Intraplate Seismicity in Northwest Iberia along the Trace of the Ventaniella Fault: A Case for Fault Intersection at Depth

Intraplate seismicity in northwest (NW) Spain, an otherwise stable continental area, is dominated by low-magnitude events and occurs both in swarms and dispersed along faults. A detailed study of one of the most active fault segments, the Ventaniella fault, has produced an accurate image of foci distribution and revealed new insights on the origin of this lingering activity. The improved location of earthquakes by a temporary seismic network has allowed us to better constrain the geometry of the seismogenic segments of the fault at depth. Between 2015 and 2017, a portable seismic array of 10 seismographs recorded 45 low-magnitude earthquakes (< 2) at depths between 9 and 18 km. These hypocenters define a tubular trend plunging to the NW. The linear seismicity pattern is interpreted as the result of the intersection at depth of two main fault planes: a northwest-southeast (NW-SE) fault reactivated in the Alpine Orogeny and the frontal thrust of the Cantabrian Mountains, running east-west (E-W). The clustering of earthquakes along this particular line of intersecting faults coincides spatially with the presence at depth of an important lateral gradient in crustal thickness, related to the termination of the crustal root beneath the Cantabrian Mountains. The mechanical constraints in the continental crust imposed by the arrangement of crustal scale faults and the gradient in crustal thickness may have reactivated seismically old faults in a context of a stable continental area.