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The Role of Subcritical Cracking during Slow Faulting of Rocks Deformed Under Well-Drained Conditions

Zachary Zega, & Wen-lu Zhu

Submitted September 11, 2022, SCEC Contribution #12534, 2022 SCEC Annual Meeting Poster #172

Regions of elevated pore fluid pressures are often collocated with slow slip events on natural faults. During undrained deformation when the timescale for deformation is faster than the timescale for fluid diffusion, dilatant microcracking in rock leads to drops in pore fluid pressure that strengthen the rock and stabilize failure. This strengthening phenomenon associated with dilatancy and undrained deformation is termed dilatant hardening. Experimentally, rock strength and failure mode are dictated by the differential pressure (i.e., the difference between the confining and pore fluid pressures). Slow faulting associated with high fluid pressure at the same differential pressure in compact rocks such as granites could be explained by dilatant hardening. Whether such a stabilizing effect at high pore fluid pressure can be observed in porous rocks where deformation is under drained conditions remains unclear.

To investigate how drainage conditions affect the stabilization effect of high pore fluid pressure on fault growth, we deformed highly permeable samples of Adamswiller and Darley Dale sandstones using a constant differential pressure of 10 MPa and pore fluid pressures from 2 to 180 MPa. The samples were deformed using constant strain rates of 10-4 s-1, 10-5 s-1, or 10-6 s-1. Experiments completed on Adamswiller samples did not show rate-dependent shear strength and similar faulting behaviors were observed at all pressure conditions, indicating negligible dilatant hardening during drained deformation. In contrast, experiments completed on Darley Dale samples exhibited rate dependent shear strength and slow failure at high pore fluid pressure at a strain rate of 10-6 s-1. Our microstructural analyses indicate slow faulting at high pore fluid pressure was associated with extensive grain crushing and higher crack densities. Slow faulting under well drained conditions that permit ample time for fluid diffusion cannot be explained by the conventional dilatant hardening model. We propose that brittle creep plays an important role during faulting in Darley Dale deformed at 10-6 s-1, and the interplay between dilatant hardening and stress corrosion-facilitated fault growth is responsible to slow faulting in porous rocks under nominally drained conditions.

Key Words
subcritical cracking, creep, pore fluid pressure, slow slip, rock deformation

Zega, Z., & Zhu, W. (2022, 09). The Role of Subcritical Cracking during Slow Faulting of Rocks Deformed Under Well-Drained Conditions . Poster Presentation at 2022 SCEC Annual Meeting.

Related Projects & Working Groups
Fault and Rupture Mechanics (FARM)