  Understanding the interplay between concurrent length scales is a fundamental
issue in many problems involving friction between sliding interfaces, from
tribology to the study of earthquakes and seismic faults. On the one hand, a
macroscopic sliding event is preceded by slip precursors with a characteristic
propagation length scale. On the other hand, the emergent frictional properties
can be modified by surface patterning depending on their geometric length
scale. This suggests that macroscopic sliding of structured surfaces is
governed by the interplay between the length scale of the slip precursors and
those characterizing the geometric features. In this paper, we investigate
these aspects by means of numerical simulations using a two-dimensional
spring-block model. We discuss the influence of the geometric features on the
occurrence and localization of slip precursors, extending the study to
interfaces characterized by two geometric length scales. We find that different
types of detachment sequences are triggered by specific surface structures,
depending on their scales and relation to sliding direction, leading to a
macroscopically smooth transition to sliding in the case of hierarchical and/or
anisotropic features. These concepts could be exploited in devices switching
from static to dynamic sliding, and can contribute to an improvement in the
understanding and interpretation of seismic data
