Ultra-low co-seismic stiffness of fault rocks at seismogenic (8-11 km) depth
Abstract
During the seismic cycle, elastic stiffness limits the amount of elastic strain energy stored in the wall rocks bordering a fault. Elastic stiffness of fault zone rocks is expected to be highly variable during the seismic cycle due to complicated damage and healing processes. In addition to longer-term alteration which may take place during exhumation, it is impossible to assess how well rock stiffness as measured in the laboratory represents in situ, coseismic rock stiffness at seismogenic depths. Here we estimate the in situ, coseismic rock stiffness of fault rocks from the pseudotachylyte-bearing Gole Larghe Fault Zone of the Adamello Batholith, Italian Southern Alps, using aspect ratio measurements of pseudotachylyte injection veins and numerical Displacement Discontinuity Method simulations. Aspect ratios of over 100 pseudotachylyte injection veins which cut across tonalite, cataclasite, or aplite show that maximum vein aperture is linearly related to vein length. To model vein opening, the fault and the injection vein are assumed to be filled with melt that has a fluid pressure P. Consistent with recent results from modeling of melt lubrication we assume that the magnitude of the fluid pressure P is exactly the same as the fault-normal normal stress such that the fault vein approximately maintains constant thickness during slip (i.e. melt extrusion exactly balances melt production). This model assumes that melt is injected into the sidewall without significant fluid overpressure, taking advantage of pre-existing planes of weakness and transiently reduced fault-parallel normal stress in the wake of the earthquake rupture tip. Numerical simulations of injection vein opening due to fluid pressure of frictional melt indicate that the average in situ coseismic stiffness of the fault rocks ranged from 2-15 GPa, about a factor of two less than typical laboratory measurements of the same rocks, and the stiffness of tonalite and cataclasite are markedly different.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2011
- Bibcode:
- 2011AGUFM.T23E2461G
- Keywords:
-
- 8010 STRUCTURAL GEOLOGY / Fractures and faults;
- 8118 TECTONOPHYSICS / Dynamics and mechanics of faulting;
- 8163 TECTONOPHYSICS / Rheology and friction of fault zones