SUMMATION AND DECOMPOSITION OF PRINCIPAL STRESSES IN THE CRUST

Abstract

In the compilation of World Stress Map, 9% of data comes from overcoring and hydraulic fracturing measurement, 23% from borehole breaking off, 63% from earthquake focal mechanism, and 5% from young geological investigation (Zoback,et al, 1989). Only overcoring and hydraulic fracturing can provide both the orientation and magnitude of the horizontal stress, all other methods can only provide the orientation, but no the magnitude of the stresses. Although some researchers tried to estimate magnitudes of stresses in earthquake mechanism research based on some additional assumptions (CHEN, Duda, 1996; ZHAO,et al, 2002). This method, however, has not been widely applied. What kind of analysis can be done with orientation-only data? What kind of incorrect operations should be avoided? These are basic important problems. However, some confusions and misunderstandings exist. For example, a simple operation is to use the average of measured orientation to represent the principal stress orientation in a specific area; or decompose a stress into a summation of long wavelength and short wave length components. Is it correct to do in this way? Some fundamental ideas are hidden in these seemingly simple problems. We will discuss these questions in this note.