Abstract: The article analyzes the change feature of source depth of small earthquakes around the source regions before mainshocks with 8 moderately strong earthquakes with magnitude greater than 5 in 7 groups from the seismic catalogue in the northern Tienshan region corrected with double-difference location method. The result shows that there exists anomalous trend change of source depth of small earthquakes before six ones of seven groups' earthquakes. There are 9 similar abnormal change images with corresponding ratio of 66.7 per cent. The anomalous image offers reference criteria for comprehensive judgment of earthquake prediction.

Key words: Source depth of small earthquake, Double-difference location, Spatial image, Northern Tienshan mountain