Frictional and Poromechanical Properties of Serpentinite: Implications for Deep Fault Slip Behavior

Abstract:

Recent observations of deep tremor and low-frequency earthquakes (LFE) have raised fundamental questions about the physics of quasidynamic rupture and the underlying fault zone processes. The presence of serpentinite at P-T conditions characteristic of deep tremor and LFE suggests that it may be an important element in complex fault slip, however, little is known about its hydrological and mechanical properties. Here, we report on experiments designed to investigate the frictional behavior of serpentinite recovered from outcrops (SO1 and SO2) and the SAFOD borehole (G27). XRD analyses reveal the presence of lizardite, kaolinite, talc and hydrotalcite in the SO1; lizardite, clinochlore and magnetite in SO2; and lizardite, quartz and calcite in G27. We powdered samples from intact blocks to a grain size <150 µm and sheared the resulting gouge in a double-direct shear configuration using a true triaxial deformation apparatus. Effective normal stress (σ’_n = σ_n – P_p) was kept constant at values ranging from 2 to 40 MPa. Shear stress was applied via a constant load point displacement rate, and velocity was increased stepwise from 0.1 to 100 μm/s, after which a series of slide-hold-slides (SHS) were performed. We measured permeability normal to the shear direction and recovered samples for microstructural analysis. Mechanical strength varied from m=0.17 for SO1, m=0.33 for SO2 and m=0.53 for G27. Gouges exhibit an overall velocity strengthening behavior, with values approaching velocity neutral at the highest effective normal stress. SHS tests show positive healing rates for S02 and G27; whereas, SO1 samples exhibit zero or negative healing rates. Permeability decreases with increasing σ’_n, with SO1 (k = 10^-20m²) showing the lowest values. Microstructural observations revealed a well-developed R-Y-P fabric in SO1, which is not observed in SO2 and G27. We posit that the development of different shear fabrics controlled by mineralogy controls frictional and hydrological properties. In this context, when serpentinite is associated with other weak phyllosilicates minerals, at typical seismogenic depths, it can control the mode of failure along subduction zones and major plate boundary faults.