Creep deformation of synthetic damage zone rocks created from Split Hopkinson Pressure Bar tests

Abstract:

Understanding the time-dependent deformational properties of rocks in fault damage zones is important to characterize fault shear stress development in the interseismic period. To measure these effects, we studied the creep behavior of synthetic fault damage zone rocks produced from Berea sandstone and cores collected from the Taiwan Chelungpu Fault Drilling Project (TCDP). To overcome the challenge of preparing cylindrical specimens from damaged rocks, we synthetically prepared damaged specimens by conducting high strain rate experiments using Split Hopkinson Pressure Bar (SHPB). This methodology is consistent with previous studies which suggest that the experimental stain rates achieved by SHPB might be similar to those experienced in the host rocks during ruptures.

In order to capture the entire spectrum of rock types surrounding the Chelungpu fault zone, we prepared core plugs of sandstone, sandy siltstone, bioturbated siltstone and siltstone from TCDP rocks. Cores were impacted at four different striker bar velocities to create pervasive failure across the specimen at different strain rates. Damage increases with striker bar velocity, as well as the proportion of diagonal shear fractures over axis-parallel splitting mode fractures.

At a constant striker bar velocity, strain rate achieved in sandstone was higher than other rock types, but the elastic modulus, peak stress and energy consumed during deformation were lower. We noted a decrease in total volume of the samples for sandstone but an increase for silt rich specimens. These results suggest difference in mode of damage, where damage in sandstone is due to pore volume compaction but damage in silt-rich rocks is caused by fracture volume creation.

Preliminary creep experiments with the damaged Berea sandstones exhibit higher volumetric creep deformation in samples impacted at higher strain rate. We observe anisotropy in creep behavior which is either caused by the anisotropy in the original rock texture and/or the anisotropy in damage. To understand the rheology of the damage rocks, we further intend to constrain the visco-plastic constitutive relations from the creep data. This study will provide insights into time dependent deformation of damage zones in the crust.