SEMINAR - Judith Chester

The macroscopic behavior of earthquake rupture depends, in part, on processes operating at the mesoscopic and microscopic scales both along rupture surfaces and in the bordering damaged rocks. Earthquake rupture propagation is strongly influenced by the balance of energy radiated as seismic waves and that associated with the breakdown in strength at the rupture tip. Creation of new fracture surfaces and frictional slip both on and off the rupture surface contribute to the breakdown energy. In spite of recent success in quantifying fracture and friction in fault zones, efforts to understand slip processes and quantify the energy budget during seismic rupture are hindered by uncertainty in the characteristics and origin of various types of damage. We are using field and laboratory data from exhumed and drilled faults, and from faults produced in the laboratory, to place constraints on the processes of dynamic weakening and the energy balance for rupture propagation. Structural observations of the Punchbowl fault, a large-displacement exhumed fault, document extreme localization of slip consistent with weakening by frictional heating. The data suggest that the creation of fracture surfaces may only account for a small fraction of the total energy budget ( < 1%), whereas the energy associated with activation of frictional slip on secondary faults away from the master fault surface is significant (3 to 10% of the total energy for a strong and weak fault model, respectively). An outstanding question is how the energy dissipated by fracture surface creation throughout the fault zone and by frictional slip off the fault surface is spatially and temporally distributed over the earthquake cycle. In particular, it is important to determine whether slip on subsidiary faults in the damage zone occurs in response to the dynamic stress concentration associated with the rupture tip, or is a result of a wear process during subsequent coseismic sliding on the main fault surface and fault creep (e.g., by sub-critical cracking) during interseismic periods. Field observations of fracture fabrics in the damage zone support the assumption that some damage is associated with dynamic rupture-tip stresses, particularly in the region near the fault surface. If a large fraction of off-fault frictional dissipation occurs during breakdown in the tip region of the earthquake rupture, then the energy available for rupture propagation and seismic radiation is diminished. Dissipation of energy by frictional slip away from the rupture surface may reduce or delay the onset of weakening processes, such as thermal fluid pressurization, at the rupture surface. Furthermore, if the damage zone of an earthquake rupture surface is characterized by significant lateral variations in fracture density, one might expect significant variations in rupture characteristics and radiation efficiency.

Judith's seminar title: "Geologic constraints on mechanisms of energy dissipation during earthquakes"; Wednesday the 24th of April 2007