The 1920 Haiyuan earthquake (M=8.5) formed a new surface left-late-ral rupture zone along the Haiyuan fault.Its length is about 215 km.The rupture zong consists of seven discontinuous echelon fault segments.The maximum horizontal component of the displacement along the rupture zone is 17m,the average displacement is 7.07 m,and the maximum verticl component of the displacement is 5 m.The discontinuity of the displacement distribution along the rupture zone corresponds with the discontinuity of the fault's trace.
Since Holocene,5 great earthquakes with magnitude about 8.5 have occured on the Haiyuan fault.Except for the 2nd event,all were dated by Lanzhou University with 14C:Event Ⅰ is 9360±75 yrs B.P.;Event Ⅱ 7830 yrs B.P.(insertion);Event Ⅲ 6300±70 yrs B.P.;Event Ⅳ 3680±60 yrs;Event Ⅴ less than 200 yrs B.P.(ie 1920 Haiyuan earthquake).The recurrence intervals of events increased progressively,and the average recurrence interval was 2325 yrs.The offset of the Haiyuan fault in the Late-Pleistocene was about 500m;since the late period of Late-Pleisto-cene,200m;and since the Holocene it was about 50m.The average slip rate is about 5mm/yr.
This paper makes an exhoustive study in the following aspects 1.the fracture zone of the 1920 Haiyuan earthquake (M=8.5);2.the Holocdne prehistoric great earthquake on the Haiyuan fault;and 3.Late-Quater-nary faulting on the Haiyuan fault.