M. La Rocca

A double seismic antenna experiment at teide Volcano: existence of local seismicity and lack of evidences of Volcanic tremor

Data analyzed in the present work correspond to a 40 days field experiment carried out in Teide Volcano (Canary Islands, Spain) with two short-period small-aperture dense seismic antennas in 1994. The objective of this experiment was to detect, analyze and locate the local seismicity. We analyzed also the background seismic noise to investigate the possible presence of volcanic tremor. From a set of 76 events, we selected 21 of them in base of their good signal-to-noise ratio and their possibility to locate their seismic source by using the seismic antennas. A visual classification based on the S‐P time and seismogram shape has permitted to establish three groups of events: local seismicity (S‐P time between 3 and 5 s), very local earthquakes (S‐P time smaller than 3 s) and artificial explosions. These earthquakes have been located by applying the Zero Lag CrossCorrelation technique and the inverse ray-tracing procedure. Those earthquakes that were recorded simultaneously by both seismic antennas were also located by intersecting both back-azimuths. The analysis of the seismicity has revealed that the amount of seismicity in Teide Volcano is moderate. This seismicity could be distributed in three main areas: inside the Caldera Edifice (below the Teide‐Pico Viejo complex), in the eastern border of the Caldera Edifice and offshore of the island. At present, this activity is the only indicator of the volcano dynamics. The analysis of the back-ground seismic noise has revealed that at frequencies lower than 2 Hz, the Oceanic Load signal is predominant over other signals, even over local earthquakes with a magnitude of 2.0. Due to this, although if in the Teide area were present a weak volcanic tremor, or other volcanic signals with predominant peaks below 2 Hz, to observe them would be a very difficult task. q 2000 Elsevier Science B.V. All rights reserved.

Array Analysis of Seismograms from Explosive Sources: Evidence for Surface Waves Scattered at the Main Topographical Features

In this article, we analyze the seismic wavefield produced by blasts fired at Mt. Vesuvius for a deep seismic sounding experiment and recorded by a dense short-period three-component seismic array. Seismograms from five explosions occurring at the bottom of wells located at different distances and azimuth with respect to the array site have been analyzed with the zero lag cross-correlation technique applied to both vertical and horizontal components in the frequency range 3–7 Hz. The aim is to analyze the portion of the seismograms long after the first P onset in order to obtain the slowness and backazimuth distribution of correlated secondary phases. Polarization analysis in time domain, carried out using the array covariance matrix technique, has also been applied to the well-correlated phases to determine their wave composition. Results show that the correlated phases detected in the coda of the seismogram shots are composed almost entirely of surface waves. Location of the scatterers shows a high concentration of the scatterers in and around the volcanic edifice of Mt. Vesuvius, confirming that topographical irregularities play an important role in the generation of the high-frequency scattering phenomena.

Location of the Source and Shallow Velocity Model Deduced from the Explosion Quakes Recorded by Two Seismic Antennas at Stromboli Volcano

Abstract The seismic wavefield associated to the ongoing eruptive activity at Stromboli volcano (Italy) is investigated using data from two small-aperture, short-period seismic arrays deployed on the northern and western flanks, located at about 1.7 km from the active craters. Two distinct approaches are used to analyze the recorded signals: 1. 1) the zero-lag cross-correlation method is used to analyze the explosion quakes data, to estimate slowness and backazimuth as a function of lapse time; 2. 2) multiple filter technique and phase matched filtering are used to estimate Rayleigh wave dispersion, to obtain a shallow velocity model of the two sites. Estimates of slowness vectors at the two different array sites show a primary (volcanic) source located at shallow depth beneath the crater region. Secondary sources associated with path effects are located in close proximity of the sector graben of Sciara del Fuoco and of the old parasitic cone of Timpone del Fuoco. The shallow velocity structure derived for the western flank depicts striking resemblance with that previously inferred for the northern flank of the volcano.

Testing Small-Aperture Array Analysis on Well-Located Earthquakes,

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.

Shallow velocity model of the northern flank of Stromboli Volcano, deduced by high frequency surface wave dispersion

The wavefield produced by the Stromboli volcano explosion quakes shows a significant amount of surface waves. Rayleigh waves recorded by a linear array have been investigated to infer the shear-wave velocity model of the Stromboli northern flank. The group velocity dispersion curve was obtained using the multiple filter technique, while the phase velocity dispersion curve was calculated both by phase-matched filtering and performing a p–ω stack on the observed waveforms. Through the inversion of these curves we were able to recover the shear-wave structure to a depth of about 190 m.

Properties of the neural network sieve bootstrap

In this paper, a sieve bootstrap scheme, the neural network sieve bootstrap, for nonlinear time series is proposed. The approach, which is nonparametric in its spirit, retains the conceptual simplicity of a classical residual bootstrap, and it has some advantages with respect to the blockwise schemes and kernel bootstrap techniques. The resampling scheme from the residuals of the feedforward neural networks is shown to be asymptotically justified. A Monte Carlo simulation study shows that the procedure performs similar to the autoregressive (AR)-sieve bootstrap for linear processes, while it outperforms the AR-sieve bootstrap, the moving block bootstrap and kernel bootstrap for nonlinear processes, both in terms of bias and variability.

Site response obtained from array techniques applied to the seismic noise: Two examples in Italy

The vertical component of the seismic noise has been recorded in two different sites near the towns of Mercato S. Severino and Benevento in Southern Italy by a small aperture array, in order to investigate the characteristics of the noise propagation and to study the site response. Three different array techniques have been applied in the two investigated sites: Beam Forming, High Resolution and Spatial Correlation methods. We used two simple array geometry for localising possible noise sources and estimating local shallow structure using ambient noise. The cross shaped array results effective for determining the phase velocity of waves in the case when the noise is from a single localised source; the circular array, on the other hand, is successfully used when the noise sources are distributed. The main results are: the analysis of a coherent component of the noise recorded in the two sites, interpreted as Rayleigh waves, results in reasonable velocity models; the noise recorded in the M.S.S. Plain is a space stationary signal, while the noise at the Benevento site is possibly produced by a stable noise source located close to the array. Due to this evidence, the correlation method does not yield satisfactory results when applied to the Benevento site. The 2–6 Hz spectral peaks of the noise recorded in the M.S.S. Plain can be interpreted as due to a site effect, considering the satisfactory agreement of the noise spectrum at those frequencies with the theoretical transfer function computed on the basis of the velocity model deduced from the Rayleigh waves dispersion analysis.

On the Imputation of Missing Values in Univariate \(PM_{10}\) Time Series

Missing data frequently happen in environmental research, usually due to faults in data acquisition, inadequate sampling or measurement error. They make difficult to determine whether the limits set by the European Community on certain indicators of air quality are fulfilled or not. Indeed, due to missing values, the number of exceedances per year of \(PM_{10}\), that is particulate matter 10 \(\upmu \)m or less in diameter, and other air quality indicators are often heavily underestimated, and no environmental policy is applied to protect citizen health.