Complex seismic sequences originated from the collective behavior of asperities: an experimental approach

Tectonic faults can slip with diverse behaviors: from aseismic creep to seismic slip. Such diverse slip behaviors of a fault are mainly controlled by the frictional stability of the rough fault interface, where a complex set of real contacts are established by numerous discrete asperities. However, understanding how these asperities precisely control the slip stability still remains elusive.

Here we develop a novel analog fault model to overcome the difficulty of imaging an exhaustive spatiotemporal variability of a natural fault interface at depth. Specifically, numerous identical rigid spherical PMMA (poly-methyl-methacrylate) beads, which are used to model the discrete frictional asperities, are embedded with height variations and random spatial distribution in a soft viscoelastic silicone block to establish numerous micro-contacts with a thick transparent rigid PMMA plate on the top. During the entire shear process of such a heterogeneous fault interface, not only the subtle motion of each local asperity can be directly measured by the high-resolution optical monitoring system, but also the seismic waves emitted from slip transients that occurred at local asperities can be captured by the acoustic monitoring system.

The synchronization of the local rapid slips at all asperities is responsible for the unstable system-size stick-slip of the macroscopic fault that generates large amplitude energetic acoustic event. It is interesting to observe that complex seismic activities initiate also early during the interseismic phases and are interpreted as the seismic signature of destabilizing transients that originate from spatiotemporal interactions of limited local asperities. We locate acoustic events at the asperity scale and correspond them with these slow transients. We further quantify the partitioning of the resolved slip taking place on the asperities as dynamic events to interpret the nature of the complex seismicity. Our results provide insights into a better understanding of the physical processes leading to the occurrence of foreshocks and complex seismic sequences.