Seismicity and potencially active faults in the Northwest and Central-West Iberian Peninsula

Abstract

The Northwest and Central-West Iberian Peninsula configure an intraplate area far from the active plate boundaries, where the Variscan basement crops out extensively (Iberian Massif). This area of the Iberian Peninsula has been traditionally considered a seismically stable region; however, it presents a moderate intraplate seismicity which indicates the presence of active structures and the occurrence of potentially damaging earthquakes. The scarcity of Mesozoic and Cenozoic deposits makes very difficult to track the record of the more recent tectonic activity and the characterization of active tectonic structures within the Iberian Massif. Nevertheless the seismic sequences of 1995-1997 in Lugo (5.1 mb; IV) and 2003 in Zamora (4.2 Mw) provided important information about the orientation of the present stress tensor, and the distribution of the hypocenters informed about the rupture geometry of the fault planes. The present work integrates geological, geomorphological, structural, and seismological data in order to define the main potentially active faults in the region. Faults trending NE-SW to N-S are potentially active as strike-slip, in some cases with a reverse component, under a NW-SE to N-S compression.

The seismogenic zone of the continental crust in Northwest Iberia and its relation to crustal structure

“…[]). The majority of events above magnitude 3 or 4 are nucleated in the Becerreá area [e.g., Martín‐González et al ., ] or nearby and in relation to similar type of structures. Focal mechanisms in those seismic events indicate that the maximum horizontal compression, σ h max , is oriented northwesterly, at very high angle to the inferred slipping faults which strike N30°E [ de Vicente et al ., ; López‐Fernández et al ., ].…”

“…However, the most significant feature in vertical profiles is the presence of several earthquake swarms, such distributions were already visible in older earthquake catalogs [e.g., Rueda and Mezcua , ]. In 3‐D, these swarms have a tubular or pipe‐like shape from the surface to around 12–15 km in depth, where they tend to merge with background regional seismicity (Figures and ) [ López‐Fernández et al ., ; Martín‐González et al ., ]. The largest of the earthquake swarms in NW Iberia, the Becerreá swarm, stands out for having a different distribution of seismicity with depth, dominated by shallower earthquakes that ultimately reduce locally the thickness of the seismogenic zone (Figure c).…”

“…Since 1979 seven seismic sequences were recorded in that area, including up to 14 events of magnitude >4 and a main event of magnitude 5.1 mbLg (21 May 1997). In general, earthquake magnitudes are below magnitude 3.5 [ Martínez‐Díaz et al ., ; López‐Fernández et al ., ; Martín‐González et al ., ]. Seismicity in Becerreá has been related directly to NE trending strike‐slip faults [ González‐Casado and Giner , ; López‐Fernández et al ., , ; Martínez‐Díaz et al ., ]; however, these faults have little topographic expression at the surface and produce a very limited offset of geological units [ Martín‐González , ].…”

“…In other words, rocks currently at the surface in Galicia were not deeper than 1.7 km by the beginning of the Cenozoic, assuming in this estimate a temperature gradient close to the surface of 28°/km. This mere 1.7 km have been denudated on average in the last 100 Ma, in some samples mostly in the last 10–20 Ma [ Grobe et al ., ; Martín‐González et al ., ]. This is true for most of the study area, although as Alpine shortening increases to the east, so does the amount of expected eroded material.…”

Exceptional river gorge formation from unexceptional floods

“…1d). This included a primary NNE left-lateral strike-slip fault system concordant with previous field-based regional structural geological analyses 34,35 . This primary pattern exerts a significant control on drainage network orientation, as observed in the lower Río Esla reach ( Supplementary Fig.…”