Ramon Macià

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.

Normal Faulting in the 1923 Berdún Earthquake and Postorogenic Extension in the Pyrenees

The 10 July 1923 earthquake near Berdun (Spain) is the largest instrumentally recorded event in the Pyrenees. We recover old analog seismograms and use 20 hand-digitized waveforms for regional moment tensor inversion. We estimate moment magnitude Mw 5.4, centroid depth of 8 km, and a pure normal faulting source with strike parallel to the mountain chain (N292°E), dip of 66° and rake of -88°. The new mechanism fits into the general predominance of normal faulting in the Pyrenees and extension inferred from Global Positioning System data. The unique location of the 1923 earthquake, near the south Pyrenean thrust front, shows that the extensional regime is not confined to the axial zone where high topography and the crustal root are located. Together with seismicity near the northern mountain front, this indicates that gravitational potential energy in the western Pyrenees is not extracted locally but induces a wide distribution of postorogenic deformation.