Numerical simulation of large shear strain drops during jog failure for echelon faults based on a heterogeneous and strain-softening model

- We model jog failure of transpressional and transtensional en echelon faults.
- Our model assumes strain-softening during shear failure.
- Heterogeneous strength is used for rock medium and faults.
- Large shear strain drops occur during the jog failure.
- Fault spacing and overlap dictate the jog failure mode.

For two kinds of echelon faults with different spacings, we numerically model the spatiotemporal distribution of large shear strain drops using a heterogeneous and strain-softening model implemented in FLAC (Fast Lagrangian Analysis of Continua). Nine quantities, regarding large shear strain drops and released energy due to shear or tensile failure, are calculated during the jog failure. It is found that the largest shear strain drops are created upon the jog failure for both kinds of echelon faults, suggesting the occurrence of large cracking or slip events. For the compressive echelon faults, the number of elements undergoing large shear strain drops, and the sum of all large drops exhibit pronounced responses upon the jog failure, but not for the extensional echelon faults. This means that whether abnormalities are apparent during the jog failure depends on types of echelon faults. Large shear strain drops have superiority over some quantities used commonly, such as the number of failed elements and corresponding energy due to failure. This is due to the fact that the failure of an element storing high elastic strain energy can lead to unloading responses of its surrounding elastic elements. Namely, an input for a system results in many outputs. Whereas, these common quantities can only reflect the information of failed elements. For the compressive echelon faults, large events occur in the vicinity of the jog, appropriate for detecting abnormalities related to the jog failure. However, for the extensional echelon faults, they occur at faults, thus the jog may not be appropriate based on the present model without shear dilatation and pore fluid. A small fault spacing equal to overlap leads to the fact that the jog fails easily and early. For the compressive echelon faults, responses of some quantities become less apparent during the jog failure as fault spacing increases.