CAN CREEPING CARBONATES CHANGE FAULT ZONE PROPERTIES? (Invited Presentation)

The physical properties of the rocks within fault zones, such as porosity, permeability, and stiffness, exert influence over the slip behavior of faults. These properties are also subject to change over time as the fault evolves with frictional healing and creep over the seismic cycle and longer term over multiple earthquake cycles. Often, these physical properties are probed using seismic velocities measured in active fault zones, but these snapshots do not capture the temporal evolution of these properties. Deformation that is active at shallow depths over long timescales, like pressure solution, brittle creep, and compaction, are likely to change the physical properties and ultimately fault mechanical and hydraulic behavior. These processes are particularly active and well-documented in carbonates. Carbonates are common input material to subduction zones that host or are incorporated along the décollement, as well as large hosting large continental faults. We conducted creep (stress-controlled) tests in a uniaxial consolidation system and shear (displacement-controlled) experiments in a triaxial deformation apparatus on carbonates to directly measure compaction and frictional healing, respectively. Samples were derived from drilling at the Hikurangi and Costa Rica margins. Intact cores and synthetic gouges produced from the same cores were tested under saturated conditions over stresses from 10–100 MPa. Systematic changes in physical properties, including porosity, seismic wave speeds (V_p, V_s), and elastic moduli were monitored with increasing strain and as a function of both normal stress and hold time. Overall, we find that carbonate content as well as the mineralogy of secondary phases influences both the evolution of stiffness and healing rates of natural carbonates.