Observations of stress relaxation before earthquakes

Theory and observations suggest that seismic shear wave splitting, caused by fluid- saturated stress- aligned microcracks, directly monitors low- level deformation before fracturing, faulting and earthquakes occur. In the past, it had been assumed that the accumulation of stress before earthquakes continued until stress was released by faulting at the time of the earthquake. However, new data and reappraisal of existing data sets now suggests that the stress begins to relax and cracks close from tens of minutes to months before the earthquake actually occurs, with the logarithm of the duration of the relaxation proportional to the magnitude of the impending earthquake. The duration of the relaxation appears to be directly correlated with earthquake magnitude, and may have implications for the earthquake source process and the ability to predict earthquakes.