Abstract: As the foundation of long term prediction for strong earthquakes, the determination of seismic zone and potential focal region is based on seismotectonic analogy and repetition of seismic activity.Current long term prediction method is helpless in tectonically "stable" area, in which there is no active structure and historic earthquakes (including paleoearthquakes).The results of high precision seismic location and focal mechanism solution inversed with wave data in Zhangbei earthquake show:under the effect of stable and uniform tectonic stress field, the small earthquakes concentrated on a zone along the direction of maximum shear stress, and kept active.When the predominant direction of focal mechanism is consistent with the direction of stress field, the zone of small earthquakes can be treated as active focal fault.This conclusion can be used in the determination of potential focal region.The earthquake occurred in tectonically "stable" area belongs to new fracture.