Abstract: In studying the focal fracture process and the specific characteristics of the body and surface waves propagating through the earth, three relations for estimating shear stress values (τ₀) and one for estimating focal dimensions 2a from the values of magnitudes M_Land M_S of minor earthquakes have been derived, namely,logτ₀=1.5M_L—M_S—0.68 M_L=2logτ₀+2log（2a）—0.06 M_S=2logτ₀+3log（2a）—0.77 log（2a）=M_S—M_L+0.71 Using these formulae, it is relatively convenient to estimate the shear stress values τ₀ and focal dimensions 2a. Based on the stress values thus estimated, the variations of the stress field of the surrounding regions before and after the major Tonghai Earthquake have been studied. Before the main shock of magnitude 7.7, there appeared very high values (up to 282 bars) in the Eshan area near Tonghai and lower values, less than 100 bars, elsewhere. After the main shock, the shear stress values dropped to nearly 100 bars in the middle part of the earthquake fault zone and a few points of higher stress values appeared at the ends of the fault. However, following the occurrence of a number of aftershocks of magnitudes greater than 5, the shear stress values all dropped to about 100 bars.