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, L_s) 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, L_c), 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, L_s increases but L_c decreases with increasing normal stress.