E. Buforn

Seismicity, source mechanisms and tectonics of the Azores-Gibraltar plate boundary

Abstract A study and partial revision of the seismicity and source mechanisms of earthquakes are presented for the oceanic plate boundary of Eurasia and Africa from the Azores islands to the Strait of Gibraltar. Focal mechanisms of 36 earthquakes, wave-form modelling of 4 and seismic moments and dimensions of 28, have been calculated. These results indicate that the boundary is divided into three sections with different characteristics: the Azores, the Central section and the East section. The Azores section has high seismicity of moderate magnitude and a low slip rate (0.76 cm/yr); the predominant mechanism is normal or transform faulting with horizontal tensions on average in a N25° E direction. In the Central section large earthquakes occur with strike-slip right-lateral motion along faults in an E-W direction; there is a high slip rate (3.39 cm/yr). The East section is of more complex nature; large earthquakes also occur, but the predominant mechanism is reverse faulting with a horizontal pressure axis in the N30°W direction, resulting in underthrusting of Africa. From the slip vectors, the pole of rotation for the African plate is located at 28.2° N, 21.1° W, nearer to the boundary than previous locations. Close to the continent, there is a continuation in the ocean crust of some structural features of the Iberian peninsula.

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.

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 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.

Intermediate and deep earthquakes in Spain

Recent improvements in the seismological networks on the Ibero-Maghrebian region have permitted estimation of hypocentral location and focal mechanisms for earthquakes which occurred at South Spain, Alboran Sea and northern Morocco of deep and intermediate depth, with magnitudes between 3.5 and 4.5. Intermediate depth shocks, range from 60 to 100 km, with greater concentration located between Granada and Málaga. Fault-plane solutions of 5 intermediate shocks have been determined; they present a vertical plane in NE-SW or E-W direction. Seismic moments of about 1015 Nm and dimensions of about 1 km have been determined from digital records of Spanish stations.P-wave forms are complex. This may be explained by the crustal structure near the station, discontinuities in the upper mantle and inhomogeneities near the source. Deep activity at about 650 km has only 3 shocks since 1954 (1954, 1973, 1990). Shocks are located at a very small region. Fault-plane solutions show a consistent direction of the pressure axis dipping 45° in E direction. For the 1990 shock seismic moment is 1016 Nm and dimensions 2.6 km. TheP-waves are of simpler form with a single pulse. The intermediate and deep activities are not connected and no activity has been detected between 100 and 650 km. The intermediate shocks may be explained in terms of a recent subduction from Africa under Iberia in SE direction. The very deep activity must be related to a sunk detached block of lithospheric material still sufficiently cold and rigid to generate earthquakes.

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.

Regional stresses along the Eurasia-Africa plate boundary derived from focal mechanisms of large earthquakes

The focal mechanism solutions of 83 European earthquakes withM>6, selected from a total of 140, have been used to derive the directions of the principal axes of stress along the plate boundary between Eurasia and Africa from the Azores islands to the Caucasus mountains. Along most of the region, the horizontalP-axes are at an angle of 45° to 90° with the trend of the plate boundary. HorizontalT-axes are concentrated in central Italy and northern Greece in association with normal faulting. Large strike-slip motion of right-lateral character takes place at the center of the Azores-Gibraltar fault and the North Anatolian fault. From Gibraltar to the Caucasus the boundary is complicated by the presence of secondary blocks and zones of extended deformations with earthquakes spread over wide areas. Intermediate and deep earthquakes are present at four areas with arc-like structure, namely, Gibraltar, Sicily-Calabria, Hellenic arc and Carpathians.

Source mechanism of intermediate and deep earthquakes in southern Spain

Focal mechanisms of 10 intermediate-depth earthquakes (30<h<150 km) and one very deep (h 650 km) which occurred in southern Spain and the Alboran Sea are studied in this paper. Distribution of epicenters with foci at intermediate depth shows a N–S alignment with a geometry parallel to the east of the Arc of Gibraltar. Focal mechanisms have been determined from first motion of P-wave and modeling wave forms of direct P arrivals. Most of the intermediate depth events present single source time functions with small time duration (smaller than 0.3 s) and only for the event of March 27, 1987 a complex source time function has been found. The very deep earthquake of March 8, 1990 has a complex focal mechanism with a long source time function (1.2 s) and two different fault-plane orientations. Scalar seismic moments and dimensions have been obtained from modeling and spectral analysis. The results are interpreted in terms of the seismotectonic framework of the region and suggest the existence of a vertical slab extending from 50 km to 150 km with strike N–S produced by a lithospheric delamination process. The existence of the very deep activity at 650 km of depth may be explained in terms of a block of lithospheric material that still cold and rigid, generates the very deep earthquakes.

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 Large Chilean Historical Earthquakes of 1647, 1657, 1730, and 1751 from Contemporary Documents

The four largest historical earthquakes of central Chile in the seventeenth and eighteenth centuries, 1647, 1657, 1730, and 1751, are studied using contemporary documents available in the Archivo General de Indias of Seville (Spain). These docu- ments provide detailed information about these earthquakes. This is the first time these documents have been used directly for a seismological study. The 1647 earthquake practically destroyed the city of Santiago. Damage to the cathedral and main buildings is given in detail. The 1657 earthquake near the city of Concepcion produced a large tsunami that further contributed to the damage. The 1730 earthquake, the largest of these events, caused damage in a large region that extended more than 1000 km from Copiapo in the north to Concepcion in the south. This event caused heavy damage in Santiago and was followed by a large tsunami that affected the region between Valparaiso and Concepcion. The 1751 Concepcion earthquake was the next largest earthquake of this period. It affected a very large region from Santiago to Valdivia, including a large tsunami that destroyed Concepcion and made it necessary to relocate the city. We suggest that this event was very similar in size and extent to that of Maule in 27 February 2010. Online Material: Documents of the Archivo de Indias about the Chilean earth- quakes of 1647, 1657, 1730, and 1751.

The 2010 Granada, Spain, Deep Earthquake

The deep earthquake ( h =650 km) that occurred on 11 April 2010 south of Granada, Spain, has been studied using the inversion of body waves at teleseismic and regional distances. We have obtained a solution of dip-slip motion on either a vertical plane or a nearly horizontal plane with the pressure axis dipping 45° to the east. The horizontal plane is chosen as the rupture plane, with rupture propagating from east to west on the basis of directivity effects at teleseismic distances and differences of the waveforms at regional distances. The comparison of these results with the focal mechanisms of four other deep earthquakes that occurred in the same area shows similar rupture processes. The origin of this deep seismic activity remains an open question. Tomographic studies have shown the existence of an anomalous body in this region that extends from 200- to 700-km depth. The olivine-spinel phase transitions or shear melting along horizontal planes inside of this body may be an explanation for the occurrence of these earthquakes.