Abstract: The 240km long Haiyuan fracture zone is a typical active fault in the inland of China. The 1920 Haiyuan earthquake (M_S8.6) has made the whole fault offset sinistrally. Our recent research on the large scale geological-geomorphological mapping and 3-D trench excavation at Gaowanzi,Haiyuan fault has revealed that at least 7 paleoearthquakes have been occurred since Holocene. And we have also studied the time intensity distribution of these events. Except the earthquake of 1920,these events have their ages about (10004±3196),(6689±169),(6120±505),(4208±577),(2763±372),(1005±465) and 30a,respectively,and the recurrence interval between two adjacent events is (3315±3200),(561±532),(1920±766),(1425±686),(1578±595) and (980±465)a,respectively. It shows that recurrence of these events is not uniform. At least 4 events comply with the Quasi periodic occurrence model and their average recurrence interval is (1641±207)a while the former 3 events and the later 2 events have the possibility of complying with the nonquasi-periodic occurrence model. This implies that the fault activity has the stage feature in the time distribution,that is,it complies with different occurrence models in different period. We have obtained the horizontal displacements of events Ⅲ,Ⅳ,Ⅴ,Ⅵ,Ⅶ. They are (5.6±2.3),(1.5±1.1),(1.5±1.1),(2±1) and (7±0.5)m,respectively. It indicates that the intensity distribution is also not uniform. The intensity of event Ⅲ is nearly the same as that of event Ⅶ while that of other events is much smaller. This suggests that the characteristic slip of the Haiyuan fault is graded,that is,large characteristic earthquakes occurred on the master segments while relatively small ones occurred on the secondary segments. The recurrence interval of the large earthquakes which have nearly the same intensity as that of 1920 earthquake is about 6000a since middle Holocene while that of small earthquakes with 1～2m horizontal displacement is 1000～2000a. There are 3 events whose displacement is 1～2m occurred between these two large earthquakes. So we estimate that the next earthquake is a relatively small one with a horizontal displacement of 1～2m.

Key words: Paleoearthquake, Recurrence interval, Fault displacement, Haiyuan fault, 3-D trench

摘要： 对海原断裂高湾子地点进行大比例尺地质地貌填图和三维探槽开挖,揭示出7次古地震事件,除1920年地震外,它们的年龄分别为距今(10004±3196),(6689±169),(6120±505),(4208±577),(2763±372)和(1005±465)a;重复间隔分别为(3315±3200),(561±532),(1920±766),(1425±686),(1578±595)和(980±465)a。事件Ⅲ,Ⅳ,Ⅴ,Ⅵ,Ⅶ的水平位移量分别为(5.6±2.3),(1.5±1.1),(1.5±1.1),(2±1)和(7±0.5)m.显示出重复间隔时间的分段性和特征行为的分级性。两次类似1920年强度的地震间隔期间,有3次位移量为1～2m的地震发生。

关键词: 古地震, 地震复发间隔, 断层位移, 海原断裂, 三维探槽