EAPS DLS Department Highlight Series: Matej Pec
Earthquake nucleation at the base of the seismogenic layer
Plate tectonics relies on the existence of rigid plates separated by narrow and long-lived shear zones that accommodate relative movement and form plate boundaries. Independent observations from seismology, structural geology and rock mechanics support the general view that a transition from “brittle” to “viscous” deformation occurs as depth increases. The peak of crustal strength is expected in the brittle-to-viscous transition region which stores the largest amount of strain energy that can be released either during creep, slow slip events or earthquakes.
In this talk, I will discuss deformation experiments on felsic rocks undergoing the brittle-to-viscous transition. I will document that their strength is governed by the development of nanocrystalline to amorphous principal slip zones that are about an order of magnitude weaker than the coarser-grained fault rocks. These slip zones behave as nearly linear-viscous fluids with a very low activation energy, suggesting that they will be “rate-strengthening” and should prevent an earthquake instability in the classical rate-and-state friction model. The rheological behavior of the nanocrystalline material, however, suggests a possible weakening mechanism conceptually different from the rate-and-state friction model that may nevertheless lead to an earthquake instability. Fault weakening occurs due to the intrinsic low viscosity of the nanocrystalline fault rocks. Shearing of coarse-grained fault rocks locally leads to comminution and patchy production of nanocrystalline material, the volume occupied by these materials increases with increasing work and homologous temperature, eventually reaching a percolation threshold. Although the extremely fine-grained material is rate-strengthening, it has a much lower viscosity than the surrounding material. Once this material is generated in sufficient quantities and forms a kinematically favorable failure plane, the fault displacement may accelerate on this lower viscosity layer at the same stress level and in the absence of significant temperature increase potentially leading to an earthquake instability. This type of earthquake instability is similar to the instabilities leading to intermediate and deep earthquakes, suggesting a possible link between crustal and deeper seismicity.
About this Series
The Department Lecture Series at EAPS at MIT is a series of Weekly talks given by leading thinkers in the areas of geology, geophysics, geobiology, geochemistry, atmospheric science, oceanography, climatology, and planetary science. For more information please contact: Maggie Cedarstrom, email@example.com.