Chin. Sci. Bull. (2014) 59(13):1416–1424 DOI 10.1007/s11434-014-0202-0
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Article
Geology
The spatial distribution pattern of landslides triggered by the 20 April 2013 Lushan earthquake of China and its implication to identification of the seismogenic fault Chong Xu • Xiwei Xu
Received: 27 June 2013 / Accepted: 10 October 2013 / Published online: 22 February 2014 Ó Science China Press and Springer-Verlag Berlin Heidelberg 2014
Abstract After the 20 April 2013 Lushan MS 6.6 earthquake occurred, investigation and identification of the seismogenic fault for this event have become a focused and debatable issue. This work prepared an initial landslide inventory map related to the Lushan earthquake based on field investigations and visual interpretation of high-resolution aerial photographs and provided evidence for solving the issue aforementioned. The analysis of three landslide-density profiles perpendicular to strike direction of the probable seismogenic fault shows that many landslides occurred on the footwall of the Shuangshi–Dachuan fault (SDF), without sudden change of landslide density near the fault. Very few landslides were detected near the Dayi fault (DF) and also no change of landslide density there. While obvious sudden change of landslide density appeared about 1–2 km from the northwest to the western Shangli fault (WSF), and the landslide density on the hanging wall of the fault is obviously higher than that of on the footwall. Therefore, we infer that the seismogenic fault for the Lushan earthquake is neither the SDF nor the DF, rather probably the WSF located between these two faults, which is an evident linear trace on the earth surface. Meanwhile, the coseismic slip did not propagate upward to the ground, implying the Lushan earthquake was spawned by a blind-thrust-fault beneath the WSF. Keywords Lushan earthquake Landslides Spatial distribution pattern Blind-thrust-fault Implication Aerial photographs
C. Xu (&) X. Xu Key Laboratory of Active Tectonics and Volcano, Institute of Geology, China Earthquake Administration, Beijing 100029, China e-mail:
[email protected];
[email protected]
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1 Introduction Coseismic surface ruptures and landslides are two important expressions of geomorphic changes caused by large earthquakes, both of which are closely related. Earthquaketriggered landslides are usually concentrated along coseismic surface rupture zones and show obvious differences with varied geometric characteristics and movement behaviors in fault segments. Therefore, the spatial distribution pattern of earthquake-triggered landslides can be used to help reveal the sources of large earthquakes [1], especially when the seismogenic fault cannot be identified by visible surface ruptures. On April 20, 2013, an MW 6.6 (or MS 7.0) earthquake occurred at Lushan County, Sichuan Province of China. Its epicenter was located at 30.3°N, 103.0°E with a focal depth of 13 km from China Earthquake Network Center (CENC). Although no consensus was reached, most studies of focal mechanism inversions suggested that this earthquake was caused by a thrust slip of the Shuangshi–Dachuan fault (SDF) [2–6]. Whereas the field investigations showed that the earthquake did not produce visible coseismic surface rupture zones, though some local fissures were seen on ground. Thus, it is concluded that the Lushan earthquake was generated by a blind-reverse fault [7], rather than the SDF. This paper attempts to address the issue above from another aspect. A preliminary inventory of landslides triggered by the Lushan earthquake was constructed based on field investigations and visual interpretation of highresolution aerial photographs. Locations of a total of 3,883 landslides were determined in an area of 2,885 km2 covered by available aerial photographs of post-earthquake. Three landslide-density profiles normal to the direction of seismic slip are analyzed to describe the spatial distribution
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pattern of landslides which provides evidence for identification of the seismogenic fault for the Lushan event.
2 Spatial distribution pattern of landslides triggered by the Lushan earthquake Using visual interpretation of high-resolution aerial photographs and field investigations, an inventory map of landslides triggered by the Lushan earthquake has been prepared. In the field, we saw a large number of landslides of various types triggered by the shock, including rock falls, soil falls, and rock sliding [8]. About 2,885 km2, available postearthquake aerial photographs with 0.6 and 0.2 m resolutions were collected for landslide visual interpretation (Fig. 1). The principles for a detailed landslide interpretation based on aerial photographs are as follows: (1) All earthquake-triggered landslides should be mapped as long as they can be recognized from images. (2) Previous studies [9, 10] tend to consider a landslide to be a pre-earthquake landslide,
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if the landslides show similar shapes on pre- and postearthquake images, conversely, the landslide is considered to be a coseismic landslide. However, images of pre- and postearthquake are from different data sources, and most of the earthquake-triggered landslides are of small scales, so it is difficult to determine landslides that occurred during the earthquake or existed before the earthquake. On post-earthquake aerial photographs, coseismic landsides triggered by the earthquake generally show relatively more dim shade and color differences from surroundings than landslides occurred prior to the earthquake because of thick vegetation cover and frequent rainfall in the earthquake struck area. Therefore, we determine a landslide is triggered by the earthquake or existed before the earthquake by observing that the slide surface is fresh or not. It should be noted that although some landslides destroy vegetation, only some trees are knocked down, while rock is not exposed. So it should be carefully discerned on images. After detailed visual interpretation of post-seismic photographs, an inventory map of landslides triggered by
Fig. 1 Spatial distribution of landslides triggered by the Lushan earthquake. SDF Shuangshi–Dachuan fault; WSF western Shangli fault; DF Dayi fault; CENC, China Earthquake Network Center (the same below)
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the Lushan earthquake was prepared (Fig. 1). Locations of 3,883 landslides were mapped on a GIS platform. Some of the landslides were verified by field observations. In fact, the aerial photographs of 0.6 and 0.2 m resolutions are almost comparable with photos taken in the field. Verification results in the field demonstrate that this approach is feasible and reliable. Figure 2 shows photos and aerial photographs of two landslides triggered by the Lushan earthquake. It indicates that most of the 3,883 landslides occurred on the hanging wall of the western Shangli fault (WSF) (Fig. 1). A large number of landslides are present on either side of the SDF. And there is no obvious sudden change of landslide density near the SDF. Landslide number density (LND) map (Fig. 3) related to the earthquake was constructed based on a 500-m search radius. It indicates that the majority areas of LND values larger than 5 km-2 are distributed in the north and
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northwest of the epicenter. Around Lushan–Longmen– Baosheng is of high LND values, despite this area is located on the footwall of the SDF. It means that the seismogenic fault of the Lushan earthquake is not the SDF, instead a fault located at east of the SDF.
3 Sudden change of landslide density across a seismogenic fault As earthquake-triggered landslides are strongly controlled by the seismogenic fault, spatial distribution patterns differ much with various faults. Landslides triggered by a thrustfault earthquake occur mostly on its hanging wall, which attenuate with the increasing distance to the seismogenic fault (surface rupture) at a rate significantly lower than that on the footwall [9]. Landslides triggered by a strike slip-
Fig. 2 Comparison of photos and aerial photographs of two landslides. a Photo of the Chabanpo rock slide; b full view of aerial photograph of the Chabanpo rock slide; c photo of the Pingtou rock avalanche in Tangjiagou drainage; d full view of aerial photograph of the Pingtou rock avalanche in Tangjiagou drainage
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Fig. 3 Map showing landslide number density values related to the Lushan earthquake
fault earthquake have similar spatial distribution patterns on both hanging wall and footwall of the fault and are usually distributed close to the seismogenic fault [11–13]. Some earthquakes with complex focal rupture mechanisms, such as the January 12, 2010 Haiti MW 7.0 earthquake, can trigger landslides with complex patterns of spatial distributions, no obvious correlation with the seismogenic fault [10, 14, 15]. The landslides triggered by reverse-fault earthquakes are largely distributed on the hanging wall. For example, most of the landslides triggered by the 2008 Wenchuan earthquake occurred on the hanging wall of the seismogenic fault (the Yingxiu–Beichuan fault) [16–20]; the landslide density on the hanging wall was higher than that of the footwall; and its decay rate with the distance to the fault is slower than that on the footwall. Figure 4 shows a sudden drop of landslide density across the fault (three proxies including landslide area percentage, landslide centroid, and landslide top point number density, LAP, LCND, and LTND) of 1-km bands parallel to the fault.
The spatial distribution of landslides triggered by the 2008 Iwate–Miyagi Nairiku, Japan MW 6.9 earthquake indicates at least 4,100 landslides occurred on the hanging wall, while almost no landsides on the footwall [21]. Sato et al. [22] reported that at least 2,424 landslides were triggered by the October 8, 2005 Kashmir earthquake, most of which occurred on the hanging wall; about 47 % of the total appeared 2 km to the coseismic surface faultrupture; and a sudden change of landslide density was presented across the surface rupture. Most of the landslides triggered by the October 23, 2004 MW 6.6, a blindreverse-fault generating event, in Chuetsu, Niigata Prefecture, Japan occurred in steep terrain areas on the hanging wall [23]. During the September 21, 1999 Chichi, Taiwan MW 7.6 earthquake, most of the landslides were also distributed on the hanging wall 20–40 km to the seismogenic fault, only a few landsides on the footwall [24]. Other examples include the 2002 M 7.9 Alaska earthquake [25] and the 1994 MW Northridge California earthquake [26, 27].
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Fig. 4 Statistics of landslide density across the seismogenic fault related to the 2008 Wenchuan earthquake. YBF Yingxiu–Beichuan fault
4 Statistics of landslides triggered by the Lushan earthquake normal to the fault Similar to the May 12, 2008 Wenchuan earthquake [28– 30], the Lushan earthquake is considered to be resulted from an abrupt slip on a low-angle thrust fault of the southern segment of the Longmenshan fault zone between the South China block and the Bayan Har block, when the Bayan Har block was moving southeastward and obstructed by the South China block [7]. This event might have filled a rupture gap of the southern segment of the Longmengshan fault zone [31]. Focal mechanisms of the Lushan earthquake indicated that the seismogenic fault is a NE trending sub-fault of the Longmenshan thrust belt. There are three candidates for the seismogenic fault: the SDF, WSF, and Dayi fault (DF). Results of field investigations indicate that some compression phenomena of northwest-southeast direction are present in Shuangshi, Taiping, and Longmen and other places; no obvious continuous coseismic surface ruptures along these three faults were found [7]. To make statistics of earthquake-triggered landslide density between these faults, 1-km bands of 41° trending from the epicenter were classified. Then, LND values of each band were calculated. Considering the limited coverage of the available aerial photographs, two approximate rectangle sub-areas I and II were also selected for statistics in addition to the overall region under statistics (Fig. 5). Because the areas of the eastern Sichuan Basin covered by aerial photographs only registered few landslides in small-scale soil falls and slides due to lower topographic relief, so these area were excluded from the subsequently statistics. As shown in Fig. 5, five landslides were excluded. The statistical result is shown in Fig. 6. It indicates that there is no sudden change of earthquake-triggered landslide density presented across the SDF, which means
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this fault is not the seismogenic fault of the Lushan earthquake. On contrast, an obvious sudden change of landslide density is present about 1–2 km from the northwest to the western Shanli fault, where LND values are larger than 3 km-2 in the northwest, and smaller than 1 km-2 in the southeast. It implies that the WSF was likely active during the earthquake. There were only a few landslides on the hanging wall and footwall of the DF, which means the area around the DF was located on the footwall of the real seismogenic fault, and the DF was not active when the Lushan earthquake happened. In order to eliminate probable limitations of incomplete coverage of aerial photographs or other regional factors (such as geology and topography), two areas I and II (Fig. 5) of approximately rectangle shape covered by continuous post-earthquake aerial photographs were also selected for statistics of landslides number density. Two more profiles of LND of 1-km bands perpendicular to the probable NE trending seismogenic fault were produced as shown in Figs. 7 and 8, respectively. The results show 1,869 landslides in the area I and 1,038 landslides in the area II, respectively, which exhibit similar patterns of landslide spatial distribution like that in Fig. 6: there registered many landsides on the footwall of the SDF and no sudden change of LND values across the fault; a sudden change of LND values appears at 1–2 km northwest of the WSF. In addition, although there are several sudden changes of LND at 2, 11, and 15 km northwest of the SDF as shown in Figs. 6 and 7, they take place in a narrow band in which landslides are highly uneven in distributed (Fig. 5). Therefore, it is inferred that these sudden changes of LND values were due to local topographic and geologic conditions rather than the result from the seismogenic fault or the SDF. Landslide abundance can be delegated as LND or landslides area density. Although landslide abundance statistics in this paper only based on LND, some previous
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Fig. 5 1-km bands for statistics of landslide number density
Fig. 6 Profile of landslide number density (LND) of 1-km bands perpendicular to the probable NE trending seismogenic fault in the area covered by available aerial photographs. LND landslide number density
publications [15, 16] showed similar curves of both LND and landslide area density with distance to the seismogenic fault. Therefore, statistical results of landslide abundancebased LND are credible in this paper.
5 Results The latest study suggests that the Lushan earthquake was generated by a blind-thrust fault [7]. From the features of the
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Fig. 7 Profile of landslide number density (LND) of 1-km bands perpendicular to the probable NE trending seismogenic fault in the area I shown in Fig 5. LND landslide number density
Fig. 8 Profile of landslide number density (LND) of 1-km bands perpendicular to the probable northeastern trend seismogenic fault in area II shown in Fig. 5. LND landslide number density
earthquake-triggered landslides, it is inferred that the SDF is not the seismogenic structure of the Lushan earthquake based on the two facts: usually a sudden change of landslide density should be present across the seismogenic fault (or coseismic surface fault-rupture), and many landslides occur in the areas Lushan, Longmen, and Baosheng which lie on the footwall of the SDF. However, considering some compression phenomena, suspected surface fissures, and eruption of water and soil [7], the important role of the SDF in the Lushan earthquake cannot be ignored. In other words, the Lushan earthquake might have activated the SDF to a certain degree. Coupled with that the values of LND are high and some compression phenomenon in areas Lushan, Longmen, and Baosheng (Fig. 3), it can be inferred that part of the earthquake energy was absorbed by the SDF and the probable Lushan–Longmen–Baosheng blind fault. Consequently, the real seismogenic fault had no enough energy to produce coseismic surface ruptures. Compression phenomena and high landside number density in the area between the SDF and the WSF indicate that this area experienced more deformation than other areas. The energy for this deformation was from the rupturing of the WSF. Figure 9 simply
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illustrates a tectonic model which suggests that the WSF is the seismogenic fault of the 2013 Lushan earthquake. This model can explain the coseismic deformation as well as the pattern of landslide distribution by this event.
6 Conclusions This work was based on field investigations and visual interpretation of available high-resolution aerial photographs. In an area of about 2,885 km2, 3,883 landslides triggered by the 2013 Lushan earthquake were located. These landslides were mainly distributed on the hanging wall of the SDF and between the SDF and the WSF. LND profiles of 1-km bands perpendicular to the probable NE trending seismogenic fault were constructed three areas, including most of the area covered by available aerial photographs (registered 3,878 landslides), and two approximately rectangle areas named I and II covered by continuous aerial photographs (registered 1,869 and 1,038 landslides, respectively). The results indicate that there was no sudden change of landslide density near the SDF, and
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Fig. 9 Tectonic model of the Lushan earthquake. Red-bold line means the probable seismogeinc fault
many landslides occurred on the footwall of this fault. Much fewer landslides occurred near the DF. Most of the landslides triggered by the Lushan earthquake were located on the hanging wall of the WSF, and a sudden change is present at about 1–2 km northwest of the WSF. Based on correlations of landside spatial distribution patterns with three NE trending faults in the earthquake struck area, this work concludes that both the SDF and the DF are not the seismogenic of the Lushan earthquake, whereas the WSF is the probable the seismogenic fault. The coseismic rupture of this seismogenic fault did not reach the earth surface, because part of the earthquake energy was likely absorbed by the SDF and by a probable existing blind fault around the areas Lushan, Longmen, and Baosheng counties. This resulted in a sudden change of landside density values about 1–2 km northwest of the WSF rather than just on the WSF proper. This study provides a piece of supportive evidence for the inference that the seismogenic fault of the Lushan earthquake is a blind-reverse fault [7]. Acknowledgments This work was supported by the National Natural Science Foundation of China (41202235 and 91214201). We thank Institute of Remote Sensing and Digital Earth, Chinese Academy of Sciences, Sichuan Bureau of Surveying, Mapping and Geoinformation, and Institute of Optics and Electronics, Chinese Academy of Sciences for providing high-resolution aerial photographs post-earthquake. We also thank two anonymous reviewers and Profs. Peizhen Zhang and Xianglin Gao from Institute of Geology, China Earthquake Administration for their helpful comments and suggestions that improved the manuscript.
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