Activity Research On The Typical Faults In The Northwestern And Southeastern Margin Of Ordos Since Late Pleistocene

This dissertation chose the typical faults,the Langshan piedmont fault(LSPF)and Luoyunshan piedmont fault(LYSPF)in the northwestern and southeastern margin of Ordos block as the study area,with using the methods of trenching,fluvial landforms and chronology,as far as possible to improve the integrity of the paleo-earthquake sequence of the faults.Using the related parameters of fault activity in order to establish the regional seismic structure model and predict the characteristics of future strong earthquakes.This study can provide basic data for regional earthquake disaster defense.The results show that based on faulted landforms,the uplift rate of the LSPF is 0.05 mm/yr ~ 0.25 mm/yr since 19 ka BP,and 0.05 mm/yr ~ 0.23 mm/yr since 50 ka BP.Based on borehole exposure,the sedimentation rate of the strata is 0.9 mm/yr ~ 3.2 mm/yr since Holocene,and 0.6 mm/yr ~ 2.4 mm/yr since Late Pleistocene.Eight paleo-earthquake events happened on the LSPF from 43,000 years ago,and six paleo-earthquake events are complete from 14,000 years ago,the latest event is 2300 a ~ 2400 a BP;the LSPF has the characteristic of quasi-periodic recurrence and the average recurrence interval is 2350 years.Therefore,there is a high probability that the LSPF will has another earthquake.In this study,the main characteristics of the LSPF were the rupture of all segments at the same time.The vertical displacement of the paleo-earthquake events is 0.2 m ~ 2.5 m and the corresponding maximum paleo-earthquake magnitude is Ms 7.4.The vertical activity rate of the LSPF is 0.19 mm/yr ~ 1.12 mm/yr since the Holocene,and 0.19 mm/yr ~ 0.77 mm/yr for last 14 thousand years.The author surveyed the longitudinal profiles of 28 gullies across the LYSPF by differential GPS and dating of sediments well preserved in the typical terraces and obtained 6-level knickpoint series of the gullies along the Luoyunshan Piedmont.The author compared it with previous studies on the paleo-earthquakes of the LYSPF revealed by trenches and found that the paleo-earthquake events from knickpoint and trench studies are in the good agreement in terms of the time of occurrence and vertical displacement of the fault.It shows studying the paleo-earthquake events of the fault by knickpoint series is feasible.At last,the author established a more complete paleo-earthquake sequence of the LYSPF supported by geomorphologic and sedimentary evidence,i.e.,six paleo-earthquake events happened on the LYSPF from about 28,000 years ago,and the latest event is 4620 a ~ 4750 a BP;the LYSPF has the characteristic of quasi-periodic recurrence and the average recurrence interval is 4585 years.The vertical displacement of the paleo-earthquake events is 2.4 m ~ 3.0 m;according to the empirical equations between co-seismic displacement and earthquake magnitude the magnitude of these events is Ms 7.0~7.5.Since Late Pleistocene,the activity of the LYSPF has been distributed along the fault,or it is a whole activity.The vertical activity rate of the LYSPF is 0.47 mm/yr ~ 0.88 mm/yr since the Holocene,and 0.44 mm/yr ~ 0.53 mm/yr since 28 thousand years.This study aims to complement and improve the traditional trenching method,provide theoretical and methodological support for the research of active tectonics,and contribute important information for the seismic hazard assessment of the densely populated Linfen area.The seismic hazard of Langshan area and Luoyunshan area cannot be ignored.There is a high probability of casualties and property damage in future earthquakes.Therefore,the LSPF and the LYSPF should be given enough attention to strengthen the necessary monitoring,in order to accurately grasp the fault activity in these areas,to reduce property loss of country and people.At the same time,the construction or large and medium-sized construction projects to be built in Langshan area and Luoyunshan area,should also be considered.The buildings should be able to withstand an 8-magnitude earthquake.