Faulting Style Controls for the Space-Time Aftershock Patterns

To understand the complexity and thus to predict earthquake occurrence in size, time, and space, seismicity patterns are characterized by two robust empirical laws: the exponential distribution of magnitude and the power law decrease of aftershock rate over time. These laws are known as the Gutenberg-Richter law and the Omori law, respectively. Using global earthquake catalogs, we resolve that on average the K (aftershock productivity) and the p-value (exponent of the power law decrease of aftershock rate over time) are dependent on the mainshock faulting style. Strike-slip events have a lower aftershock rate (N) and K-values and a larger p-value than thrust and normal events, respectively. Within the epidemic-type aftershock sequence model, strong K, N values are driven by a high-branching ratio value (n). Within the same framework, a relatively higher n value for the thrust events also predicts the lower p-value we observe for thrust events as compared to strike-slip and normal-faulting events, respectively. Furthermore, we observed that earthquake interactions through time and space are a function of the faulting style when measured by mu(t), the exponent of the power law decrease of earthquake density over space. The mu(t) values of thrust events for different time windows always remain smaller than those of the strike-slip events. When changes in faulting styles are driven by stress pattern, the Anderson faulting theory predicts thrust faulting that requires somewhat larger stresses, in absolute magnitude, than do normal and strike-slip faulting. Within the framework of rate-and-state friction law, changes in the stress heterogeneity patterns reproduce the p-value changes we observe. Our results suggest that only stress perturbations associated with mainshock rupture affect the productivity and decay rate over time of aftershocks.