Nucleation process of laboratory earthquakes on a submeter fault under different normal stresses

We conducted friction experiments on a 669 mm long granodiorite simulated fault to study the detailed spatiotemporal evolutions of nucleation zones (characterized by the strain-releasing zones) of stick-slip events under four different normal stresses via a dense array of strain rosettes (with spacing of 21 mm between the adjacent rosettes and offset distance of 8 mm from the fault). Cumulative shear strain relative to the strain at 1 s prior to the stick-slip instability is calculated for each strain rosette. By doing this, the strain-releasing zone can be detected in the spatiotemporal evolution of the cumulative shear strain. We present a typical pattern of the nucleation process under each normal stress. Under the normal stresses of 7.5 and 10 MPa, a quasi-static expanding nucleation zone initiates at x ~ -80 mm and expands slowly and then shrinks to a critical length (referred to as the contraction nucleation length, Ls) before it merges into a subsequently accelerated expanding nucleation zone. The accelerated expanding nucleation zone initiates in the middle of the fault (x ~ 0) near the outer edge of the quasi-static expanding nucleation zone, which first expands bilaterally at a speed of the order of m/s and then gradually accelerates towards the fault ends. When the length of the accelerated nucleation zone expands to another critical length (referred to as the ultimate nucleation length, Lc), the dynamic rupture initiates (t = 0). Under normal stresses of 2.5 and 5.0 MPa, the similar accelerated expanding nucleation zone is also detected but the quasi-static expanding nucleation zone is unobservable. Moreover, Ls increases but Lc decreases with increasing normal stress.