Abstract
The Daqingshan Piedmont Fault (DPF) is one of the major active normal faults in the Hetao depression zone in the northern part of Ordos Block, North China. It extends in NEE direction along the Daqingshan piedmont zone in the eastern part of the depression, dipping to the south, for a length of 223 km. The fault formed in the Eocene and underwent strong movement during the Cenozoic time. Its vertical displacement amplitude has exceeded 2400 m since the Quaternary. The fault can be divided into 5 active segments. Paleoseismological studies were concentrated on its western part from Baotou to Tumdzuoqi whereas the Hohhot Segment to the east was scarcely studied. To fill this gap of knowlegde, the authors carried out in-depth study on the Daqingshan piedmont fault during recent years. Excavation of trenches at Kuisu, Ulanblang, and Bakouzi sites on the Hohhot Segment of the Daqingshan piedmont fault and study of geomorphic surfaces allow us to identify and date paleoearthquakes and to evaluate the completeness of paleoseismic activity history. This was done both for the individual sites and for the entire segment since the Late Quaternary using the «method for displacement confining» along the fault and «method for correlation between multiple trenches». In this paper we present the geological loggings of two trenches at Kuisu site, provide the evidence for 6 events since 19 ka BP and the cumulative displacement amount produced by them is around 7 m. But the cumulative displacement amount obtained from difference in heights of geomorphic surfaces is 5.??.5.5 m. Results of tests using the method of displacement confining show that the event sequence revealed at this site can be considered complete. The data supplemented with information obtained in the Ulanblang and Bakouzi trenches show that 7 paleoseismic events occurred on the Hohhot Fault Segment since 19 ka BP, i.e. they occurred at 18.75 ± 0.75 ka, 16.97 ± ± 0.96 ka, 14.65 ± 0.67 ka, 11.82 ± 0.69 ka, 9.45 ± 0.26 ka, 6.83 ± 0.26 ka, and 4.50 ± 0.23 ka BP, respectively, and the average recurrence interval is 2.375 ± 0.432 ka. These results basically reflects the history of paleoseismic activity on the fault segment in this period of time.
Highlights
The Daqingshan Piedmont Fault (DPF) is one of the major active normal faults in HetaoYongkan Ran, Peizhen Zhang and Lichun Chen earthquakes along the fault (Research Group on Active Fault System around Ordos Block, 1988; Li et al, 1994; Wu et al, 1995; Nie et al, 1996; Jiang et al, 2001)
According to the characteristics of surface ruptures produced by normal faults and their evolution, we propose to use methods for «confining displacement amount» and for «correction between events in many trenches» to determine the completeness of paleoseismic activity history along the fault segment
It indicates that in all 7 paleoseismic events occurred on the Hohhot Segment since about 19 ka BP, i.e. they occurred in 18.75 ± 0.75 ka, 16.97 ± 0.96 ka, 14.65 ± 0.67 ka, 11.82 ± 0.69 ka, 9.45 ± 0.26 ka, 7.41 ± 0.315 ka, and 4.50 ± 0.23 ka BP, respectively
Summary
The Daqingshan Piedmont Fault (DPF) is one of the major active normal faults in Hetao. A large number of data have been accumulated and provided a good basis for new studies. There exist some weakly studied problems for the fault, for example, the previous paleoseismological studies were largely concentrated on its western part from Baotou to Tumdzuoqi and less on the other segments of the fault. Can the revealed paleoearthquakes be considered representative of a complete history of paleoseismic activity on the fault in the Late Quaternary? These questions led the authors to conduct a detailed study on the Daqingshan piedmont fault during recent years, in particular, on the Hohhot Segment. This paper deals with the paleoseismic events revealed along the segment since the Late Quaternary and discusses the completeness of its paleoseismic activity history
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