Mechanism of 2003, 2007 and 2009 earthquakes (S. Vicente Cape) and implications for the 1755 Lisbon earthquake

The San Vicente Cape region (SW Iberia) is of great seismological interest due to its tectonic complexity and for the occurrence of the 1755 Lisbon mega-earthquake. A structure capable of generating such large earthquake has not been convincingly found but authors agree with the possible occurrence in the future of a similar earthquake offshore of San Vicente Cape.We have studied the mechanism of three earthquakes in this area: 29 July 2003 (Mw = 5.3), 12 February 2007 (Mw = 6.1) and 17 December 2009 (Mw = 5.5) which throw light on the dynamics of the region. These earthquakes are the largest occurred in the last 40 years at the western of San Vicente Cape. From inversion of body waves and kinematic slip distribution, we have obtained that the three shocks have similar characteristics (dimensions, maximum slip, stress drop, source time function, focal depth and rupture velocity), but we can observe differences on geometry of the rupture that reflect the great seismotectonics complexity of the zone. The 2003 and 2007 focal mechanisms are similar, corresponding to thrusting motion but the 2009 earthquake has dip-slip motion on a vertical plane. The ruptures planes for the three shocks, deduced from the slip distribution, show ruptures on NE-SW planes, with the released energy propagating to NE direction, compatible with the regional horizontal compression in the NW-SE direction produced by the convergence between the Eurasian and African plates. This direction of faulting may be applied to the generation of the 1755 Lisbon earthquake, in terms of a complex rupture along NE-SW trending thrust faults at the Gorringe Bank, the Horseshoe Scarp and the Marques de Pombal Fault, with rupture propagating in NE direction toward the coast of Portugal and which may explain the large damage at Lisbon city

The 1980, 1997 and 1998 Azores earthquakes and its seismotectonic implications

We have studied the focal mechanisms of the 1980, 1997 and 1998 earthquakes in the Azores region from body-wave inversion of digital GDSN (Global Digital Seismograph Network) and broadband data. For the 1980 and 1998 shocks, we have obtained strike– slip faulting, with the rupture process made up of two sub-events in both shocks, with total scalar seismic moments of 1.9 × 1019 Nm (Mw = 6.8) and 1.4 × 1018 Nm (Mw = 6.0), respectively. For the 1997 shock, we have obtained a normal faulting mechanism, with the rupture process made up of three sub-events, with a total scalar seismic moment of 7.7 × 1017 Nm (Mw = 5.9). A common characteristic of these three earthquakes was the shallow focal depth, less than 10 km, in agreement with the oceanic-type crust. From the directivity function of Rayleigh (LR) waves, we have identified the NW–SE plane as the rupture plane for the 1980 and 1998 earthquakes with the rupture propagating to the SE. Slow rupture velocity, about of 1.5 km/s, has been estimated from directivity function for the 1980 and 1998 earthquakes. From spectral analysis and body-wave inversion, fault dimensions, stress drop and average slip have been estimated. Focal mechanisms of the three earthquakes we have studied, together with focal mechanisms obtained by other authors, have been used in order to obtain a seismotectonic model for the Azores region. We have found different types of behaviour present along the region. It can be divided into two zones: Zone I, from 30°W to 27°W; Zone II, from 27°W to 23°W, with a change in the seismicity and stress direction from Zone I. In Zone I, the total seismic moment tensor obtained corresponded to left-lateral strike–slip faulting with horizontal pressure and tension axes in the E–W and N–S directions, respectively. In Zone II, the total seismic moment tensor corresponded to normal faulting, with a horizontal tension axis trending NE–SW, normal to the Terceira Ridge. The stress pattern for the whole region corresponds to horizontal extension with an average seismic slip rate of 4.4 mm/yr

Intractable policy failure: the case of bovine TB and badgers

The failure to eliminate bovine TB from the English and Welsh cattle herd represents a long-term intractable policy failure. Cattle-to-cattle transmission of the disease has been underemphasised in the debate compared with transmission from badgers despite a contested evidence base. Archival evidence shows that mythical constructions of the badger have shaped the policy debate. Relevant evidence was incomplete and contested; alternative framings of the policy problem were polarised and difficult to reconcile; and this rendered normal techniques of stakeholder management through co-option and mediation of little assistance

The Gulf of Cádiz: thrusting or strike-slip motion?

In the Gulf of Cádiz (SW Iberian Peninsula) the boundary between Eurasia and Nubia plates corresponds to a narrow band well defined by the seismicity, where large earthquakes (M>7.0) may be associated to N-S to NNW-SSE horizontal compression due to the convergence between the two plates. Most of these earthquakes are at shallow depth (less than 40 km), with some foci at intermediate-depth, showing E-W distribution and limited by a narrow band less than 20 km wide that broadens as we move to the Strait of Gibraltar. In this area the lithospheric material is relatively rigid and the stresses are released by larger earthquakes. General tectonic models proposed for the Azores-Tunisia plate boundary explain fairly well the nature of its seismicity and tectonic motions; however, details of some of its aspects are still poorly understood and controversial. Zittelini et al. (2009) has recently proposed for the Gulf of Cádiz, transcurrent-transpressional motion along a long strike slip fault based on multichannel seismic reflection surveys. However, this contradicts the compressional motion and reverse faulting of large 1755 Lisbon earthquake (~Mw9), which generate a large tsunami and the recent moderate (Mw>6.0) in 1964, 2007 and 2009 and large (Mw= 8.0) in 1969 earthquakes occurred in the region. These earthquakes show thrusting motion along E-W faults with the southern block going under, corresponding to horizontal NW-SE compression, and they can be related directly to the plate convergence between Nubia and Iberia. References Zitellini et al., 2009. The quest for the Africa–Eurasia plate boundary west of the Strait of Gibraltar. Earth and Planetary Science Letters 280 (2009) 13–50

Focal Mechanism and Rupture Process of 2004 Alhoceima (Morocco, Mw=6.2) Earthquake from Teleseismic and Regional Broad-Band Data.

We have study the focal mechanism of the 2004 Alhoceima (Morocco, Mw=6.2) earthquake using teleseismic and regional broad-band data. The solution obtained shows strike slip motion with planes striking respectively on NNE-SSW and WNW-ESE direction and horizontal pressure axes in NNW-SSE direction. We inverted body waves at teleseismic distances using as initial orientation the solution obtained from 126 P polarities. A model of extended source with rupture velocity between 2.5-3.0 km was used for the inversion. We find a complex rupture with four events at shallow depth (2-8 km). The rupture started at 6 km depth and propagated toward the south with maximum seismic moment releases at the first step (80% over a total of 1.8 x10e18 Nm). Similar result was obtained from slip inversion. An aftershock occurred on 12/03/04 (Mw=4.8) was used as empirical green function using broad-band data at re gional distances (40 to 300 km) to estimate the source time function. Comparison of these results with those obtained for the 1994 earthquake show similar behaviour, namely, a complex rupture process and apparently no relation of the 1994 and 2004 shocks with the Nekor fault, the most important geological feature in the studied area. The stress pattern derived from the 1994 and 2004 focal mechanisms are in agreement with the regional stress pattern in the Alboran Sea: horizontal compres- sion in NNW-SSE and horizontal extension in E-W direction

Reply to Comment on “The earthquakes of 29 July 2003, 12 February 2007, and 17 December 2009 in the region of Cape Saint Vincent (SW Iberia) and their relation with the 1755 Lisbon earthquake

Indeed, the Lisbon earthquake is a very unusual seismic event and an exception to the rule because most great tsunami-generating earth- quakes are related to well-defined subduction zones. The epicentral region, focal mechanism and the structures involved at the origin of this earthquake are still a matter of debate, with several models having been proposed (e.g. Baptista et al., 2003; Grandin et al., 2007a,b; Gutscher et al., 2006; Ribeiro et al., 2006; Vilanova et al., 2003; Zitellini et al., 2001)

Viabilidad de las propuestas didácticas planteadas en la formación inicial : opiniones, dificultades y necesidades de profesores principiantes

El trabajo estudia cómo perciben unos profesores principiantes la viabilidad de las propuestas didácticas que se plantean en un curso de formación inicial del profesorado de educación secundaria de física y química cuando se encuentran ante aulas y centros reales. Además se profundiza en las causas de la escasa viabilidad de algunos conocimientos, que se agrupan en cuatro ámbitos: conocimientos y creencias, demandas de formación, características de los alumnos y factores contextuales. Finalmente se establecen algunas implicaciones para la formación del profesorado.This study explores how fi ve secondary beginning teachers perceive the viability of didactic proposals that had been worked during one course of initial Physics and Chemistry teacher training, when they fi nd themselves in the presence of real classrooms in real schools. Besides we analyse the reasons for the scarce viability of some proposals, grouped in four fi elds: knowledge and beliefs, demands of training, students characteristics and contextual factors. Finally we establish some implications for teacher training