F. De Miguel

Basement structure of the Granada basin, Betic Cordilleras, Southern Spain

Abstract The analysis and interpretation of geophysical data (gravity and seismic reflection) has facilitated the definition of the Granada basin structure. The reflector showing the contact between the Betic-Subbetic basement and the Neogene-Quaternary sedimentary filling has been identified. Mapping of the basement in two and three dimensions is presented. The presence of four important depocenters (Genil, Chimeneas, Cubillas and Granada) has been determined. These troughs are limited by ridge areas through important sets of fractures. In some cases the accumulation of Neogene-Quaternary sediments reaches a thickness exceeding 3 km as in the Genii and Cubillas depocenters. The mapping of the most important fractures affecting the basement has been achieved, defining four systems that have influenced and conditioned the genesis and late evolution of the Granada basin. The directions of the most important groups of fractures are: NE-SW, N70W to E-W, N45W and N10-30E.

Space clustering properties of the Betic-Alboran earthquakes in the period 1962–1989

Abstract In order to obtain a way to quantify the spatial dependence between earthquakes in the Betic-Alboran (south Spain) region, the Single Link Clustering ( slc ) method has been applied to a catalogue of earthquakes, with magnitude greater or equal to 4.0, that occurred in this region from 1962 to 1989. The significant distances of links obtained, which show alignments whose direction is similar to the direction of some surface faults, are 15, 30 and 40 km, and the limit of inconsistency appears at distances greater than or equal to 70 km. Most of the historical destructive earthquakes have taken place within the clusters obtained, and the majority within the dc = 30 km cluster. This implies that the most relevant seismic activity occurs in some source areas where there is a strong nesting of activity.

Effect of a sedimentary basin on estimations of Qc and QLg

Abstract The effect which a sedimentary basin (Granada basin, southern Spain) exercises on the estimated Q c and Q Lg attenuation values within a determined frequency range are described. It has been shown that for the values of Q c there is a decrease in the number of fits, with a correlation coefficient > 0.8 in the 1.5 Hz frequency band, in comparison with the other frequency bands. On the other hand, an overestimation of the values of the coda- Q is observed between a station located on the sediment in the interior of the basin and the stations situated on the borders of the basin. The values of the attenuation for Lg waves in the paths which cross the Granada basin show a series of peaks centred in the 1–3 Hz frequency band, which overestimate the values of Q Lg with respect to other frequency bands. All these phenomena indicate that the presence of a low-velocity sedimentary basin modifies the processes of generation of coda waves and the propagation of Lg waves.

Geometrical spreading function for short-period S and coda waves recorded in southern Spain

Two methods to estimate the geometrical spreading factor n are described; the first is applied to S waves recorded at various distances from the source, whereas the second is applied to coda waves. The direct S-wave method is a new technique which provides the geometrical spreading factor independently of the quality factor Q. This method is based on the double spectral ratio. The coda method provides the geometrical spreading factor along with the Q value. To check their reliability, these methods are applied to real data (from the Andalusian Seismic Network) and to synthetic data. The synthetic test of the coda method indicates that this method to obtain n must not be used to calculate the attenuation factor Q. The results obtained with real data show a geometrical spreading coefficient that is greater than unity and slightly frequency dependent. For S waves, the following frequency dependence was obtained: n=(1.19 ± 0.14)+(0.052 ± 0.007)ƒ and for coda waves, n=1.33 ± 0.19 at 1.5 Hz n=1.57 ± 0.19 at 3 Hz n=1.29 ± 0.24 at 6 Hz The results obtained in this paper (n > 1) suggest a velocity increase with depth and/or strong lateral inhomogeneities which modify the wavefront geometry.

Characteristics of the Seismic Attenuation in Two Tectonically Active Zones of Southern Europe

The seismic energy attenuation in the frequency range of 1–18 Hz was studied in the two tectonically active zones of Irno Valley (Southern Italy) and Granada Basin (South-East Spain). Data were recorded by short period vertical components seismographs for low-magnitude local earthquakes. The method of coda waves, assuming singleS toS scattering approximation, was used to calculate the quality factorQ from the two data set. Results show a quality factor increasing with frequency, following the empirical lawQ=Qofn.Qo andn are lower for the Irno Valley than for Granada. This result is interpreted in terms of different scattering environments present in the two investigated areas.

Efficiency of a seismic network from the viewpoint of mathematical information theory: application to the Andalucia (Spain) seismic network

Abstract Concepts of information theory applied to data from the Andalucia (Spain) seismic network permit the discussion of whether data are correctly used in determining epicentres. The elementary definition of Shannons information allows a discussion of the coverage of the epicentres in terms of azimuths and distances to the seismic stations. The contributions of all the stations of the network to the coverage is also investigated. Data were obtained from 13 seismic stations and 765 epicentral determinations corresponding to the seismic activity of the years 1983-84 and some months of 1985 in the Central Belies area (Southern Spain). Information theory concepts were applied to the data after a distribution of epicentres according to their m o values and hypocentral depths. The obtained results show a better treatment of very shallow earthquakes, especially of those with values of m b less than or equal to 2.5. No significant different coverages were obtained for the sets of earthquakes classified according to their hypocentral depths and m b values.