Brittle sedimentary strata focus a multimodal depth distribution of seismicity during hydraulic fracturing in the Sichuan basin, southwest China

The number of background earthquakes (ML ≥ 0) in the southern Sichuan basin, southwest China, has increased thirtyfold as a result of hydraulic fracturing. Background events are originally deep (4-6 km) within the sedimentary section but build into a multimodal distribution both at depth and in the shallow stimulated reservoir (2-4 km) - representing a counterpoint to the usual triggering of seismicity on deep sub-reservoir basement faults. Surprisingly, the largest events (ML ≥ 3) evolve in the deep sedimentary strata (4-6 km) that are hydraulically isolated from the injection zone (2-4 km) by low permeability layers. We evaluate the friction-stability rheology of the strata within the full stratigraphic section to define the feasibility of nucleation within these shallow and deep strata. These show velocity-neutral to velocity-weakening behavior in the shallow reservoir transitioning to more strongly velocity-weakening with increase in both depth and temperature. Poroelastic stress ca..., , , # Brittle sedimentary strata focus a multimodal depth distribution of seismicity during hydraulic fracturing in the Sichuan basin, southwest China

**Mengke An, Derek Elsworth, Fengshou Zhang, Rui Huang, Junlun Li, Zhengyu Xu, Zhen Zhong, and Manchao He. Brittle Sedimentary Strata Focus a Multimodal Depth Distribution of Seismicity during Hydraulic Fracturing in the Sichuan Basin, Southwest China. *Tectonophysics*.**

**Data S1**

**The friction-displacement data for the nine shear experiments on rock gouges from Xixiangchi to Doushantuo Formations in Sichuan Basin.**

**Authors**

Mengke An

Department of Civil and Environmental Engineering, The Hong Kong Polytechnic University, Hung Hom, Kowloon, Hong Kong 100872, China.

Department of Energy and Mineral Engineering, EMS Energy Institute and G3 Center, The Pennsylvania State University, University Park, PA 16802, USA.

<[e-mail]>

Derek Elsworth

Department of Energy and Mineral Engineering, EMS Energy Inst...