Spatial variations of b-value and crustal stress in the Pyrenees

Abstract

The seismicity in the Pyrenees is continuous and well surveyed since more than 20 years. We use the catalogue of seismicity between 1997 and 2013 to explore the spatial variations of the b-value, which corresponds to the slope of the frequency-magnitude distribution of the earthquakes. Especially, variations of the b-value characterise the state of stress of the crust, possibly highlighting a deficit of large earthquake occurrence. We estimate the differential crustal stress from the b-value using a relationship published by Scholz (Geophys Res Lett 42:1399–1402, 2015). We also estimate the stress drop variations by determining a power law which links the magnitude to the seismic source radius in the Pyrenees. We focus on the depth variations and we analyse vertical profiles of b-value, differential stress and stress drop, first in the whole Pyrenean belt, then in 10 subregions. The b-values are generally smaller than 1, except in the uppermost 3–5 km where the obtained high values could be linked to the presence of fluids. Downward, the b-values decrease slowly or remain constant until a depth of increase, which could correspond to the brittle-ductile limit of the crust. We propose that this depth and the regional and vertical variations of the b-values are related to the regional tectonic context and possibly to the density heterogeneities. We also suggest that stress drop and differential stress are linearly correlated and that the stress drop is at least 1.8‰ of the differential stress.