b-value variations and fracture pinning

Abstract

We propose to interpret possible fluctuations of the b-value on the basis of a simple fault model. It relies on the observation that many deep reservoirs host only a very limited number of major faults, e.g. in Soultz-sous-Forêts, Alsace, where most of the induced seismicity is related to one major fault. Our numerical model consists of a weak and fragile fault with a discrete set of asperities embedded in a stiff elastic matrix on which the fracture front intermittently pins and depins. Depinning periods are related to bursts that are considered as analogues of micro-seismic events. We show that the relative stiffness of the fault with respect to that of the matrix is the key parameter which controls the development of two different regimes of b-values of the magnitude distribution of these events.