Damage stratigraphy as a controlling mechanism of earthquakes distribution and fault patterns

Decollement zones are sub-horizontal weak zones in the earth's crust that strongly affect tectonic deformation. The weakness of a decollement zone is commonly attributed to thermal and composition layering that is relatively static in time. Here, 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. We argue that this earthquake distribution may pose an increased geohazard to the Chengdu area by shallow, moderate earthquakes. •Damage from tectonic processes continuously weakens stratigraphic units to transform them into thick decollement zones.•The depth distribution of seismic activity in the western Sichuan Basin is controlled by a damaged Triassic sequence.•The existence of the ‘damage stratigraphic’ unit may pose a seismic hazard to the inhabitants of the Chengdu area.