Deciphering the Origins of Transient Seismic Moment Accelerations by Realistic Dynamic Rupture Simulations

Properties of earthquake source physics can be inferred from the comparison between seismic observations and results of dynamic rupture models. Although simple self-similar rupture models naturally explain the space and time observations at the scale of the whole earthquake, several observational studies based on the analysis of source time functions (STFs) suggest that they are unable to reproduce the initial accelerating phases of the rupture. We here propose to reproduce the observed transient moment accelerations, without affecting the global self-similarity of the rupture, to constrain their possible physical origins. Simulated STFs are generated from dynamic simulations with heterogeneous slip-weakening distance Dc. Heterogeneity is introduced on the fault plane through a fractal number-size distribution of circular patches, in which Dc takes a value proportional to their radius. As a consequence of the stochastic spatial distribution of the patches, rupture development exhibits a large variability, and delays between initiation and main rupture activation frequently occur. This variability, together with the dynamic correlation between rupture velocity and slip velocity inside each broken patch, successfully perturbs the self-similar properties: rather than growing quadratically with time, STFs have an higher apparent time exponent, close to the observed value of 2.7. In a broader perspective, our simulations show that to respect observed STF shapes, realistic dynamic models should generate bursts of seismic moment, most likely by episodes where slip and rupture velocity are correlated. Such a behavior appears to emerge more naturally when considering heterogeneities in the friction parameters rather than in the initial stress.