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Tectonic uplift and landslides triggered by the Wenchuan earthquake and constraints on orogenic growth: A case study from Hongchun Gully, Longmen Mountains, Sichuan, China
Quaternary International xxx (2014) 1e11
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Tectonic uplift and landslides triggered by the Wenchuan earthquake and constraints on orogenic growth: A case study from Hongchun Gully, Longmen Mountains, Sichuan, China Yong Li a, *, Rongjun Zhou b, Guohua Zhao a, Haibing Li c, Decheng Su c, Hairong Ding a, Zhaokun Yan a, Liang Yan a, Kung Yun a, Chao Ma a a b c
National Key Laboratory of Oil and Gas Reservoir Geology and Exploitation, Chengdu University of Technology, Chengdu 610059, China Institute of Earthquake Engineering, Seismological Bureau of Sichuan Province, 610041, China Institute of Geology, Chinese Academy of Geological Science, Beijing 100037, China
a r t i c l e i n f o
a b s t r a c t
Article history: Available online xxx
Many uncertainties need to be taken into account to estimate the relationship between tectonic uplift, the landslide volume triggered by the Wenchuan earthquake, and constraints on the orogenic growth of the Longmen Shan (Mountain). In this research, coseismic tectonic uplift, landslides, and postseismic debris-flows in Hongchun Gully located at the earthquake epicenter have been studied. Field data, aerial photographs, and digital elevation data were used to estimate the quantitative relationship between the tectonic uplift and landslides, debris-flows and their constraints on the orogenic growth of the Longmen Mountains. The coseismic tectonic uplift volume (667 104 m3) was more than the coseismic landslide volume (380.01 104 m3). Only 57% of the volume of the tectonic uplift has been converted to the volume of landslides in Hongchun Gully, indicating new uplift and orogenic growth of the Longmen Shan. The volume of postseismic debris flows was 70.5 104 m3, indicating that 20% of the volume of coseismic landslides was converted into postseismic debris flows by heavy rainfall. The volume of materials discharged by Minjiang River (the main river) is 48.5 104 m3, indicating that only 13% of coseismic landslide sediments has been removed from the range by fluvial processes, and at least 86% of landslide sediments still remain in the Hongchun Gully. Coseismic tectonic uplift volume exceeds coseismic landslides volume, and the Wenchuan Earthquake caused new uplift and geomorphic growth of the Longmen Mountains. This is not consistent with a previous conclusion that mass wasting triggered by the 2008 Wenchuan earthquake exceeds orogenic growth and led to a net material deficit in the Longmen Shan region. © 2014 Elsevier Ltd and INQUA. All rights reserved.
Keywords: Wenchuan earthquake Coseismic tectonic uplift Coseismic landslides Postseismic debris flows Orogenic growth Longmen Shan
1. Introduction Longmen Shan (Mountain) is a linear, asymmetric border range between the Tibetan Plateau and the Sichuan Basin and is marked by an extremely steep slope with relief of over 5 km (Figs. 1 and 2)(Densmore et al., 2005; Li et al., 2006; Godard et al., 2009). It extends in a NEeSW direction with a total length of 500 km and a width of 30 km. This border mountain shows as a positive isostatic gravity anomaly zone and the lower crust has been lifted by 11 km
* Corresponding author. National Key Laboratory of Oil and Gas Reservoir Geology and Exploitation, Institute of Sedimentary Geology, Chengdu University of Technology, Chengdu 610059, China E-mail addresses: [email protected], [email protected] (Y. Li).
(Li et al., 2006). This may be caused by tectonic shortening (Hubbard and Shaw, 2009), crustal isostatic rebound related to denudation (Densmore et al., 2005; Li et al., 2006; Fu et al., 2011) or channel flow (Royden et al., 1997; Clark et al., 2005; Burchfiel et al., 2008). The denudation thickness of the range is 6e10 km (Xu and Kamp, 2000; Li et al., 2006). Its current landforms resulted from continuous competition between earthquake-driven uplift and surface process-driven erosion. On May 12, 2008, the Wenchuan earthquake (Ms 8.0) took place in the middle and northern segments of the Longmen Mountains (Figs. 1 and 2), and on April 20, 2013, the Lushan (Ms 7.0) earthquake occurred in the southern segment. The tectonic uplift triggered by the Wenchuan earthquake changed the slope gradient instantly and caused massive landslides and debris flows. As a consequence, the landforms and the river system also were modified. The role of the huge earthquake and its
http://dx.doi.org/10.1016/j.quaint.2014.05.005 1040-6182/© 2014 Elsevier Ltd and INQUA. All rights reserved.
Please cite this article in press as: Li, Y., et al., Tectonic uplift and landslides triggered by the Wenchuan earthquake and constraints on orogenic growth: A case study from Hongchun Gully, Longmen Mountains, Sichuan, China, Quaternary International (2014), http://dx.doi.org/10.1016/ j.quaint.2014.05.005
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surface processes in the geomorphic evolution of Longmen Shan has become a topic of scientific concern. Therefore, the interplay between coseismic rock uplift and coseismic landslides thus raises a fundamental question: do large earthquakes e and the landslides they trigger e create or destroy mountainous topography? It is usually assumed that thrust earthquakes will not only lead to new uplift and orogenic growth of mountains, but also lead to massive landslides and debris flows, which reduces the height of the mountains. 1.1. Objective of the study Parker et al. (2011) stated that mass wasting triggered by the 2008 Wenchuan earthquake exceeded orogenic growth and led to a net material deficit. However, the effects of tectonic uplift,
landslides, and debris-flows driven by the Wenchuan earthquake on the orogenic growth of Longmen Shan are a current subject of debate (Molnar, 2012). So there are many uncertainties to be taken into account to estimate the relationship between tectonic uplift and landsliding. After the Wenchuan earthquake, heavy rainfall induced catastrophic debris flows in Hongchun Gully. After their discharge from the gully entrance, massive debris flows quickly blocked the Minjiang River. This triggered a silting and flooding disaster. This is a typical case for quantitative study of the relationships between the volume of landslide sediments and that of tectonic uplift. The aim of this study is to estimate the relationship between tectonic uplift and mass wasting triggered by the Wenchuan earthquake and its constraint on the growth of the Longmen Shan. Therefore, we chose Hongchun Gully, located at the epicenter of the earthquake
Fig. 1. Distribution map of landslides, debris flows and precipitation of heavy rainfall in the Longmen Shan region on 13th August 2010 after the Wenchuan earthquake.
Please cite this article in press as: Li, Y., et al., Tectonic uplift and landslides triggered by the Wenchuan earthquake and constraints on orogenic growth: A case study from Hongchun Gully, Longmen Mountains, Sichuan, China, Quaternary International (2014), http://dx.doi.org/10.1016/ j.quaint.2014.05.005
Y. Li et al. / Quaternary International xxx (2014) 1e11
(Figs. 1e4), as a small and typical drainage basin to study the relationship between coseismic tectonic uplift, landslides, and debris flows in the Longmen Shan (Figs. 1e4). This provides some constraints for material-transfer processes in river systems. 1.2. Methods This study has involved quantitative research on coseismic landslides, post-earthquake debris flow, and the river unloading process in Hongchun Gully, which is located at the epicenter of the Wenchuan earthquake. A framework has been established for the
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transfer pathways of coseismic landslide and debris-flow sediments after the earthquake. Based on field measurements and remote-sensing techniques, this study identified the distribution and volume of coseismic landslides, debris flows, and tectonic uplift. Firstly, based on field investigation data and image data interpretation, including EO-1 images, aerial photographs, and digital elevation maps with SPOT accuracy of 5 m and precision of 10 m, the spatial distribution and volume of landslide and debrisflow sediments and sedimentation flux have been accurately calculated in Hongchun Gully. Secondly, the volume of the coseismic tectonic uplift was calculated. Thirdly, a quantitative
Fig. 2. Geological map of the Longmen Mts. and adjacent region (F1: Maowen Fault; F2: Beichuan Fault; F3: Pengguan Fault).
Please cite this article in press as: Li, Y., et al., Tectonic uplift and landslides triggered by the Wenchuan earthquake and constraints on orogenic growth: A case study from Hongchun Gully, Longmen Mountains, Sichuan, China, Quaternary International (2014), http://dx.doi.org/10.1016/ j.quaint.2014.05.005
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Fig. 3. Drainage Systems in upstream of Minjiang River and location of Hongchun Gully.
relationship between the volume of coseismic tectonic uplift and the volume of coseismic and postseismic landslides and debrisflows has been evaluated. 2. Geomorphology and active tectonics of Hongchun Gully Hongchun Gully (entrance coordinates: N31040 01.100, E103 290 32.700 ) is situated in the upper reaches of the Minjiang River (Figs. 1e3). The Minjiang River originates from the Min Mountain and flows through Longmen Mountain into the Sichuan Basin.
Hongchun Gully is located on the left bank of the Minjiang River northeast of Yingxiu Town, at the epicenter of the Wenchuan earthquake. The valley trend is NE45 and parallels the Beichuan fault, but it is at 90 to the main channel of the Minjiang River, trending SE135 . Therefore, the confluence angle between the Hongchun Gully and the Minjiang River (the main river) is close to 90 . The seismogenic fault of the Wenchuan earthquake (Beichuan Fault) trend is NE45 cuts through the gully with an inclination of 300e315 and a dip of 35e60 . Its right bank of the gully is a hanging wall of the fault, composed of the Pre-Sinian “Pengguan complex”. The left bank of the gully is on the footwall of the fault,
Please cite this article in press as: Li, Y., et al., Tectonic uplift and landslides triggered by the Wenchuan earthquake and constraints on orogenic growth: A case study from Hongchun Gully, Longmen Mountains, Sichuan, China, Quaternary International (2014), http://dx.doi.org/10.1016/ j.quaint.2014.05.005
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Fig. 4. Landforms and coseismic landslides induced by the 2008 Wenchuan Earthquake in Hongchun gully (taken 15 May, 2010).
Fig. 5. Distribution of coseismic surface ruptures and landslides in Hongchun Gully and Yingxiu town during the Wenchuan earthquake (from Fu et al., 2008, taken 15 May, 2008).
Please cite this article in press as: Li, Y., et al., Tectonic uplift and landslides triggered by the Wenchuan earthquake and constraints on orogenic growth: A case study from Hongchun Gully, Longmen Mountains, Sichuan, China, Quaternary International (2014), http://dx.doi.org/10.1016/ j.quaint.2014.05.005
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composed of Sinian (Neo-Proterozoic) to Triassic dolomite, limestone, and sandstone. Hongchun Gully is a fault gully located at the earthquake epicenter, cut by the Beichuan fault. The Hongchun Gully has a fan-shaped valley (Figs. 2 and 6), and the basin area is approximately 560 104 m2. There are many coseismic and postseismic landslides and debris flows from loose surface soil and fractured rock. The width of the gully entrance is 80e110 m. It shows a deep-cut “V” shape in profile, and the slope of the bank is generally 40e50 . The length of the main gully is 3.62 km. The valley is steep, with the average vertical gradient of the riverbed at 31.25‰, and the relative elevation difference from the highest point (2168.4 m) to the entrance (880 m) is 1288.4 m. The runoff in Hongchun Gully is supplied mainly by precipitation. Downstream flow in the rainy season is up to 2.0e5.0 m3/s. 3. The volume of coseismic rock uplift in Hongchun Gully By using Synthetic Aperture Radar (SAR) amplitude data, de Michele et al. (2010) derived the three-component coseismic surface displacement field due to the Wenchuan earthquake. Based on this data, Parker et al. (2011) have calculated the net volume added to the orogen in the earthquake via coseismic rock uplift, as 2.6 109 m3 in the Longmen Shan region. Because the surface rupture of the seismogenic Beichuan Fault passes through Hongchun Gully and vertically offset Highway G213 by 2.3 ± 0.1 m vertically and by 0.8 ± 0.2 m horizontally at the gully entrance (Li et al., 2010, 2011; Xu et al., 2009) we used the up or vertical component of a hanging wall on the north side of the main gully and the area of the right bank in the river basin (290 104 m2)
to calculate the net coseismic tectonic uplift volume change. The net coseismic volume is 667 104 m3 (2.3 m (tectonic uplift thickness) 290 104 m2). This is the volume of material added to the orogen in the earthquake by coseismic rock uplift. Findings on active tectonics (Li et al., 2006; Densmore et al., 2007) and the Wenchuan earthquake surface rupture (Xu et al., 2009; Li et al., 2010, 2011) have suggested that the surface rupture in this area is consistent with the historical earthquake scarp of active faults. During the past 70,000 years, a fault scarp of approximately 40 m was formed at the same place, indicating that locations where earthquakes and surface ruptures have occurred during the Quaternary are still major earthquake zones for the present and the future. Given the overlapping of the earthquake surface rupture and the fault scarp on the Beichuan fault, we believe that the earthquake was an in-situ recurrence (Li et al., 2006; Lin et al., 2010). Since 40 ka, more than 30 paleoseismic records of strong earthquakes have been discovered in the Longmen Shan region, with the latest strong earthquake occurring at about 930 ± 40 BP (Li et al., 2006). This indicates that the recurrent time interval of strong earthquakes in Longmen Shan region is about 1000 years (Lin et al., 2010, Li et al., 2012). 4. The volume of pre-seismic landslides and debris-flows in Hongchun Gully According to the field survey, Hongchun Gully was a debris-flow gully before the Wenchuan earthquake. Loose sediments were the main deposits in the gully, and the total sediment volume in the basin was approximately 90 104 m3. According to historical
Fig. 6. Map of coseismic landslides in Hongchun Gully.
Please cite this article in press as: Li, Y., et al., Tectonic uplift and landslides triggered by the Wenchuan earthquake and constraints on orogenic growth: A case study from Hongchun Gully, Longmen Mountains, Sichuan, China, Quaternary International (2014), http://dx.doi.org/10.1016/ j.quaint.2014.05.005
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records, two debris flows had occurred previously. The first outbreak of debris flows took place in the early 1930s, and the second one occurred in 1962. The floods carried a large amount of silt and rocks, with a volume of about 2 104 m3, out of the gully entrance, accounting for 2.22% of the volume of the Hongchun Gully landslide (90 104 m3). This indicates that before the Wenchuan earthquake, the conversion rate from landslides to sediment discharged into the Minjiang river was very small. Therefore, the total volume of pre-seismic landslides and the area of the drainage were compared, indicating a representative thickness of landslideinduced denudation of only 0.16 m before the earthquake. According to historical records, pre-seismic landslides were discharged into the Minjiang River only in 1962, and the discharge volume was about 2 104 m3. Therefore, the conversion ratio be-
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Table 1 The volume of landslides and debris flows in the Hongchun Gully. Time
Volume of sediments （104 m3）
Erosion rate
Volume of pre-seismic sediments Volume of coseismic sediments Total volume of sediments Volume of postseismic Volume of residual sediments at the entrance sediments Volume of sediments discharged into the river on August 13, 2010 Volume of sediments discharged into the river on August 18, 2010
90 380.01 70.5 30
0.16 mm/y 0.68 mm/y
40.5 8
Table 2 Monthly statistics of rainfall between 2007 and 2010, Hongchun Gully (Yingxiu) (mm). Month
Jan
Feb
Mar
Apr
May
Jun
Jul
Aug
Sep
Oct
Nov
Dec
Annual
14 16.1 12 2.9
9.4 20.8 10.3 7.3
16.7 69.7 54 36.8
36.1 107.3 102.5 60.2
69.8 93 68.3 101.9
90.7 149.6 38 73.2
160.4 143.2 170.8 142.6
221 245 102.1 507.2
95.5 256.2 182.3 248.5
131.8 99.2 72.4 58.2
17.5 17.3 7.1 7.2
16.8 5 15 21.8
879.7 1222.4 834.8 2545.8
Year 2007 2008 2009 2010
(Data Source: website of Sichuan Meteorological Stations).
tween pre-seismic landslides and sediments discharged by the river is very small. Assuming the 1000-year recurrent cycle, Hongchun Gully had a rate of landslide-induced erosion of 0.16 mm/y before the earthquake. This result is basically consistent with the millennial-scale erosion rate of the Minjiang River before the earthquake (0.2e0.3 mm/y according to the cosmogenic nuclides method, Ouimet, 2010), but is significantly less than the million-year-scale average denudation rate of Longmen Mountain (0.5e0.7 mm/y according to fission track, Kirby et al., 2008; Godard et al., 2009). 5. Volume of coseismic landslides in Hongchun Gully Massive collapses, landslides, and debris flows were induced by the Wenchuan earthquake in Hongchun Gully (Figs. 2, 4e6). The field survey showed that there were 40 new collapse or landslide initiation points. The largest landslide is located on the right bank of the upper reaches of the gully, with a width of 180 m, length of 300 m, thickness of 10e20 m, and total volume of about 36 104 m3. The total volume of loose solid materials in the gully caused by the earthquake was 380.01 104 m3. Compared to the pre-seismic landslide volume (Table 2), the increased landslide volume was 290.01 104 m3 and its growth rate was 322%, indicating the coseismic landslide volume is three times the pre-seismic landslide volume. According to the drainage area, the average erosion thickness of coseismic landslides is 0.68 m in the gully. According to the 1000-year recurrent cycle (Lin et al., 2010), the erosion rate of coseismic landslides is 0.68 mm/y. This is significantly greater than the millennial-scale erosion rate of the Minjiang River before the earthquake (Table 1), but is basically consistent with that for the Longmen Mountains on a million-year scale. This indicates that coseismic landslides erosion are the main external forces in long-term surface erosion processes in these areas and may therefore play an important role in the long-term geomorphic evolution of Longmen Shan. The erosion rate of seismic landslides is equivalent to an average reduction rate of the surface elevation within an earthquake recurrence cycle.
6. Volume of postseismic debris-flows caused by heavy rain on August 13, 2010 in Hongchun Gully 6.1. Orographic rain on Longmen Shan Orographic rain is a basic driver for heavy rainfall in the Longmen Shan (Fig. 9). Orographic rain is controlled mainly by the East Asian monsoon, which moves from southeast to northwest from July to September. The monsoon passes over Longmen Shan, and the east side of the mountain is the windward slope. Rainfall is enhanced, resulting in a rain-shadow effect and orographic rain. Precipitation is therefore increased tremendously from Sichuan basin to Longmen Shan. A northeast-trending belt of heavy rainfall is formed, with an average annual precipitation of 1600e2000 mm. During 2008, 2009, there were no heavy rainfall events in the Minjiang river basin. However, during August 12e19, 2010, there was a heavy rainfall event in the Longmen Mountain region. The precipitation during this heavy rainfall exceeded many times than the historical average daily, monthly, and yearly precipitation (Table 2), which caused debris flows in the upstream parts of Hongchun Gully. Because the heavy rainfall zones are basically consistent with NE-trending earthquake fault zones (Fig. 1), this leads to the current situation in which seismic landslides and debris flows are distributed along the fault zone. Therefore, the Wenchuan earthquake was a major factor triggering coseismic landslides, and regional heavy rainfall is another major factor triggering postseismic landslides and debris flows. These interactions represent positive and negative feedback mechanisms among the processes of earthquake tectonic uplift, heavy rainfall, and surface erosion (Fig. 9). 6.2. The volume of postseismic debris-flows in Hongchun Gully A huge quantity of debris-flows sediments in Hongchun Gully, caused by heavy rainfall on August 13, 2010, blocked the Minjiang River. The debris flows formed a fan at the entrance of Hongchun Gully (Figs. 6 and 7) with volume of 70.5 104 m3, length of 900 m, area of 0.06 km2, and thickness of 10e20 m (Gan et al., 2012).
Please cite this article in press as: Li, Y., et al., Tectonic uplift and landslides triggered by the Wenchuan earthquake and constraints on orogenic growth: A case study from Hongchun Gully, Longmen Mountains, Sichuan, China, Quaternary International (2014), http://dx.doi.org/10.1016/ j.quaint.2014.05.005
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Compared with the volume of coseismic landslides (Table 1), the conversion ratio between coseismic landslides and postseismic debris-flows sediments in Hongchun Gully was 19.67%, which is close to 20%. The conversion rate of seismic landslides into debris flow was quite fast, more than 10 times the value before the Wenchuan earthquake. 6.3. Blockage by postseismic debris-flows
Fig. 7. Orographic rain in the Longmen Shan. (F1: Maowen fault; F2: Yingxiu-Beichuan fault; F3: Pengguan fault).
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The postseismic debris-flow fan blocked three-quarters of the main Minjiang river (only 80e100 m wide) (Figs. 7 and 8). The postseismic debris-flow fan has a fan shape, with a wide front tapering to its end, thus forming a long and narrow tail. Vertically, it shows a wedge shape, with a high, steep, and thick head which lowers and thins towards the bottom. In cross section, the head is a curved bulge, the middle part is flat, and the tail is a sunken arc. The main components of the debris flows were stone fragments mixed with sand and gravel. Stone fragments accounted for about 60% of the debris flows, with grain sizes of 20e50 cm, followed by boulders (approximately 15%), boulder gravel (approximately 10%), angular pebbles (approximately 10%), and sand (about 5%). The cobbles in the sediments were generally less than 1.0 m in diameter. The debris flowed as a kind of non-Newtonian fluid in tributaries and came together with the Newtonian fluid in the main river, changing the sand-water composition of the main river and the local boundary conditions in a short time. The confluence had a significant impact on the sediment characteristics, kinetic characteristics, and evolution of the main river. The impact was felt mainly in the following three ways: (1) the confluence area is a zone of strong turbulence, where the movement of water is complex. Water mixing, swirl, side-wall resistance, internal collision, and separation of silt, as well as the shear effects of two fluids, caused energy loss in the confluence area, leading to high water levels near the confluence point and silt deposition in the confluence area. (2) The channel was narrowed. Debris flows were directly discharged into the Minjiang River and occupied and compressed the channel on one side. The debris flow led to river blocking and intensified bending and resulted in flooding near the other side of the main river, which affected the new district of Yingxiu town. (3) The riverbed was elevated. Debris flows blocked the channel, resulting in diversion of the Minjiang River by about 300 m and causing river blockage. The debris flows elevated the upstream section of the riverbed, with a cumulative accretion of approximately 8e10 m. In addition, high-strength sand was transported downstream, and therefore the sand content of the river increased substantially. The debris flows elevated the downstream riverbed, with a cumulative accretion of approximately 3.5 m. (4) The gradient of the channel increased significantly. The debris flow caused changes in the river on the vertical section, forming rapids and a pit-type cascade riverbed. 7. Volume of sediments removed from the range by fluvial processes The debris flows sediments of the Hongchun Gully discharged into the Minjiang River formed a fan-shaped accumulation body with volume of 48.5 104 m3, length of 600 m, width of 320 m, and an average thickness of 20 m. Compared to the volume of coseismic landslides (Table 1), the conversion ratio was approximately 13%. Compared to the volume of postseismic debris-flows, the conversion ratio was 69.64%, or close to 70%. The following two conclusions can be reached: (1) after the Wenchuan earthquake, the conversion rate from landslides and debris-flows sediments river sediments was 35 times that before the earthquake, and (2) after
Please cite this article in press as: Li, Y., et al., Tectonic uplift and landslides triggered by the Wenchuan earthquake and constraints on orogenic growth: A case study from Hongchun Gully, Longmen Mountains, Sichuan, China, Quaternary International (2014), http://dx.doi.org/10.1016/ j.quaint.2014.05.005
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Fig. 8. Postseismic debris flows fan driven by heavy rainfall in Hongchun Gully on August 13, 2010 (taken 14 August, 2010).
the earthquake, the occurrence frequency of heavy rainfall above the critical rainfall intensity could be one to three years. According to the volume of sediments (49.1 104 m3/y) removed from the range by fluvial processes, most coseismic landslide sediments could be converted into debris flows and river sediments in the next 7e21 years. It can therefore be hypothesized that in the next
10e20 years, the majority of coseismic landslide and debris-flow sediments will be transported away by the river. Dadson et al. (2004) noted that seismic landslides led to the increase of river sedimentation flux. Within a very short time, the postseismic debris-flows in Hongchun Gully discharged a large amount of solid materials into the Minjiang River, increasing the
Fig. 9. Debris flows on August 13, 2012 Hongchun Gully (aerial photograph from the Sichuan Geological Environment Monitoring Station).
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sand content of the river substantially. In 2010, the sediment input into the Minjiang River from Hongchun Gully debris flows was 49.1 104 m3, equivalent to 37 104 t. In comparison with the average annual sediment discharge of the Minjiang River (764 104 t, Li et al., 2006), the contribution to the increment of sediment discharge in the Minjiang River was 5% of the sediment yield in August 2010. More than 10 debris-flows were triggered by heavy rainfall in August 2010 (Fig. 8), among which the largest was the Longchi debris-flow, when sediments discharged into the Minjiang River reached 400 104 m3. Therefore, it is estimated that in August 2010, the total sediment discharge into the Minjiang River due to postseismic debris-flows was about 500 104 m3, equivalent to 384.62 104 t. The contribution to the increment of sediment discharge in the Minjiang Basin was 50% and caused an increase of 50% in river sedimentation flux. 8. Discussion and conclusions In an active orogenic belt, coseismic uplift caused by periodic earthquakes is an important driving factor for the mountainbuilding. At the same time, coseismic and postseismic landslides and debris flows also reduce the height of mountains (Hovius et al., 2000; Parker et al., 2011). When the volume of the coseismic tectonic uplift is greater than the volume denuded by coseismic landslides and postseismic debris flows, the result is an increase in the average elevation; otherwise, there will be a decrease in the average elevation. Because the volume of coseismic landslides (~5e15 km3) was greater than the volume of coseismic tectonic uplifts (2.6 ± 1.2 km3, de Michele et al., 2010), our group (Parker et al., 2011) stated that the Wenchuan earthquake will lead to a net material deficit and reduce the height of the Longmen Mountains. However, there are many uncertainties to be taken into account to estimate the relationship between the total landslide volume and the net growth of the Longmen Shan in the Wenchuan earthquake. The first is to estimate the volume of coseismic and postseismic landslides and debris flows. The second is to estimate the volume of tectonic uplift. The third is to estimate river unloading time for seismic landslide sediments and the length of the earthquake recurrence cycle (Quimet et al., 2007). Therefore, quantitative analyses of the river unloading time for seismic landslide sediments are meaningful to an understanding of the relationship between earthquakes and geomorphic evolution in orogenic belts. The calculation of the river unloading time for seismic landslide sediments in Hongchun Gully in this paper will be conducive to a new understanding of the relationship between Longmen Mountain earthquakes, landslide erosion, and geomorphic evolution. Using quantitative data for the volume of coseismic tectonic uplift, and the volume of coseismic and postseismic landslides and debris flows in Hongchun Gully, the quantitative relationship between them has been determined. The coseismic landslide volume (380.01 104 m3) was less than the coseismic tectonic uplift volume (667 104 m3) in Hongchun Gully. Only 57% of the volume of the tectonic uplift has been converted to the volume of landslides in Hongchun Gully, indicating new uplift and geomorphic growth of Longmen. The volume of debris flows was 70.5 104 m3, indicating that 20% of the volume of coseismic landslides was converted into debris flows by heavy rainfall. The volume of materials discharged by Minjiang River (the main river) is 48.5 104 m3, and only 13% of coseismic landslide sediments has been removed from the range by fluvial processes. At least 86% of landslide sediments remain in the Hongchun Gully. Coseismic and postseismic landslides cannot be directly and quickly transported out of the Longmen Shan region in a very short period of time. Only in the next 10e30 years, the coseismic landslide and debris-flow sediments will be discharged
by fluvial processes. The current landslides and debris flows cannot cause a material deficit on Longmen Shan region. The Wenchuan Earthquake caused new uplift and geomorphic growth of the Longmen mountains. The volume of landslides did not exceed the incremental rock volume due to coseismic uplift, and cannot lead to a net material deficit in the Longmen Shan region. This conclusion is not consistent with our previous result that mass wasting triggered by the 2008 Wenchuan earthquake exceeds orogenic growth (Parker et al., 2011), is consistent with studies of the Taiwan Chi-Chi (Mw 7.6) earthquake (Hovius et al., 2000). Acknowledgements This study was a project funded by the National Natural Science Foundation of China (grant nos. 40841010, 40972083, 41172162, 41372114, 41340005), a geological survey project funded by the Ministry of Land and Resources (grant no. 1212011121268), and an autonomous research project of the National Key Laboratory of Oil and Gas Reservoir Geology and Exploitation (grant no. SK -0801). References Burchfiel, B.C., Royden, L.H., van der Hilst, R.D., Hager, B.H., 2008. A geological and geophysical context for the Wenchuan earthquake of 12 May 2008, Sichuan, People's Republic of China. GSA Today 18 (7), 4e11. Clark, M.K., House, M.A., Royden, L.H., Whipple, K.X., Burchfiel, B.C., Zhang, X., Tang, W., 2005. Late Cenozoic uplift of southeastern Tibet. Geology 33, 525e528. Dadson, S.J., Hovius, N., Hongey, Chen, Dade, W.B., Lin, J.C., Hsu, M.L., Lin, C.W., Horng, M.J., Chen, T.C., Milliman, J., Stark, P.C., 2004. Earthquake-triggered increase in sediment delivery from an active mountain belt. Geology 32, 733e736. Densmore, A.L., Li, Y., Ellis, M., Zhou, R.J., 2005. Active tectonics and erosional unloading of eastern margin. Journal of Mountain Science 2 (2), 146e154. Densmore, A.L., Ellis, M.A., Li, Y., Zhou, R.J., Hancock, S.G., Richardson, N.J., 2007. Active tectonics of the Beichuan and Pengguan faults at the eastern margin of the Tibetan Plateau. Tectonics 80 (8), 113e127. de Michele, M., Raucoules, D., de Sigoyer, J., Pubellier, M., Chamot-Rooke, N., 2010. Three-dimensional surface displacement of the 2008 May 12 Sichuan earthquake (China) derived from Synthetic Aperture Radar: evidence for rupture on a blind thrust. Geophysical Journal International 183, 1097e1103. Fu, B.H., Shi, P.L., Zhang, Z.W., 2008. Spatial characteristics of the surface rupture produced by the Ms8.0 Wenchuan Earthquake-using high resolution remote sensing imagery. Acta Geologica Sinica 82 (12), 1679e1687 (in Chinese). Fu, B.H., Shi, P.L., Guo, H.D., Satoshi, O., Yoshiki, N., Sarah, W., 2011. Surface deformation related to the 2008 Wenchuan earthquake, and mountain building of the Longmen Shan, eastern Tibetan Plateau. Journal of Asian Earth Sciences 40, 805e824. Gan, J.J., Sun, H.Y., Huang, R.Q., Tang, Y., Fan, C.R., Li, Q.Y., Xu, X.N., 2012. Study on mechanism of formation and river blocking of Hongchuangou giant debris flow at Yingxiu of Wenchuan County. Journal of Catastrophology 27 (1), 5e11 (in Chinese). , J., Tibari, B., de Sigoyer, J., Pubellier, M., Zhu, J., 2009. Late Godard, V., Pik, R., Lave Cenozoic evolution of the central Longmen Shan, eastern Tibet: Insight from(UTh)/He thermochronometry. Tectonics 28. http://dx.doi.org/10.1029/ 2008TC002407. TC5009. Hubbard, J., Shaw, J., 2009. Uplift of the Longmen Shan and Tibetan plateau, and the 2008Wenchuan (M¼7.9) earthquake. Nature 458, 194e197. Hovius, N., Stark, C.P., Chu, H.T., Lin, J.C., 2000. Supply and removal of sediment in a landslide-dominated mountain belt: Central Range, Taiwan. Journal of Geology 108 (1), 73e89. Kirby, E., Whipple, K., Harkins, N., 2008. Topography reveals seismic hazard. Nature Geoscience 1 (8), 485e487. Li, Y., Zhou, R.J., Densmore, A.L., Ellis, M., Li, Y.Z., Allen, P.A., Steffen, D., Li, B., Zhang, Y., Richardson, N.J., Wang, F.L., He, Y.L., Xi, X.H., Li, X.G., Si, G.Y., Wang, M., 2006. The Geology of the Eastern Margin of the Qinghai-Tiben Plateau. Geological Publishing House, Beijing, pp. 1e168. Li, Y., Zhou, R.J., Densmore, A.L., Cao, S.Y., Liu, Y.P., 2011. Spatial relationship between surface ruptures in the Ms 8.0 Wenchuan earthquake ,the Longmen Shan region,Sichuan, China. Journal of Earthquake and Tsunami 5 (4), 329e342. Li, Y., Huang, R.Q., Yan, L., Lei, J.C., Zhang, Y., He, Y.L., Chen, L.S., Li, X.G., Wang, S.Y., Ye, Y.Q., Liu, Y.F., Kang, C.C., Ge, T.Y., He, Q., Huang, Wei, 2010. Surface ruprure and huazard of Wenchuan Ms 8.0 earthquake, Sichuan, China. International Journal of Geosciences 1 (1), 21e31. Lin, A.M., Ren, Z.K., Jia, D., Yosuke, M., 2010. Evidence for a Tang-Song Dynasty great earthquake along the Longmen Shan thrust belt prior to the 2008 Mw 7.9 Wenchuan earthquake. Journal of Seismology 14, 615e628. Molnar, P., 2012. Isostasy can't be ignored. Nature Geoscience 5, 83.
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Y. Li et al. / Quaternary International xxx (2014) 1e11 Ouimet, W.B., 2010. Landslides associated with the May12, 2008 Wenchuan earthquake: implications for the erosion and tectonic evolution of the Longmen Shan. Tectonophysics 491, 244e252. Ouimet, W.B., Whipple, K.X., Royden, L.H., Sun, Z.M., Chen, Z.L., 2007. The influence of large landslides on river incision in a transient landscape: eastern margin of the Tibetan Plateau (Sichuan, China). Geological Society of America Bulletin 119, 1462e1476. Parker, R.N., Densmore, A.L., Rosser, N.J., de Michele, M., Li, Y., Huang, R.Q., Whadcoat, L., Petley, N.D., 2011. Mass wasting triggered by the 2008 Wenchuan earthquake is greater than orogenic growth. Nature Geoscience. http:// dx.doi.org/10.1038/NGEO1554.
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Royden, L.H., Burchfiel, B.C., King, R.W., Wang, E., Chen, Z.L., Shen, F., Liu, Y.P., 1997. Surface deformation and lower crustal flow in eastern Tibet. Science 276, 788e790. Xu, X.W., Wen, X.D., Yu, G., Cao, G.H., Klinger, Y., 2009. Coseismic reverse and oblique-slip surface faulting generated by the 2008Mw7.9 Wenchuan earthquake, China. Geology 37, 515e518. Xu, G., Kamp, P., 2000. Tectonics and denudation adjacent to the Xianshuihe fault, eastern Tibetan Plateau: constraints from fission-track thermochronology. Journal of Geophysical Research 105, 19231e19251.
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Tectonic uplift and landslides triggered by the Wenchuan earthquake and constraints on orogenic growth: A case study from Hongchun Gully, Longmen Mountains, Sichuan, China Yong Li a, *, Rongjun Zhou b, Guohua Zhao a, Haibing Li c, Decheng Su c, Hairong Ding a, Zhaokun Yan a, Liang Yan a, Kung Yun a, Chao Ma a a b c
National Key Laboratory of Oil and Gas Reservoir Geology and Exploitation, Chengdu University of Technology, Chengdu 610059, China Institute of Earthquake Engineering, Seismological Bureau of Sichuan Province, 610041, China Institute of Geology, Chinese Academy of Geological Science, Beijing 100037, China
a r t i c l e i n f o
a b s t r a c t
Article history: Available online xxx
Many uncertainties need to be taken into account to estimate the relationship between tectonic uplift, the landslide volume triggered by the Wenchuan earthquake, and constraints on the orogenic growth of the Longmen Shan (Mountain). In this research, coseismic tectonic uplift, landslides, and postseismic debris-flows in Hongchun Gully located at the earthquake epicenter have been studied. Field data, aerial photographs, and digital elevation data were used to estimate the quantitative relationship between the tectonic uplift and landslides, debris-flows and their constraints on the orogenic growth of the Longmen Mountains. The coseismic tectonic uplift volume (667 104 m3) was more than the coseismic landslide volume (380.01 104 m3). Only 57% of the volume of the tectonic uplift has been converted to the volume of landslides in Hongchun Gully, indicating new uplift and orogenic growth of the Longmen Shan. The volume of postseismic debris flows was 70.5 104 m3, indicating that 20% of the volume of coseismic landslides was converted into postseismic debris flows by heavy rainfall. The volume of materials discharged by Minjiang River (the main river) is 48.5 104 m3, indicating that only 13% of coseismic landslide sediments has been removed from the range by fluvial processes, and at least 86% of landslide sediments still remain in the Hongchun Gully. Coseismic tectonic uplift volume exceeds coseismic landslides volume, and the Wenchuan Earthquake caused new uplift and geomorphic growth of the Longmen Mountains. This is not consistent with a previous conclusion that mass wasting triggered by the 2008 Wenchuan earthquake exceeds orogenic growth and led to a net material deficit in the Longmen Shan region. © 2014 Elsevier Ltd and INQUA. All rights reserved.
Keywords: Wenchuan earthquake Coseismic tectonic uplift Coseismic landslides Postseismic debris flows Orogenic growth Longmen Shan
1. Introduction Longmen Shan (Mountain) is a linear, asymmetric border range between the Tibetan Plateau and the Sichuan Basin and is marked by an extremely steep slope with relief of over 5 km (Figs. 1 and 2)(Densmore et al., 2005; Li et al., 2006; Godard et al., 2009). It extends in a NEeSW direction with a total length of 500 km and a width of 30 km. This border mountain shows as a positive isostatic gravity anomaly zone and the lower crust has been lifted by 11 km
* Corresponding author. National Key Laboratory of Oil and Gas Reservoir Geology and Exploitation, Institute of Sedimentary Geology, Chengdu University of Technology, Chengdu 610059, China E-mail addresses: [email protected], [email protected] (Y. Li).
(Li et al., 2006). This may be caused by tectonic shortening (Hubbard and Shaw, 2009), crustal isostatic rebound related to denudation (Densmore et al., 2005; Li et al., 2006; Fu et al., 2011) or channel flow (Royden et al., 1997; Clark et al., 2005; Burchfiel et al., 2008). The denudation thickness of the range is 6e10 km (Xu and Kamp, 2000; Li et al., 2006). Its current landforms resulted from continuous competition between earthquake-driven uplift and surface process-driven erosion. On May 12, 2008, the Wenchuan earthquake (Ms 8.0) took place in the middle and northern segments of the Longmen Mountains (Figs. 1 and 2), and on April 20, 2013, the Lushan (Ms 7.0) earthquake occurred in the southern segment. The tectonic uplift triggered by the Wenchuan earthquake changed the slope gradient instantly and caused massive landslides and debris flows. As a consequence, the landforms and the river system also were modified. The role of the huge earthquake and its
http://dx.doi.org/10.1016/j.quaint.2014.05.005 1040-6182/© 2014 Elsevier Ltd and INQUA. All rights reserved.
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surface processes in the geomorphic evolution of Longmen Shan has become a topic of scientific concern. Therefore, the interplay between coseismic rock uplift and coseismic landslides thus raises a fundamental question: do large earthquakes e and the landslides they trigger e create or destroy mountainous topography? It is usually assumed that thrust earthquakes will not only lead to new uplift and orogenic growth of mountains, but also lead to massive landslides and debris flows, which reduces the height of the mountains. 1.1. Objective of the study Parker et al. (2011) stated that mass wasting triggered by the 2008 Wenchuan earthquake exceeded orogenic growth and led to a net material deficit. However, the effects of tectonic uplift,
landslides, and debris-flows driven by the Wenchuan earthquake on the orogenic growth of Longmen Shan are a current subject of debate (Molnar, 2012). So there are many uncertainties to be taken into account to estimate the relationship between tectonic uplift and landsliding. After the Wenchuan earthquake, heavy rainfall induced catastrophic debris flows in Hongchun Gully. After their discharge from the gully entrance, massive debris flows quickly blocked the Minjiang River. This triggered a silting and flooding disaster. This is a typical case for quantitative study of the relationships between the volume of landslide sediments and that of tectonic uplift. The aim of this study is to estimate the relationship between tectonic uplift and mass wasting triggered by the Wenchuan earthquake and its constraint on the growth of the Longmen Shan. Therefore, we chose Hongchun Gully, located at the epicenter of the earthquake
Fig. 1. Distribution map of landslides, debris flows and precipitation of heavy rainfall in the Longmen Shan region on 13th August 2010 after the Wenchuan earthquake.
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(Figs. 1e4), as a small and typical drainage basin to study the relationship between coseismic tectonic uplift, landslides, and debris flows in the Longmen Shan (Figs. 1e4). This provides some constraints for material-transfer processes in river systems. 1.2. Methods This study has involved quantitative research on coseismic landslides, post-earthquake debris flow, and the river unloading process in Hongchun Gully, which is located at the epicenter of the Wenchuan earthquake. A framework has been established for the
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transfer pathways of coseismic landslide and debris-flow sediments after the earthquake. Based on field measurements and remote-sensing techniques, this study identified the distribution and volume of coseismic landslides, debris flows, and tectonic uplift. Firstly, based on field investigation data and image data interpretation, including EO-1 images, aerial photographs, and digital elevation maps with SPOT accuracy of 5 m and precision of 10 m, the spatial distribution and volume of landslide and debrisflow sediments and sedimentation flux have been accurately calculated in Hongchun Gully. Secondly, the volume of the coseismic tectonic uplift was calculated. Thirdly, a quantitative
Fig. 2. Geological map of the Longmen Mts. and adjacent region (F1: Maowen Fault; F2: Beichuan Fault; F3: Pengguan Fault).
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Fig. 3. Drainage Systems in upstream of Minjiang River and location of Hongchun Gully.
relationship between the volume of coseismic tectonic uplift and the volume of coseismic and postseismic landslides and debrisflows has been evaluated. 2. Geomorphology and active tectonics of Hongchun Gully Hongchun Gully (entrance coordinates: N31040 01.100, E103 290 32.700 ) is situated in the upper reaches of the Minjiang River (Figs. 1e3). The Minjiang River originates from the Min Mountain and flows through Longmen Mountain into the Sichuan Basin.
Hongchun Gully is located on the left bank of the Minjiang River northeast of Yingxiu Town, at the epicenter of the Wenchuan earthquake. The valley trend is NE45 and parallels the Beichuan fault, but it is at 90 to the main channel of the Minjiang River, trending SE135 . Therefore, the confluence angle between the Hongchun Gully and the Minjiang River (the main river) is close to 90 . The seismogenic fault of the Wenchuan earthquake (Beichuan Fault) trend is NE45 cuts through the gully with an inclination of 300e315 and a dip of 35e60 . Its right bank of the gully is a hanging wall of the fault, composed of the Pre-Sinian “Pengguan complex”. The left bank of the gully is on the footwall of the fault,
Please cite this article in press as: Li, Y., et al., Tectonic uplift and landslides triggered by the Wenchuan earthquake and constraints on orogenic growth: A case study from Hongchun Gully, Longmen Mountains, Sichuan, China, Quaternary International (2014), http://dx.doi.org/10.1016/ j.quaint.2014.05.005
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Fig. 4. Landforms and coseismic landslides induced by the 2008 Wenchuan Earthquake in Hongchun gully (taken 15 May, 2010).
Fig. 5. Distribution of coseismic surface ruptures and landslides in Hongchun Gully and Yingxiu town during the Wenchuan earthquake (from Fu et al., 2008, taken 15 May, 2008).
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composed of Sinian (Neo-Proterozoic) to Triassic dolomite, limestone, and sandstone. Hongchun Gully is a fault gully located at the earthquake epicenter, cut by the Beichuan fault. The Hongchun Gully has a fan-shaped valley (Figs. 2 and 6), and the basin area is approximately 560 104 m2. There are many coseismic and postseismic landslides and debris flows from loose surface soil and fractured rock. The width of the gully entrance is 80e110 m. It shows a deep-cut “V” shape in profile, and the slope of the bank is generally 40e50 . The length of the main gully is 3.62 km. The valley is steep, with the average vertical gradient of the riverbed at 31.25‰, and the relative elevation difference from the highest point (2168.4 m) to the entrance (880 m) is 1288.4 m. The runoff in Hongchun Gully is supplied mainly by precipitation. Downstream flow in the rainy season is up to 2.0e5.0 m3/s. 3. The volume of coseismic rock uplift in Hongchun Gully By using Synthetic Aperture Radar (SAR) amplitude data, de Michele et al. (2010) derived the three-component coseismic surface displacement field due to the Wenchuan earthquake. Based on this data, Parker et al. (2011) have calculated the net volume added to the orogen in the earthquake via coseismic rock uplift, as 2.6 109 m3 in the Longmen Shan region. Because the surface rupture of the seismogenic Beichuan Fault passes through Hongchun Gully and vertically offset Highway G213 by 2.3 ± 0.1 m vertically and by 0.8 ± 0.2 m horizontally at the gully entrance (Li et al., 2010, 2011; Xu et al., 2009) we used the up or vertical component of a hanging wall on the north side of the main gully and the area of the right bank in the river basin (290 104 m2)
to calculate the net coseismic tectonic uplift volume change. The net coseismic volume is 667 104 m3 (2.3 m (tectonic uplift thickness) 290 104 m2). This is the volume of material added to the orogen in the earthquake by coseismic rock uplift. Findings on active tectonics (Li et al., 2006; Densmore et al., 2007) and the Wenchuan earthquake surface rupture (Xu et al., 2009; Li et al., 2010, 2011) have suggested that the surface rupture in this area is consistent with the historical earthquake scarp of active faults. During the past 70,000 years, a fault scarp of approximately 40 m was formed at the same place, indicating that locations where earthquakes and surface ruptures have occurred during the Quaternary are still major earthquake zones for the present and the future. Given the overlapping of the earthquake surface rupture and the fault scarp on the Beichuan fault, we believe that the earthquake was an in-situ recurrence (Li et al., 2006; Lin et al., 2010). Since 40 ka, more than 30 paleoseismic records of strong earthquakes have been discovered in the Longmen Shan region, with the latest strong earthquake occurring at about 930 ± 40 BP (Li et al., 2006). This indicates that the recurrent time interval of strong earthquakes in Longmen Shan region is about 1000 years (Lin et al., 2010, Li et al., 2012). 4. The volume of pre-seismic landslides and debris-flows in Hongchun Gully According to the field survey, Hongchun Gully was a debris-flow gully before the Wenchuan earthquake. Loose sediments were the main deposits in the gully, and the total sediment volume in the basin was approximately 90 104 m3. According to historical
Fig. 6. Map of coseismic landslides in Hongchun Gully.
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records, two debris flows had occurred previously. The first outbreak of debris flows took place in the early 1930s, and the second one occurred in 1962. The floods carried a large amount of silt and rocks, with a volume of about 2 104 m3, out of the gully entrance, accounting for 2.22% of the volume of the Hongchun Gully landslide (90 104 m3). This indicates that before the Wenchuan earthquake, the conversion rate from landslides to sediment discharged into the Minjiang river was very small. Therefore, the total volume of pre-seismic landslides and the area of the drainage were compared, indicating a representative thickness of landslideinduced denudation of only 0.16 m before the earthquake. According to historical records, pre-seismic landslides were discharged into the Minjiang River only in 1962, and the discharge volume was about 2 104 m3. Therefore, the conversion ratio be-
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Table 1 The volume of landslides and debris flows in the Hongchun Gully. Time
Volume of sediments （104 m3）
Erosion rate
Volume of pre-seismic sediments Volume of coseismic sediments Total volume of sediments Volume of postseismic Volume of residual sediments at the entrance sediments Volume of sediments discharged into the river on August 13, 2010 Volume of sediments discharged into the river on August 18, 2010
90 380.01 70.5 30
0.16 mm/y 0.68 mm/y
40.5 8
Table 2 Monthly statistics of rainfall between 2007 and 2010, Hongchun Gully (Yingxiu) (mm). Month
Jan
Feb
Mar
Apr
May
Jun
Jul
Aug
Sep
Oct
Nov
Dec
Annual
14 16.1 12 2.9
9.4 20.8 10.3 7.3
16.7 69.7 54 36.8
36.1 107.3 102.5 60.2
69.8 93 68.3 101.9
90.7 149.6 38 73.2
160.4 143.2 170.8 142.6
221 245 102.1 507.2
95.5 256.2 182.3 248.5
131.8 99.2 72.4 58.2
17.5 17.3 7.1 7.2
16.8 5 15 21.8
879.7 1222.4 834.8 2545.8
Year 2007 2008 2009 2010
(Data Source: website of Sichuan Meteorological Stations).
tween pre-seismic landslides and sediments discharged by the river is very small. Assuming the 1000-year recurrent cycle, Hongchun Gully had a rate of landslide-induced erosion of 0.16 mm/y before the earthquake. This result is basically consistent with the millennial-scale erosion rate of the Minjiang River before the earthquake (0.2e0.3 mm/y according to the cosmogenic nuclides method, Ouimet, 2010), but is significantly less than the million-year-scale average denudation rate of Longmen Mountain (0.5e0.7 mm/y according to fission track, Kirby et al., 2008; Godard et al., 2009). 5. Volume of coseismic landslides in Hongchun Gully Massive collapses, landslides, and debris flows were induced by the Wenchuan earthquake in Hongchun Gully (Figs. 2, 4e6). The field survey showed that there were 40 new collapse or landslide initiation points. The largest landslide is located on the right bank of the upper reaches of the gully, with a width of 180 m, length of 300 m, thickness of 10e20 m, and total volume of about 36 104 m3. The total volume of loose solid materials in the gully caused by the earthquake was 380.01 104 m3. Compared to the pre-seismic landslide volume (Table 2), the increased landslide volume was 290.01 104 m3 and its growth rate was 322%, indicating the coseismic landslide volume is three times the pre-seismic landslide volume. According to the drainage area, the average erosion thickness of coseismic landslides is 0.68 m in the gully. According to the 1000-year recurrent cycle (Lin et al., 2010), the erosion rate of coseismic landslides is 0.68 mm/y. This is significantly greater than the millennial-scale erosion rate of the Minjiang River before the earthquake (Table 1), but is basically consistent with that for the Longmen Mountains on a million-year scale. This indicates that coseismic landslides erosion are the main external forces in long-term surface erosion processes in these areas and may therefore play an important role in the long-term geomorphic evolution of Longmen Shan. The erosion rate of seismic landslides is equivalent to an average reduction rate of the surface elevation within an earthquake recurrence cycle.
6. Volume of postseismic debris-flows caused by heavy rain on August 13, 2010 in Hongchun Gully 6.1. Orographic rain on Longmen Shan Orographic rain is a basic driver for heavy rainfall in the Longmen Shan (Fig. 9). Orographic rain is controlled mainly by the East Asian monsoon, which moves from southeast to northwest from July to September. The monsoon passes over Longmen Shan, and the east side of the mountain is the windward slope. Rainfall is enhanced, resulting in a rain-shadow effect and orographic rain. Precipitation is therefore increased tremendously from Sichuan basin to Longmen Shan. A northeast-trending belt of heavy rainfall is formed, with an average annual precipitation of 1600e2000 mm. During 2008, 2009, there were no heavy rainfall events in the Minjiang river basin. However, during August 12e19, 2010, there was a heavy rainfall event in the Longmen Mountain region. The precipitation during this heavy rainfall exceeded many times than the historical average daily, monthly, and yearly precipitation (Table 2), which caused debris flows in the upstream parts of Hongchun Gully. Because the heavy rainfall zones are basically consistent with NE-trending earthquake fault zones (Fig. 1), this leads to the current situation in which seismic landslides and debris flows are distributed along the fault zone. Therefore, the Wenchuan earthquake was a major factor triggering coseismic landslides, and regional heavy rainfall is another major factor triggering postseismic landslides and debris flows. These interactions represent positive and negative feedback mechanisms among the processes of earthquake tectonic uplift, heavy rainfall, and surface erosion (Fig. 9). 6.2. The volume of postseismic debris-flows in Hongchun Gully A huge quantity of debris-flows sediments in Hongchun Gully, caused by heavy rainfall on August 13, 2010, blocked the Minjiang River. The debris flows formed a fan at the entrance of Hongchun Gully (Figs. 6 and 7) with volume of 70.5 104 m3, length of 900 m, area of 0.06 km2, and thickness of 10e20 m (Gan et al., 2012).
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Y. Li et al. / Quaternary International xxx (2014) 1e11
Compared with the volume of coseismic landslides (Table 1), the conversion ratio between coseismic landslides and postseismic debris-flows sediments in Hongchun Gully was 19.67%, which is close to 20%. The conversion rate of seismic landslides into debris flow was quite fast, more than 10 times the value before the Wenchuan earthquake. 6.3. Blockage by postseismic debris-flows
Fig. 7. Orographic rain in the Longmen Shan. (F1: Maowen fault; F2: Yingxiu-Beichuan fault; F3: Pengguan fault).
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The postseismic debris-flow fan blocked three-quarters of the main Minjiang river (only 80e100 m wide) (Figs. 7 and 8). The postseismic debris-flow fan has a fan shape, with a wide front tapering to its end, thus forming a long and narrow tail. Vertically, it shows a wedge shape, with a high, steep, and thick head which lowers and thins towards the bottom. In cross section, the head is a curved bulge, the middle part is flat, and the tail is a sunken arc. The main components of the debris flows were stone fragments mixed with sand and gravel. Stone fragments accounted for about 60% of the debris flows, with grain sizes of 20e50 cm, followed by boulders (approximately 15%), boulder gravel (approximately 10%), angular pebbles (approximately 10%), and sand (about 5%). The cobbles in the sediments were generally less than 1.0 m in diameter. The debris flowed as a kind of non-Newtonian fluid in tributaries and came together with the Newtonian fluid in the main river, changing the sand-water composition of the main river and the local boundary conditions in a short time. The confluence had a significant impact on the sediment characteristics, kinetic characteristics, and evolution of the main river. The impact was felt mainly in the following three ways: (1) the confluence area is a zone of strong turbulence, where the movement of water is complex. Water mixing, swirl, side-wall resistance, internal collision, and separation of silt, as well as the shear effects of two fluids, caused energy loss in the confluence area, leading to high water levels near the confluence point and silt deposition in the confluence area. (2) The channel was narrowed. Debris flows were directly discharged into the Minjiang River and occupied and compressed the channel on one side. The debris flow led to river blocking and intensified bending and resulted in flooding near the other side of the main river, which affected the new district of Yingxiu town. (3) The riverbed was elevated. Debris flows blocked the channel, resulting in diversion of the Minjiang River by about 300 m and causing river blockage. The debris flows elevated the upstream section of the riverbed, with a cumulative accretion of approximately 8e10 m. In addition, high-strength sand was transported downstream, and therefore the sand content of the river increased substantially. The debris flows elevated the downstream riverbed, with a cumulative accretion of approximately 3.5 m. (4) The gradient of the channel increased significantly. The debris flow caused changes in the river on the vertical section, forming rapids and a pit-type cascade riverbed. 7. Volume of sediments removed from the range by fluvial processes The debris flows sediments of the Hongchun Gully discharged into the Minjiang River formed a fan-shaped accumulation body with volume of 48.5 104 m3, length of 600 m, width of 320 m, and an average thickness of 20 m. Compared to the volume of coseismic landslides (Table 1), the conversion ratio was approximately 13%. Compared to the volume of postseismic debris-flows, the conversion ratio was 69.64%, or close to 70%. The following two conclusions can be reached: (1) after the Wenchuan earthquake, the conversion rate from landslides and debris-flows sediments river sediments was 35 times that before the earthquake, and (2) after
Please cite this article in press as: Li, Y., et al., Tectonic uplift and landslides triggered by the Wenchuan earthquake and constraints on orogenic growth: A case study from Hongchun Gully, Longmen Mountains, Sichuan, China, Quaternary International (2014), http://dx.doi.org/10.1016/ j.quaint.2014.05.005
Y. Li et al. / Quaternary International xxx (2014) 1e11
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Fig. 8. Postseismic debris flows fan driven by heavy rainfall in Hongchun Gully on August 13, 2010 (taken 14 August, 2010).
the earthquake, the occurrence frequency of heavy rainfall above the critical rainfall intensity could be one to three years. According to the volume of sediments (49.1 104 m3/y) removed from the range by fluvial processes, most coseismic landslide sediments could be converted into debris flows and river sediments in the next 7e21 years. It can therefore be hypothesized that in the next
10e20 years, the majority of coseismic landslide and debris-flow sediments will be transported away by the river. Dadson et al. (2004) noted that seismic landslides led to the increase of river sedimentation flux. Within a very short time, the postseismic debris-flows in Hongchun Gully discharged a large amount of solid materials into the Minjiang River, increasing the
Fig. 9. Debris flows on August 13, 2012 Hongchun Gully (aerial photograph from the Sichuan Geological Environment Monitoring Station).
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Y. Li et al. / Quaternary International xxx (2014) 1e11
sand content of the river substantially. In 2010, the sediment input into the Minjiang River from Hongchun Gully debris flows was 49.1 104 m3, equivalent to 37 104 t. In comparison with the average annual sediment discharge of the Minjiang River (764 104 t, Li et al., 2006), the contribution to the increment of sediment discharge in the Minjiang River was 5% of the sediment yield in August 2010. More than 10 debris-flows were triggered by heavy rainfall in August 2010 (Fig. 8), among which the largest was the Longchi debris-flow, when sediments discharged into the Minjiang River reached 400 104 m3. Therefore, it is estimated that in August 2010, the total sediment discharge into the Minjiang River due to postseismic debris-flows was about 500 104 m3, equivalent to 384.62 104 t. The contribution to the increment of sediment discharge in the Minjiang Basin was 50% and caused an increase of 50% in river sedimentation flux. 8. Discussion and conclusions In an active orogenic belt, coseismic uplift caused by periodic earthquakes is an important driving factor for the mountainbuilding. At the same time, coseismic and postseismic landslides and debris flows also reduce the height of mountains (Hovius et al., 2000; Parker et al., 2011). When the volume of the coseismic tectonic uplift is greater than the volume denuded by coseismic landslides and postseismic debris flows, the result is an increase in the average elevation; otherwise, there will be a decrease in the average elevation. Because the volume of coseismic landslides (~5e15 km3) was greater than the volume of coseismic tectonic uplifts (2.6 ± 1.2 km3, de Michele et al., 2010), our group (Parker et al., 2011) stated that the Wenchuan earthquake will lead to a net material deficit and reduce the height of the Longmen Mountains. However, there are many uncertainties to be taken into account to estimate the relationship between the total landslide volume and the net growth of the Longmen Shan in the Wenchuan earthquake. The first is to estimate the volume of coseismic and postseismic landslides and debris flows. The second is to estimate the volume of tectonic uplift. The third is to estimate river unloading time for seismic landslide sediments and the length of the earthquake recurrence cycle (Quimet et al., 2007). Therefore, quantitative analyses of the river unloading time for seismic landslide sediments are meaningful to an understanding of the relationship between earthquakes and geomorphic evolution in orogenic belts. The calculation of the river unloading time for seismic landslide sediments in Hongchun Gully in this paper will be conducive to a new understanding of the relationship between Longmen Mountain earthquakes, landslide erosion, and geomorphic evolution. Using quantitative data for the volume of coseismic tectonic uplift, and the volume of coseismic and postseismic landslides and debris flows in Hongchun Gully, the quantitative relationship between them has been determined. The coseismic landslide volume (380.01 104 m3) was less than the coseismic tectonic uplift volume (667 104 m3) in Hongchun Gully. Only 57% of the volume of the tectonic uplift has been converted to the volume of landslides in Hongchun Gully, indicating new uplift and geomorphic growth of Longmen. The volume of debris flows was 70.5 104 m3, indicating that 20% of the volume of coseismic landslides was converted into debris flows by heavy rainfall. The volume of materials discharged by Minjiang River (the main river) is 48.5 104 m3, and only 13% of coseismic landslide sediments has been removed from the range by fluvial processes. At least 86% of landslide sediments remain in the Hongchun Gully. Coseismic and postseismic landslides cannot be directly and quickly transported out of the Longmen Shan region in a very short period of time. Only in the next 10e30 years, the coseismic landslide and debris-flow sediments will be discharged
by fluvial processes. The current landslides and debris flows cannot cause a material deficit on Longmen Shan region. The Wenchuan Earthquake caused new uplift and geomorphic growth of the Longmen mountains. The volume of landslides did not exceed the incremental rock volume due to coseismic uplift, and cannot lead to a net material deficit in the Longmen Shan region. This conclusion is not consistent with our previous result that mass wasting triggered by the 2008 Wenchuan earthquake exceeds orogenic growth (Parker et al., 2011), is consistent with studies of the Taiwan Chi-Chi (Mw 7.6) earthquake (Hovius et al., 2000). Acknowledgements This study was a project funded by the National Natural Science Foundation of China (grant nos. 40841010, 40972083, 41172162, 41372114, 41340005), a geological survey project funded by the Ministry of Land and Resources (grant no. 1212011121268), and an autonomous research project of the National Key Laboratory of Oil and Gas Reservoir Geology and Exploitation (grant no. SK -0801). References Burchfiel, B.C., Royden, L.H., van der Hilst, R.D., Hager, B.H., 2008. A geological and geophysical context for the Wenchuan earthquake of 12 May 2008, Sichuan, People's Republic of China. GSA Today 18 (7), 4e11. Clark, M.K., House, M.A., Royden, L.H., Whipple, K.X., Burchfiel, B.C., Zhang, X., Tang, W., 2005. Late Cenozoic uplift of southeastern Tibet. Geology 33, 525e528. Dadson, S.J., Hovius, N., Hongey, Chen, Dade, W.B., Lin, J.C., Hsu, M.L., Lin, C.W., Horng, M.J., Chen, T.C., Milliman, J., Stark, P.C., 2004. Earthquake-triggered increase in sediment delivery from an active mountain belt. Geology 32, 733e736. Densmore, A.L., Li, Y., Ellis, M., Zhou, R.J., 2005. Active tectonics and erosional unloading of eastern margin. Journal of Mountain Science 2 (2), 146e154. Densmore, A.L., Ellis, M.A., Li, Y., Zhou, R.J., Hancock, S.G., Richardson, N.J., 2007. Active tectonics of the Beichuan and Pengguan faults at the eastern margin of the Tibetan Plateau. Tectonics 80 (8), 113e127. de Michele, M., Raucoules, D., de Sigoyer, J., Pubellier, M., Chamot-Rooke, N., 2010. Three-dimensional surface displacement of the 2008 May 12 Sichuan earthquake (China) derived from Synthetic Aperture Radar: evidence for rupture on a blind thrust. Geophysical Journal International 183, 1097e1103. Fu, B.H., Shi, P.L., Zhang, Z.W., 2008. Spatial characteristics of the surface rupture produced by the Ms8.0 Wenchuan Earthquake-using high resolution remote sensing imagery. Acta Geologica Sinica 82 (12), 1679e1687 (in Chinese). Fu, B.H., Shi, P.L., Guo, H.D., Satoshi, O., Yoshiki, N., Sarah, W., 2011. Surface deformation related to the 2008 Wenchuan earthquake, and mountain building of the Longmen Shan, eastern Tibetan Plateau. Journal of Asian Earth Sciences 40, 805e824. Gan, J.J., Sun, H.Y., Huang, R.Q., Tang, Y., Fan, C.R., Li, Q.Y., Xu, X.N., 2012. Study on mechanism of formation and river blocking of Hongchuangou giant debris flow at Yingxiu of Wenchuan County. Journal of Catastrophology 27 (1), 5e11 (in Chinese). , J., Tibari, B., de Sigoyer, J., Pubellier, M., Zhu, J., 2009. Late Godard, V., Pik, R., Lave Cenozoic evolution of the central Longmen Shan, eastern Tibet: Insight from(UTh)/He thermochronometry. Tectonics 28. http://dx.doi.org/10.1029/ 2008TC002407. TC5009. Hubbard, J., Shaw, J., 2009. Uplift of the Longmen Shan and Tibetan plateau, and the 2008Wenchuan (M¼7.9) earthquake. Nature 458, 194e197. Hovius, N., Stark, C.P., Chu, H.T., Lin, J.C., 2000. Supply and removal of sediment in a landslide-dominated mountain belt: Central Range, Taiwan. Journal of Geology 108 (1), 73e89. Kirby, E., Whipple, K., Harkins, N., 2008. Topography reveals seismic hazard. Nature Geoscience 1 (8), 485e487. Li, Y., Zhou, R.J., Densmore, A.L., Ellis, M., Li, Y.Z., Allen, P.A., Steffen, D., Li, B., Zhang, Y., Richardson, N.J., Wang, F.L., He, Y.L., Xi, X.H., Li, X.G., Si, G.Y., Wang, M., 2006. The Geology of the Eastern Margin of the Qinghai-Tiben Plateau. Geological Publishing House, Beijing, pp. 1e168. Li, Y., Zhou, R.J., Densmore, A.L., Cao, S.Y., Liu, Y.P., 2011. Spatial relationship between surface ruptures in the Ms 8.0 Wenchuan earthquake ,the Longmen Shan region,Sichuan, China. Journal of Earthquake and Tsunami 5 (4), 329e342. Li, Y., Huang, R.Q., Yan, L., Lei, J.C., Zhang, Y., He, Y.L., Chen, L.S., Li, X.G., Wang, S.Y., Ye, Y.Q., Liu, Y.F., Kang, C.C., Ge, T.Y., He, Q., Huang, Wei, 2010. Surface ruprure and huazard of Wenchuan Ms 8.0 earthquake, Sichuan, China. International Journal of Geosciences 1 (1), 21e31. Lin, A.M., Ren, Z.K., Jia, D., Yosuke, M., 2010. Evidence for a Tang-Song Dynasty great earthquake along the Longmen Shan thrust belt prior to the 2008 Mw 7.9 Wenchuan earthquake. Journal of Seismology 14, 615e628. Molnar, P., 2012. Isostasy can't be ignored. Nature Geoscience 5, 83.
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Please cite this article in press as: Li, Y., et al., Tectonic uplift and landslides triggered by the Wenchuan earthquake and constraints on orogenic growth: A case study from Hongchun Gully, Longmen Mountains, Sichuan, China, Quaternary International (2014), http://dx.doi.org/10.1016/ j.quaint.2014.05.005