关键词: 活动断裂, 古地震, 活动习性, 古浪

Abstract: The Tianqiaogou Huangyangchuan Fault (called also Gulang Fault) lies to the east of Lenglongling,the highest peak on the eastern section of the Qilianshan Mountains. It is one of the important active faults in eastern Qilianshan. From west to east,the fault is about 86km long,initiating from Hongyaoxian,extending eastward along Tainqiaogou,Qianjin,Guanjiatai,and Huangyangchuan villages,and ending at Jiapigou. The western segment of the fault is NWW-trending,the middle is nearly E-W-trending and the eastern is NEE-trending. Among them,the middle segment is convex slightly to the south. The fault is basically continuous,except that a small step-over exists near Guanjiatai village,dividing the fault into two sub-segments:the Tianqiaogou sub-segment in the west and the Huangyangchuan sub-segment in the east. Field investigation revealed that offset landforms are well developed along the fault,such as offset gully,fault scarp,sag pond,offset terrace and alluvial fans. All the offset landforms indicate that the fault was dominated by left lateral strike-slip with a component of normal faulting. Tracing along the fault and trenching across 6 typical offset landforms revealed the evidence of Holocene activities and several palaeo earthquakes along the fault. By the use of progressive constraining method,7 Holocene palaeo-earthquakes and one historic earthquake are identified. The occurrence time of these events are determined to be 10,743±342a BP (Event Ⅰ),9038±39a BP (Event Ⅱ),6910±438a BP (Event Ⅲ),4847±185a BP (Event Ⅳ),3562±190a BP (Event Ⅴ),2476±194a BP (Event Ⅵ),1505±253a BP (Event Ⅶ) and 1927AD (Event Ⅷ,possibly the Gulang earthquake). The distribution and recurrence behavior of strong earthquakes along the fault show a distinct linearity and sub-periodical recurrence,for which the linear regression equation of event-time sequence is T_i=1516i-11734. Moreover,during field investigation we found new evidence indicating that surface rupture occurred along the Tianqiaogou-Huangyangchuan fault during the Gulang M_S8.0 in 1927.

Key words: active fault, palaeo-earthquake, activity, Gulang