Graça Silveira

Anisotropic tomography of the Atlantic Ocean from Rayleigh surface waves

Abstract The depth extent of the Mid Atlantic Ridge and the role of hotspots in the Atlantic opening are still a matter of debate. In order to constrain the structure and the geodynamic processes below the Atlantic Ocean, we provide the first anisotropic phase velocity maps of this area, obtained at a regional scale. We have determined Rayleigh wave phase velocities along 1311 direct epicentre to station paths. For each path, phase velocities are calculated by a technique of cross-correlation with a synthetic seismogram. These phase velocities are corrected for the effect of shallow layers. They are then inverted, without a priori constraints, to obtain maps of the lateral variations of the anisotropic phase velocities in the period range 50–250 s. The ridge axis corresponds to a low velocity anomaly, mainly visible at short periods. A good correlation between hotspot locations and low velocity anomalies is obtained for the whole period range. Furthermore, a low velocity anomaly elongated along a North–South direction is visible for every period and seems to be correlated with hotspot positions. On average, the North Atlantic is associated with higher velocities than the South Atlantic. The shields below Canada, Brazil and Africa display high velocity anomaly at short periods and only the Brazilian and African shields are still visible for a period of 200 s, thus suggesting that the Canadian shield is a shallower structure. The maps of phase velocity anisotropy under the Atlantic Ocean are interpreted in the Mid-Atlantic area, where we have the best resolution. Close to the ridge, the fast axis of Rayleigh wave phase velocity is found perpendicular to the ridge axis. A comparison of anisotropy directions and plate motion shows that seismic anisotropy integrates also deeper phenomena such as mantle convection.

Educating for earthquake science and risk in a tectonically slowly deforming region

Over the past decade, scientists have been called to participate more actively in public education and outreach (E&O). This is particularly true in fields of significant societal impact, such as earthquake science. Local earthquake risk culture plays a role in the way that the public engages in educational efforts. In this article, we describe an adapted E&O program for earthquake science and risk. The program is tailored for a region of slow tectonic deformation, where large earthquakes are extreme events that occur with long return periods. The adapted program has two main goals: (1) to increase the awareness and preparedness of the population to earthquake and related risks (tsunami, liquefaction, fires, etc.), and (2) to increase the quality of earthquake science education, so as to attract talented students to geosciences. Our integrated program relies on activities tuned for different population groups who have different interests and abilities, namely young children, teenagers, young adults, and professionals.

An Active Seismic Zone in Intraplate West Iberia Inferred From High‐Resolution Geophysical Data

Intraplate Iberia is a region of slow lithopsheric deformation (<1 mm/yr) with significant historical earthquake activity. Recent high-quality instrumental data have shown that small-magnitude earthquakes collapse along clusters and lineaments, which however do not bear a clear relationship to geologically mapped active structures. In this article, we investigate the controls of these earthquake clusters. In particular, we study two of the identified clusters—the Arraiolos and the Évora seismic zones (ASZ and ESZ), located in the Western Ossa Morena Zone, southwest Iberia. The ASZ marks a sharp boundary between a seismically active region to its south and a more quiet region to its north. We revise historical earthquakes in order to clarify whether earthquake activity in the region is persistent. We use data from a local network to compute accurate epicenters, focal depth, focal mechanisms, and spatiotemporal clustering, thus characterizing ongoing small-scale fracturing. Finally, we analyze complementary data sets, including tomographic models, Global Navigation Satellite Systems data, magnetic anomalies, and gravity anomalies, in order to discuss the factors that control seismogenesis in the two seismic zones. Consistency between earthquake locations, focal mechanisms and Global Navigation Satellite Systems data suggests that the ASZ is an active right-lateral shear zone, which divides two blocks within the Western Ossa Morena Zone. The ESZ seems to localize microseismicity due to its granitic lithology. These results suggest that high-resolution geophysical data have the potential to reveal blocks with different seismogenic and rheological behaviors, which may be used to improve our understanding of fault systems and the assessment of earthquake hazard in slowly deforming regions. Plain Language Summary Mainland Portugal is a region of slow lithospheric deformation. This means that changes in Earth’s outmost layer—the lithosphere—occur at very low rates (<1 mm/yr). In such environments, faults producing earthquakes are not easy to identify at the Earth’s surface, both because their evidence can be gradually erased by wind and water or simply because they do not reach the surface. Recent studies have shown that small earthquakes in mainland Portugal group together delineating seismically more active regions. In this article we focus in two particular groups of earthquakes—the Arraiolos and the Évora seismic zones (ASZ and ESZ) and we investigate why they occur in these particular locations. We obtain precise maps of earthquake epicenters. When possible, we also analyze the direction of slip during the earthquake and the orientation of the fracture on which it occurred. We compare our results with other data sets, such as images of the Earth’s interior, that could give hints about the constitution of crust beneath the ASZ and the ESZ. Earthquakes epicenters show fault sections at depth in the ASZ. These faults separate two crustal blocks with distinct material properties. In the ESZ earthquakes are associated to contrasts in crustal materials.