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A PRELIMINARY STUDY ON SPATIAL DISTRIBUTION PATTERNS OF LANDSLIDES TRIGGERED BY JIUZHAIGOU EARTHQUAKE IN SICHUAN ON AUGUST 8TH, 2017 AND THEIR SUSCEPTIBILITY ASSESSMENT
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摘要: 2017年8月8日四川省九寨沟发生7.0级地震，是继2008年汶川大地震和2013年芦山地震之后，四川省发生的又一次7.0级以上的强震。为了给现场救援和震后地质灾害防治提供科学依据，本文作者第一时间赶赴现场，并基于震前和震后高精度遥感影像，完成了震区地质灾害解译和复核工作，共解译地质灾害1883处，主要以中小型浅层滑坡和崩塌为主。基于解译结果，对同震灾害的空间分布规律和控制因素（距断层距离、地面峰值加速度PGA、高程、坡度和坡向等）进行了分析，研究表明地质灾害主要沿北西-南东向呈带状分布，且沿公路、沟谷较为发育，在野外推测发震断层2km范围内高度集中，呈现明显的断层效应，但与塔藏断层之间的空间相关性相对较弱。在上述分析的基础上，采用逻辑回归模型，利用地震、地形和地质3大因素（8个因子）对地质灾害易发性进行了快速评价和分区，经统计校验证明该模型的准确率达0.851，模型精度较高。Abstract: A magnitude 7.0 earthquake struck Jiuzhaigou on August 8th, 2017 in Sichuan province, which is another strong earthquake happened in Sichuan province after the 2008 great Wenchuan earthquake and the 2013 Lushan earthquake. In order to provide scientific evidence for on-the-spot rescue work and geologic hazard prevention after the earthquake, the author arrived quickly on the field and finished geologic hazard interpretation and review work in the earth-stricken areas based on high resolution remote sensing images before and after the earthquake. As a result, 1833 earthquake-induced geohazards have been obtained in the study area of 840km2, which are mainly medium-scale and small-scale shallow landslides and collapses. In terms of the results of the interpretation, the spatial distribution patterns and control factors in the cosesmic geohazards (distance from the fault, PGA, elevation, slope, aspect etc.) are analyzed, the research shows that the geohazards triggered by earthquake have the features of zonal distribution mainly distributed along north-west and east-south, especially along the roads and valleys. The width of strong development zone along the field inferred seismogenic fault is about 2km, showing a distinct faulting effect, but the spatial correlation betweem geohazards and Tazang fault is weak. Based on the above-described analysis, logistic regression model is taken quick comments and divisions of geologic hazard susceptibility are made. After statists and testing, the accuracy of this model has reached 0.851, a comparatively high precise data.
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图 1 九寨沟地震震中区域地震地质构造与研究区位置图
Figure 1. Geological structure and location of study area of Jiuzhaigou earthquake epicentral area
图 2 遥感影像分布图
Figure 2. Pre-earthquake and post-earthquake remote sensing imagery coverage map
图 3 基于震前和震后高精度遥感影像的同震灾害解译实例
A.大型滑坡, B.崩塌, C和D.震前已有滑坡复活, E.震后熊猫海周围地质灾害解译; abcde为震前
Figure 3. Examples of coseismic landslide interpretation based on high resolution pre-earthquake and post-earthquake images
图 4 九寨沟地震诱发地质灾害分布图
Figure 4. Inventory map of landslide triggered by the Jiuzhaigou earthquake
图 6 汶川地震、芦山地震和九寨沟地震诱发滑坡面积－概率密度图
Figure 6. The size-frequency distribution of landslides triggered by the Wenchuan earthquake, Lushan earthquake and the Jiuzhaigou earthquake
图 7 地质灾害空间分布规律研究所选因子栅格图
a.距可能发震断层距离；b.地面峰值加速度PGA；c.高程；d.坡度；e.坡向；f.地质灾害面密度图
Figure 7. Factor maps:a.distance to possible seismogenic fault, b.PGA, c.elevation, d.slope, e.aspect, f.coseismic landslide area density
图 8 同震地质灾害与距可能发震断层距离的关系图
Figure 8. Relationship between co-seismic landslide and distance from possible seismogenic fault
图 9 同震灾害面密度（a）与距现场调查发震断层关系（b）和距塔藏断层关系（c）对比图
Figure 9. The comparison chart of relationships between coseismic landslide areal density (a) with distance from field investigation of seismogenic fault (b) and distance from Tazang fault (c)
图 10 同震地质灾害与PGA和地形因素的关系图
Figure 10. Relationships between co-seismic landslide with PGA and topographic factors (elevation, slope and aspect)
图 11 同震地质灾害与地层岩性关系图
Figure 11. Relationship between co-seismic landslide and lithology
图 12 九寨沟地震诱发地质灾害易发性评价分区图
Figure 12. Susceptibility zoning map of geological landslides induced by Jiuzhaigou earthquake
图 13 九寨沟地震诱发灾害易发性评价ROC曲线
Figure 13. ROC curve of Jiuzhaigou earthquake induce landslides susceptibility assessment
表 1 遥感影像及其它地理信息数据
Table 1. Remote sensing images and other geographical information data
数据类型 获取时间 数据源 分辨率/m 20151021 SPOT5 2.5 20151207 SPOT5 2.5 影像数据 20170811 无人机航拍影像 0.2 20170809 高分2号 1 20170816 高分1号 1 地形数据（DEM) 震前 四川省测绘局 5 地质图 震前 四川省地质局 1:20万 地震数据 20170809 中国地震台网中心 — 
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表 2 地质灾害空间分布规律研究因子选取
Table 2. Selection of impact factors of geological hazards spatial distribution research
因子名称 选取依据与意义 获取方法 地震（诱发）因素 距可能发震断层距离 本次地震发震断层不明显，目前仍存在争议，因此本文对所有可能的发震断层进行分析，从地质灾害空间分布的角度为发震断层的确定提供依据（图 7a) 野外调查断层位置与国家活断层研究中心提供断层信息（矢量文件）；通过GIS的空间缓冲（buffer)功能，进行距离分析 地面峰值加速度（PGA) PGA，地面峰值加速度，是反映地震时地表震动强度的重要参数，部分强震的PGA可达到1g以上，本次地震的观测值为0. 08g~0. 26g，震中附近值较高，随着距震中距离增加，逐渐衰减（图 7b) 美国地质调查局（USGS)公布数据（矢量文件），通过GIS转换成5m×5m的栅格数据 地形因素 高程 研究区高程为149~4580m(图 7c)，以200m为间距将其划分为12级 高程信息来自测绘局震前数字高程模型(DEM)，分辨率为5m。地形因子均可通过GIS的3D Analyst功能，自动从DEM获取 坡度 研究区内坡度起伏较大，最大坡度达84. 5°，平均坡度32°(图 7d) 坡向 坡向共分为平坦、N、NE、E、SE、S、SW、W、NW 9级（图 7e) 地质因素 地层岩性 研究区内地层岩性按照地层年代岩组共分为1类（图 1 :第三系（N2)、三叠系（T1、T2、T3)、二叠系（P1、P2)、石炭系（C1、C2、C3)、泥盆系（D3) 四川省地质局1:20万地质图
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表 3 各评价因子分级与标准化值
Table 3. Classification and normalization of evaluation factors
影响因子 因子分级 Sij Zij 指标值 标准化值 坡度/(。) < 10 0.0405 0.0028 0.0700 0.0045 10~20 0.0996 0.0078 0.0781 0.0050 2 0 ~30 0.1845 0.0234 0.1266 0.0081 30 ~40 0.3753 0.1984 0.5285 0.0339 40 ~50 0.2297 0.469 1.9458 0.1247 50 ~60 0.0586 0.2611 4.4578 0.2857 60 ~7 0 0.0109 0.0566 5.2046 0.3335 > 70 0.0010 0.0031 3.1924 0.2046 坡向 平坦 0.0665 0.0557 0.8363 0.0951 北 0.1432 0.1701 1.1881 0.1351 东北 0.1471 0.2587 1.7581 0.1999 东 0.1069 0.1687 1.5772 0.1793 东南 0.1065 0.1229 1.1534 0.1312 南 0.1299 0.0630 0.4851 0.0552 西南 0.1423 0.0656 0.4606 0.0524 西 0.1059 0.0518 0.4890 0.0556 西北 0.0516 0.0437 0.8465 0.0963 高程/m < 2000 0.0028 0.0001 0.0335 0.0127 2000 ~2500 0.1185 0.1027 0.8666 0.3280 2500 ~3500 0.6141 0.8324 1.3555 0.5131 3500 ~4000 0.2212 0.0598 0.2704 0.1024 > 4000 0.0434 0.0050 0.1157 0.0438 曲率 < -0.5 0.4318 0.4426 1.0249 0.3279 -0.5~5 0.1496 0.1791 1.1972 0.3830 > 0.5 0.4186 0.3784 0.9038 0.2891 岩性 三叠系岩组 0.4145 0.1333 0.3215 0.0804 二叠系岩组 0.1708 0.1321 0.7736 0.1935 第三系土坡组 0.0025 0.0016 0.6508 0.1628 石炭系岩组 0.3726 0.7204 1.9336 0.4837 泥盆系岩组 0.0396 0.0126 0.3177 0.0795 PGA /g 0. 12 0.0630 0.0054 0.0859 0.0194 0. 16 0.2094 0.1332 0.6358 0.1436 0.20 0.3112 0.3771 1.2117 0.2736 0. 24 0.3284 0.3619 1.1018 0.2488 0.26 0.0879 0.1225 1.3936 0.3147 距断层距离/m 0~1000 0.4667 0.5390 1.1549 0.3968 1000 ~2000 0.2575 0.3275 1.2719 0.4370 > 2000 0.2758 0.1335 0.4840 0.1663 距水系距离/m 0~300 0.1697 0.1395 0.8221 0.2395 300-600 0.1500 0.2588 1.7258 0.5028 > 600 0.6803 0.6017 0.8844 0.2577
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表 4 易发性评价模型各因子回归系数与权重系数
Table 4. Logistic regression coefficient and weight coefficient
因子类别 坡度 坡向 高程 曲率 PGA 岩性 距断层距离 距水系距离 回归系数 9.356 7.984 3.569 -0.993 1.499 3.354 2.596 1.963 权重系数 0.2990 0.2552 0.1141 0.0317 0.0479 0.1072 0.0830 0.0619
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表 5 基于逻辑回归模型的研究区灾害易发性分区面积及其百分比
Table 5. Area and percentage of landslides susceptibility zoning based on the logistic regression model
易发区分类 各分区面积/km2 各分区面积占研究区面积百分比/% 75 %测试样本各分区灾害面积占总灾害面积百分比/% 25%检验样本各分区灾害面积占总灾害面积百比/% 较高易发区 19.35 4.49 42.91 44.48 高易发区 48.43 11.24 28.58 30.94 中易发区 110.27 25.59 20.81 17.59 低易发区 208.75 48.45 6.07 5.57 较低易发区 44.06 10.23 1.58 1.19
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