We estimate the absolute stress value at the earthquake source region of a small cubic area in Homestead valley fault segment which broke during the Landers earthquake, using seismic stress drop and rotations of stress axes during the event. We obtained the pre-seismic compressive, intermediate and extensional principal stress values as 323, 319 and 312MPa in the depth of 8∼12 km. The shear stresses on the fault plane before and after the event are 6MPa and 1MPa respectively. The results show that normal stress increased after the earthquake, which helps to terminate the rupture process. The pre-seismic shear-stress is greater than that after the quake and their differences roughly correspond to the seismic stress drop. The post-seismic shear-stress is positive, which means no overshooting in co-seismic slip. The differential stress value is less than the absolute stress value, and the shear stress is also relatively small.
The study of tectonic stress field plays an important role in the research of geodynamics. The world stress map plan collects global tectonic stress measurements and research results to establish global stress database (Zoback, 1992). Although we have some understanding on stress field, relatively little is known about the stress value. Stress is a key factor in a variety of seismic dynamic and geodynamic problems, so geophysicists always seek a method to estimate crustal stress value. Using rotations of principal stress axes and seismic stress change, Wesson & Boyd (2007) proposed the absolute partial stress tensor determination method and applied to 2002 Denali earthquake. Their results show that the spatially averaged shear stress resolved onto the fault plane is 1∼4MPa before the earthquake, and nearly zero after the quake. Fialko et al. (2005) determined the differential stress at San Andreas fault is on the order of 50MPa.
