More than 3000 small earthquakes in Gansu region from Jan.1977 to uly 1984 were relocated with layered crust model.It could be seen that:
1.These events are located commonly in upper crust less than 15Km deep,concentrating mainly on three layers near 1.5Km,7.5Km and 11.0Km deep,respectively;
2.The distribution features of foci,such as layers divided,the active evel of upper layer and "source" depth of seismic activity etc.,relate to great historical earthquakes,most of which are located about 15Km deep,fracturing the media close to 40Km deep;
3.The magnitude of moderate and small events show no change with the depth of earthquake sources.
Finally,it is disscussd the affection of crust model used in location to the result of depth determination.