Daqingshan Piedmont Research Articles

Classification and Semiquantitative Evaluation of Paleoearthquake Identification From Trenches on Normal Faults: A Case Study of Holocene Paleoearthquake Events From the Northern Margin of the Hetao Basin, China

AbstractLong paleoearthquake series are crucial for understanding the recurrence patterns of active faults and evaluating the seismic hazards of faults. In this study, the first semiquantitative evaluation of paleoearthquake events from trenches was applied to normal faults, and classification criteria for identifying paleoearthquakes in trenches on normal faults are proposed to demonstrate the feasibility of semiquantitative evaluations of paleoearthquake events on normal faults. In this study, 72 paleoearthquake trench sites on normal faults around the world were analyzed, and the main indicators for identifying paleoearthquakes on normal faults include the presence of vertical offset (VO), collapse wedge (CW), fissure (FIS), buried paleosol (BP), angular unconformity (AU), upward termination (UT), and sand liquefaction (LF) features. To describe and apply this semiquantitative evaluation method for paleoearthquake events on normal faults, 33 trenches for paleoearthquakes on a fault system in the northern margin of the Hetao Basin were comprehensively analyzed to determine Holocene paleoearthquake events on the Sertengshan, Wulashan, and Daqingshan piedmont faults, and the reliability of paleoearthquake events is discussed. The integrity of the paleoearthquake events obtained was tested by the displacement limit method. The Holocene paleoearthquake recurrence on the three faults was quasiperiodic; the coefficients of variation (COVs) were 0.44, 0.58, and 0.4.

Vertical Slip Rates of Normal Faults Constrained by Both Fault Walls: A Case Study of the Hetao Fault System in Northern China

The slip rate is a fundamental kinematic parameter of active faults. Traditional methods using fault scarps or trenches may produce inaccurate estimates of a fault’s vertical slip rate. A normal fault’s vertical slip rate requires constraints from the hanging wall and footwall. Here, the vertical slip rate at each measuring point along the fault was calculated by the joint constraints of terraces in the footwall and boreholes in the hanging wall. Nine measuring points were selected along the Sertengshan piedmont fault. The vertical slip rates of this fault since 65 and 12 ka were 0.74–1.81 and 0.86–2.28 mm/a, respectively. Four measuring points were selected along the Wulashan piedmont fault. The vertical slip rates of this fault since 60 and 12 ka were 2.14–3.11 and 1.84–2.91 mm/a, respectively. Seven measuring points were selected along the Daqingshan piedmont fault; the vertical slip rates were 2.5–3.88 and 1.78–2.83 mm/a since 58 and 11 ka, respectively. Analysis of the slip rates, theelapsed timesince the last palaeoearthquake and the mean recurrence interval of palaeoearthquakes on each fault segment on the northern margin of the Hetao Basin suggests that the Langshankou and Hongqicun segments of the Sertengshan piedmont fault are at higher risk of earthquakes than the other segments. Among the fault segments of the Wulashan piedmont fault, the Baotou segment is at the highest seismic risk. The seismic risk of the Tuyouxi segment of the Daqingshan piedmont fault should not be ignored, and the Tuzuoxi, Bikeqi and Hohhot segments have high seismic risk. Based on the findings and a dynamic model of the formation and evolution of the Ordos block, it is concluded that the depositional centre of the Hetao Basin has tended to migrate from west to east. The vertical force generated by deep material movement is the dominant factor leading to a greater vertical slip rate in the eastern portion of the northern margin of the Hetao Basin. The modern stress field in the Hetao Basin results in an increase in the vertical slip rate of active faults from west to east along the northern margin of the basin.

Responses of Stream Geomorphic Indices to Piedmont Fault Activity in the Daqingshan Area of China

A recent correlation of stream geomorphic indices to fault activity has revealed that stream geomorphologies in bedrock mountain areas are good records of local fault movements. The Daqingshan piedmont fault is one of the main active faults in the fault system on the northern margin of the Hetao Basin and has produced frequent large-scale earthquakes since the Late Pleistocene. In the present study, following the segmentation regime of previous studies, we divide the fault zone into five segments, namely, the Baotou, Tuyouqi West, Tuzuoqi West, Bikeqi, and Hohhot segments, and we discuss the relationship between the drainage basin geomorphology and the piedmont fault activity in the Daqingshan area using 30 m spatial resolution Shuttle Radar Topography Mission (SRTM) digital elevation model (DEM) data. We use a range of geomorphic indices to examine the drainage basins in the Daqingshan area, including the channel steepness index (ksn), slope, hypsometric integral (HI), relief degree of land surface (RDLS), and stream lengthgradient index (SL), extracted with ArcGIS and MATLAB, and we also consider local lithologic and climate aspects. Furthermore, we compare the geomorphic indices with the slip rates of individual segments of the Daqingshan piedmont fault and paleoseismic data. The results show that the geomorphic indices of drainage basins in the Daqingshan area are primarily affected by the piedmont fault activity in the Daqingshan area. The geomorphic indices also demonstrate that piedmont fault activity has been the most intense in the middle segment of this fault system since the Late Quaternary and decreases towards the two sides.

Palaeoseismic records in lacustrine sediments—A case study of the Daqingshan piedmont fault and Hasuhai Lake in Inner Mongolia, China

AbstractAs a traditional method for palaeoseismic studies, trenching can be combined with dating techniques to identify palaeoseismic events and the earthquake recurrence interval. However, when using trenches to study palaeoearthquakes, factors such as the active tectonic background of the earthquake‐caused structure, the lithology on both sides of the fault, the geomorphology location and type and the samples and methods for dating will affect the location of the trench. Thus, trenches should be carefully selected and used to identify the impact of ancient earthquakes. The results have substantial uncertainties and limitations. In recent years, scholars have made considerable progress in using other methods to reveal the palaeoseismic information of faults. Moreover, the history of fault activity may have been recorded in the lacustrine sediment adjacent to the fault. Hasuhai Lake is adjacent to the middle segment of the Daqingshan piedmont fault in Inner Mongolia. Since the Holocene, the region has experienced a temperate continental semi‐arid climate with little interference, and Hasuhai Lake and peripheral waters present weak hydrodynamic conditions that provide an ideal location for the study of palaeoseismic records in lacustrine sediments. Sediment samples and samples for dating were collected from three trenches excavated on the periphery of the Hasuhai Lake. Their variations in grain size and magnetic susceptibility revealed that wind and flowing water jointly produced the sedimentary conditions of Hasuhai sediments. The 14C dating results and variations in the grain size distribution, grain size components and magnetic susceptibility of sediments caused by seismic events obtained in this study were compared with those caused by a series of palaeoseismic events at the middle segment of the Daqingshan piedmont fault reported by previous studies using trenches, knickpoints and palaeosol records. The results identified seven palaeoseismic events recorded near Hasuhai Lake since 12,000 years. The combined use of lacustrine sediment variation characteristics and dating techniques is an effective method for studying palaeoseismic events.

Surface rupture geomorphology and vertical slip rates constrained by terraces along the Wulashan piedmont fault in the Hetao Basin, China

The Wulashan piedmont fault is a typical normal fault in the northern Ordos Block and has exhibited intense activity since the Late Pleistocene. It is located among the Ordos Block, Alashan Block, Yanshan Block and North China Block and between the NE expansion of the Tibetan Plateau and the subduction of the Pacific Plate. This area is important as a key location to study the remote effects of the Tibetan Plateau NE expansion. Historical M 8 earthquakes in 7 BCE and 849 CE occurred near this fault and thus the fault poses a potential earthquake threat but has received little research attention. We observed the surface ruptures, measured the heights of terraces created by fault offsets, excavated trenches and used optically stimulated luminescence (OSL) dating of samples to study the late Quaternary fault activity. This study shows that the fault is divided into three segments. Three levels of terraces were produced by movement on the fault. The T1 terrace formed at approximately 10 ka, the T2 terrace formed between 30 and 40 ka, and the T3 terrace formed between 40 and 50 ka. Based on the average terrace heights along the fault, we calculate that the vertical fault slip rates have been approximately 1.12–1.34 mm/a in the Holocene and 2.2–2.28 mm/a since 50 ka. Our analysis shows that the Daqingshan piedmont fault has the highest vertical slip rate, the Wulashan piedmont fault has an intermediate slip rate and the Sertengshan piedmont fault has the lowest vertical slip rate. The area between the Wulashan and Daqingshan piedmont faults readily accumulates seismic stress and represents a potential future earthquake site. Trenches provide visible evidence of vertical rupture and slip movement in subsurface profiles, revealing six events on the Gongmiaozi segment since 25 ka and 14 events on the Baotou segment since 120 ka. The analysis shows that the Ordos peripheral fault system formed due to the subduction of the Pacific Plate. The northward expansion of the Tibetan Plateau and the movement of the Ordos Block have caused fault activity in the northern part of the Hetao Basin since the Late Pleistocene. The modern tectonic stress field and the upwelling of asthenospheric material east of the Hetao fault zone have resulted in an increase in the slip rate on the Hetao fault zone from west to east.

Holocene paleoearthquakes of the Daqingshan fault detected from knickpoint identification and alluvial soil profile

Are there any effective methods to reveal paleoearthquakes on normal faults except traditional trenching technique? In this paper, we study Holocene paleoearthquakes of the Daqingshan fault which is a normal fault along the Daqingshan piedmont of Inner Mongolia in China. We identify knickpoints from stream profiles and study alluvial soil profiles to reconstruct the Holocene paleoearthquakes of the fault. From the fault’s footwall we extract 25 gullies from IRS-P5 DEM data, and identify knickpoints in the profile that result from fault motion disturbing each channel. We combine the retreat distances and the knickpoint retreat rates to determine each knickpoint’s forming time. We study alluvial fan outcrops that contain various paleosol sequences. As three distinct Holocene paleosols developed in the Daqingshan piedmont alluvial fans, we assume that the soil profile development was interrupted by fault activity preserved by interbedded gravel between the paleosols. The gravel layer between two adjacent paleosol layers represents material transported there after a paleoseismic event. Thus we date paleosol layers which are above and below the gravel layer to constrain paleoseismic events. Since trenches had been made by our predecessors along the fault to reveal the Holocene paleoearthquakes, we identify the Holocene paleoearthquake records from both sides of the fault, and then compare the results with the results from the trenches. The final result demonstrates that the knickpoints’ sequence in the footwall and the paleosols’ ages in the hanging wall correspond very closely with the Holocene paleoearthquakes along the Daqingshan piedmont fault. Methods in this paper have future application value to study paleoearthquakes on other normal faults with similar structure to the Daqingshan fault.

Late Quaternary history of paleoseismic activity along the Hohhot Segmentof the Daqingshan piedmont faultin Hetao depression zone, North China

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.

Recurrence characteristics of late-quaternary strong earthquakes on the major active faults along the northern border of Ordos block

Through study on trenches, analysis of recurrence characteristics and recurrence interval cluster/gap of strong earthquakes along the major active faults on the northern border of Ordos block, we found 62 paleoearthquakes that occurred in the late Quaternary, including 33 earthquakes occurring in the Holocene. The recurrence characteristics of the paleoearthquakes are different at three levels, segments, faults, and fault zones. The strong seismic sequence on the independent segments is mostly characterized by long- and short-interval recurrences, while that on the faults and in fault zone is characterized clearly by random and cluster recurrences. Results of the moving window test indicate that the probabilities of “temporal cluster or gap”, caused by random coincidence as opposed to intersegment contagion, are 64% and 70% for the Serteng piedmont fault and for the south-border fault of Wula Mountains, respectively, no clear interaction among the segments of each fault; while the probability is 26.8% for the whole fault zone, suggesting a clear interaction among the faults of this fault zone. These recurrence characteristics may imply an effect of the entire block motion on the recurrence of strong earthquakes. Moreover, the elapsed time for the Wujumeng Pass-Dongfeng Village segment of Serteng piedmont fault and the Tuzuo Banner-Wusutu and the Hohhot segments of Daqingshan piedmont fault has exceeded the average recurrence interval, hence these three segments may be the possible places for future strong earthquakes.