La Cerdanya Research Articles

A numerical filter for the restitution of digital seismograms

Based on the low-pass Butterworth filter, a digital recursive filter has been designed for the restitution of digital seismograms and is applicable in general to any type of digital record. It is a powerful, simple, and easily implemented filter, all parameters of which can be varied. A reduced number of operations permits the filter to be applied in real time to any transient signal of low frequency. As an example, it has been applied to digital registers recorded by seismic networks at Phlegraean Fields (Italy) and La Cerdanya (Spain).

Seismicity and codaQ values in the eastern Pyrenees: First results from the La Cerdanya seismic network

Seismicity in the La Cerdanya region of the eastern Pyrenees has been accurately mapped for the first time using data from a local seismic network. The majority of earthquakes lies on or near the La Cerdanya fault or secondary faults to the south. Coda magnitudes determined for these earthquakes, using magnitude relations from other regions, range between −0.5 and 2.2. These are, however, presumed to be underdetermined values sinceQ values appear to be very low in the La Cerdanya region. CodaQ values at a frequency of 1.5 Hz range between 17 and 120, the lowest values being obtained for the most seismically active regions. CodaQ values also increase with increasing distance, a result which indicates decreasing seismic attenuation with increasing depth in the crust.

Attenuative body wave dispersion at La Cerdanya, eastern Pyrenees

Coda- Q for P- and S-waves has been measured from digitally recorded events occurring in the La Cerdanya region of the eastern Pyrenees. Interpreted in terms of a power law, Q( f) = Q 0 f η , Q-coda for P-waves is characterized by Q 0 = 14 and η = 1.07, and S-waves by Q 0 = 14 and η = 1.13. Using a generalization of a model due to Dainty (1981), we obtain a Q model for S-waves in which intrinsic- Q is 23, the frequency dependence (ζ) of intrinsic- Q is 1.17, and the turbidity factor is 0.051. Interpreted in terms of a continuous relaxation model, where Q m is minimum Q, and τ 1 and τ 2 are high- and low-frequency cutoffs, respectively, the values of the parameters are Q m = 5 and τ 1 = 0.37 when τ 2 is assumed to be 10 000. Body wave dispersion, as computed from the differences in arrival times of the wave filtered at 3, 6, 12 and 24 Hz relative to that at 6 Hz has been measured and found to range from 0.067 at 3 Hz to −0.075 at 24 Hz. This dispersion constrains τ 2 to be 43.