Antonio Posadas

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.

Spatial‐temporal analysis of a seismic series using the principal components method: The Antequera Series, Spain, 1989

A method for the characterization of a seismically active zone from a distribution of hypocenters is presented. This method is based on principal components analysis, a powerful multivariant statistical technique that is used to find the rupture local ellipsoid (RLE). The ellipsoid is a planar structure with which two variations of the method are developed: the spatial principal components analysis and the spatial-temporal principal components analysis; using these methods, it is possible to find the dominating tendencies in the fracturing of the seismically active volume as well as the temporal evolution of the process. The methodology developed has been applied to a series of earthquakes that occurred near Antequera, Spain, with the result that the main fracture series trends N70°–80°E. Moreover, the temporal evolution of the system from the most relevant RLE has been characterized.

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.

Information Theory to Characterize Spatiotemporal Patterns of Seismicity in the Kanto Region

This article proposes a quantitative method to characterize the spatiotemporal evolution of the seismic activity of a region for a given time interval. The technique is based on the fact that the model whose parameters maximize the mutual information is the best model. These parameters can provide a probabilistic characterization of the evolution of the earthquakes of a region in space and time. To do this, it is necessary to establish a propagation model (cross template [CT] model in this article) and reasonable criteria to decide when an area (whose size β will be found by the model) is considered seismically activated in a period of time τ (whose value is also the result of the application of the method). The results found with the CT model are highly satisfactory and show that our model is a powerful tool to quantitatively characterize spatiotemporal seismic patterns.

Spatial and temporal analysis of a seismic series using a new version of the three point method: application to the 1989 Antequera (Spain) earthquakes

Abstract The three point method (TPM) has been successfully applied to several seismic series and has provided information about the spatial characteristics (azimuth and dip) of the fault planes activated in the rupture process. A new development of the TPM to determine temporal characteristics, is presented, to obtain the evolution of the fracturing process of an active fault system. For the analysis of the 158 microearthquakes and earthquakes that took place in the seismic series of Antequera in June 1989, the choice of a threshold magnitude (mu = 2.5) has permitted the events related to the most relevant fractures to be distinguished. Secondly, only the events between two concentric spheres (here named Spatial Crown) with respect to a given earthquake, have been used in order to avoid taking into account earthquakes that are too close to each other, together with the very distant events that have little relation to the event analysed. The Spatial Crown has permitted some clear results in the Antequera series, where we have found that the fracturing process began fundamentally with N 80° E planes and evolved to N 65° W planes. Finally, an error analysis enables an estimation of the uncertainty in the results to be obtained from the errors in the data.