C. Sanz de Galdeano

Geologic evolution of the Betic Cordilleras in the Western Mediterranean, Miocene to the present

Abstract The Betic Cordilleras were formed during the Eo-, Meso- and Neo-Alpine evolution of the Western Mediterranean area. During the Neo-Alpine stage the northward subduction of the African plate took place and the Algerian-ProvenCal Basin opened, with the creation of oceanic floor. Associated space problems produced the expulsion and extensive stretching of the Internal Zones to the west, which in turn disrupted and deformed the External Zones of the Betic and Rif Cordilleras. These occurrences essentially took place in the Burdigalian, and continued with diminished intensity into the Middle Miocene. Following the suturing of the transcurrent contact between the Internal and External Zones, important N60°–90° E and NW—SE faults occurred. The Betic Cordilleras are characterized as having been deformed by regional horizontal shortening (compression) oriented WNW—ESE or NW—SE, which gradually rotated to the NNW—SSE in the Late Miocene, when its most important intramontane Neogene basins were formed. The Alboran Basin, with a thinned continental crust, was formed as a western prolongation of the opening of the Algerian—Provencal Basin, with its basic features already apparent in the Burdigalian. The volcanism present in the southeast of the Betic Cordilleras and in northeast Morocco, crossing the Alboran Basin, is mainly Late Miocene in age. It is linked to fracturing and exhibits chemistry related to the different thickness of the crust.

Seismotectonics of the Ibero-Maghrebian region

Abstract A study of the seismotectonics of the Ibero-Maghrebian region is presented as deduced from main geological features, seismicity and focal mechanism data. The hypocentral distribution of shallow, intermediate-depth and deep earthquakes is analyzed. The direction of the stress distribution is inferred from the focal mechanism of 24 selected events. A seismotectonic framework summarizing these results together with the main geological features shows that a NW-SE general direction of horizontal compressive stresses is present in the region with a localized zone of horizontal tensions in the Betics and the Alboran Sea. The presence of intermediate-depth and deep earthquakes is tentatively explained as caused by two consecutive processes of subduction.

Continental fragmentation along the South Scotia Ridge transcurrent plate boundary (NE Antarctic Peninsula)

Abstract The study of the South Scotia Ridge on the basis of swath bathymetry, multichannel seismic and magnetometry profiles, obtained during the HESANT92/93 cruise and complemented with satellite gravimetry and seismicity data illustrates the tectonics of the region. The thinned continental crust fragments of the ridge are bounded by oceanic crust of the Scotia Sea to the North and Powell Basin to the South. The northern boundary represents the contact between the Scotia and Antarctic plates. This boundary is a sinistral transpressional fault with transtensional segments and moderate recent tectonic activity. Another fault located at the southern boundary appears inactive and does not reveal any features that would enable the kinematics to be determined. Both faults have associated steep scarps since they separate oceanic and continental crust types. The most significant active deformation lies in the axial depression of the ridge, within a band delineated by fault systems with WSW-ENE and SW-NE strikes. These faults develop pull-apart basins, which separate the northern and southern blocks of the ridge. The northern block is being fragmented from the Antarctic Plate by a zone of transtensive faults, and is probably a crustal element independent of the Antarctic Plate. The axial depression, which crosses the ridge slightly obliquely, is characterized by deep basins locally more than 5000 m deep and associated high seismicity. The fault geometry and earthquake focal mechanisms indicate an active sinistral transtensive regime for the fault system, although it may locally have transpressive regimes depending on the fault plane and the stress field orientations. The internal basins are characterized by an asymmetric development showing itself as depositional wedges generally thickening northward. Deposits onlap the southern margins and are affected by normal faults in the northern margins. The seismicity around the Scotia Plate shows that the present stresses are compressive along the northern boundary with the South American Plate (σ 1 SW-NE and subhorizontal) and along the western boundary with the Antarctic Plate (σ 1 WNW-ESE and subhorizontal). For the South Scotia Ridge, however,σ 1 is steeply inclined andσ 3 is subhorizontal with a NW-SE trend. The stress distribution in Bransfield Strait is similar to the ridge and the recent extension could be partially explained by the westward continuation of the active fault system of the central South Scotia Ridge. The fragmentation of continental crustal blocks, due to the tectonic activity along the transcurrent plate boundaries, is a mechanism that contributes to the deformation of the northeastern end of the Antarctic Peninsula. This area appears appropriate for the analysis of continental plate fragmentation processes.

Active faulting in the internal zones of the central Betic Cordilleras (SE, Spain)

The internal zones of the Betic Cordilleras show a present-day relief that is mainly controlled by kilo- metre-size, symmetrical or north-vergent folds which developed mostly since Middle Miocene times. The Sierra Nevada, Sierra Alhamilla, Sierra de Los Filabres, Sierra Tejeda and Sierra de Gador, among others, are roughly E-W trending high mountain ranges, corresponding to antiforms where metamorphic rocks crop out. The surrounding depressions are located in synforms, where Neogene rocks are preserved from erosion. Field evidence shows that the growth of the folds is coeval with fault development, and that at least three of them, i.e. the Padul Fault, the Zafarraya Fault, and the Balanegra Fault, may be considered to be active seismogenetic structures. The Zafarraya Fault, in particular, is thought to be responsible for the 1884 Andalucia Earthquake. The fault is located at the northern limb of the Sierra Tejeda antiform, and could be interpreted as a collapse structure developed along the external arch of the uplifted fold. The Padul and Balanegra faults are located at the southeastern border of the Granada Basin and south of the Sierra de Gador, respectively. They belong to a set of NW-SE oriented faults that are mainly normal in character and indicate NE-SW extension. The set up, since 1999, of a GPS network within and around the Granada Basin and the planed installation of a new network in the Sierra Tejeda, will give us new insights on the present-day deformation behaviour of both folds and faults in the area.

Shallow seismicity and active faults in the Betic Cordillera. A preliminary approach to seismic sources associated with specific faults

Abstract Seismicity in the Betic Cordillera is quite active but in general moderate. The maps of I ≥ VI or M1 ≥ 3.5 show some significant groupings. In turn, the network of Neogene and Quaternary faults in the Cordillera is very dense, with three main sets striking N60–70° to east-west, northwest-southeast and northeast-southwest. The fault segments in this network which are active or potentially active from a neotectonic perspective have been marked. We have also compared the fracture network with the epicentral positions of the major seisms, as there is often considerable uncertainty about the location of these positions, and we have indicated the clearest correlations with various fault segments. By using both neotectonic and seismic data, we obtain a series of fault segments considered to be active or potentially active in the Betic Cordillera. These results, while open to future improvements, represent a preliminary step towards establishing the seismic sources associated with segments of specific faults.

The b parameter in the Betic Cordillera, Rif and nearby sectors. Relations with the tectonics of the region

Abstract The a and b parameters of the Gutenberg-Richter ratio for intensities and magnitudes are evaluated according to the methods of calculation, treatment and quality of the data used. When these variability factors have been controlled by the corresponding confidence indices, the mean values of these parameters are used to regionalise the Iberian-Maghreb area as regards shallow and intermediate seismicity. Analysis of the various sectors under consideration (seismic groupings, differentiated seismic sources in the Betic Cordillera, crustal domains and lithospheric domains) provides values of the afore-mentioned parameters coherent with the known tectonic evolution of this area. We also show that the b parameter is not reliable for small-size seismic sources because of the poor statistical representativity of the sample (very few years for small regions). The b parameter values obtained for both crustal and lithospheric domains are remarkably similar and can be linked with what is to be expected in highly fractured sectors. This supports the hypotheses that explain the existence of intermediate seismicity by movement of faults cutting almost the entire lithosphere, or even by the possible presence of a zone of incipient subduction.

Study of the Damaging Earthquakes of 1911, 1999, and 2002 in the Murcia, Southeastern Spain, Region: Seismotectonic and Seismic-Risk Implications

A detailed study of four earthquakes that occurred in the Murcia region (southeastern Spain) in 1911, 1999, and 2002 has been carried out. New intensity maps have been plotted for the March and April 1911 shocks. These show maximum values of VII-VIII (EMS). We have found values of VI and V, respectively, for the 1999 and 2002 earthquakes. Surface wave magnitudes range from 5.2 for the Bullas 2002 event to 5.7 for the March 1911 event. Focal mechanisms for the Mula 1999 and Bullas 2002 events indicate reverse and strike slip motions, with scalar seismic moments of 5.9 � 10 16 N m and 8.6 � 10 15 N m, respectively, and focal dimensions of 4.0 and 1.5 km. Both earthquakes were recorded at epicentral distances greater than 21 km, showing low values of peak ground acceleration (PGA )( 0.020 g). These records involve a ground motion less than the ones expected in the region, according to different hazard studies. The seismic hazard map that we obtained for the Murcia region (for a return period of 475 years) shows higher values than those given in the Spanish Building Code NCSE-02 (2002) (e.g., 0.24 g for PGA in the city of Murcia, compared with 0.16 g in the code).

The structure of the Alboran Sea: an interpretation from seismological and geological data

Abstract Almost all the earthquakes included in the catalog for the Iberian–Maghrebian area up to 2000 have been used in order to know the structure of the Alboran Sea area after the verification of the aleatory nature of the errors in their spatial localization. A new focal mechanisms catalog has also been used as well as many of the available geological data. During the Mesozoic and till the Oligocene, the Betic–Rif Internal Zone was situated further E, but with the opening of the Algero-Provencal basin in the early Miocene, the Betic–Rif Internal Zone moved to the W. Contemporary, the Alboran Sea was created as the western prolongation of the Algero-Provencal basin. The Betic–Rif Internal Zone overthrused part of the Iberian and African plates, producing the partial sinking of both plates and being responsible of the intermediate seismicity existing in the western sector of the Alboran Sea. The intermediate earthquakes in the Atlas towards the NE and WSW are not related to lithospheric sinking but to significant deep faults limiting a subplate in NW Africa. In the Atlantic, the intermediate earthquakes between the Gorringe sector and Gibraltar are produced in the contact between the Iberian and African plates and by the important faults crossing it. The existence of four very deep earthquakes is related to the previous sinking of the lithosphere originally associated to the domain in which the Betic–Rif Internal Zone was situated.

Seismic Potential of the Main Active Faults in the Granada Basin (Southern Spain)

Abstract — The main active faults of the Granada Basin are located in its central-eastern sector, where the most important tectonic activity is concentrated, uplifting its eastern part and sinking the western border. Several parameters related to the seismic potentiality of these active, or in some cases probably active, faults in this basin are used for the first time. Many of these faults can generate earthquakes with magnitudes larger than 6.0 MW, although this is not the general case. The fault situated to the N of Sierra Tejeda, probably the one responsible for the big earthquake of 25/12/1884, stands out, because it could generate an earthquake with magnitude 6.9 MW. Although at present all the data needed are not fully known, we consider that the final results show, as a whole, the average expected return periods of the faults in the Granada Basin.

Stress fields in the Iberian-Maghrebi region

This study concerns the present stressfield between the Eurasian and Africanplates in the Iberian-Maghrebi region(Portugal, Spain, Morocco, Algeria andTunisia). In addition to an up-to-datecatalogue of earthquakes in this area, acatalogue of the focal mechanisms composedof 486 solutions of fault planes,standardized in terms of notation andinformation type, was used. These data wereused applying the right-dihedron method ofAngelier and Mechler (1977), to obtaindifferent zones with homogeneous stress.The results obtained for shallowearthquakes (h < 30 km) coincide, in themajority of cases, with the general stressfields proposed by numerous authors forthis region, according to which there isNW-SE compression. However, the stressorientation appears to vary in certainareas, perhaps perturbed by the opening ofthe Atlantic Ocean, the approach of Iberiaand Africa, or the extension of the AlboranSea.For the intermediate earthquakes (30 < h< 150 km) no general pattern was found,and the P and T axes seem to be randomlyoriented for the depth intervalsconsidered. For the very deep earthquakes(h > 600 km), however, the P axis lies ina NNW-SSE direction, dipping towards theSSE, while the T axis is subhorizontal in aNE-SW direction.The determinations from the focalmechanisms highlight the existence of aregional stress field with a subhorizontalcompression axis trending NW-SE. Superimposed are others that specificallyaffect particular sectors; these arerelated to the opening of the AtlanticOcean, the extension of the BeticCordillera and the Alboran Sea, and eventhe present compression between the Iberianand European plates.