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

Abstract

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.

PRISM3D: a 3-D reference seismic model for Iberia and adjacent areas

“…region. This has been exemplified in a previous study of seismicity in Alentejo (southern Portugal) by Matos et al (2018). This study showed that epicentral locations calculated with PRISM3D, using NonLinLoc to compute traveltimes on a full 3-D grid, could be shifted by up to ∼10 km with respect to those obtained with the IGN 1-D model.…”

Reevaluation of the 11 November 1858 Earthquake and Tsunami in Setúbal: A Contribution to the Seismic and Tsunami Hazard Assessment in Southwest Iberia

“…Those faults are approximately perpendicular to the faults proposed in this study and most likely related to Mesozoic rifting. Thus, they are not necessarily contradictory to the results of our analysis especially when considering the world stress data (Heidbach et al, 2016;Stich et al, 2020) and GNSS data (Matos et al, 2018;Palano et al, 2015) in the propsosed source area. Our findings, the missing geological evidence, and the ambiguities in the proposed source zone require further field investigation.…”

Mapping of Eastern North Atlantic Ocean seismicity from Po/So observations at a mid-aperture seismological broad-band deep sea array

“…The stations were oriented with magnetic compass. The array is situated on metamorphic rocks of neoproterozoic and lower palaeozoic age of the Ossa Morena zone near the seismically active east-west striking Moura-Vidigueira fault and the longer Messejana fault striking NE-SW (Borges et al 2001;Matos et al 2018). A few focal mechanisms obtained for this area show strike-slip characteristics (Borges et al 2001).…”