Non-steady state flow and the seismic cycle a new challenge for structural geology
During large earthquakes, stresses are re-distributed from the brittle seismogenic upper crustal layer into the deeper crust, which on a long term deforms by thermally activated creep. The imposed coseismic strain rate causes a downshift of the brittle-ductile transition, with a switch to brittle failure accompanied by localized crystal plastic deformation at high differential stress. The high stresses are subsequently relaxed by creep during the interseismic period, while the fault in the seismogenic layer above is loaded again and the brittle ductile transition returns towards its original position. The record of such mid-crustal stress cycles in exhumed metamorphic rocks is discussed. Its analysis provides detailed information on processes and conditions in the middle and deeper crust related to the earthquake cycle. Based on the principle of uniformitarianism, this information is important for the understanding of the seismic phenomena and geodetic observations as a function of time.