The northern region of Sichuan province in China is located at the central and northern segment of the north-south seismic belt.A series of active faults are developed in this region;they have a complex distribution and have been historically responsible for a series of strong (large) earthquakes,extending to the present-day.However,it is difficult to confirm the basic parameters and causative structures relating to earthquakes that occurred many years ago.Therefore,this study verifies the basic parameters and discusses the causative structures of three moderate-strong earthquakes that occurred before 1900 A.D. in the northern region of Sichuan province (the Songpan earthquake in 638 A.D.,Nanping earthquake in 1738 A.D.,and an earthquake in the north of Songpan in 1748 A.D.),based on detailed textual research using historical earthquake records and field investigations.The results indicate that:(1) the magnitude of the Songpan earthquake in 638 A.D.should be amended from a magnitude of 5 3/4 to that of about 6 1/4 .In addition,the epicenter intensity was about 8,and the heavily-damaged area of this earthquake was located in (Gonggangling) and Hulugou,near the boundary of Songpan county and Nanping county (now Jiuzhaigou county).The long axis direction of the isoseismal line had an approximate N-S orientation,which is consistent with the strike of the northern segment of the Minjiang active fault zone.(2) The magnitude of the Nanping earthquake in 1738 A.D. should be amended from a magnitude of 5 3/4 to one of about 6;the epicenter intensity was approximately 7~8,and the heavily-damaged area of this earthquake was located northwest of Huilongbu.The long axis direction of the isoseismal line has an approximate NW orientation,which is consistent with the strike of the southeastern segment of the Tazang active fault zone.(3) The intensity of northern Songpan earthquake in 1748 A.D. should be amended to 8,and the heavily-damaged area relating to this earthquake was located near Huangshengguan.The long axis direction of the isoseismal line had an approximate N-S orientation,which is consistent with the strike of the northern segment of the Minjiang active fault zone.These results can be used as an important reference to analyze features of seismic activity and to judge areas at risk from strong earthquakes in the future.