EARTHQUAKE STRESS DROP AND THE STRESS DROPS OF ROCK FRACTURE

The uncertainties of earthquake stress drop based on static dislocation theory were discussed. The stress drops of ten earthquakes having surface breakage were studied in detail and calculated using the fault parameters obtained from field observations and the earthquake moment obtained from seismic waves. Comparison of the stress drops obtained by diferent authors was made. It is clear that the value of a stress drop may vary by factor of two. But from the statistical point of view, the stress drops were distributed in a range from a few bars to a few hundred bars and most of them lie within the range of 20-60 bars. The stress drops for stick-slip and brittle shear fracture in rock experiments carried out under conditions similar to the crust for typical rocks in the crust were reviewed. It shows that the stress drops are dependent on the pressure, temperature, type of rocks, pore pressure, and the existence of the hydrous minerals etc., their values lie in the range from zero to a few thousand bars. The physical state of the earthquake faults have been discussed, the earthquake stress drop is an average value of the various stress drop's values on the fault plane. It was also pointed out that the earthquake stress drop obtained from static theory is lower than the actual value because the static theory did not take account of the energy dissipated in the fracture of fault plane.