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- An active seismic zone in intraplate West Iberia inferred from high-resolution geophysical dataPublication . Custodio, Susana; Batlló, Josep; Zahradnik, Jiri; Arroucau, Pierre; Silveira, Graça; Heimann, Sebastian; Matos, CatarinaIntraplate Iberia is a region of slow lithopsheric deformation (<1mm/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 clustersthe Arraiolos and the Evora 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 layerthe lithosphereoccur at very low rates (<1mm/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 earthquakesthe Arraiolos and the Evora 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.
- New study on the 1941 Gloria Fault earthquake and tsunamiPublication . Baptista, Maria Ana Carvalho Viana; Miranda, Jorge Miguel; Batlló, Josep; Lisboa, Filipe; Luís, Joaquim; Maciá, RamonThe M ∼ 8.3–8.4 25 November 1941 was one of the largest submarine strike-slip earthquakes ever recorded in the Northeast (NE) Atlantic basin. This event occurred along the Eurasia–Nubia plate boundary between the Azores and the Strait of Gibraltar. After the earthquake, the tide stations in the NE Atlantic recorded a small tsunami with maximum amplitudes of 40 cm peak to through in the Azores and Madeira islands. In this study, we present a re-evaluation of the earthquake epicentre location using seismological data not included in previous studies. We invert the tsunami travel times to obtain a preliminary tsunami source location using the backward ray tracing (BRT) technique. We invert the tsunami waveforms to infer the initial sea surface displacement using empirical Green’s functions, without prior assumptions about the geometry of the source. The results of the BRT simulation locate the tsunami source quite close to the new epicentre. This fact suggests that the co-seismic deformation of the earthquake induced the tsunami. The waveform inversion of tsunami data favours the conclusion that the earthquake ruptured an approximately 160 km segment of the plate boundary, in the eastern section of the Gloria Fault between −20.249 and −18.630◦ E. The results presented here contribute to the evaluation of tsunami hazard in the Northeast Atlantic basin.