Preliminary analysis of the first digital recordings obtained in the Portuguese seismographic network — Attenuation studies

1573-0840

acceleration ; attenuation ; digital recording ; dispersion ; displacement ; frequencies ; path ; ratios ; site ; source ; velocity

Springer Online Journal Archives 1860-2000

Energy, Environment Protection, Nuclear Power Engineering

Geography

Geosciences

Abstract During experiments with digital stations in the period 1985–1987, twenty five earthquakes with magnitudes m b in the range 2.9 to 4.8 and epicentres located within the area 36°–42.3° (N) and 4.5°–13.6° (W) were recorded at Montemor (MOE) and Montachique (MTH). The three-component recordings were obtained by Geotech S13 instruments with 1 second period. A preliminary analysis of the recordings consisted in the determination of amplitudes and spectral contents of P and S waves, and led to the following observations: (1) The attenuation of waves is expressed by the equation V = exp(C 2).R C 1. exp(C 3.M), where V stands for acceleration, velocity or displacement; M-magnitude; R-focal distance; C 1, C 2 and C 3 are constants to be obtained by least square fitting. The application of this equation led to C 1 of the order 1.7 for displacement, 1.8 for velocity and 2.0 for acceleration, with an average mean square error σ ≈ 0.8. (2) The ratios L/T (longitudinal/transversal amplitudes), for velocity and displacement, showed a tendency to reduce with increasing focal distance, being 2 for short distances (〈50 km) and 1 for long distances (≈400 km). (3) The ratios S/P (S-wave/P-wave amplitudes), although with a large dispersion, showed a slight tendency for increasing with focal distance. (4) The predominant frequencies also showed a slight tendency to decrease with increasing focal distance and with magnitude. (5) The dependence of C 1 with frequency (3 to 12 Hz) is well behaved from 0.95 to 1.75 (for the velocity trace).

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