Current Research in EEPS Seminar: Dr. Matej Pec – Massachusetts Institute of Technology
The Influence of Grain Size on Fault Strength and Stability at the Base of the Seismogenic Layer
Field and experimental studies document that fault slip under a broad range of conditions is accommodated in a thin volume of nano-crystalline to amorphous materials derived from the wall rock. These materials hence contribute to the strength and stability of a fault, or in other words whether the fault will creep or fail in an earthquake. However, the origin of nano-crystalline to amorphous fault rocks is currently unclear; hypothesis range from temperature induced melting due to fast slip during an earthquake to amorphization by comminution at tectonic creep rates. The rheological properties of these materials are unknown as well. Fluid like microstructures indicate that the materials can flow, however its viscosity is yet to be measured.
In this talk, I will first present results from experiments on microcrystalline (initial grain size <= 250 µm) granitoid fault rocks deformed under a broad range of conditions (pressures of 300 – 1500 MPa and temperatures of 150˚ – 600˚C corresponding to depths of about 10 – 55 km). Under these conditions, narrow slip zones containing nano-crystalline to amorphous materials develop by comminution at high stresses as a result of strain localization and partitioning. These slip zones delimit larger lenses containing coarser grained material. The volume occupied by the slip zones increases with increasing temperature and strain. The initially microcrystalline fault rocks creep steadily at temperatures >300˚C and strain rates <10-3 s-1 but fail abruptly in a series of stick-slip events accompanied by bursts of acoustic emissions (AE) at lower temperatures and higher strain rates.
Second, I will present results from experiments on nano-crystalline fault rocks (initial grain size ≈ 100 nm) of the same granitoid composition that were obtained by high-energy planetary ball milling. These fault rocks creep steadily without any recorded AEs at all explored conditions (200˚, 300˚ & 500˚C at 500 MPa confining pressure) and are about an order of magnitude weaker than the microcrystalline fault rocks deformed at identical conditions. Stress-stepping experiments to determine the rheology indicate that the nanocrystalline fault rocks exhibit a linear stress – strain rate relationship with a stress exponent, n of ≈1. Hence, it appears that fine grained fault rocks can deform by stable diffusion creep even at low temperatures due to the very short diffusional distances. Stick-slip behavior in microcrystalline fault rocks presumably occurs when the volume occupied by the slip zones is not sufficient to accommodate the imposed strain and where strain incompatibility leads to loading of the larger microcrystalline lenses. Alternatively abrupt failure can occur if a kinematically favorable failure plane develops allowing for rapid slip in the nanocrystalline to amorphous material.