Seismic source in the Iberian-African plate boundary

The plate boundary between Iberia and Africa has been studied using data on seismicity and focal mechanisms. The region has been divided into three areas: A; the Gulf of Cadiz; B, the Betics, Alboran Sea and northern Morocco; and C, Algeria. Seismicity shows a complex behavior, large shallow earthquakes (h < 30 km) occur in areas A and C and moderate shocks in area B; intermediate-depth activity (30 < h < 150 km) is located in the depth earthquakes (h » 650 km) are located to the south of Granada. Moment rate, slip velocity and b values have been estimated for shallow shocks, and show similar characteristics for the Gulf of Cadiz and Algeria, and quite different ones for the central region. Focal mechanisms of 80 selected shallow earthquakes (8 ‡ mb ‡ 4) show thrust faulting in the Gulf of Cadiz and Algeria with horizontal NNW-SSE compression, and normal faulting in the Alboran Sea with E-W extension. Focal mechanisms of 26 intermediate-depth earthquakes in the Alboran Sea display vertical motions, with a predominant plane trending E-W. Solutions for very deep shocks correspond to vertical dip-slip along N-S trends. Frohlich diagrams and seismic moment tensors show different behavior in the Gulf of Cadiz, Betic-Alboran Sea and northern Morocco, and northern Algeria for shallow events. The stress pattern of intermediate-depth and very deep earthquakes has different directions: vertical extension in the NW-SE direction for intermediate depth earthquakes, and tension and pressure axes dipping about 45 ° for very deep earthquakes. Regional stress pattern may result from the collision between the African plate and Iberia, with extension and subduction of lithospheric material in the Alboran Sea at intermediate depth. The very deep seismicity may be correlated with older subduction processes

From strike-slip to reverse reactivation: The Crevillente Fault System and seismicity in the Bullas-Mula area (Betic Cordillera, SE Spain)

Several major N80ºE faults occur in the Bullas-Mula area (SE Spain). These faults and the numerous thrust slices of the Sierra de Ricote can be related to the movements of the Crevillente (or Cadiz-Alicante) fault system, which causes the westward displacement of the Betic Internal Zone and part of the External Zone. These faults moved with dextral strike-slip from Late Burdigalian to Early Tortonian. From this time on, the 1 position changed from a WNW-ESE direction to approximately N-S, giving rise to movements with a new reverse character. The focal mechanisms of the 1999 Mula earthquakes indicate a N80ºE nodal plane, and their pressure axes also coincide with the 1 direction existing from the Late Miocene and deduced from mesotectonic analysis at many points of the region. There is also good coincidence between their epicentral position and the fault traces. The Bullas earthquakes that occurred in 2002 are not directly related to the Crevillente faults, although their stress-pressure axes coincide with the 1 direction reported there.This study corresponds to the project REN2003-05178-C03- 01, the research group RNM217 (Junta de Andalucía) and the “Acción Especial BTE2001-5230-E”. The figures have been drawn by A. Caballero. P. Ruano helped in the program of stress determinations. We are grateful for the accurate correction and the suggestions of Drs. González Lodeiro (Granada), Martínez Díaz (Madrid) and Santanach (Barcelona), together with the editor’s corrections which have greatly improved the former version of the manuscript.Peer reviewe

Estudio estadístico de la dirección de esfuerzos principales en terremotos

Tesis Univ. Complutense de Madrid, 1982.Depto. de Física de la Tierra y AstrofísicaFac. de Ciencias FísicasTRUEProQuestpu

Estudio estadístico de la dirección de esfuerzos principales en terremotos

Tesis Univ. Complutense de Madrid, 1982.Depto. de Física de la Tierra y AstrofísicaFac. de Ciencias FísicasTRUEProQuestpu

Submarine activities of the royal naval observatory

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Estudo do mecanismo de rotura dos sismos de 1939 (M7) e 2007 (M6) na faixa S. Miguel – Glória (Açores): uma contribuição para a compreensão da sismotectónica local

O presente trabalho tem como objetivo o estudo de 3 dos mais significativos sismos instrumentais ocorridos na faixa S. Miguel-Glória, Açores: o de 8 de Maio de 1939 e os de 5 e 7 de Abril de 2007. A investigação envolveu a concepção e desenvolvimento de um método que permite recuperar os movimentos reais do solo em formato digital a partir de sismogramas antigos. Os resultados obtidos para o sismo de 1939 relocalizam o evento numa posição mais próxima da fronteira de placas e com mecanismo também mais próximo da generalidade dos eventos estudados. O estudo da fonte dos sismos de 2007 reforçam o mecanismo padrão da zona

From strike-slip to reverse reactivation: The Crevillente Fault System and seismicity in the Bullas-Mula area (Betic Cordillera, SE Spain)

Several major N80ºE faults occur in the Bullas-Mula area (SE Spain). These faults and the numerous thrust slices of the Sierra de Ricote can be related to the movements of the Crevillente (or Cadiz-Alicante) fault system, which causes the westward displacement of the Betic Internal Zone and part of the External Zone. These faults moved with dextral strike-slip from Late Burdigalian to Early Tortonian. From this time on, the σ1 position changed from a WNW-ESE direction to approximately N-S, giving rise to movements with a new reverse character. The focal mechanisms of the 1999 Mula earthquakes indicate a N80ºE nodal plane, and their pressure axes also coincide with the σ1 direction existing from the Late Miocene and deduced from mesotectonic analysis at many points of the region. There is also good coincidence between their epicentral position and the fault traces. The Bullas earthquakes that occurred in 2002 are not directly related to the Crevillente faults, although their stress-pressure axes coincide with the σ1 direction reported there

New fault-plane solutions of Moroccan earthquakes for the 2005-2008 period

[EN] We present new fault-plane solutions, based on P wave first motion, of 10 moderate shocks that occurred in Morocco during the period 2005-2008. In the Middle Atlas and adjacent area, the 3 fault-plane solutions correspond either to reverse or to strike-slip faulting with a normal component, in good agreement with the state of stress determined in this area, reflected by NW-SE horizontal pressure axes. In the central and eastern Rif area, the 3 strike-slip and normal solutions (with ENE-WSW T-axes) are in agreement with the local state of stress. Finally, in the western Rif front, which is undergoing increased seismic activity, the 2 fault-plane solutions obtained (reverse faulting) show unusual NE-trending P-axes that support the model of lateral escape of the Central Rif.[FR] Nous présentons de nouveaux mécanismes au foyer, basés sur le premier sens de mouvement des ondes P, pour 10 séismes de magnitudes modérées ayant eu lieu de 2005 et 2008. Dans le Moyen Atlas et les régions avoisinantes, les 3 mécanismes correspondent soit à un jeu inverse, soit à un jeu décrochant à composante normale, en bon accord avec l’état de contrainte déterminé dans cette région, reflété par l’orientation NW-SE des axes de pression. Dans le Rif central et oriental, les 3 mécanismes déterminés, à jeu décrochant ou normal, avec des axes de tension orientés ENE-WSW, sont également compatibles avec l’état de contrainte local. Enfin, au front du Rif occidental, qui a enregistré une activité sismique accrue au cours des dernières années, les solutions montrent des axes P orientés NE-SW. Cette direction inhabituelle conforte le modèle tectonique d’échappement du Rif.Peer reviewe

Neotectonic modeling of the Ibero-Maghrebian region

15 páginas, 12 figuras.Thin-shell finite element techniques have been applied to model the neotectonics of the Ibero-Maghrebian region, in the westernmost Mediterranean. This region is characterized by a complex seismotectonic pattern and moderate seismic activity associated with the convergence between Africa and Eurasia. We compare two end-member models using different fault networks. Model predictions, including anelastic strain rates, vertically integrated stresses and velocity fields, are compared to the seismicity map and to data on directions of maximum horizontal compression. Best results are obtained assuming a low fault friction coefficient (0.05) and when the Betics and Rif are modeled as parts of the same arc-shaped chain. The highest predicted fault slip rates are in the Tell mountains. Farther to the west, fault slip is more homogeneously distributed over the Betic-Rif chain, Gulf of Cadiz, and Alboran Sea, indicating a diffuse geometry of the plate boundary in this area. The areas of highest predicted strain rates coincide with the most seismically active regions, located in northern Algeria and northeastern Morocco. Our best model also reproduces a major change of the stress regime, from thrusting in the east (Tell mountains) to predominantly strike-slip and normal faulting in the west (Betic-Rif chain and Alboran Sea). The Alboran basin is shown to be undergoing significant internal transpression and therefore cannot be considered as a rigid microplate. The western part of the Alboran Sea and surrounding areas are being extruded to the WNW with respect to Iberia.Spanish research programs Ramón y Cajal, BTE2002-02462, REN2000-0777-C02-01/RIES, DGESIC PB97-1267-C03-01, and a grant from the Foundation Jaime del Amo (University Complutense of Madrid) have partially supported this work.Peer reviewe