ALBERT

Description: Over fifty eruptive episodes with Strombolian activity, lava fountains, and lava flows occurred at Mt Etna volcano between 2006 and 2013. Namely, there were seven paroxysmal lava fountains at the South-East Crater in 2007-2008 and 46 at the New South-East Crater between 2011 and 2013. Lava emissions lasting months affected the upper eastern flank of the volcano in 2006 and 2008-2009. Effective monitoring and forecast of such volcanic phenomena are particularly relevant for their potential socio-economic impact in densely populated regions like Catania and its surroundings. For example, explosive activity has often formed thick ash clouds with widespread tephra fall able to disrupt the air traffic, as well as to cause severe problems at infrastructures, such as highways and roads. Timely information about changes in the state of the volcano and possible onset of dangerous eruptive phenomena requires efficacious surveillance methods. The analysis of the continuous background seismic signal, the so-called volcanic tremor, turned out of paramount importance to follow the evolution of volcanic activity [e.g., Alparone et al., 2003; Falsaperla et al., 2005]. Changes in the state of the volcano as well as in its eruptive style are usually concurrent with variations of the spectral characteristics (amplitude and frequency) of tremor. The huge amount of digital data continuously acquired by INGV’s broadband seismic stations every day makes a manual analysis difficult. In order to tackle this problem, techniques of automatic classification of the tremor signal are applied. In a comparative study, the robustness of different methods for the identification of regimes in volcanic activity were examined [Langer et al., 2009]. In particular, Langer et al. [2011] applied unsupervised classification techniques to the tremor data recorded at one station during seven paroxysmal episodes in 2007-2008. Their results revealed significant changes in the pattern classification well before the onset of the eruptive episodes. This evidence led to the development of specific software packages, such as the program KKAnalysis [Messina and Langer, 2011], a software that combines an unsupervised classification method (Kohonen Maps) with fuzzy cluster analysis. The operational characteristics of these tools - fail-safe, robustness with respect to noise and data outages, as well as computational efficiency - allowed on-line processing at the operative centre of the INGV-Osservatorio Etneo in 2010 and the identification of criteria for automatic alarm flagging. The system is hitherto one of the main automatic alerting tools to identify impending eruptive events at Etna. The software carries out the on-line processing of the new data stream coming from two seismic stations, merged with reference datasets of past eruptive episodes. In doing so, results obtained for new data are immediately compared to previous eruptive scenarios. Given the rich material collected in recent years, we are able to apply the alert system to eleven stations at different elevations (1200-3050 m) and distances (1-8 km) from the summit craters. Critical alert parameters were empirically defined to obtain an optimal tuning of the alert system for each station. To verify the robustness of this new, multistation alert system, a dataset encompassing about eight years of continuous seismic records (since 2006) was processed automatically using KKAnalysis and collateral software off-line. Then, we analyzed the performance of the classifier in terms of timing and spatial distribution of the stations. We also investigated the performance of the new alert system based on KKAnalysis in case of activation of whatever eruptive centre. Intriguing results were obtained in 2010 throughout periods characterized by the renewal of volcanic activity at Bocca Nuova-Voragine and North-East Crater, and in the absence of paroxysmal phenomena at South-East Crater and New South-East Crater. Despite the low-energy phenomena reported by volcanologists (i.e., degassing, low-to moderate explosions), the triggered alarms demonstrate the robustness of the classifier and its potential: i) to identify even subtle changes within the volcanic system using tremor, and ii) to highlight the activation of a single eruptive centre, even though different from the one for which the classifier was initially tested. It is worth noting that in case of activation of weak sources, the successful performance of the classifier depends upon the general level of signals originating from other sources in that specific time span.

Description: Published

Description: Nicolosi (Catania, Italy)

Description: 2V. Dinamiche di unrest e scenari pre-eruttivi

Description: open

Description: Mt. Vesuvius is a worldwide known active volcano located in Southern Italy, east of the city of Napoli. Its last eruption occurred in 1944, ending a continuous open-conduit activity that lasted for more than three centuries. After then the volcano entered into a quiescent phase. Nonetheless, the explosive style of its past activity and its proximity to densely populated areas make Mt. Vesuvius one of the most dangerous volcanoes in the world. For this reason, a comprehensive monitoring program was established, with the goal of early detection of any possible signals of unrest.

Description: Published

Description: 809-816

Description: 5V. Sorveglianza vulcanica ed emergenze

Description: JCR Journal

Description: restricted

Description: Mt. Etna is permanently active requiring a continuous data acquisition a multidisciplinary monitoring system where huge data masses accumulate and pose severe difficulties of interpretation. Therefore the INGV staff has developed a number of software tools for data mining, aiming at identifying structures in the data which can be related to the volcanic activity and furnish criteria for the definition of alert systems. We tackle the problem by applying methods of clustering and classification. We identify data groups by defining a measure of similarity or distance. Data groups may assume various shapes, once forming convex clouds once complex concave bodies. The tool “KKAanalysis” is a basket of clustering methods and forms the backbone of the tremor-based automatic alarm system of INGV-OE. It exploits both SOM and Fuzzy Clustering. Besides seismic data the concept has been applied to petrochemic data as well as in a combined analysis of gas-emission data and seismic data. The software “DBSCAN” focuses on density-based clustering that allows discovering clusters with arbitrary shape. Here, clusters are defined as dense regions of objects in the data space separated by regions of low density. In DBSCAN a cluster grows guaranteeing that the density within a group of objects exceeds some threshold. In the context of volcano monitoring the method is particularly promising in the recognition of ash particles as they have a rather irregular shape. The “MOTIF” software allows identifying typical wave forms in time series. It overcomes shortages of methods like cross- correlation, which entail a high computational effort. MOTIF on the other hand can recognize non-similarity of two patterns on a small number of data points without going through the whole length of the data vectors. The development includes modules for feature extraction and post-processing verifying the validity of the results obtained by the classifiers.

Description: Published

Description: Nicolosi (Catania, Italy)

Description: 2V. Dinamiche di unrest e scenari pre-eruttivi

Description: open

Description: Timely identification of changes in the state of volcanoes and onset of potentially dangerous eruptive phenomena requires efficacious surveillance methods. In the case of an active volcano like Mt Etna, the continuous background seismic signal called volcanic tremor is of paramount importance. The huge amount of continuously acquired digital data entails the necessity of data reduction and parameter extraction. For this purpose, techniques of automatic analysis of volcanic tremor were applied by INGV for the real time monitoring of this signal. We checked the possibility to identify regimes of volcanic activity based on pattern classification of volcanic tremor. A specific software named “KKAnalysis” was developed. It combines various unsupervised classification methods (Kohonen Maps and fuzzy cluster analysis) and forms the backbone of an automatic alert system at INGV-OE. Besides its near real time application, it can be operated off-line, allowing an efficient a-posteriori processing of data and tuning of the alarm criteria to match specific needs of sensitivity and robustness. An ongoing development of this tool will allow us to include a large number of seismic stations in a multistation-alarm system. The new system will be more robust in case of failure of single sensors, and will achieve a better coverage of the various eruptive craters. In an off-line test, we exploited a dataset covering eight years of seismic records, and analysed the performance of the new system in terms of “trigger timing” and spatial distribution of the stations. Intriguing results were obtained throughout periods of renewal of volcanic activity at Bocca Nuova-Voragine and North East Crater, and in the absence of paroxysmal phenomena at South East Crater and New South East Crater.

Description: Published

Description: Nicolosi (Catania, Italy)

Description: 2V. Dinamiche di unrest e scenari pre-eruttivi

Description: open

Description: Twenty-five lava fountains occurred on Mt. Etna from January 2011 to April 2012. In summer 2012 volcanic activity resumed in a milder form within the Bocca Nuova crater, before it came to an essential halt in August 2012. All these unrests offer rich material for testing automatic procedures of data processing and alert systems, running 24/7, in the context of volcano surveillance. We focus on the seismic background radiation – volcanic tremor – which plays a key role in the monitoring of Mt. Etna. Since 2006 a multistation alert system has been established in the INGV operative centre of Catania exploiting STA/LTA ratios. Besides, also the spectral characteristics of the signal, which change correspondingly to the type of volcanic activity, can be exploited for warning purposes. Here we apply Self Organizing Maps and Fuzzy Clustering which offer an efficient way to visualize signal characteristics and its development with time. All these techniques allow to identify early stages of eruptive events, and automatically flag a critical status before this becomes evident in conventional monitoring techniques. Changes of tremor characteristics are related to the position of the source of the signal. The location of the sources exploits the distribution of the amplitudes across the seismic network. The locations were extremely useful for warning, throughout both the flank eruption in 2008 as well as the 2011 lava fountains, during which a clear migration of tremor sources towards the eruptive centres could be noticed in advance. The location of the sources completes the picture of an imminent volcanic unrest, and corroborates early warnings flagged by the changes of signal characteristics. Real time data processing requires computational efficiency, robustness of the methods and stability of data acquisition. The amplitude based multi-station approach is not sensitive to the failure of single stations and therefore offers a good stability. The single station approach, exploiting unsupervised classification techniques, limits logistic efforts, as only one or few key stations are necessary. Both strategies have proven to be insensitive to disturbances (undesired transients like earthquakes, noise, short gaps in the continuous data flow). False alarms were not encountered so far. Stable data acquisition and processing come with a properly designed data storage solution. The reliability of data storage and its access is a critical issue. A cluster architecture has been realized for failover protection, including a Storage Area Network system, which allow easy data access following predefined user policies. We present concepts of the software architectures deployed at INGV Osservatorio Etneo in order to implement this tremor-based multi approach system. We envisage the integration of seismic data and those originating from other scientific fields (e. g., volcano imagery, geochemistry, deformation, gravity, magneto-telluric). This will facilitate cross-checking of evidences encountered from the single data streams, in particular allow their immediate verification with respect to ground truth.

Description: Published

Description: Nicolosi (Catania, Italy)

Description: 1.4. TTC - Sorveglianza sismologica delle aree vulcaniche attive

Description: 5.6. TTC - Attività di Sala Operativa

Description: open

Description: In the present paper, we will describe the field survey (Fig. 1) and the data analysis of an experiment carried out to put constraints on the magnitude detection threshold in the area of Campi Flegrei. Results show that seismic radiation emitted from VT seismic events at frequency lower than 2 Hz has a high detection threshold (minimum magnitude around 1.5). In the range between 2 and 20 Hz, VT events with magnitudes smaller than about 0.5 have a high probability to be undetected. This result indicates that noise reduction through borehole stations and/or small arrays is essential for an accurate seismic monitoring in the Campi Flegrei area.

Description: Published

Description: 190-198

Description: 3.1. Fisica dei terremoti

Description: JCR Journal

Description: restricted

Description: The 2001 Etna eruption occurred from July 17th to August 9th, 2001 and was preceded by several days of intense seismicity and ground deformation. We investigated the seismic activity recorded during November 2000 - June 2001 interval time preceding the eruption, to understand the meaning of the seismicity connected to the dike intrusion, that locally modified the stress field acting in the area. The earthquakes were recorded by the permanent local networks operating during that time and run by the Istituto Internazionale di Vulcanologia (IIV-CNR) and the Sistema POSEIDON. During the analyzed period, 683 earthquakes have been firstly localized by means of a 1D velocity model derived from Hirn et al., 1991 using the software HypoEllipse [Lahr, U. S. Geol. Survey, Open-File Report, 89/116, 81 pp., 1989]. In order to further improve the quality of the seismic dataset, we extracted 522 earthquakes with Gap less than 200°, Erh 〈 1.5 km, Erz 〈 2 km, RMS less than 0.5 sec, and a minimum number of S phases equal to 2. This latter seismic dataset was relocated using TomoDD code [Zhang and Thurber, BSSA, 93, 1875-1889. 2003] and a 3D velocity model [Patanè et al., Science, 313, 821- 823, 2006 after modified]. Using first motion polarity data, 3D fault plane solutions were computed by means of the software FPFIT [Reasenberg and Oppenheimer, U.S. Geological Survey Open File Report, 85/739, 109 pp, 1985]. Then, adopting restricted selection criteria (Npol more than 12; focal plane uncertainties less than 20°; number of solutions 〈 2; number of discrepancies less than 15%), we selected 116 FPSs. This dataset represented the input file for the stress and strain tensors computation using the inversion codes developed by Gephart and Forsyth,[ JGR 89: 9305-9320, 1984] and by Kostrov [Izv Acad Sci USSR Phys Solid Earth, 1, 23-40], respectively. On the basis of P and T axes distribution and the orientation of the main seismogenic stress and strain axes, we put some seismological constraints on the recharging phase leading to the 2001 Etna eruption.

Description: Published

Description: Salina

Description: 1.4. TTC - Sorveglianza sismologica delle aree vulcaniche attive

Description: open

Description: We present the first application of a time reverse location method in a volcanic setting, for a family of long-period (LP) events recorded on Mt Etna. Results are compared with locations determined using a full moment tensor grid search inversion and cross-correlation method. From 2008 June 18 to July 3, 50 broad-band seismic stations were deployed on Mt Etna, Italy, in close proximity to the summit. Two families of LP events were detected with dominant spectral peaks around 0.9 Hz. The large number of stations close to the summit allowed us to locate all events in both families using a time reversal location method. The method involves taking the seismic signal, reversing it in time, and using it as a seismic source in a numerical seismic wave simulator where the reversed signals propagate through the numerical model, interfere constructively and destructively, and focus on the original source location. The source location is the computational cell with the largest displacement magnitude at the time of maximum energy current density inside the grid. Before we located the two LP families we first applied the method to two synthetic data sets and found a good fit between the time reverse location and true synthetic location for a known velocity model. The time reverse location results of the two families show a shallow seismic region close to the summit in agreement with the locations using a moment tensor full waveform inversion method and a cross-correlation location method.

Description: Published

Description: 452-462

Description: 1.4. TTC - Sorveglianza sismologica delle aree vulcaniche attive

Description: JCR Journal

Description: reserved

Description: (extended abstract)

Description: INGV, Regione Sicilia, Ministero Sviluppo Economico

Description: Published

Description: Ettore Majorana Foundation and Centre for Scientific Culture, Erice, Sicily

Description: open

Description: In this paper the history of the Mobile Seismic Network of the Osservatorio Vesuviano (at present Department of Napoli of the Istituto Nazionale di Geofisica e Vulcanologia – INGV) is described. The instrumental development and the main research and monitoring activities since the early 1980’ are reported. Starting from the analog stations with magnetic tape recording to the first digital stations with trigger recording, until the modern multichannel 24bit devices, the technological development of the last 30 years has given a strong contribution to increase the knowledge in the seismology and seismotectonics fields. The Mobile Seismic Network has been engaged in active and passive seismic studies both on volcanic and tectonic areas. Particularly, the contribution of the Mobile Seismic Network for the monitoring of Neapolitan active volcanoes has been very important to improve the Permanent Network. During seismic crises high quality data have been gathered allowing detailed analyses of the seismic activity. Moreover, the time synchronization by means of GPS time code allowed the deployment of the Mobile Network abroad and the collaboration with the main international research institutes. At present, the Mobile Seismic Network of the Osservatorio Vesuviano is developing on multichannel acquisition systems, also in array configuration, to gathered simultaneously seismic signals with a large frequency band.

Description: Published

Description: 1-51

Description: 1.4. TTC - Sorveglianza sismologica delle aree vulcaniche attive

Description: 3.2. Tettonica attiva

Description: N/A or not JCR

Description: open

Description: We present the first application of a time reverse location method in a volcanic setting, for a family of long-period (LP) events recorded on Mt Etna. Results are compared with locations determined using a full moment tensor grid search inversion and cross-correlation method. From 2008 June 18 to July 3, 50 broad-band seismic stations were deployed on Mt Etna, Italy, in close proximity to the summit. Two families of LP events were detected with dominant spectral peaks around 0.9 Hz. The large number of stations close to the summit allowed us to locate all events in both families using a time reversal location method. The method involves taking the seismic signal, reversing it in time, and using it as a seismic source in a numerical seismic wave simulator where the reversed signals propagate through the numerical model, interfere constructively and destructively, and focus on the original source location. The source location is the computational cell with the largest displacement magnitude at the time of maximum energy current density inside the grid. Before we located the two LP families we first applied the method to two synthetic data sets and found a good fit between the time reverse location and true synthetic location for a known velocity model. The time reverse location results of the two families show a shallow seismic region close to the summit in agreement with the locations using a moment tensor full waveform inversion method and a cross-correlation location method.

Description: In press

Description: (11)

Description: 1.4. TTC - Sorveglianza sismologica delle aree vulcaniche attive

Description: JCR Journal

Description: reserved

Description: Mt. Vesuvius (southern Italy) is one of the volcanoes that poses the greatest risk in the world because of its highly explosive eruptive style and its proximity to densely populated areas. The urbanization around Mt. Vesuvius began in ancient times, and the impact of eruptions on human activities has been severe. This is testified to by the ruins of Pompeii, which are covered by the products of the plinian eruption that took place in A.D. 79 (Sigurdsson et al. 1985), and more recently by the published reports of the eruptions that occurred from 1631 to 1944. For these reasons, Mt. Vesuvius was also one of the first volcanoes to be equipped with monitoring instruments. Pioneering instrumental observations began just before the second half of the 1800s, when the Vesuvius Observatory was founded in 1841 (Imbò 1949). At that time, Vesuvius was very active (Ricciardi 2009), and its effusive and explosive eruptions often caused damage to the surrounding areas. At the same time, it was a famous tourist attraction that drew travelers from all over the world (Gasparini and Musella 1991). Since the middle of the 1800s, at least 12 eruptions have occurred that have been superimposed on persistent intra-crater activity that has been characterized by Strombolian explosions and by the formation of small lava lakes. The last eruption occurred on 18 March 1944 and marked a change in the status of Mt. Vesuvius, as it entered a closed-conduit phase that persists today. Following this last eruption, a change occurred in the 1960s, as documented by an increase in the occurrence rate of earthquakes. Since 1972, the monitoring of Mt. Vesuvius has improved over time and become more systematic, so that there is a remarkable dataset relating to the current phase of quiescence. Over more than a century and a half of observations, many monitoring instruments have been used for Mt. Vesuvius, including early seismometers, several of which are now kept in the Museum of Volcanology of the Vesuvius Observatory. The present monitoring system is based on seismological, geodetical geodetical, and geochemical observations performed using an instrumental network that was designed on the basis of the current state of the volcano while also taking into account the likely scenario of future unrest.

Description: Published

Description: 625-634

Description: 1.4. TTC - Sorveglianza sismologica delle aree vulcaniche attive

Description: JCR Journal

Description: reserved

Description: Stress can undergo rapid temporal changes in volcanic environments, and this is particularly true during eruptions. We use two independent methods, coda wave interferometry (CWI) and shear wave splitting (SWS) analysis to track stress related wave propagation effects during the waning phase of the 2002 NE fissure eruption at Mt Etna. CWI is used to estimate temporal changes in seismic wave velocity, while SWS is employed to monitor changes in elastic anisotropy. We analyse seismic doublets, detecting temporal changes both in wave velocities and anisotropy, consistent with observed eruptive activity. In particular, syn-eruptive wave propagation changes indicate a depressurization of the system, heralding the termination of the eruption, which occurs three days later.

Description: Published

Description: 1779-1788

Description: 1.4. TTC - Sorveglianza sismologica delle aree vulcaniche attive

Description: JCR Journal

Description: reserved

Description: We propose a method for analyzing the polarization of three-component digital recordings using the discrete wavelet transform (DWT). This method allows for the automatic detection and separation of seismic phases that have a coherent linear or elliptical polarization. It can be correctly used in the analysis of seismic signals relating to volcanic activity because they arise from a complex wave field that consists of near-field and far-field components that have frequency-dependent polarization. First, the analytic extension of the signal is decomposed using DWT, then each single component is used to determine a local complex polarization vector in the timescale domain. This analysis reveals the presence of seismic phases with coherent polarization over a range of DWT scales and finite temporal intervals. Using the orthogonality property of the DWT, it is possible to isolate a single coherent component, reconstructing it in the time domain and computing the full polarization tensor. This procedure can be fully automated, introducing a quantitative definition of wavelet polarization coherence on the DWT dyadic grid. A recursive algorithm (called POLWAV) starts from the wavelet coefficient with the highest modulus, and then selects all of the neighbors that show coherence with it above a given threshold. We show how the POLWAValgorithm can be used for separating wave-field components and for detecting coherent seismic phases on continuous recordings. Example applications to actual seismic recordings at Stromboli Volcano (Tyrrhenian Sea) are presented.

Description: Published

Description: 670–683

Description: 1.4. TTC - Sorveglianza sismologica delle aree vulcaniche attive

Description: JCR Journal

Description: reserved

Description: A seismic noise recording experiment has been carried out in the Campi Flegrei volcanic area from March 9 to 26, 2009 in the framework of the INGV-DPC 2007-2009 agreement (Project V1 – UNREST). The project aimed at the realization of an integrated method for the definition of the unrest phases at Campi Flegrei. 21 digital three-component seismic stations equipped with broad band seismometers have been added to the existing 11 digital stations already deployed in the area. The preliminary results show a correlation between the seismic noise level and the anthropic activity, whereas the meteorological conditions seem affecting the low frequency seismic noise. These results are important to define the detection thresholds of the seismic signals generated during a possible renewal phase of the volcanic activity.

Description: Published

Description: 1-21

Description: N/A or not JCR

Description: open

Description: IEEE Standard for Local and Metropolitan Area Networks (hereafter IEEE 802.16; online at http://standards.ieee.org/getieee802/802.16.html) is one of the most promising mobile and fixed broadband wireless access technologies for next-generation all-IP networks in the 3.5 GHz band (European spectrum profile). Commonly known as Universal WiMAX (worldwide inter-operability for microwave access), this access technology reaches a high bit rate and covers large areas with a single base station, making it possible to offer connectivity to end users in a cost-effective way. A further useful property of the WiMAX technology is that the transmission can be used both in line-of-sight (LOS) and non-line-of-sight (NLOS) environments, allowing highly feasible communications (WiMAX Forum 2004). Thanks to these features, IEEE 802.16 opens the way to the use of wireless technologies in the environmental monitoring of areas such as seismic and volcanic zones.

Description: European Community’s Sixth Framework Programme, Contract no. IST-034622-IP

Description: Published

Description: 411-419

Description: 1.4. TTC - Sorveglianza sismologica delle aree vulcaniche attive

Description: JCR Journal

Description: reserved

Description: We have implemented a method based on an unsupervised neural network to cluster the waveforms of very-long-period (VLP) events associated with explosive activity at the Stromboli volcano (southern Italy). Stromboli has several active vents in the summit area producing together more than 200 explosions=day. We applied this method to investigate the relationship between each vent and its associated VLP explosive waveform. We selected 147 VLP events recorded between November and December 2005, when digital infrared camera recordings were available. From a visual inspection of the infrared camera images, we classified the VLPs on the basis of which vent produced each explosion. We then applied the self-organizing map (SOM), an unsupervised neural technique widely applied in data exploratory analysis, to cluster the VLPs on the basis of their waveform similarity. Our analysis demonstrates that the most recurrent VLP waveforms are usually generated by the same vent. Some exceptions occurred, however, in which different waveforms are associated with the same vent, as well as different vents generating similar waveforms. This suggests that the geometry of the upper conduit-vent system plays a role in shaping the recurring VLP events, whereas occasional modest changes in the source process dynamics produce the observed exceptions.

Description: Published

Description: 2449–2459

Description: 1.4. TTC - Sorveglianza sismologica delle aree vulcaniche attive

Description: JCR Journal

Description: reserved

Description: The surface-wave field associated with the explosive activity at Stromboli volcano is investigated using data recorded by two short-period seismic arrays, deployed on the north and west flanks of the volcano. The group-velocity dispersion curves for Rayleigh waves are derived using the multiple filter technique. The phasevelocity dispersion curves are recovered using a phase match filter and compared with that inferred from zero-lag cross-correlation analysis applied to the array data. These analyses indicate Rayleigh-wave group velocities ranging from 0.29 to 0.24 km/sec in the 1.5- to 8.0-Hz frequency band, and phase velocities ranging from 1 km/sec at 1.5 Hz to about 0.3 km/sec at frequencies above 5 Hz. In addition, the dispersive properties of the attenuation coefficient (c) for Rayleigh waves are inferred from application of the multiple filter technique to seismograms recorded at different distances from the source. These results are validated through examination of the spectral amplitude decay with distance for both body and Rayleigh waves. The values of the body-wave quality factor thus obtained are Qa=20 and Qa=6 for the north and west side of the island, respectively. The velocity and attenuation dispersion curves are inverted for the shear-wave velocity and Qb structures down to a depth of about 200 m. Shear-wave velocities for the west flank range from about 0.3 km/ sec for the uppermost 17-m-thick layer to 1.9 km/sec at depths greater than 200 m. Comparison with previous studies indicates a similar velocity structure for the north and west flanks. The attenuation structure for the west flank is described by a shallower, 36-m-thick layer with Qb=9, underlain by a half-space with Qb=50. On the north flank, Qb=40 for the shallower 30-m-thick layer and Qb=44 for the underlying half-space. Residuals from analysis of the spectral decay with distance are used to quantify site effects affecting the different array elements on the west flank. Local amplifications at that array are interpreted in terms of an edge effect associated with concave topography. Velocity similarities observed at the north and west flanks are compatible with surface geologic data. Discrepancies in attenuation properties at the two sites are interpreted in terms of different degrees of heterogeneity and crack density controlling the scattering quality factor Qs.

Description: Published

Description: 1102-1116

Description: JCR Journal

Description: reserved

Description: The Colfiorito Basin is a small intramountain depression in the southern section of the Northern Apennine chain that is filled with Quaternary alluvial deposits. The presence of soft alluvial deposits has significantly influenced the level of local damage that was caused by two major earthquakes (ML 5.6 and 5.8) belonging to the swarm that started in September 1997. To verify the effects of the basin structure on the predominant frequency of seismic motion, ambient noise measurements were carried out in the Colfiorito Basin during two experiments in May and July of 2002. The horizontal-to-vertical spectral ratios (HVSRs) were calculated for data collected at four profiles in the basin. Array techniques were applied to determine the wave types that composed the noise, to estimate their apparent velocity and azimuth of propagation, and to calculate a velocity-dispersion curve from which a velocity-depth structure was derived. The data analysis shows a high amplification in the HVSR at low frequency. This feature is common to most of the sites, including the reference site, and it is interpreted as being due to weather disturbances. The peak frequencies of the spectral ratio calculated at the sites located in the center of the basin coincide with the theoretically estimated resonance frequencies. The arrayaveraged HVSR calculated for the array located in the middle of the plain has a pronounced peak at 0.9 Hz. This corresponds to the peak of the amplification function calculated on the basis of the velocity model deduced from the dispersion analysis. The HVSR method is instead unsuitable for the prediction of the resonance frequencies of sediments in the sites where strong lateral variations of basement topography are present. We measured apparent velocities in the range of 0.3–0.8 km/sec by applying f-k methods to array recordings. These values are compatible with the predominance of surface waves in the noise, as also confirmed by polarization analysis. Both Rayleigh and Love waves are present in the background seismic noise. The results obtained by applying the spatial autocorrelation method to the vertical component of the ground motion recorded at a 240-m-wide circular array deployed in the middle of the basin revealed the presence of Rayleigh waves, and f-k methods combined with polarization techniques revealed the presence of polarized Love waves. The wave-field analysis indicates two main propagation directions: the first is around N100 E in the frequency band of 1.0–2.0 Hz; this radiation can be interpreted as being generated at the east-southeast step borders of the basin. The second main direction is around N300 E in the frequency band of 2.0–3.0 Hz; its source may be a 180-m-deep depression located at the southwest corner of the basin.

Description: Published

Description: 490–505

Description: reserved

Description: 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.

Description: Published

Description: 440-456

Description: 3.6. Fisica del vulcanismo

Description: JCR Journal

Description: reserved

Description: The properties of the tremor wave field at Stromboli are analyzed using data from small-aperture arrays of short-period seismometers deployed on the north flank of the volcano. The seismometers are configured in two semi-circular arrays with radii of 60 and 150 m and a linear array with length of 600 m. The data are analyzed using a spatiotemporal correlation technique specifically designed for the study of the stationary stochastic wave field of Rayleigh and Love waves generated by volcanic activity and by scattering sources distributed within the island. The correlation coefficients derived as a function of frequency for the three components of motion clearly define the dispersion characteristics for both Rayleigh and Love waves. Love and Rayleigh waves contribute 70% and 30%, respectively, of the surface-wave power. The phase velocities of Rayleigh waves range from 1000 m/sec at 2 Hz to 350 m/sec at 9 Hz, and those for Love waves range from 800 to 400 m/sec over the same frequency band. These velocities are similar to those measured near Puu Oo on the east rift of Kilauea Volcano, Hawaii, although the dispersion characteristics of Rayleigh waves at Stromboli show a stronger dependence on frequency. Such low velocities are consistent with values expected for densely cracked solidified basalt. The dispersion curves are inverted for a velocity model beneath the arrays, assuming those dispersions represent the fundamental modes of Rayleigh and Love waves.

Description: Published

Description: 653-666

Description: reserved

Description: In this article we report on the implementation of an automatic system for discriminating landslide seismic signals on Stromboli island (southern Italy). This is a critical point for monitoring the evolution of this volcanic island, where at the end of 2002 a violent tsunami occurred, triggered by a big landslide. We have devised a supervised neural system to discriminate among landslide, explosion-quake, and volcanic microtremor signals. We first preprocess the data using a compact representation of the seismic records. Both spectral features and amplitude-versus-time information have been extracted from the data to characterize the different types of events. As a second step, we have set up a supervised classification system, trained using a subset of data (the training set) and tested on another data set (the test set) not used during the training stage. The automatic system that we have realized is able to correctly classify 99% of the events in the test set for both explosion-quake/ landslide and explosion-quake/microtremor couples of classes, 96% for landslide/ microtremor discrimination, and 97% for three-class discrimination (landslides/ explosion-quakes/microtremor). Finally, to determine the intrinsic structure of the data and to test the efficiency of our parameterization strategy, we have analyzed the preprocessed data using an unsupervised neural method. We apply this method to the entire dataset composed of landslide, microtremor, and explosion-quake signals. The unsupervised method is able to distinguish three clusters corresponding to the three classes of signals classified by the analysts, demonstrating that the parameterization technique characterizes the different classes of data appropriately.

Description: Published

Description: 1230-1240

Description: reserved

Description: In order to verify the duration magnitude MD we calculated local magnitude ML values of 288 earthquakes occurring from October 2002 to April 2003 at Mt. Etna. The analysis was computed at three digital stations of the permanent seismic network of Istituto Nazionale di Geofisica e Vulcanologia of Catania, using the relationship ML = logA+alog?-b, where A is maximum half-amplitude of the horizontal component of the seismic recording measured in mm and the term «+alog?-b» takes the place of the term «-logA0» of Richter relationship. In particular, a = 0.15 for ?〈200 km, b=0.16 for ?〈200 km. Duration magnitude MD values, moment magnitude MW values and other local magnitude values were compared. Differences between ML and MD were obtained for the strong seismic swarms occurring on October 27, during the onset of 2002-2003 Mt. Etna eruption, characterized by a high earthquake rate, with very strong events (seismogram results clipped in amplitude on drum recorder trace) and high level of volcanic tremor, which not permit us to estimate the duration of the earthquakes correctly. ML and MD relationships were related and therefore a new relationship for MD is proposed. Cumulative strain release calculated after the eruption using ML values is about 1.75E+06 J1/2 higher than the one calculated using MD values.

Description: Published

Description: JCR Journal

Description: open

Description: The Campanian volcanism develops near the sea. Therefore, the geophysical study of the marine environment is a key to a better understanding of the tectonic evolution and the origin of volcanism in the area. An abundance of high quality seismic data in the marine sector, where little direct information is available, is critical to the study of Campanian volcanism. This paper concerns the reprocessing of a seismic reflection dataset acquired in Naples Bay and processed during 1973. Even though the overall data quality was high for that time, of course their acquisition technological limits have been overcome by the new processing. Our reprocessing aimed at: 1) reduction of random noise in the data; 2) removal of unwanted coherent events; 3) reduction of spatial aliasing by means of trace interpolation on Commod Shot Point (CSP) gathering; 4) improvement of resolution of the seismic wavelet with spiking deconvolution algorithms and finally 5) reposition of reflectors in their correct locations in the space-TWT domain by means of dip moveout and post-stack time migration. A comparison between the new and old data shows that the new sections are characterized by a much higher S/N ratio. Diffraction hyperbole has been collapsed. Reverberations, ghosts and multiples have been removed or greatly attenuated, especially between the reflectors of interest, allowing us to follow them with more detail and with greater continuity. Furthermore, data resolution has been boosted by the reprocessing, allowing the interpreter to evaluate reflector position and continuity in greater detail. The reinterpretation phase of such lines, that is already in an advanced stage, will therefore allow us to gain new insights into the structural setting of the bay, with the aim of exploring the connection between tectonics and volcanism.

Description: Published

Description: JCR Journal

Description: open