New Mechanisms for Earthquakes Distribution and Fault Patterns

In this chapter, we present evidence for damage stratigraphy in which a stratigraphic unit evolves dynamically into a weak decollement zone. The analysis is focused on the internal structure of a deep, tectonically active, decollement zone in the Sichuan Basin, China. The analyzed zone is about a 1 km thick sequence of Triassic rocks that extends across most of the Sichuan Basin at a depth range of 5.8–6.8 km. This sequence is composed of evaporites (mostly anhydrite) and carbonates (dolomite and limestone). We used three-dimensional seismic surveys, borehole observations, and rock-mechanics experiments to document pervasive internal damage revealed by fractures and flow features within this unit. As this damaged unit is defined by its stratigraphic position, we term it a Damaged Stratigraphic Unit (DSU). The damage-induced weakness of the DSU controls two active tectonic features in the western Sichuan Basin. First, large, intra-basin, frontal thrust faults terminate against the DSU from above and from below. Second, regional earthquakes display a profound decrease in frequency and seismic moment at the depth interval of 5.8–6.8 km that coincides with the DSU.