A STATISTICAL ANALYSIS FOR CHARACTERIZING LANDSLIDE CAUSED BY HEAVY RAINFALL AND SEVERE EARTHQUAKE

Kuan-Tsung Chang J*, lin-King Liu 2, Wei-Chen Hsu3, Tian-Yuan Shih 4

'*Corresponding Author, Assistant Professor, Department of Civil Engineering and Environmental Informatics, Minghsin University of Science and Technology; Tel:+886-3-5593 142 ext. 3282, E-mail:

ktchang1 2 1 6@gmail.com 2Secretary General, Taiwan Group on Earth Observations and CEO, LIDAR Technology Co., Ltd.

jkliu@lidar.com.tw 3 Ph. D Candidate, Department of Civil Engineering, National Chiao Tung University and General Manager,

LIDAR Technology Co. , Ltd. ianhsu@lidar.com.tw 4 Professor, Department of Civil Engineering, National Chiao Tung University; tyshih@nctu.edu.tw

ABSTRACT

Heavy rainfall and earthquakes are the two major factors inducing landslides in Taiwan. The distribution of area size is the most basic quantitative parameter of landslides . Therefore, the purpose of this study is to characterize the scale and spatial difference of rainfall-induced as compared with those of earthquake-induced landslides. Two representative landslide cases, Toraji typhoon occurred in 2001 and 921 earthquake occurred in 1 999, are used to analyze the causes of different kinds of landslide disasters in the paper. The test area for the 921 earthquake case is 3700 km2 , the recognized number of landslide in the event is 7279, and total area for the landslides is 1 4766 Ha. Moreover, the maximum area of one landslide is 532 Ha, the average area for the landslides is 2 Ha, and its corresponding standard deviation is 1 3 Ha. In the test case of Toraji typhoon, the total study area is 8847 krn2, the number of landslides is 1 0359 , and total area for the landslide is 22305 Ha. The maximum area of a landslide is 232 Ha, the average area for the landslides is 2.2 Ha, and its corresponding standard deviation is 6.2 Ha.

Index Terms-Geological Hazard, Remote Sensing, Typhoon, Earthquakes

1. INTRODUCTION

The World Bank (2005) released a report entitled "Natural Disaster Hotspots: A Global Risk Analysis," stating that approximately 73% of the Taiwan land area and population is exposed to 3 or more risks of natural disasters [ 1 ] . According t o the statistics o f the National Fire Agency (NFA), 270 natural disaster events have occurred in Taiwan from 1958 to 2007. These include typhoons (7 1 . 1 %) , flooding ( 15%) , earthquakes (8 .5%), torrential rainfall (2.2%), wind storms ( 1 .5%), mountain flooding (0.7%), and landslides (0.7%) [2] . Taiwan is located in the northwest of

the Pacific Ocean, on the major tracks of typhoons . On average, approximately 5 typhoons are likely to affect Taiwan per year. In addition, Taiwan is located on the convergent boundary between the Eurasian and the Philippine Sea tectonic plates . Due to the active collision between these two plates , earthquakes take place every day. Landslides triggered by earthquakes become very common. The frequency of natural disasters is on an increasing trend. There are two ultimate purposes in the landslide hazard study including prevention and mitigation. Before the disaster occurring, the location and scale of disasters are needed to be estimated. Preparation should be sound and as early as possible to prevent the disaster. After landslides take places, the status of disaster has to be assessed and subsequent secondary hazards should be evaluated. To minimize the impact of disasters, the effective mitigation procedures should be conducted. Different approaches of remotely sensing data exist, including aerial photography, optical satellites, synthetic aperture radar imagery, and topographic data acquisition. They can aLI be used for landslide inventory [2-4] . The distribution of area size is the most basic quantitative parameter of landslides. The total area of the landslide is an important indicator of the damage caused by the disaster event.

Therefore, the purpose of this study is to characterize the scale and spatial difference of rainfall-induced as compared with those of earthquake-induced landslide. Two representative landslide cases, Toraji typhoon occurred in 2001 and 921 earthquake occurred in 1 999, are used to analyze the causes of different kinds of landslide disaster in the paper.

2. METHODOLOGY

Landslides are a natural phenomenon for the dynamic balance of the earth's surface. The potential or intrinsic factors of landslides include geological, hydrological and morphological factors. The external or triggering factors

978- 1 - 4799- 1 1 1 4- 1 / 1 3 /$3 1 .00 ©20 1 3 IEEE 3678 IGARSS 201 3

include earthquakes, climate, hydrology, and human activities. In Taiwan, the geology is highly fractured and landforms are in high relief. In addition, frequent earthquakes combined with heavy rainfall impose further stress to the earth's surface, with mass movements such as landslides , slumping, and mudflows occurring frequently. Three types of landslide survey methods exist: ground, aerial, and space-borne, or a combination [6] . Ground survey can be highly accurate, but is slow. When hazards occur, accessibility is low. Therefore, it is impossible to make the survey in near real-time or in a large coverage area after the disaster event. The photographic or image interpretation approach can be adopted and implemented manually, automatically, or semi-automatically. Manual interpretation requires that well-trained geologists delineate landslides under a stereoscopic environment, which is time­consuming and labor-intensive [6] . Satellite imagery can also be used to extract information of geological features , geomorphology, land use , hydrology, and so on. However, most landslide detection is based mainly on spectral features of remotely sensed images other than topographic features . Because the spectral features of buildings and roads are similar to those of landslides, serious misjudgments can occur [6] . If topographic information can be employed, misjudgements can be substantially reduced. The procedures adopted in this study are as follows :

First, the original digital image containing narrow distribution of spectral reflectance values (Digital value) can be enhanced to the range of 0 and 255 in this work. Then a Tasselled Cap transformation is used to capture the bare areas with a higher brightness in the color-composited image consisted of three bands of SPOT image. The formula used as follow:

BI index=O.60539*XS 1 +O.61 922*XS2+0.50008*XS3 ( 1 )

This process can lead landslide o r non-vegetative cover areas turn into the khaki color. The transfonned image so called landslide-enhanced image. It helps experts interpreting landslide in the GIS environment. Through the use of criteria for visual interpretation, e.g. tone, location, shape, direction, and shadow criteria, artificial intelligence of expert systems and automatic procedures can be developed to improve the efficiency and accuracy of landslide mapping [5, 6] .

Subsequently, statistics of landslides are deduced for both major triggering factors of landslides in Tiawan, namely earthquake and rainfall.

3. RESULTS AND DISCUSSION

The satellite images used for 921 earthquake and Toraji typhoon event were taken on September 27 , 1 999 and on October 22, 200 1 , respectively. The spatial distribution of landslide area for the two events is shown as Fig. 1 . The coverage extent of study area for the 921 earthquake is shown as the polygon in blue color in this figure. Other one

in red color is the coverage extent of Toraji typhoon. Moreover, total area for each cases is 3700 km2 and 8847km2, respectively. For the statistical analysis, the size of landslides (A) is divided into five categories : A;;; 1 ha, 1 ha < A;;; 10 ha, 1 0 ha < A;;; 50 ha, 50 ha < A;;; 100 ha, and A> 100 ha. In the earthquake-induced landslide case, the statistic of landslide size is summarized as Table 1 . The vast majority of earthquake-induced landslides is small landslides ( ;;; 1 ha) , which accounts for 95%. The statistics of landslide slopes are shown in Table 2. The results indicate that most of the landslides occurred in the slope range of 30 -45 (-30.9%), the second slope range is 45 �60 (-22.7%), and the third slope range is 1 5 � 30 (-19 .0%). From the landslide area statistic in the table 2, the majority of landslides are in the slope range of 1 5 �30 (- 42. 1 %), the second is 30 -45 (-28 .7%). The results show that the most of small area landslide occurred in steep slopes. However, most of landslides occurred in the gentler slopes.

In the other hand, the statistic of landslide size for the Toraji typhoon case is summarized as Table 3. The results are similar to those by earthquake-induced landslides. The vast majority of rainfall-induced landslides is small landslides (;;; 1 ha) which accounts for 59.75%. However, the landslide size is mostly belonging to class 2 (1 ha < A;;; 10 ha) (-46.30%). In the statistics of landslide slope of rainfall-induced landslides, the statistics of each slope class are shown as Table 4 . The slope class of most of landslides occurred in the range of 30 �45 , the percentage for the landslide number and landslide area are 60.68% and 62.72%, respectively. Moreover, the second majority class is 15 -30 .

A comparison of the results in Table 1 and Table 3 indicates that the average size of landslides caused by the 921 Earthquake is less than those caused by the Toraji typhoon. The landslides with an area size less than 10 hectares for two cases are 99% and 96%, respectively. However, the average slope of landslide caused by the earthquake is greater than those in the typhoon case if the results of Table 2 and Table 4 are compared. The fraction of landslides with a slope larger than 45 degree approach to 33% in the earthquake case, whereas the fraction of landslides with a slope larger than 30 degree caused by the typhoon is only 1 2%.

To analyze the location of landslides triggered by the 921 earthquake, it is found that they are mostly occurred at the top of hill (46.8%) and others at abdominal parts of the slopes (44.8%) (shown as Table 4). Moreover, landslide occurred at the top of hill in terms of area is 62.6%. It shows that landslides trigged by earthquake occurred mainly in top of the hill, then followed by abdominal parts. However, rainfall-induced landslides mainly occurred at the abdominal positIOn. As mentioning in statistical analysis for the landslide aspect, mainly favor direction of the West and

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Northwest in area statistics accounted for 62.8% shown as Fig. 2(a). It seems to be related to geological formations or the seismic peak ground acceleration (PGA) (shown as Fig. 2(b)). Further study is required for more understanding. Finally, the comparison of the size and slope of both rainfall-induced and earthquake-triggered landslide is summarized in Table 5.

Table 1. The statistics of landslide size for the 921 Earthquake

Area(ha) Number of Percentage Landslide Percentage

landslides Area(ha)

0�1 20,819 94.77 2,550.87 38.75

1�1O 1,085 4.94 2,739.61 4l.61

1O�50 61 0.28 1,067.90 16.22

50�100 4 0.02 224.9 3.42

>100 0 0.00 0 0.00

Total 21,969 100.00 6,583.28 100.00

Table 2. The statistics of landslide slope categories for the 921 Earthquake

Slope(O) Number of Percentage Landslide Percentage

accounts Area(ha)

< 5 2,146 9.77 56.14 0.50

5�15 1,606 7.31 2,03l.33 17.98

15�30 4,175 19.00 4,759.41 42.13

30�45 6,785 30.88 3,242.78 28.70

45�60 4,983 22.68 1,060.85 9.39

60�75 2,035 9.26 130.82 l.l6

>75 239 l.09 16.11 0.14

Total 21,969 100.00 11,297.4 100.00

Table 3. The statistics of landslide size for the Toraji typhoon

Area(ha) Number of Percentage Landslide Percentage

accounts Area(ha)

0-1 6 , 1 9 1 59.75 2,629 .495 1 1 .63 1-10 3 ,783 36.5 1 10 ,47 1 .27 46.30

10-50 355 3 .43 6,737 .250 29.79

50-100 25 0.24 1 ,697 .467 7.50

>100 7 0.07 1 ,082 .945 4.79

Total 10 ,361 100.00 22,6 1 8 .42 100.00

Table 4. The statistics of landslide slope categories for the Toraji typhoon

Slope(O) Number of Percentage Landslide Percentage

accounts Area(ha)

<5 40 0.39 39 .098 0 . 17 5-5 296 2 .86 359.358 1 .59

1 5-30 2 ,441 23.56 4 , 1 8 3 . 1 6 1 8 .49

30-45 6,287 60 .68 14 , 1 86.62 62.72

45-60 1 ,279 1 2 .34 3 ,832 .95 1 6 .95

60-75 1 8 0 . 17 17 .235 0.08

>75 0 0.00 0.00 0.00

Total 10361 100.00 226 1 8 .425 100.00

Table 5. Comparison of sizes and slopes of landslides for both rainfall-induced and earthquake-triggered landslides

Factor

Location

Slope

Slope shape

Rainfal l - i nd uced Ea r thq u a ke- t r iggered

abdominal parts top of hill

30 �40 35 �55

Concave slope or valley Convex slope or ridge terrain

terrain

D The extent of landslides for the 921 Earthquake

The extent of landslides for the Toraji typhoon

Figure 1. The spatial distribution of landslides for the 921 Earthquake and Toraji typhoon

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<5 400J

JOOO 5-15

(a) A radar chart for the landslide aspect (b) Horizontal PGA contour map for the 921 Earthquake

Figure 2. The statistical chart of landslide aspect and a PGA contour map for the 921 Earthquake

4. CONCLUSIONS

(1) Using SPOT satellite images and expert interpretation method, two representative landslide cases are performed to analyze the causes of different kinds of landslide disaster in the paper. The results of the landslide mapping indicate that the average area size of the landslides only around two hectares, and a half amount of landslides has an area less than ten hectares.

(2) The area sizes and slope gradients of landslides are different for rainfall-induced and earthquake-induced factors. In general, the average size of earthquake­induced landslides are smaller than that of rainfall­induced landslides. The general slope gradients of earthquake-induced landslide are larger than those of rainfall-induced landsl ide.

(3) Further study can be conducted in different geological environments and geomorphological regions. And, thus we can gain further understanding in the size and frequency of landslides in various geological and geomorphological conditions.

5. REFERENCES

[1] M. Dilley, R. S. Chen, U. Deichmann, A. L. Lerner-Lam, and M. Arnold, "Natural Disaster Hotspots: A Global

Risk Analysis . Disater Risk Management," Series No.5, The World Bank, 145 pages, 2005. ISSN: 0821359304. http://www.preventionweb. net/files/1100_Hotspots.pdf.

[2] NFA, "Historical records of natural disasters of Taiwan from 1958 to 2007," National Fire Agency, Ministry of the Interior, Access date: 31 December

[3] J. K. Liu, K. T. Chang, J. Y. Rau, W. C. Hsu, Z. Y. Liao, C. C. Lau, and T. Y. Shih, "The Geomorphometry of Rainfall-Induced Landslides in Taiwan Obtained by Airborne Lidar and Digital Photography," Geoscience and Remote Sensing, In-Teh, Inc., 2009.

[4] K. T. Chang, Q. X. Kao, Z. Y. Wang, and J. K. Liu, "Automatic Rainfall-induced Landslide Interpretation and Features Analysis," Special issue for Disaster Prevention, Journal of Photogrammetry and Remote Sensing, 15(1): 79-95,2010. (in Chinese)

[5] J. K. Liu, C. M. Tseng, C. W. Lin, C. Y. Chen, and H. Y. Yin, "The Applications of Airborne Lidar in Disaster Assessment in Wide-Area Slope lands of Taiwan," Sino­Geotechnics , 129: 35-44, 2011.

[6] K. T. Chang, J. K. Liu, and C. I. Wang, "An Object­oriented Analysis for Characterizing the Rainfall­induced Shallow Landslide," Journal of Marine Science and Technology, 20(6): 647-656, 2012. DOl: 1O.6119/JMST-012-0430-2.

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