23 May 2023
 | 23 May 2023
Status: this preprint is open for discussion.

Offshore seismicity clusters in the West iberian Margin illustrated by two decades of events

Gabriela Fernandez-Viejo, Carlos Lopez-Fernandez, and Patricia Cadenas

Abstract. An analysis of two decades (2003–2022) of seismicity recorded by the Spanish and Portuguese seismic networks along the West Iberian passive margin has resulted in a better understanding of the distribution of moderate seismic activity in this intraplate submarine area. The study provides a precise trend of specific alignments inferred from the density maps of seismicity, giving an accurate depiction of event distribution along two wide stripes that extend 700 km long through the ocean floor in a WNW-ESE direction. These bands are parallel to the Africa-Eurasia plate boundary but are distinctly separated from its related seismicity by approximately 300 km and 700 km, respectively. This is a sufficient distance to be considered as intraplate activity. When trying to relate this seismicity to structural and geophysical features, a conclusive picture doesn't emerge. The earthquakes occur indiscriminately across thinned continental, hyperextended, and exhumed mantle rift domains. They fade out in the proximity of undisputed oceanic crust, but some events extend beyond. The hypocentral depths signal a considerable amount of events nucleating in the upper mantle. The focal mechanisms, although scarce, are predominantly strike-slip. Considering these observations, hypothesis ranging from subduction initiation, development of strained corridors or local structures of the margin, have been discussed in order to explain this relatively anomalous seismicity. However, some of them do not portray convincing arguments, while others are too unspecific. None of them are flawless, suggesting that several factors may be at play. Despite being one of the most probed passive margins in the world, the present geodynamical status of the West Iberian Margin manifested in its modern seismicity remains unknown. Interpreting this data within a global tectonic plate framework, together with the potential addition of seafloor seismometers, may provide the key to understanding this activity along one of the most archetypical margins of the Atlantic Ocean.

Gabriela Fernandez-Viejo et al.

Status: open (until 07 Jul 2023)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse

Gabriela Fernandez-Viejo et al.

Gabriela Fernandez-Viejo et al.


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Short summary
The seismic activity of the last 20 years that occurs within the West Iberian Margin has been analyzed through the spanish and portuguese seismic networks databases. As traditionally viewed as diffuse seismicity, the study reveal two distinct broad clusters in the NW-SE direction. Based on the characteristics of the events some hypothesis for this activity are put forward and discussed.