Seismic Sources on the Iberia-African Plate Boundary and their Tectonic Implications

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 area B; the depth earthquakes (h = 650 km) are located to the south of Granada. Moment rate, slip velocity andhvalues 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 o 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.

[1]  A. Ibenbrahim,et al.  Geophysical evidence for lithospheric delamination beneath the Alboran Sea and Rif–Betic mountains , 1996, Nature.

[2]  C. Frohlich,et al.  Earthquake focal mechanisms, moment tensors, and the consistency of seismic activity near plate boundaries , 1992 .

[3]  H. Zeck.  Betic-Rif orogeny: subduction of Mesozoic Tethys lithosphere under eastward drifting Iberia, slab detachment shortly before 22 Ma, and subsequent uplift and extensional tectonics , 1996 .

[4]  J. M. M. Solares,et al.  Sismicidad del Area Ibero-Mogrebi , 1983 .

[5]  島崎 邦彦,et al.  Historical seismograms and earthquakes of the world , 1988 .

[6]  E. Buforn,et al.  Mecanismos focales en el sur de España: periodo 1965-1985 , 1994 .

[7]  E. Buforn,et al.  A deep earthquake under South Spain, 8 March 1990 , 1991, Bulletin of the Seismological Society of America.

[8]  M. Bezzeghoud,et al.  The Ain Temouchent (Algeria) Earthquake of December 22 nd , 1999 , 2004 .

[9]  W. Spakman,et al.  The P-wave velocity structure of the mantle below the Iberian Peninsula: evidence for subducted lithosphere below southern Spain , 1993 .

[10]  N. L. Grimison,et al.  The Azores-Gibraltar Plate Boundary: Focal mechanisms, depths of earthquakes, and their tectonic implications , 1986 .

[11]  J. Morales,et al.  Active continental subduction beneath the Betic Cordillera and the Alborán Sea , 1999 .

[12]  J. Mezcua,et al.  Seismological evidence for a delamination process in the lithosphere under the Alboran Sea , 1997 .

[13]  D. Hatzfeld.  Etude sismotectonique de la zone de collision Ibero-Maghrebine. , 1978 .

[14]  P. Bird,et al.  Thin‐shell modeling of neotectonics in the Azores‐Gibraltar Region , 2001 .

[15]  N. White,et al.  Origin of the Betic‐Rif mountain belt , 1997 .

[16]  J. Mezcua,et al.  Seismotectonic of the Azores-Alboran region , 1976 .

[17]  Richard G. Gordon,et al.  Closure of the Africa‐Eurasia‐North America Plate motion circuit and tectonics of the Gloria Fault , 1989 .

[18]  G. Houseman.  From mountains to basin , 1996, Nature.

[19]  Göran Ekström,et al.  Evidence of bias in estimations of earthquake size , 1988, Nature.

[20]  John P. Platt,et al.  Extensional collapse of thickened continental lithosphere: A working hypothesis for the Alboran Sea and Gibraltar arc , 1989 .

[21]  R. Madariaga,et al.  Intermediate and deep earthquakes in Spain , 1991 .

[22]  J. Badal,et al.  Moment Magnitudes for Early (1923–1961) Instrumental Iberian Earthquakes , 2000 .

[23]  Elisa Buforn,et al.  Seismotectonics of the Ibero-Maghrebian region , 1995 .

[24]  A. Udías,et al.  Source mechanism of intermediate and deep earthquakes in southern Spain , 1997 .

[25]  C. Morley.  Discussion of origins of hinterland basins to the Rif-Betic Cordillera and Carpathians , 1993 .

[26]  F. Medina,et al.  Mécanismes au foyer des séismes du Maroc et des régions voisines (1959-1986). Conséquences tectoniques , 1992 .

[27]  Enric Banda,et al.  Evidence for the eastward migration of the Alboran Sea based on regional subsidence analysis: A case for basin formation by delamination of the subcrustal lithosphere? , 1995 .

[28]  Mourad Bezzeghoud,et al.  Source parameters of the 1992 Melilla (Spain, MW = 4.8), 1994 Alhoceima (Morocco, MW = 5.8), and 1994 Mascara (Algeria, MW = 5.7) earthquakes and seismotectonic implications , 1999, Bulletin of the Seismological Society of America.

[29]  D. McKenzie.  Active Tectonics of the Mediterranean Region , 1972 .

[30]  F. Gomez,et al.  An integrated geophysical investigation of recent seismicity in the Al-Hoceima region of north Morocco , 1997, Bulletin of the Seismological Society of America.

[31]  P‐wave tomographic images in the Central Betics‐Alborán Sea (South Spain) using local earthquakes: Contribution for a continental collision , 1998 .

[32]  P. Bird,et al.  Neotectonic modeling of the Ibero-Maghrebian region , 2002 .

[33]  J. Soto,et al.  Lithospheric Structure Beneath the Alboran Basin: Results from 3D Gravity Modeling and Tectonic Relevance , 2000 .

[34]  C. Sanz de Galdeano,et al.  Focal mechanism of Mula (Murcia, Spain) earthquake of February 2, 1999 , 2001 .

[35]  J. Mezcua,et al.  Seismicity and focal mechanisms in South Spain , 1988 .

[36]  Elisa Buforn,et al.  Seismicity, source mechanisms and tectonics of the Azores-Gibraltar plate boundary , 1988 .

[37]  M. Meghraoui,et al.  Goringe-Alboran-Tell tectonic zone: A transpression system along the Africa-Eurasia plate boundary , 1996 .

[38]  M. Bezzeghoud,et al.  Postseismic deformation at El Asnam (Algeria) in the seismotectonic context of northwestern Algeria , 1997 .

[39]  J. Borges,et al.  Seismotectonics of Portugal and its adjacent Atlantic area , 2001 .

[40]  John H. Woodhouse,et al.  CENTROID-MOMENT TENSOR SOLUTIONS FOR 201 MODERATE AND LARGE EARTHQUAKES OF 1981 , 1983 .

[41]  G. Westbrook,et al.  A seismic reflection and GLORIA study of compressional deformation in the Gorringe Bank region, eastern North Atlantic , 1999 .

[42]  B. Goffé,et al.  How does the Alpine belt end between Spain and Morocco , 2002 .

[43]  H. Kanamori,et al.  Source process and tectonic implications of the Spanish deep-focus earthquake of March 29, 1954☆ , 1976 .