period seismic stations. These stations (BKE and BKN) were in operation in two periods, BKE (digital) from January 1999 to the middle of 2000 and BKN from January 1999 to the end of 1999; the hypocentral distances were not greater than 4 km. We found evidence of time changes of measured at high frequency 1מ 1מ 1מ
Computers & Geosciences, 2002
Geophysical Journal International, 2005
We investigated the intrinsic dissipation and scattering properties of the lithosphere under the ... more We investigated the intrinsic dissipation and scattering properties of the lithosphere under the Friuli region (northeastern Italy) using two hypotheses: (i) a uniform earth model and (ii) two 'reasonable' non-uniform, layered crustal models. For case (i) we measured the coda Q, and used the multiple-lapse time window analysis (MLTWA) technique to obtain separate estimates of intrinsic absorption and scattering attenuation.
Geophysical Journal International, 2002
Scattered waves observed at the seismographs of the National Italy's seismic network have been us... more Scattered waves observed at the seismographs of the National Italy's seismic network have been used to investigate the intrinsic dissipation and scattering properties of the lithosphere under the Southern Apennines, Italy. First, we investigate the coda-Q properties, then we apply the MLTW analysis in the hypothesis of velocity and scattering coefficient constant with depth, and finally we interpret these results with the aid of numerical simulations in a medium with depth dependent velocity and scattering coefficient.Results obtained in the hypothesis of a uniform model show that a low scattering-Q−1 and a relatively higher intrinsic-Q−1 characterize the lithosphere of the Southern Apennines. Numerical simulations of the seismogram energy envelopes were performed hypothesizing a strongly scattering crust and trasparent upper mantle, both with reasonable intrinsic dissipation coefficients. In these symplifying assumptions the theoretical curves calculated for the homogeneous model fit to the synthetic envelopes with scattering attenuation coefficients always greater than the synthetic values. This results lead to the consideration that scattering-Q−1 obtained using MLTW analysis under the assumption of uniform medium are overestimated. The values of the scattering-Q−1 estimated for Apennines at low frequency (1–2 Hz) in the hypothesis of uniform medium are of the same order of those obtained in several areas around the world. The estimates obtained for frequencies ranging from 2 to 12 Hz are very low if compared with those obtained in the same hypothesis for other areas around the world. Coda Q−1 closely resembles intrinsic Q−1.
Geophysical Research Letters, 2002
1] A high resolution P-wave image of Mt. Vesuvius edifice has been derived from simultaneous inve... more 1] A high resolution P-wave image of Mt. Vesuvius edifice has been derived from simultaneous inversion of travel times and hypocentral parameters of local earthquakes, land based shots and small aperture array data. The results give details down to 300-500 m. The relocated local seismicity appears to extend down to 5 km below the central crater, distributed in a major cluster, centered at 3 km below the central crater and in a minor group, with diffuse hypocenters inside the volcanic edifice. The two clusters are separated by an anomalously high Vp region at around 1 km depth. A zone with high Vp/Vs in the upper layers is interpreted as produced by the presence of intense fluid circulation. The highest energy quakes (up to M = 3.6) are located in the deeper cluster, in a high P-wave velocity zone. Our results favor an interpretation in terms of absence of shallow magma reservoirs.
Journal of Volcanology and Geothermal Research, 1998
A detailed structural and geophysical study of the Somma-Vesuvius volcanic complex was carried ou... more A detailed structural and geophysical study of the Somma-Vesuvius volcanic complex was carried out by integrating mesostructural measurements, focal mechanisms and shear-wave splitting analysis. Fault-slip and focal mechanism analysis indicate that the volcano is affected by NW-SE-, NE-SW-trending oblique-slip faults and by E-W-trending normal faults.
Physics and Chemistry of The Earth Part A-solid Earth and Geodesy, 1999
... F. Bianco 1 , M. Castellano 1 , Corresponding Author Contact Information , G. Milano 1 , G. V... more ... F. Bianco 1 , M. Castellano 1 , Corresponding Author Contact Information , G. Milano 1 , G. Vilardo 1 , F. Ferrucci 2 and S. Gresta 3. ... A. Zollo, P. Gasparini, J. Virieux, H. Le Meur, G. De Natale, G. Biella, E. Boschi, P. Capuano, R. De Franco, P. Dell'Aversana, R. De Matteis, I ...
Geophysical Research Letters, 2001
Bulletin of The Seismological Society of America, 2009
The site-corrected source scaling pattern is estimated for local earthquakes (0:9 ≤ M D ≤ 3:6) at... more The site-corrected source scaling pattern is estimated for local earthquakes (0:9 ≤ M D ≤ 3:6) at Mt. Vesuvius. The dataset comprises 35 low-to-moderate local earthquakes recorded by 14 three-component seismic stations during 1993, 1996, and 1999. Site-transfer functions in the frequency range 1 Hz-25 Hz are estimated from the spectra of S waves and coda waves and from the horizontal-to-vertical (H=V) spectral ratios. We applied the direct spectral ratios method to S waves, considering as a reference the average spectrum and the inversion method to S waves and coda waves. The site amplification on the coda waves was also compared with that evaluated using the wavelet transform. The standard deviation associated with the experimental results is computed for all of the used methods.
Physics of The Earth and Planetary Interiors, 2006
A scattering image of the Campi Flegrei caldera has been obtained using coda wave envelopes from ... more A scattering image of the Campi Flegrei caldera has been obtained using coda wave envelopes from local microearthquake data recorded at a portable 12-station digital network. This was operated during the paroxysmic phase of the ground uplift (bradyseism) that occurred in the years 1983-1984. The Nishigami technique has been used to obtain the 3D distribution of the strong scatterers in an earth volume with a horizontal extension of 20 km × 20 km and a vertical thickness of 6 km. This earth volume was segmented in blocks of 2 km × 2 km × 2 km for the outer section, and in blocks of 1 km × 1 km × 1 km for the inner section. The inversion was performed using a hybrid jackknife-SVD method that has allowed qualitative control of the stability and robustness of the solution. The results show that the scatterers revealed by the coda envelopes, which were band-pass filtered at a center frequency of 18 Hz, match the inner border of the caldera rim, which is well outlined on land by geological observations. The scatterer position also delineates this border well beneath sea level. These results are in close agreement with recently obtained seismic velocity tomography. The scatterers imaged at the 6 and 12 Hz band-pass frequencies are located around the zone of maximum uplift, beneath Solfatara crater, where most of the present residual volcanic activity is concentrated.
Physics of The Earth and Planetary Interiors, 1997
Journal of Volcanology and Geothermal Research, 2004
On October 9, 1999 an earthquake of M L = 3.6 occurred about 3 km beneath the central cone of Mt.... more On October 9, 1999 an earthquake of M L = 3.6 occurred about 3 km beneath the central cone of Mt. Vesuvius, near Naples, Italy. The event had the highest magnitude recorded for at least 25 years, and possibly since the last eruption of this volcano (1944), and was not accompanied by other geophysical or geochemical changes. The present paper essentially deals with the seismological data collected at Mt. Vesuvius for 29 years before the October 9 earthquake till the end of 2001, and describes the time pattern distribution of seismic slip release and the b-parameter of the Gutenberg^Richter distribution. The self-similarity of the source process is investigated through the scaling law of the seismic spectrum. Results indicate a two-fold pattern of stress release, with high values (up to 100 bar) for earthquakes occurring close to the top of the carbonate basement that underlies the volcano at 2^3 km of depth, and low values (down to 0.1 bar) for the shallow events occurring within the volcanic edifice. The scaling law of the seismic spectrum is non-self-similar, indicating that the source dimensions do not scale with the seismic moment. For this reason the low-magnitude events substantially contribute to the overall cumulative seismic slip release. The bparameter of the Gutenberg^Richter distribution shows a variation around 1980, and a substantial constancy in the other time periods. The presence of extended aquifers, with their tops at about 1 km beneath the crater, favors the hypothesis of the triggering of the shallowest events by water-level changes. This hypothesis is in agreement with the low values of the stress drop measured for the shallowest seismic events. The existence of a carbonate basement with its top at about 2.5 km beneath the crater and the higher stress drops for the deeper events make reasonable the hypothesis that the pre-fractured carbonate basement may be the site of tectonic stress release. ß
Physics of The Earth and Planetary Interiors, 2006
Seismic attenuation in the area of Mt. Vesuvius is reappraised by studying more than 400 S-coda e... more Seismic attenuation in the area of Mt. Vesuvius is reappraised by studying more than 400 S-coda envelopes of small local VT earthquakes recorded at Mt. Vesuvius from 1996 to 2002 at the three-component stations of OVO and BKE. The purpose is to obtain a stable separate estimate of intrinsic and scattering quality factors for shear waves. We investigate in the present paper four frequency bands, centered respectively at f c = 3, 6, 12 and 18 Hz with a bandwidth of 0.6f c . Stacked coda envelopes are fit to the multiple scattering model according to the Zeng approximation in the hypothesis of constant velocity half space. Results show that the diffusion regime is a good approximation as the scattering attenuation (proportional to Q −1 S , the inverse scattering-quality factor) is much stronger than the intrinsic dissipation (proportional to Q −1 I ). Q −1 S decreases with frequency while intrinsic attenuation is much less frequency-dependent. We also fit the stacked coda envelopes at BKE to the diffusion equation solved with the boundary condition of a 2 km thick diffusive layer over a homogeneous half space. Results show that the diffusivity, D, estimated in the assumption of reflecting boundary condition is greater than that estimated in the assumption of uniform half space, whereas the diffusivity estimated with the absorbing boundary condition is close to the estimate done in the assumption of half space. OVO station shows results different from those obtained at BKE and at a group of five stations located on Mt. Vesuvius for the frequency bands centered at 12 and 18 Hz. In these two bands, scattering attenuation at OVO is comparable to the intrinsic dissipation, and is much smaller than that measured at the other stations. We interpret this anomaly as due to an effect of strong lateral heterogeneity which modifies the redistribution of the seismic energy into the coda at OVO. A comparison of the results obtained using passive data (the present data set) and the active data obtained in the same area during TOMOVES experiment by Wegler show that the diffusivity estimated with shot data during TOMOVES is smaller of a factor greater than 4. This discrepancy is interpreted as due to different earth volumes sampled by the coda waves in the two cases.
Uploads
Papers by Francesca Bianco