Dynamic fault weakening during earthquakes: Rupture or friction?

• Published in: Earth and planetary science letters 2021-12, Vol.575, p.117165, Article 117165
• Main Authors: Chen, Xiaofeng, Chitta, Sai Sandeep, Zu, Ximeng, Reches, Ze'ev
• Format: Article
• Language: English
• Subjects: earthquake nucleation; fault weakening; frictional sliding; rupture-front; stick-slip
• ISSN: 0012-821X; 1385-013X
• DOI: 10.1016/j.epsl.2021.117165

An earthquake is an event of dynamic, unstable slip that releases elastic energy stored in the earth's crust. This abrupt energy release requires the weakening of the slipping fault that is manifested by a strength drop from a static level to a dynamic level. The fault weakening occurs in two general zones: The rupture-front at the earthquake tip, and the frictional zone in the wake of the propagating front. Here, we analyze the weakening processes of the rupture-front in experimental stick-slips, and compare these processes to the weakening in the frictional zone. Our experimental analysis focuses on high-speed monitoring (1 MHz) of the strain-fields associated with spontaneous, unstable ruptures propagating bilaterally from a nucleation site along a circular fault. The ruptures propagated at velocities ranging from slow (∼100 m/s) to supershear, and the fault weakening by the rupture-front, defined by WS = [strength drop/peak strength], is proportional to the rupture propagation velocity. WS ranged 0.08-0.58 during tiny slip-displacements of 3.7-43 microns, in agreement with similar weakening intensity in a few sets of dynamic rupture experiments. In contrast, frictional weakening, which is traditionally analyzed in constant-velocity experiments, requires slip-displacements in the range of 0.1-10 m to reach similar weakening intensity. Therefore, in terms of slip-displacements, rupture-front weakening is more efficient than frictional weakening by orders of magnitude. Further, we observed that a rupture-front transported its intense weakening from a nucleation site to the entire experimental fault, and we argue that the equivalent process can occur along natural faults. If the rupture of a tiny earthquake, e.g., M < −5, is of similar scale and intensity as the experimental ruptures, it can locally weaken the host fault, and by doing so could serve as an earthquake nucleus. However, these tiny earthquakes could nucleate large ones only if sufficient elastic energy is available in the surrounding crust for the frictional sliding dissipation. •Dynamic fault weakening by rupture-fronts proportional to rupture velocity.•Stick-slip reduce fault strength up to 70% during slip