Edoardo Del Pezzo

Seismovolcanic signals at Deception Island volcano, Antarctica: Wave field analysis and source modeling

The seismovolcanic signals associated with the volcanic activity of Deception Island (Antarctica), recorded during three Antarctic summers (1994–1995, 1995–1996 and 1996–1997), are analyzed using a dense small-aperture (500 m) seismic array. The visual and spectral classification of the seismic events shows the existence of long-period and hybrid isolated seismic events, and of low-frequency, quasi-monochromatic and spasmodic continuous tremors. All spectra have the highest amplitudes in the frequency band between 1 and 4 Hz, while hybrids and spasmodic tremors have also significant amplitudes in the high-frequency band (4–10 Hz). The array analysis indicates that almost all the well-correlated low-frequency signals share similar array parameters (slowness and back azimuth) and have the same source area, close to the array site. The polarization analysis shows that phases at high-frequency are mostly composed of P waves, and those phases dominated by low frequencies can be interpreted as surface waves. No clear shear waves are evidenced. From the energy evaluation, we have found that the reduced displacement values for surface and body waves are confined in a narrow interval. Volcano-tectonic seismicity is located close to the array, at a depth shallower than 1 km. The wave-field properties of the seismovolcanic signals allow us to assume a unique source model, a shallow resonating fluid-filled crack system at a depth of some hundreds of meters. All of the seismic activity is interpreted as the response of a reasonably stable stationary geothermal process. The differences observed in the back azimuth between low and high frequencies are a near-field effect. A few episodes of the degassification process in an open conduit were observed and modeled with a simple organ pipe.

Discrimination of Earthquakes and Underwater Explosions Using Neural Networks

We report on the implementation of an automatic system able to discriminate between explosion-generated artificial seismic events and local earthquakes in the Phlegraean Fields (Italy). The explosions are fired weekly at the sea bottom (tens of meters below sea level) by fishermen in Pozzuoli bay; earthquakes are volcano-tectonic quakes with depths shallower than 4 km. The discrimination system is based on an artificial neural network and is composed of two modules. The first is devoted to the extraction of the seismogram signatures and the second to the classification of the seismic events into two classes. For the features extraction (preprocessing stage), instead of the conventional Fourier spectral analysis, we use a Linear Prediction Coding (LPC) algorithm. This approach compresses the data from 256 samples to only 7 parameters and can extract robust features for the spectrogram representation. The classification is performed using a supervised neural algorithm based on a Multilayer Neural Network (MLP) architecture. We applied the method to a set of 30 seismic events recorded by the stations of the local seismic network, 15 of which were generated by the fishermens explosions and 15 were volcano-tectonic earthquakes. We dealt with a total of 280 records from different stations, 121 relating to explosions and 159 to earthquakes. Data were divided in a training set containing 120 traces for earthquakes and 90 for explosions, and a test set containing 70 traces corresponding to 39 records for earthquakes and 31 records for explosions. On the test set the neural net gave a classification performance of 92%, indicating a good ability of the net to generalize. Manuscript received 10 January 2002.

Seismic Signals Associated with Landslides and with a Tsunami at Stromboli Volcano, Italy

In this article, we analyze the seismic signals produced by two landslides that occurred at the Stromboli volcano on 30 December 2002, recorded by both broadband and short-period seismic stations located in the 2.5-22-km distance range from the source. For both landslides, the characteristics of the low-frequency seismograms indicate a complex time history in the release of seismic energy. The first landslide occurred over the submerged part of the northwest sector of the volcano and had associated a large-amplitude, low-frequency pulse representative of the abrupt detachment of a large mass. Lower amplitude phases in the following 3 minutes possibly indicate minor detachment events. The highest amplitude, low-frequency signals are well described by a single-force source model. The second mass-failure episode is also characterized by a complex source and can be interpreted as a multiple event, with a less abrupt onset and at least four detachments occurring during 4-5 minutes and producing low-frequency signals. Synthetic seismograms generated by a shallow single force located in the submerged area of Sciara del Fuoco and directed upslope, fit well the first low-frequency seismic pulse recorded at Stromboli and Panarea by three-component stations. From this simulation, we estimated the force exerted by the first mass failure. The estimate of the volume through two different procedures, gives values in the range of 1.0-1.5 million m3 and about 14 million m3, respectively. The landslides, which involved both the submarine and the subaerial northwest flank of the volcano, produced a tsunami that struck the coast of Stromboli Island and in a few minutes reached the other islands of the Aeolian Archipelago. Three broadband seismic stations installed on land about 100 m from the coastline at Panarea Island, located 20 km southwest of Stromboli, recorded very long period seismic signals produced by the tsunami waves. Analysis of these signals gives invaluable information on the spectral content and propagation properties of tsunami waves and on their interaction with the ground at a short distance from the coast. Synthetic tsunami waves, obtained by a landslide source model and taking into account the bathymetry of the sea surrounding Stromboli and Panarea Islands, fit the observed phenomena and the experimental data very well. Manuscript received 19 November 2003.

Source parameters of microearthquakes at Phlegraean Fields (Southern Italy) volcanic area

The seismic activity that occurred at the Phlegraean Fields (Southern Italy) volcanic area during a pronounced episode of ground uplift has been analysed. One hundred and eighty-one three component seismograms from a digital network operating in the period January–May 1984 were processed to obtain seismic moments, source radii and stress drops for 32 microearthquakes (0.7 < ML < 3.2). An inverse method based on a least squares fitting of displacement spectra with a theoretical model has been applied to infer the spectral parameters of P-radial and SH records. This method has the advantage of giving statistical errors on the spectral estimates. The computed seismic moments range between 0.5 × 1018 and 0.13 × 1021 dyne-cm. Stress drops are approximately constant, and do not clearly depend on seismic moments, which is in contrast with other reported data from microearthquake sequences. Small values of source radii (few tens of metres) are found.

The July-August 2000 seismic swarms at Campi Flegrei volcanic complex, Italy

Two swarms of microearthquakes (ML ≤ 2.2), occurred on July 2–7 and August 22, 2000 at Campi Flegrei, accompained by a ground uplift episode (4 cm) which interrupted on early March 2000 the descending trend started on 1985. Spectral analysis indicates a direct involvement of magmatic/hydrothermal fluids in the source process of the July swarm, while the August events are typical of shear failure, similar to most of the earthquakes that occurred during the last (1982–1984) bradyseismic crisis. Precise 3-D relative location applied to similar earthquakes allows for the recognition of two parallel alignments trending NE-SW at depths of 1.7 and 3.2 Km. This trend is consistent with the direction of the main focal plane obtained from fault plane solutions and evidences tensile failure in close proximity to the zone of maximum uplift as depicted by geodetic measurements. A fault weakening mechanism triggered by increasing pore pressure is invoked as the cause of these earthquakes.

Seismic source dynamics at Vesuvius volcano, Italy

Abstract 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 b -parameter 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.

Probabilistic source location of explosion quakes at Stromboli volcano estimated with double array data

Data from two dense arrays of short-period seismometers are used to retrieve source locations of the explosion quakes at Stromboli volcano. Slowness vectors estimated at both arrays with the zero-lag cross-correlation technique constitute the experimental data set. A probabilistic approach based on a grid search spanning the volcano interior is used to calculate the probability of the source location. Results depict a shallow source, located beneath the crater area, at depths not greater than 500 m below the surface. Results are slightly different from, but comparable to, those obtained in a companion experiment carried out in the same time period using a broad-band seismometer network, which show a source shifted some hundreds of meters northwest of the crater area. The method is revealed to be effective and useful for future studies having the purpose of real-time tracking of the explosion quakes and tremor.

Testing small-aperture array analysis on well-located earthquakes, and application to the location of deep tremor

We have here analyzed local and regional earthquakes using array tech- niques with the double aim of quantifying the errors associated with the estimation of propagation parameters of seismic signals and testing the suitability of a probabilistic location method for the analysis of nonimpulsive signals. We have applied the zero-lag cross-correlation method to earthquakes recorded by three dense arrays in Puget Sound and Vancouver Island to estimate the slowness and back azimuth of direct P waves and S waves. The results are compared with the slowness and back azimuth computed from the source location obtained by the analysis of data recorded by the Pacific Northwest seismic network (PNSN). This comparison has allowed a quanti- fication of the errors associated with the estimation of slowness and back azimuth obtained through the analysis of array data. The statistical analysis gives σBP � 10° and σBS � 8° as standard deviations for the back azimuth and σSP � 0:021 sec= km and σSS � 0:033 sec=km for the slowness results of the P and S phases, respec- tively. These values are consistent with the theoretical relationship between slowness and back azimuth and their uncertainties. We have tested a probabilistic source location method on the local earthquakes based on the use of the slowness estimated for two or three arrays without taking into account travel-time information. Then we applied the probabilistic method to the deep, nonvolcanic tremor recorded by the arrays during July 2004. The results of the tremor location using the probabilistic method are in good agreement with those obtained by other techniques. Thewide depth range, of between 10 and 70 km, and the source migration with time are evident in our results. The method is useful for locating the source of signals characterized by the absence of pickable seismic phases.

Changes in the Coda Decay Rate and Shear-Wave Splitting Parameters Associated with Seismic Swarms at Mt. Vesuvius, Italy

We study the time changes of (1) the b -value of the Gutenberg-Richter distribution, (2) the inverse coda (\batchmode \documentclass[fleqn,10pt,legalpaper]{article} \usepackage{amssymb} \usepackage{amsfonts} \usepackage{amsmath} \pagestyle{empty} \begin{document} \(Q(Q_{\mathrm{C}}^{-1})\) \end{document}, and (3) the shear-wave splitting parameters (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 \batchmode \documentclass[fleqn,10pt,legalpaper]{article} \usepackage{amssymb} \usepackage{amsfonts} \usepackage{amsmath} \pagestyle{empty} \begin{document} \(Q_{\mathrm{C}}^{-1}\) \end{document} measured at high frequency (6, 12, and 18 Hz). The changes seem to be correlated with the occurrence of two swarms with largest magnitudes of 3.4 and 3.6, respectively in April 1996 and October 1999. The earthquake with the largest magnitude in the second swarm appears to be the largest event since the latest eruption in 1944. An increase in \batchmode \documentclass[fleqn,10pt,legalpaper]{article} \usepackage{amssymb} \usepackage{amsfonts} \usepackage{amsmath} \pagestyle{empty} \begin{document} \(Q_{\mathrm{C}}^{-1}\) \end{document} starts (msec/strain units) for after the occurrence of both swarms, reaching a maximum after more than 1 yr for the first swarm and after 6 months for the second swarm. These two changes were not accompained by any corresponding variation of the b -value, which shows an almost constant (inside the statistical uncertainty) pattern. The last swarm ( M 3.6) was preceeded by an increase of T d at both stations, indicating a possible change of the stress state before the M 3.6 earthquake. The qS 1 polarization direction also shows a variation in correspondence to the same earthquake, which was interpreted as generated by an increase of the differential stress acting at a regional scale in the north-south direction shortly before the M 3.6 event. The strain change associated to this earthquake was estimated to be of the order of 10-9 using data from the straingram recorded at a Sacks-Evertson dilatometer located about 3 km from the epicenter. The given information allows us to estimate the sensitivity of the the measured parameters to the strain change induced by the M 3.6 earthquake. The sensitivity is of the order of \batchmode \documentclass[fleqn,10pt,legalpaper]{article} \usepackage{amssymb} \usepackage{amsfonts} \usepackage{amsmath} \pagestyle{empty} \begin{document} \(1.4{\times}10^{9}{\ }(Q_{\mathrm{C}}^{-1}{/}\mathrm{strain\ units})\) \end{document} for \batchmode \documentclass[fleqn,10pt,legalpaper]{article} \usepackage{amssymb} \usepackage{amsfonts} \usepackage{amsmath} \pagestyle{empty} \begin{document} \(Q_{\mathrm{C}}^{-1}\) \end{document} and is of the order of 2 × 1010 (msec/strain units) for T d. Manuscript received 15 July 2003.

Separation of intrinsic and scattering Q for volcanic tremor: An application to Etna and Masaya volcanoes

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.