Seismicity and seismotectonics of the Basque-Cantabrian Zone (northern Iberia) from six years of observations using a dense temporary network of broadband seismic stations

The safe management of subsurface-related economic activities, such as fluid extraction or storage (groundwater, hydrocarbon, H2, CO2, etc.), requires a reliable assessment of local seismicity. In intraplate regions, such assessments are difficult because earthquakes are often scattered and difficult to associate with active structures. Thus, studies of the local seismicity in intraplate settings often require detailed long-term seismic surveys. 

In this work, we present the results of more than six years of seismic monitoring in the Basque-Cantabrian Zone (BCZ), a region of great economic and geological interest in the eastern continuation of the Pyrenees along northern Iberia. Although the BCZ has long been an area of intensive subsurface use and resource exploitation, knowledge of its background seismicity and active structures remains limited. During our six-year long survey, we recorded more than 1200 earthquakes and computed 42 new focal mechanisms. 

The observed seismicity is generally dispersed and concentrates primarily to the east of the studied area, in the transition between the BCZ and the Western Pyrenees (WP). Within the BCZ, seismicity is associated with salt diapirs and blind faults that likely affect the Paleozoic basement, as well as with a major south-dipping Mesozoic normal fault. In the WP, seismicity primarily clusters along a steeply dipping fault that we interpret as the Ollín fault, reaching ~40 km depth. In the Southern Pyrenean Zone, we observed two seismic crises that appear to be related to blind faults. In the northern Iberian Range, seismicity is scattered over a wide range of depths, both all of them occurring above and below the frontal thrust (Cameros thrust). 

Finally, we analyzed the regional stress regime by inversting the newly-derived focal mechanisms. Our results indicate a predominantly extensional stress regime in the BCZ, with localized strike-slip components in several areas, including the South Pyrenean Zone.