Josep Vila

Attenuation and source parameters at Deception Island (South Shetland Islands, Antarctica)

Deception Island is the most active volcano of the South Shetland Islands-Antarctic Peninsula group, experiencing eruptions in 1967, 1969 and 1970. Local attenuation derived from coda analysis and source parameters derived from Brunes model, for well located seismic events, have been studied in order to complement the available geophysical information. Results show abnormally lowQ0 values and an abnormally high frequency dependence, as well as large dispersion. These factors are strongly dependent on the path travelled by the seismic wave. Retrieved values of the source parameters (stress drop, seismic moment and source radius), are again abnormally low compared to world-wide average values, for example, those obtained for the Oroville, California aftershock series between June and September, 1976. These results are consistent with some aspects of the geology of Deception Island, such as the very high degree of fracturing and faulting, and the existence of a strong hydrothermal alteration affecting most of the subaerial volcanic rocks. Moreover, the pattern defined for the lateral variations ofQ0 shows minimum values in the inner bay of the island, close to the most recent eruption vents. A large reduction in spectral amplitudes over a particular frequency range occurs in several observations, corresponding to the path crossing the zone of highest attenuation. This observation suggests the existence of a hot magmatic intrusion produced during the most recent eruption, and coincides with the superficial low density mass distribution obtained from the gravimetric model and the long wave magnetic field component obtained from magnetic surveys. The width of this intrusion is estimated to be about 200 m, in agreement with the previous results obtained analyzing residuals of the location of seismic events.

The Broadband Seismic Station CADI (Túnel del Cadí, Eastern Pyrenees), Part II: Long-Period Variations of Background Noise

The aim of this work is to complement a previous article by Vila (1998) presenting long-term observations of the behavior of seismic noise. We start by computing the Fourier transforms (FFTs) of time series (30-min length, one series per day) of the last 6 years of operation in an attempt to confirm whether the observations follow the pattern observed in the previous analysis. The analysis of the microseismic peak using 6 years of data confirms a long time (seasonal) variation of the amplitude, showing in addition the same variation in the frequency at which the maximum occurs. This points out the necessity of clarifying some of the observations in Vila (1998), and some modification in the conclusions is also necessary. A detailed analysis of the behavior of several spectral components of the seismic noise reveals that the seasonal component is presented in the frequency range 0.05-2.0 Hz. The amplitude of the seasonal component has a clear frequency dependence, being higher for frequencies close to the microseismic peak. The main differences between the spectral components are in accordance with its own amplitude level. No significant differences between the behavior of noise of the three components have been reported. Although for the geographical location of the station, the seasonal component is present in the atmospheric pressure, all attempts of correlation with seasonal components for diverse frequencies of the noise spectra have failed. This indicates that the pressure seasonal component cannot be related directly to the same component of the very long term noise spectral evolution. Manuscript received 18 January 2002.

Microseism Activity and Equilibrium Fluctuations

A study is carried out on the seismic wave field recorded in the absence of earthquakes, the unrest of the solid earth, in the frequency band of microseism activity. Two phenomenological models are presented to explain the main characteristics of the observed time series from the point of view of both the power spectrum and the phase space representation, corresponding to the cases of strong external forces (well developed microseism activity) and very low ones (minimum energy spectra, termed equilibrium fluctuations). Observations and model predictions of equilibrium fluctuations strongly suggest that the information contained in the power spectrum corresponds to medium resonances, that may strongly fluctuate, through a competition process due to the action of external forces.

Lateral variations of the local magnitude at Ebre station, Northeastern Iberian Peninsula

Since 1984, a seismic surveillance at the Baix Ebre Region (NE of the Iberian Peninsula) has been carried out from a seismic network consisting of two digital and one analog stations. Large discrepancies between the recorded amplitudes and the magnitude estimates given by different agencies have been observed. To explain these discrepancies, that consist of a large reduction of amplitude and a strong dependence of the azimuth, a local magnitude formula for the area has been computed, using the analog records obtained at EBR short-period station. Global results show an average station correction factor between the computed local magnitude and the reported magnitude of the order of 1, presenting large variations around the mean value with a clear dependence on the azimuth. A detailed analysis of this variation around the mean value reveals that earthquakes can be classified according to their location in different geological units, thus obtaining for each zone a distinct station correction factor that can fluctuate as much as 0.6 around the average. These large lateral variations of the magnitude are in good agreement with previous studies of coda-Q attenuation.

Measurement of body-wave dispersion and estimation of related attenuation from broadband stations

Abstract Attenuative parameters of the Earths mantle are obtained for the frequency range 0.5–2 Hz by means of an iterative nonlinear fit to observed body-wave dispersion. Dispersion curves are obtained by directly measuring the arrival time of some selected frequency components of P- and S-wave pulses. The frequency components are obtained by narrow band-pass filtering the signal, and the arrival time is determined as the expectation value of the first pulse, which can be interpreted as the arrival time of the center of gravity of its energy contents. Before any interpretation, arrival time must be corrected by the group delay of filter and instrumental response. The apparentQ, as a function of frequency, is retrieved from broadband records of the Network of Autonomously Recording Stations array corresponding to earthquakes which occurred in the Japan-Kurile Islands region, and interpreted in terms of the continuous relaxation model (CRM) and the power law model (PLM). For the CRM the following parameters are obtained, corresponding to average values of the mantle: for P waves,τ2 = 0.064s andQm = 587; for S waves,τ2 = 0.281andQm = 399. In both cases,τ1 has been given an arbitrary value of 1.0E + 05. Using these parameters, the values ofQP = 608andQS = 464 are obtained for a period of 1 s. For the PLM, the values ofQ0 = 611andγ = 0.07 are obtained for P waves, andQ0 = 714andγ = 0.6 for S waves, with the same numerical values forQ at 1 s. The predicted values at 1 s agree with average published data for the mantle, except the valueQS = 714 predicted by the PLM, which is 50% higher. Thus, the measurement of body-wave dispersion proves to be an independent method and a powerful complement to classical well-established methods for the study of attenuation based on amplitude analysis.