Abstract: At some stage of earthquake preparation, the focal mechanisms of small and medium-sized earthquake have roughly the same direction with the tectonic stress field. According to this feature, we define the angle between P, B, and T axises of focal mechanisms and three stress axises of tectonic stress field as the consistency parameter a, to research the dynamic changes of stress field in earthquake preparation area before strong earthquakes. In this paper, we mainly studied the changes of the consistency parameter a of the Kuril island arc M_W8.3 earthquake and Peru M_W8.4 earthquake. Our study found that before strong earthquake occurred, the focal mechanisms of a series of small and medium-sized earthquakes get small differences with the tectonic stress field, this indicates that those foreshocks are under the control of stress field, the focal mechanisms of shocks occurred at this time have the same direction with tectonic stress field; while the higher consistency means that the focal mechanisms of aftershocks are scattered, and have big difference with tectonic field, which means the control of background stress field start weakening.

Key words: Focal mechanism, Consistency parameter, Stress field, Kuril island, Peru