30 years of coda observations: Qc, Qi and Qs

A summary of the main results obtained worldwide, in memory of Keiiti Aki

Inside Mt. Vesuvius: a new method to look at the seismic (velocity and attenuation) tomographic imaging

Mt. Vesuvius have been obtained using the programming facilities as well as the enhanced graphical power of Mathematica8TM. The velocity and attenuation space distributions, already calculated inverting respectively P-wave travel times and amplitude spectra of local VT quakes, are first optimally interpolated and then graphically represented in a new Mathematica8TM code notebook (a powerful computational document with more facilities than a simple code) developed by the present authors. The notebook aims at interactively and friendly representing 3D volume distributions of velocity and attenuation parameters. The user can easily obtain vertical sections (N-S, E-W, NE-SW and NW-SE oriented) and define color scales to represent velocity or attenuation variations or prefer iso-surface plots to represent the pattern of peculiar geological structures. The use of dynamic graphical representation, allowing the sliding of any (horizontal and/or vertical) slice through the volume under study, gives an unusual and powerful vision of any small velocity or attenuation anomaly. The (open source) code, coupled with the friendly use of internal routines of Mathematica, allows to adapt the graphical representation to any user necessity. The method appears to be particularly adapt to represent attenuation images, where the space variations of the parameters are strong with respect to their average. The 3-D plots of the interpolated velocity and attenuation fields enhance the image of Mt. Vesuvius structure, evidencing low-velocity associated with high attenuation anomalies which appeared unfocused in the plots reported by Scarpa et al. [2002] and De Siena et al. [2009]

Shear wave splitting changes associated with the 2001 volcanic eruption on Mt. Etna

The time delays and polarizations of shear wave splitting above small earthquakes show variations before the 2001 July 17–August 9 2001 flank eruption on Mt Etna, Sicily. Normalized time delays, measured by singular value decomposition, show a systematic increase starting several days before the onset of the eruption. On several occasions before the eruption, the polarization directions of the shear waves at Station MNT, closest to the eruption, show 90◦- flips where the faster and slower split shear waves exchange polarizations. The last 90◦-flip being 5 days before the onset of the eruption. The time delays also exhibit a sudden decrease shortly before the start of the eruption suggesting the possible occurrence of a ‘relaxation’ phenomena, due to crack coalescence. This behaviour has many similarities to that observed before a number of earthquakes elsewhere

Seismic Q estimates in Umbria-Marche (central Italy): hints for the retrieval of a new attenuation law for seismic risk

In the Umbria Marche (Central Italy) region an important earthquake sequence occurred in 1997, characterized by nine earthquakes with magnitudes in the range between 5 and 6, that caused important damages and causalities. In the present paper we separately estimate intrinsic- and scattering- Q −1 parameters, using the classical MLTWA approach in the assumption of a half space model. The results clearly show that the attenuation parameters Qi −1 and Qs −1 are frequency dependent. This estimate is compared with other attenuation studies carried out in the same area, and with all the other MLTWA estimates obtained till now in other tectonic environments in the Earth. The bias introduced by the half space assumption is investigated through numerical solutions of the Energy Transport equation in the more realistic assumption of a heterogeneous crust overlying a transparent mantle, with a Moho located at a depth ranging between 35 and 45 km below the surface. The bias introduced by the half space assumption is significant only at high frequency. We finally show how the attenuation estimates, calculated with different techniques, lead to different PGA decay with distance relationships, using the well known and well proven Boore’s method. This last result indicates that care must be used in selecting the correct estimate of the attenuation parameters for seismic risk purposes. We also discuss the reason why MLTWA may be chosen among all the other available techniques, due to its intrinsic stability, to obtain the right attenuation parameters

The first Long Period earthquake detected in the background seismicity at Mt. Vesuvius

The typical earthquakes occurring at Mt. Vesuvius are Volcano-Tectonic. On July 20, 2003, an unusual earthquake with low and narrow frequency content was detected. The seismograms presented an emergent onset and a nearly monochromatic spectrum at all stations of the Osservatorio Vesuviano(Istituto Nazionale di Geofisica e Vulcanologia) seismic network. The event was located at about 4 km b.s.l. close to the crater axis and an equivalent duration magnitude of 0.6 was estimated. The nature of this event was investigated by comparing its features with those of two typical Volcano-Tectonic earthquakes occurred inside the same source volume. We compared the spectral content calculating the spectrograms and the coda patterns using the Hilbert Transform. A Seismic Moment Tensor inversion was performed on the low frequency earthquake. The focal mechanisms for the two Volcano-Tectonic earthquakes were estimated with a classical technique and resulted compatible with the stress field acting on the volcano. Taking into account the clear differences with the typical Volcano-Tectonic events as well as the peculiarities retrieved from our analyses (monochromatic, low frequency spectral content, and sustained coda) and also some geochemical observations, we classify the unusual low frequency seismic event detected at Mt. Vesuvius as Long Period earthquake and propose that its origin could be linked to a pressure drop in the deep hydrothermal system

The stress-field variation on the italian active volcanic areas: the shear wave splitting monitoring

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Shear wave splitting time variation by stress-induced magma uprising at Mount Etna volcano

Shear wave splitting exhibits clear time variations before the July 17th – August 9th, 2001 flanK eruption at Mount Etna. The normalized time delays, Tn, detected through an orthogonal transformation of singular value decomposition, exhibit a clearincrease starting 20 days before the occurrence of the eruption (July 17th); the qS1 polarization direction, obtained using a 3D covariance matrix decomposition, shows a 90°-flip several times during the analyzed period: the last flip 5 days before the occurrence of the eruption. Both splitting parameters also exhibit a relaxation phase shortly before the starting of the eruption. Our observations seem in agreement with Anisotropic Poro Elasticity (APE) modelling, suggesting a tool for the temporal monitoring of the build up of the stress leading to the occurrence of the 2001 eruption at Mt. Etna

Observation of coda signals from regional and local earthquakes recorded from a downhole-uphole couple of broad-band sensors at Mt Etna

Seismic coda of regional and local earthquakes recorded at a couple of broad band seismometers located at the bottom of a 125 m deep borehole and up-hole at surface show interesting spectral features. We observe strong similarity between the waveforms recorded from up- and down-hole sensors at low frequency (0.1 -3 Hz) and measurable differences in the higher frequency limit. We interpret this observation assuming that at high frequency the up-hole coda is produced by body-tosurface wave scattering in the near surface. We compare the experimental results with numerical simulations done using the Monte Carlo scheme of Yoshimoto et al. (2000) carried out in the assumption of velocity and scattering coefficient which smoothly vary with depth, with the addition of a body-to-surface wave conversion for the energy particles which reach the surface. The comparison of the experimental coda envelopes with those obtained through numerical simulation allow for a quantification of the turbidity parameter at surface

Multiple resolution seismic attenuation imaging at Mt. Vesuvius

A three-dimensional S wave attenuation tomography of Mt. Vesuvius has been ob- tained with multiple measurements of coda-normalized S-wave spectra of local small magnitude earthquakes. We used 6609 waveforms, relative to 826 volcano-tectonic earthquakes, located close to the crater axis in a depth range between 1 and 4 km (below the sea level), recorded at seven 3-component digital seismic stations. We adopted a two-point ray-tracing; rays were traced in an high resolution 3-D velocity model. The spatial resolution achieved in the attenuation tomography is comparable with that of the velocity tomography (we resolve 300 m side cubic cells). We statisti- cally tested that the results are almost independent from the radiation pattern. We also applied an improvement of the ordinary spectral-slope method to both P- and S-waves, assuming that the di¤erences between the theoretical and the experimental high frequency spectral-slope are only due to the attenuation e¤ects.We could check the coda-normalization method comparing the S attenuation image obtained with the two methods. The images were obtained with a multiple resolution approach. Results show the general coincidence of low attenuation with high velocity zones. The joint interpretation of velocity and attenuation images allows us to interpret the low attenuation zone intruding toward the surface until a depth of 500 meters below the sea level as related to the residual part of solidi ed magma from the last eruption. In the depth range between -700 and -2300 meters above sea level, the images are consistent with the presence of multiple acquifer layers. No evidence of magma patches greater than the minimum cell dimension (300m) has been found. A shallow P wave attenuation anomaly (beneath the southern ank of the volcano) is consitent with the presence of gas saturated rocks. The zone characterized by the maximum seismic energy release cohincides with a high attenuation and low velocity volume, interpreted as a cracked medium

New insights into Mt. Vesuvius hydrothermal system and its dynamic based on a critical review of seismic tomography and geochemical features

The seismic velocity and attenuation tomography images, calculated inverting respectively P-wave travel times and amplitude spectra of local VT quakes at Mt. Vesuvius have been reviewed and graphically represented using a new software recently developed using Mathematica8TM. The 3-D plots of the interpolated velocity and attenuation fields obtained through this software evidence low-velocity volumes associated with high attenuation anomalies in the depth range from about 1 km to 3 km below the sea level. The heterogeneity in the distribution of the velocity and attenuation values increases in the volume centred around the crater axis and laterally extended about 4 km, where the geochemical interpretation of the data from fumarole emissions reveals the presence of a hydrothermal system with temperatures as high as 400-450°C roughly in the same depth range (1.5 km to 4 km). The zone where the hydrothermal system is space-confined possibly hosted the residual magma erupted by Mt. Vesuvius during the recent eruptions, and is the site where most of the seismic energy release has occurred since the last 1944 eruption