Julio Mezcua

Seismotectonics of the Canary Islands

Abstract A revision of the seismicity, both historical and instrumental for the Canary Islands is presented. The occurrence on May 9, 1989 of an earthquake of magnitude 5.2 between the islands Gran Canaria and Tenerife, followed by a large number of aftershocks have been interpreted on seismotectonic grounds. The main conclusion is that a horizontal compressional stress regime in NW-SE direction is present in the region which is compatible with the tectonics in the northwestern part of the African continent.

Microseismicity and tectonics in the Granada Basin (Spain)

A microseismic experiment carried out in 1994 in the Granada Basin (Spain) permitted the precise recording of more than 80 local earthquakes. The dense distribution of the local network, with 40 to 50 instrumental records for each event, enabled us to have well-controlled hypocenters, and also 10 reliable focal mechanisms. The above observations are interpreted together with topographic data, neotectonics, and sub-surface information. Microtectonic observations in Sierra Elvira, Padul and Zafarraya gave a set of fault planes and striae, which were interpreted in terms of the recent regional stress tensor. The actual stress tensor obtained from the microseismic campaign data gives a regime in radial extension, with r1 vertical and r3 oriented NS to NNE. Microtectonic information is coherent with these orientations, but closer to 3-axial extension. A set of 64 mechanisms obtained from the permanent Andalusian network favors a NS orientation for r3. This results are interpreted in terms of the general model implying the lateral ejection of the Betic ranges towards the Atlantic. D 2002 Elsevier Science B.V. All rights reserved.

Interpretation of an aeromagnetic survey of the Spanish mainland

Abstract An aeromagnetic survey of the Spanish mainland was carried out in 1987. A first interpretation of this survey is given here, with particular reference to the structure and tectonic development of the Spanish part of the Iberian Peninsula. This involves the integration of the geophysical interpretation, where possible, with the current understanding of the tectonic framework as determined from previous geological and geophysical investigations. Of particular interest is the apparent structural symmetry and the possible manner in which this might be associated with the generally acknowledged structural units of Spain; this gross structural symmetry even seems to extend to the northern and southern coastlines where the magnetic responses are somewhat similar in character. From a magnetic point of view, the Spanish mainland appears to be dominated by a series of sub-circular structures about an ESE-WNW axis, centered on the Gredos Mountains and remaining open in the axial direction into Portugal.

On the Strong Ground Motion Attenuation in Spain

In the last two decades, all earthquake hazard studies in Spain have been done using the attenuation deduced from the isoseismal maps of the most important earthquakes. However, it is well known that the preferred parameter to measure the size of any shock is magnitude and, consequently, the knowledge of acceleration as a function of magnitude and distance is basic to any new hazard determination. The installation of a strong-motion network in Spain, with more than a hundred digital instruments set up in the last 15 years, made possible the recording of more than 447 shocks in the moment magnitude range of 1:3 ≤ M ≤ 6:3. However, only 149 shocks were processed corresponding to magnitudes greater than 3 and a distance of less than 100 km. Values of the maximum horizontal acceleration recorded on firm soil permitted us to make a regression analysis, which has been compared with similar previously published relationships. Additionally, the southwestern part of Spain is also affected by a marine source area of different very low attenuation, but there are not enough recordings to attempt a regression analysis. In this case, the only strong-motion data gathered for the area was checked against the two intensity attenuation relationships available after conversion of maximum intensity to moment magnitude, and intensity to acceleration, showing a reasonable behavior of the newest intensity attenuation law gathered for the area.

A site amplification factor map of the Iberian Peninsula and the Balearic Islands

Considering the lack of site effect cartography in the Iberian Peninsula region to be related to the strong motion data, we present a site amplification factor map covering the Iberian Peninsula and the Balearic Islands. Steps leading to produce the map have included the classification of geological units from the 1:1,000,000 scale Geological Map of Spain into six site classes characterized on a seismic response basis, and the calculation, for every site class, of both short-period and mid-period amplification factors. In order to test the validity and applicability of the map, we have calculated the synthetic seismic intensity in the particular localities where several past earthquakes were felt, supposing a point source approximation. The synthetic intensities have been obtained with and without site amplification factors, resulting in two types of synthetic isoseismals maps that have been compared with the corresponding observed isoseismals. As the amplification factors obtained are only applicable to the linear domain, the synthetic intensities greater than VII are only illustrative. A main conclusion has been drawn about the fact that synthetic isoseismals with site amplification factor approach to the related observed isoseismals in a higher degree than synthetic isoseismals without site amplification factor. In addition, the resemblances between the synthetic isoseismals produced with site amplification factors and the corresponding observed isoseismals have been found to be more evident at shorter epicentral distances, provided that intensities are lower than VIII, and therefore, we remain in the linear domain.

Deep seismic sounding experiments in Spain

Abstract Since 1974, under the Geodynamic Project and with the collaboration of institutions of several countries, a long-range program of deep seismic sounding experiments has been carried out in Spain. In the first three years of the program the following profiles have been made: 1974—Cartagena—Cadiz, Alboran Sea (two N-S and one E-W); 1975—Adra—Cartagena, Adra—Ubeda, Lanzarote (Canary Islands); 1976—Balearic Islands, Pyrenees and the Iberic system. For 1977 complementary lines are programmed in the Betica region, Alboran Sea and Canary Islands. Interpretation of the data has resulted in some preliminary models of the crust under South Spain and the Alboran Sea. Strong dips and lateral variations of the structures are present. Thickness of the crust varies from 16 km in the Alboran Sea to 38 km under the Sierra Nevada.