The 1920 Haiyuan Earthquake Rupture and the Paleoearthquakes Feature of the Haiyuan Fault in China

One of the most destructive great earthquakes of this century occurred on the Haiyuan fault at the NE margin of the Tibetan Plateau in north-central China on Dec. 16, 1920 (M=8.6). More than 230,000 people were killed during the event. The Haiyuan fault appears as a sinistral strike-slip fault since early Quaternary. The Holocene slip rate along this fault is 8+/-2mm/yr. The 1920 earthquake-generated surface rupture elongates 237 Km and the maximum sinistral offset reaches 11 m along the Haiyuan fault. One 3-D trench and other 7 trenches were excavated and altogether 13 paleoevents were distinguished since (13615+/-1105)yr B.P. The distribution of paleoearthquakes on Haiyuan fault is not uniform in time and segmentation in space during Holocene. The activity of paleoearthquakes is weak in the early Holocene and strong in the middle-late period. The shortest recurrence interval is only 100 yr. While the longest interval is about 2000 yr. The characteristic slip of the Haiyuan fault is graded. In the space, 2 grades of seismogenic units (master and secondary rupture segment) are existed on the Haiyuan fault, that is, large characteristic earthquakes occurred on the master segments while relatively small ones occurred on the secondary segments.