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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

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Tectonics
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Dongsheng Xu
Dongsheng Xu
Dongsheng Xu
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Zhongtai He at Institute of Geology, China Earthquake Administration
Zhongtai He
Baoqi Ma at Insititute of Crustal Dynamics, China Earthquake Administration
Baoqi Ma
  • Insititute of Crustal Dynamics, China Earthquake Administration
Kuan Liang
Kuan Liang
Kuan Liang
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Abstract and Figures

Long 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.
Regional seismic tectonic setting: (a) rift basins, active faults, and earthquake distribution around the Ordos block (modified from Deng, 2007); (b) locations of the three faults, earthquake distribution in the basin, and the thickness of Quaternary sediments (km); the thickness is from the (Research Group of the National Seismological Administration, 1988); SPF, Sertengshan piedmont fault; WPF, Wulashan piedmont fault; DPF, Daqingshan piedmont fault; (c) AA', Quaternary isopach map from the Hetao Basin; BB', Cenozoic isopach map from the Hetao Basin; the profiles are modified from Li and Nie (1987) based on petroleum geology, hydrology, and seismic data.
Regional seismic tectonic setting: (a) rift basins, active faults, and earthquake distribution around the Ordos block (modified from Deng, 2007); (b) locations of the three faults, earthquake distribution in the basin, and the thickness of Quaternary sediments (km); the thickness is from the (Research Group of the National Seismological Administration, 1988); SPF, Sertengshan piedmont fault; WPF, Wulashan piedmont fault; DPF, Daqingshan piedmont fault; (c) AA', Quaternary isopach map from the Hetao Basin; BB', Cenozoic isopach map from the Hetao Basin; the profiles are modified from Li and Nie (1987) based on petroleum geology, hydrology, and seismic data.
… 
Occurrence frequency of the identification indicators for normal and strike‐slip faults. (a) Statistical diagram of the occurrence frequency of paleoearthquake indicators for normal faults. (b) Occurrence frequency diagram of identification indicators for strike‐slip faults (Xu et al., 2019); VO‐vertical offset; CW‐collapse wedge; FIS‐fissure; BP‐buried paleosol; AU‐angular unconformity; UT‐upward termination; FD‐flood; LF‐sand liquefaction; GS‐growth strata.
Occurrence frequency of the identification indicators for normal and strike‐slip faults. (a) Statistical diagram of the occurrence frequency of paleoearthquake indicators for normal faults. (b) Occurrence frequency diagram of identification indicators for strike‐slip faults (Xu et al., 2019); VO‐vertical offset; CW‐collapse wedge; FIS‐fissure; BP‐buried paleosol; AU‐angular unconformity; UT‐upward termination; FD‐flood; LF‐sand liquefaction; GS‐growth strata.
… 
Typical identification indicators for normal fault paleoearthquake events. (a) I‐III, normal fault CW formation pattern; nearly horizontal wedge‐shaped accumulation body formed by the rapid dislocation of the fault in the hanging wall; one CW represents a paleoearthquake event (Deng et al., 1984; Li & Cao, 1989; Liu & Zhang, 2007; Swan et al., 1980); this indicator is rated as strong. (b) I‐III, normal fault buried paleosol pattern; in a normal fault trench, the paleosurface is identified as a paleosol layer in the hanging wall, while the footwall shows a lack of strata, and the paleosol layer is the seismic horizon (Weber & Cotton, 1981). The layers of the paleosols represent the number of events (Chen, 2002; He et al., 2015; McCalpin, 1996); this indicator was rated as fair to strong. (c) VO of the Trench B fault of the Roccapreturo fault (Falcucci et al., 2015), the most common event identification indicator in normal fault trenches; this indicator is rated as moderate to fair according to the dip separation and clarity of the fault trace. (d) Fissure patterns; the filling material is primarily from the upper young strata, and the latest strata dislocated by the faults is the earthquake event layer; this indicator is rated as strong. (e) Sand liquefaction pattern; during an earthquake, the increase in pore water pressure can lead to the liquefaction of water‐saturated noncohesive sediments (Yuan, 2018, 2019), but sand liquefaction can also be caused by other faults far from the exploration trench (Shao, 2018; Yuan, 2018); therefore, sand liquefaction was rated as weak evidence. (f) San Demetrio fault ne’ Vestini historical center trench angle unconformity (Blumetti et al., 2017); AU is related to the stratum tilt formed during the fault activity or the normal drag phenomenon along the fault plane (McCalpin, 2009); this indicator was classified as moderate to fair. (g) The Toprak Mahsulleri Ofisi trench fault in the Dinar fault terminates upwards, and a low‐quality UT of a single fault was an identification indicator (Akın 65 Kürçer et al., 2021); when this indicator is linked with other evidence, such as fractures, it can be considered moderate evidence of paleoearthquakes (Scharer et al., 2007; Shao, 2018; Xu et al., 2019).
Typical identification indicators for normal fault paleoearthquake events. (a) I‐III, normal fault CW formation pattern; nearly horizontal wedge‐shaped accumulation body formed by the rapid dislocation of the fault in the hanging wall; one CW represents a paleoearthquake event (Deng et al., 1984; Li & Cao, 1989; Liu & Zhang, 2007; Swan et al., 1980); this indicator is rated as strong. (b) I‐III, normal fault buried paleosol pattern; in a normal fault trench, the paleosurface is identified as a paleosol layer in the hanging wall, while the footwall shows a lack of strata, and the paleosol layer is the seismic horizon (Weber & Cotton, 1981). The layers of the paleosols represent the number of events (Chen, 2002; He et al., 2015; McCalpin, 1996); this indicator was rated as fair to strong. (c) VO of the Trench B fault of the Roccapreturo fault (Falcucci et al., 2015), the most common event identification indicator in normal fault trenches; this indicator is rated as moderate to fair according to the dip separation and clarity of the fault trace. (d) Fissure patterns; the filling material is primarily from the upper young strata, and the latest strata dislocated by the faults is the earthquake event layer; this indicator is rated as strong. (e) Sand liquefaction pattern; during an earthquake, the increase in pore water pressure can lead to the liquefaction of water‐saturated noncohesive sediments (Yuan, 2018, 2019), but sand liquefaction can also be caused by other faults far from the exploration trench (Shao, 2018; Yuan, 2018); therefore, sand liquefaction was rated as weak evidence. (f) San Demetrio fault ne’ Vestini historical center trench angle unconformity (Blumetti et al., 2017); AU is related to the stratum tilt formed during the fault activity or the normal drag phenomenon along the fault plane (McCalpin, 2009); this indicator was classified as moderate to fair. (g) The Toprak Mahsulleri Ofisi trench fault in the Dinar fault terminates upwards, and a low‐quality UT of a single fault was an identification indicator (Akın 65 Kürçer et al., 2021); when this indicator is linked with other evidence, such as fractures, it can be considered moderate evidence of paleoearthquakes (Scharer et al., 2007; Shao, 2018; Xu et al., 2019).
… 
Paleoearthquake information about the Sertengshan piedmont fault. (a) Distribution map of trenches on the Sertengshan piedmont fault; the black rectangle represents the trench. (b) Holocene spatiotemporal distribution map of paleoearthquakes in Sertengshan; the time axis is in the vertical direction, and the distance from each trench to Dongwugaigou is in the horizontal direction; the different colored rectangular boxes represent the event era defined by multiple trenches; and the 15 trenches from west to east are numbered TS1–TS15.
Paleoearthquake information about the Sertengshan piedmont fault. (a) Distribution map of trenches on the Sertengshan piedmont fault; the black rectangle represents the trench. (b) Holocene spatiotemporal distribution map of paleoearthquakes in Sertengshan; the time axis is in the vertical direction, and the distance from each trench to Dongwugaigou is in the horizontal direction; the different colored rectangular boxes represent the event era defined by multiple trenches; and the 15 trenches from west to east are numbered TS1–TS15.
… 
Statistical diagram of the number and quality of Holocene paleoearthquake event indicators on the Sertengshan piedmont fault. (a) Statistical diagram of the number of paleoearthquake event indicators; the red circles indicate the number of indicators at different ranks, and the blue solid line indicates that the evidence is sufficient for event identification and therefore represents a reliable paleoearthquake event. The abscissa is the mean occurrence time for each event, and the ordinate is the assigned score in Table S2 in Supporting Information S1. (b) The yellow triangle represents the sum of the rank of each event indicator (Sum Rank = Event indicator rank * Number of indicators).
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Statistical diagram of the number and quality of Holocene paleoearthquake event indicators on the Sertengshan piedmont fault. (a) Statistical diagram of the number of paleoearthquake event indicators; the red circles indicate the number of indicators at different ranks, and the blue solid line indicates that the evidence is sufficient for event identification and therefore represents a reliable paleoearthquake event. The abscissa is the mean occurrence time for each event, and the ordinate is the assigned score in Table S2 in Supporting Information S1. (b) The yellow triangle represents the sum of the rank of each event indicator (Sum Rank = Event indicator rank * Number of indicators).
… 
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1. Introduction
Understanding the earthquake recurrence patterns of active faults and evaluating regional seismic hazards require
long-term paleoearthquake records. Deformation recovery analyses of trenches and quantifications of various
evidence related to earthquake events can more objectively evaluate the reliability of earthquake events without
omitting information on sedimentology and tectonic deformation (Liu etal.,2021; Xu etal.,2019). Seitz(1999)
first classified evidence for earthquake events at a trench site on the San Andreas fault at Pitman Canyon to
discuss the credibility of paleoearthquake events. Scharer etal. (2007, 2017) further classified evidence for
paleoearthquake event identification and described in detail the characteristics of this evidence at different levels.
This method is currently primarily applied to large strike-slip faults, such as the San Andreas fault (Scharer
etal.,2017), Haiyuan fault (Liu-Zeng etal.,2015), and Altyn Tagh fault (Yuan etal.,2018), and has not yet been
used to study paleoearthquakes on normal faults. The classification and scoring of evidence of paleoearthquake
events on normal faults can enhance the objectivity of paleoearthquake event identification, which is conducive
to applying and evaluating the research results of later researchers. The Hetao Basin is an important seismic
Abstract Long 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.
Plain Language Summary In this study, 72 paleoearthquake trench sites on normal faults around
the world are analyzed, and summarized the main indicators for identifying paleoearthquakes on normal
faults. Classification criteria for identifying paleoearthquakes in trenches on normal faults are proposed. Using
the faults of the northern margin of the Hetao Basin as an example, this study described a semiquantitative
evaluation method for normal fault paleoearthquake events, and discussed the reliability of paleoearthquake
events. The integrity of the paleoearthquake events obtained was tested by the displacement limit method.
Combining with studies on the reliability and integrity of paleoearthquake events, we argue that it is feasible
to apply semiquantitative evaluations of paleoearthquake events from trenches to paleoearthquake research on
normal faults.
XU ETAL.
© 2022. American Geophysical Union.
All Rights Reserved.
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
Dongsheng Xu1 , Zhongtai He1,2,3 , Baoqi Ma1, Kuan Liang1 , Jianyu Long4, and Hao Zhang5
1National Institute of Natural Hazards, Ministry of Emergency Management of China, Beijing, China, 2State Key Laboratory
of Earthquake Dynamics, Institute of Geology, China Earthquake Administration, Beijing, China, 3Shanxi Taiyuan
Continental Rift Dynamics National Observation and Research Station, Beijing, China, 4Earthquake Administration of
Sichuan Province, Chengdu, China, 5Earthquake Administration of Jiangsu Province, Nanjing, China
Key Points:
• Semiquantitative evaluation of
paleoearthquake identification from
trenches to normal faults is applied for
the first time
• Classification criteria are proposed
for identifying paleoearthquakes in
trenches on normal faults
• This study demonstrates the feasibility
of semiquantitative evaluation of
paleoearthquake events on normal
faults
Supporting Information:
Supporting Information may be found in
the online version of this article.
Correspondence to:
Z. He,
hezhongtai@126.com
Citation:
Xu, D., He, Z., Ma, B., Liang, K., Long,
J., & Zhang, H. (2023). 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. Tectonics, 42, e2022TC007443.
https://doi.org/10.1029/2022TC007443
Received 4 JUN 2022
Accepted 13 DEC 2022
10.1029/2022TC007443
RESEARCH ARTICLE
1 of 27
... A comprehensive analysis of paleoearthquake markers on both trench walls provided robust and abundant evidence for identifying paleoearthquakes. Additionally, to objectively assess the confidence in each event, we use the scheme of Scharer et al. (2007), D. Y. Yuan et al. (2018), and D. Xu et al. (2023) to rank indicators per event by confidence with a scale of 1-4. ...
... Summarizing the Evidence for Each Event in the Trench TC1 at the Shangyaodonggou SiteEvent ID Trench poor, 1 = low. Rank definitions refer to D.Xu et al. (2023). ...
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... This Special Issue was organized by Guest Editors Zhikun Ren, Peizhen Zhang, Takashi Oguchi, and Zhongtai He, and they have engaged in active tectonic studies for many years [1][2][3][4][5][6][7][8][9][10]. ...
... Below, we aim to study active tectonic geomorphological processes using high-resolution data acquired by different remote sensing platforms and sensors. This Special Issue was organized by Guest Editors Zhikun Ren, Peizhen Zhang, Takashi Oguchi, and Zhongtai He, and they have engaged in active tectonic studies for many years [1][2][3][4][5][6][7][8][9][10]. ...
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... Addressing the challenge of rupturing the overburdened soil body during a severe earthquake requires expensive engineering interventions, such as digging deep trenches. However, this method may not guarantee precision in locating the rupture or determining the overall affected area [34][35][36]. Hence, understanding the rupture traces and widths within the overburdened soil body's interior profile is crucial. ...
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Vertical Slip Rates of Normal Faults Constrained by Both Fault Walls: A Case Study of the Hetao Fault System in Northern China
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Dinar Fault is an active breakaway fault that plays an essential role in the southwestern Anatolia extensional tectonic regime's neotectonic development. The Dinar Fault's seismicity was proven by October 1, 1995, Dinar Earthquake (Mw 6.2). It consists of three geometric segments called Gümüşsu, Kızıllı and Dikici, respectively, from northwest to southeast. The October 1, 1995 Dinar Earthquake occurred on the Kızıllı Segment, approximately 10 km long, located in the middle of the Dinar Fault. In this study, active tectonic and kinematic characteristics and paleoseismic history of the Dinar Fault were investigated. Based on the fault plane slip data obtained from the Dinar Fault, it is a normal fault with a minor right-lateral strike-slip component. A total of four trenches were excavated across the 1995 surface rupture. According to paleoseismological trench studies, the Dinar Fault's slip rate was estimated as 0.37–0.56 mm/year and the average earthquake recurrence interval was 523 years in the last 3660 years. A number of these earthquakes destroyed the ancient city of Apameia Kibotos (ancient Dinar). The last earthquake that occurred on the Dinar Fault is the October 1, 1995 Dinar Earthquake. The date of the penultimate event occurring on the Dinar Fault was determined as AD 1318–1486, while the antepenultimate event was determined as AD 916–1155. The oldest event occurring on the Dinar Fault was determined as AD 916–111 BC. Structural and paleoseismological data indicate that the other sub-segments of the Dinar Fault, which did not rupture in the 1995 earthquake, have the potential to produce earthquakes with magnitudes varying between Mw: 6.2 to Mw: 6.6.
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Rapid strain release on the Bear River fault zone, Utah–Wyoming—The impact of preexisting structure on the rupture behavior of a new normal fault
Article
Full-text available
  • Mar 2021
  • TECTONOPHYSICS
Earthquake clustering (grouping in space and time) is a widely observed mode of strain release in the upper crust, although this behavior on individual faults is a departure from classic elastic rebound theory. In this study, we consider factors responsible for a cluster of earthquakes on the Bear River fault zone (BRF), a recently activated, 44-km-long normal fault on the eastern margin of Basin and Range extension in the Rocky Mountains. The entire surface-rupturing history of the BRF, as gleaned from paleoseismic and geomorphic observations, began only 4500 years ago and consists of at least three large events. Rupture of the BRF is spatially complex and is clearly conditioned by preexisting structure. In particular, where the south end of the fault intersects older thrust faults and upturned strata along the south-dipping flank of the Precambrian basement-cored Uinta arch, the main trace ends abruptly in a set of orthogonal splays that accommodate down-dropping of a large hanging-wall graben against the arch. We hypothesize that the geomechanically strong Uinta arch crustal block impeded the development of the BRF and, over time, enabled a significant accumulation of elastic strain energy, eventually giving rise to a pulse of strain release in the mid- to late Holocene. We surmise that variations in fault strength, both in space and time, is a cause of earthquake clustering on the BRF and on other faults that are structurally and tectonically immature. The first two earthquakes on the BRF occurred during the same period of time as a regional cluster of earthquakes in the Middle Rocky Mountains, suggesting that isolated faults in this slowly extending region interact through widespread changes in stress conditions.
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Geomorphology and Paleoseismology of the Weinan Fault, Shaanxi, Central China, and the Source of the 1556 Huaxian Earthquake
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Full-text available
  • Dec 2020
The 1556 CE Huaxian earthquake resulted in an estimated 830,000 deaths and caused widespread devastation in the Weihe Basin, China. Seismic intensities from historical accounts yield, via magnitude‐intensity relations, a commonly quoted magnitude of 8¼ to 8½. The maximum recorded shaking was confined to a zone close to the Huashan and Weinan faults, which exhibit fresh scarps up to 7–8 m high. Recent palaeoseismic studies have suggested, however, that the Weinan fault has not ruptured at the surface for several thousand years. Furthermore, the 90‐km combined length of the Huashan and Weinan faults is short for an earthquake of magnitude 8¼ to 8½. We present a detailed analysis of the Weinan fault at one well‐preserved site, combining field observations and age constraints from fluvial terraces displaced by faulting, analysis of a high‐resolution digital elevation model (DEM), interpretation of the walls of a quarry that cuts through the fault zone, and from a profile of borehole cores across the fault. We find that the fault ruptured within the last ~900 years and is likely, along with the Huashan segment, to be the causative fault for the 1556 earthquake. The magnitude remains uncertain, with Mw ~ 7.5 being a plausible estimate given the fault length, and no more than Mw 8.0 if we use the maximum estimates of slip. These estimates are considerably smaller than magnitudes estimated from intensities, with importance in estimating the recurrence intervals between destructive earthquakes and hazard across central China.
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Quaternary faulting and soil formation on the County Dump Fault, Albuquerque, New Mexico
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  • Jan 2006
The Albuquerque-Santa Fe urban corridor is one of the fastest growing areas in the western United States. Twenty-seven Quaternary-age (1.8 million years ago to the present) faults have been identified within 40 km of downtown Albuquerque. The faults are associated with the Rio Grande rift and are responsible for 10 earthquakes of MMI (Modified Mercalli Intensity) V or greater since 1849. The largest of these was the MMI VII-VIII earthquake at Cerrillos on May 18, 1918. The County Dump fault is a 35-km-long, north-trending normal fault that is located in an area of suburban development west of the Albuquerque metropolitan area. The fault dips eastward under the city. To determine how often the County Dump fault ruptures the surface and to more fully understand the fault's paleoseismic history, the authors undertook a study that included: (1) measuring the height of the scarp formed by the fault, (2) opening multiple trenches across the fault, (3) describing stratigraphic and structural features in the trenches and sampling soil horizons exposed, and (4) collecting samples for thermoluminescence dating. The trench data indicate that perhaps as many as 14 earthquakes have ruptured along the County Dump fault in the past one million years. The pattern observed in the trenches suggests that the fault moves at intervals of about 20,000-40,000 years and results in displacements of approximately 1 m or less. The last three ground ruptures have estimated ages of 30,000, 45,000, and 80,000 years ago. If surface rupture spanned the entire 35-km length of the fault, a magnitude 6.9 earthquake would be possible.
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Tectonic geomorphology and paleoseismology of the northern East Franklin Mountains fault, El Paso, Texas, USA
Article
  • May 2021
  • QUATERN INT
The 45 km-long, north-trending East Franklin Mountains Fault lies in the southern Rio Grande rift (Texas). Late Quaternary normal fault scarps up to 20 m high traverse the eastern range front, but also bulge up to 2 km out onto the piedmont in lobes as much as 3.5 km wide. In the lobe areas the range-front fault strand displays no fault scarps, as if displacement has shifted to the piedmont strand since ca. 400 ka. We divide the faulted piedmont alluvial fans into seven age classes (from Middle Pleistocene to Holocene), based on erosional dissection and soil profile development. Piedmont scarps in Qf1 deposits (250–400 ka) range from 18 to 20 m high; in Qf2-3 deposits (65–100 ka), 9–10 m high; and in Qf3-4 deposits (20 ka), 2.7–4 m high (the latest displacement event). Scarp heights underestimate true vertical displacement of the footwall surface, which lies buried beneath younger fan deposits of unknown thickness on the hanging wall. Hanging-wall aggradation also causes variable scarp heights along strike on single-age landforms, with burial thickness a second-degree polynomial function (parabolic) of distance to the nearest active stream. Our paleoseismic trench on a 9.8 m-high scarp exposed evidence for five displacement events in the past 44 ka, the latest three of which had vertical displacements of 3–4 m. Such displacements imply surface ruptures longer than the 26.5 km fault length previously mapped in the USA, indicating rupture extended onto adjacent faults. The minimum long-term slip rate (>11.2 m in 44 ka) is 0.25 mm/yr; over last full seismic cycle (ca. 11–29 ka) it is 0.20–0.21 mm/yr. Optically-stimulated luminescence (OSL) dating shows that fine sand-silt grains in colluvial deposits were not all zeroed during deposition. Single-aliquot dating of quartz grains yielded multiple OSL age peaks, the youngest peaks of which yield a chronology compatible with stratigraphy and soil development in the colluvial wedge sequence.
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Surface rupture geomorphology and vertical slip rates constrained by terraces along the Wulashan piedmont fault in the Hetao Basin, China
Article
  • Feb 2020
  • GEOMORPHOLOGY
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.
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Morphotectonic and paleoseismological studies of Late Holocene deformation along the Primorsky Fault, Baikal Rift
Article
  • Jun 2019
  • GEOMORPHOLOGY
The Baikal Rift is the largest Cenozoic continental rift in Asia and has developed at the boundary between the Siberian craton and a collage of microplates composing the Amurian lithospheric plate. GPS data show that the Amurian plate moves southeastward at a rate of 3–4 mm/yr with respect to stable Eurasia but the slip rates along the main faults that control the opening of basins within the Baikal Rift System, are still poorly quantified. The recent graben system of the Olkhon region of the central Baikal Rift is a key example for studying of synrift faulting. We discovered a previously unknown paleoseismic structure in the course of our morphometric study of the Primorsky fault scarp, one of the youngest grabens within the Baikal Rift. Detailed topographic mapping of the deformation area and trenching across the fault scarp revealed two paleoseismic events responsible for scarp formation. Dating of organic remnants found in the trench walls allowed us to estimate a late Holocene age for the paleoearthquakes and bracket the vertical slip rates along the Primorsky Fault to between 0.5 ± 0.1 mm/yr (min) and 0.9 ± 0.2 mm/yr (max) over the last ~2.5 kyr. The geomorphological and paleoseismological data, in combination with the analysis of recent seismicity and horizontal geodetic rates, suggest that the Primorsky Fault has accumulated elastic stress with the potential for a ~M 6.6 earthquake. Large earthquakes of this magnitude have occurred at least twice on the investigated fault segment during the last ~1.4 kyr.
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