Based on matched filter technique (MFT) and earthquake relative location technique, a new method is proposed to detect fault structure. The weak matched filter technique (WMFT) is modified from MFT to find more similar earthquakes, which is now used to detected events different from known templates in location. After new events are detected, their locations relative to templates can be given by using time differences from cross correlation. Due to low requirement of the WMFT in wave similarity and its low computational cost, more events can be detected and might help depict the structure of faults clearly. The feasibility of this method is primarily proved by the study of Bohai Bay area using records of 40 days.