Nucleation process of magnitude 2 repeating earthquakes on the San Andreas Fault predicted by rate-and-state fault models with SAFOD drill core data

Recent laboratory shear-slip experiments conducted on a nominally flat fric tional interfacerepor ted the intriguing details of a two-phase nucleation of stick-slip motion that precedes the dynamicrupture propagation. This behavior was subsequently reproduced by a physics-based model incorporatinglaboratory-derived rate-and-state friction laws. However, applying the laboratory and theoretical results tothe nucleation of crustal earthquakes remains challenging due to poorly constrained physical and frictionproper ties of fault zone rocks at seismogenic depths. Here we apply the same physics-based model tosimulate the nucleation process of crustal earthquakes using unique data acquired during the San AndreasFault Observatory at Depth (SAFOD) experiment and new and existing measurements of friction propertiesof SAFOD drill core samples. Using this well-constrained model, we predict what the nucleation phasewill look like for magnitude ∼2 repeating earthquakes on segments of the San Andreas Fault at a 2.8 kmdepth. We find that despite up to 3 orders of magnitude difference in the physical and friction parametersand stress conditions, the behavior of the modeled nucleation is qualitatively similar to that of laboratoryearthquakes, with the nucleation consisting of two distinct phases. Our results further suggest thatprecursory slow slip associated with the earthquake nucleation phase may be observable in the hoursbefore the occurrence of the magnitude ∼2 earthquakes by strain measurements close (a few hundredmeters) to the hypocenter, in a position reached by the existing borehole