Distributions of landslides triggered by the Chi-chi Earthquake in Central Taiwan on September 21, 1999

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1 Landslides-Journal of the Japan Landslide Society Vol. 38, No. 4 (2002), March, pp , Original article Distributions of landslides triggered by the Chi-chi Earthquake in Central Taiwan on September 21, 1999 Wen-Neng WANG*) EHiroyuki NAKAMURA**) ESatoshi TSUCHIYA***) EChih-Ching CHEN****) Abstract An earthquake (M7. 3) took place in central Taiwan on the early morning of September 21, This disastrous ground shaking is called the Chi-chi earthquake, with its hypocenter about 10km below land surface. Earthquake-induced landslides were surveyed in order to mitigate secondary hazards. The area studied is about 375,000 ha, in which landslides were identified by aerial photo interpretation accompanied by SPOT images and field surveys. The color aerial photos used in this study are on the scale of 1/17,000 and were taken between September 22-26, More than twenty thousand Chi-chi earthquake-induced landslides have been recognized by aerial photo interpretation. The average landslide density and landslide rate are 0.1 pieces/ha and 4.7%, respectively. Over 90% of the landslides were less than one hectare in average and categorized as shallow landslides and rock falls. Most of the landslides occurred in a region between the Chelungpu active fault to the west and the Lishan fault to the east or in regions with a horizontal peak-ground acceleration (PGA) of more than 300 gal. Of the four geological regions in the area studied, the inner foothill zone had the greatest average landslide rate (11.6%) and the greatest number of landslides (9,090 landslides). The occurrence of earthquake-induced landslides has a close relation to their geology and distances from the earthquake fault. As to geologic formation and PGA, the Ptk2 (Plio-Pleistocene period) formation has the greatest landslides rate, 20% in a gal area, and the Pcl formation (Pliocene period) has a landslide rate, 9.2% in a gal area. By dividing into six 5-km-wide belts paralleling the Chelungpu fault, the landslide amount and its rate in the Pcl formation area have the greatest value in the 5-10km belt (about 6,000 amount) and the 10-15km belt (12.7%), respectively. In the Mkc formation area, the landslide amount and its rate have the greatest value in the 0-5km belt, about 1,400 amount, 4.7%, respectively, and then generally decrease with an increase of 5km. Keywords: landslides, earthquake, fault, SPOT, photo interpretation 1. Introduction Earthquakes are the detectable shaking of the earth's surface resulting from seismic waves caused by the sudden release of energy from the earth's crust. Surface effects associated with earthquakes include faults, subsidence, liquefaction, landslides, etc. These effects can cause severe and widespread damage. Landslides are well-known effects induced by the shaking of earthquakes. Detailed studies of earthquake-induced landslides can lead to understanding, predicting, and mitigating slope hazards during and after earthquakes in many areas. An earthquake took place in central Taiwan in the early morning (01:47 local time) of September 21, The epicenter was located at N, E near Chi-chi ( W W), a small town 12km west of Sun Moon Lake ( úœžàk). The magnitude was ML7. 3 as measured by the Central Weather Bureau (CWB), or Ms7.6 by the United States Geological Survey (USGS). The focal depth was about 10km. The Chi-chi earthquake killed *) Geoengineering Department of Land and Mineral Resources Division, Industrial Technology Research Institute **) Faculty of Agriculture, Tokyo University of Agriculture and Technology ***) Faculty of Agriculture, Shizuoka University ****) Geotechnical engineering Department of Soil Conservation Division, 921 Earthquake Reconstruction Comittee about 2,400 persons, injured 8,700, destroyed over 6,000 buildings, and made 100,000 homeless. This event caused a loss of 9.2 billion US dollars. Tens of thousands of landslides spread over mountain terrain, especially near the epicenter and the zone relatively shallow slides, typically ranging 1 to 5m deep. Although deep and broader landslides were less numerous than were shallow and disrupted ones, they contribute a great total volume of landslide material due to their enlarge size. This earthquake triggered two huge landslides yielding an estimated volume of x108 m3. We focus our study on the distribution of the landslides induced by the Chi-chi earthquake in relation to geology, and seismic ground acceleration. We anticipate to use the data produced by this study to determine zones that are prone to earthquake-induced landslides. The term landslide used in this paper includes debris slide, steep-slope failure, and rock fall, while landslide rate refers to the percentage of the landslide area to the area identified. 18

2 Wang, W-N. et al.: Distributions of landslides triggered by the Chi-chi Earthquake in Central Taiwan on September 21, Geological setting 2.1 Plate tectonics Taiwan is situated on a convergent and compressive boundary between the Eurasian plate and the Philippine Sea plate. The Philippine Sea plate subducts northward along the Ryukyu Trench in the north, and the Eurasian plate underthrusts the Philippine Sea plate along the Manila Trench in the south (Fig. 1). The Philippine Sea plate moves 7-8 mm/year northwestward in relation to the Eurasian plate (Ho, 1982). The plate collision will stress the active faults in western Taiwan; when the faults are reactivated, a large amount of strain energy is released to generate earth- vation from the west toward the east, with plains and rolling hills to the west and high mountains on the east. This area is composed mostly of Tertiary sedimentary and sub-metamorphic rocks (Table-1). The rocks generally age and indurate eastward. Longitudinal faults, from west to east, called the Chelungpu fault, the Shu- tive fault in western Taiwan, was reactivated to trigger the Chi-chi earthquake in 1999 (Fig. 2). 2.2 Topography and regional geology There are four main rivers in central Taiwan: the alluvial terraces are found along the rivers. The surface fault from the Chi-chi earthquake extends for about 100 km along the north-south Chelungpu fault. The surface fault is a reverse fault dipping toward the east, and its hanging wall uplifts 1-10m. At about 45km from the northwest of the epicenter, the fault toward the northeast by east and across the Tachiashi River, forming a waterfall 8-m in height and destroying a bridge and a dam. Therefore, the maximum slip along the fault plane could exceed 15m. The topographic relief of the studied area gains ele- Fig. 2 Geological map of studied area (revised from Ho, 1976) Table 1 Rocks exposed in the studied area (revised from Ho C. S. and Tan L. P., 1960) Fig. 1 Block diagram showing plate tectonic setting of Taiwan (Ho, 1982) J. of the Jpn. Landslide Soc., Vol.38, No (2002) 19

3 (1) Taichung basin The west region of the Chelungpu fault is composed of the Taichung basin, having a 65,450 ha area. The main part of the region is composed of alluvial fans (Qa, Table-1) and river terraces (Qte, Qlt) from the Tachiashi River, the Wushi River, and the Choshuishi River as shown in Fig. 2. A small area of hills, about 10,000 ha, appears in the southern end of the region; the rocks in the hills are mainly Plio-Pleistocene (Ptk 2) and loose sandstone or shale (Ptk 1). conglomerate (2) Outer foothill zone The outer foothill zone, about 76,800 ha, is bordered by the eastern edge of the Taichung basin (Fig. 2). This region is expressed by a series of rolling hills whose average elevation is about 400 m. The western part of this hilly area is formed of mostly gentle dipping thick sandstone with thin shale of the Pliocene to Pleistocene age (Ptk 1, Pcl and Pcs), exhibiting a typical cuesta or hogback landform. The eastern part of the hilly area, 4894 ha, is made of Plio-Pleistocene conglomerates (Ptk 2) and forms irregular sharp ridges, called Chiu-chiu- (3) Inner foothill zone The inner foothill zone of about 140,500 ha is separated from the outer foothill zone on the west by the Shuangtung fault and from the Husehshan range on the east by the Shuili fault (Fig. 2). The rocks exposed in this region are mostly Miocene sandstone and shale (Mtk, Msl, Mnc, Mkc) and few Oligocene tuffaceous sediments (0t). The slightly hard character of the rocks and high relief landforms may serve to distinguish the inner foothill zone from the outer foothill zone. (4) Husehshan range The Husehshan range, about 99,000ha, neighbors on the eastern edge of the inner foothill zone. This zone is covered by Paleocene rocks including quartzitic sandstone or quartzite, argillite, and slate (Oscl, Opl, and Ehk). The rocks have been weathered and subjected to strong tectonic deformation, resulting in complex folding, faulting and jointing. 3. Earthquake damage and ground acceleration 3.1 Earthquake damage The Chi-chi earthquake disaster area is located mainly in the western foothills of the Central range ( Taipei. As the focal depth was relatively shallow with high population, earthquakes causing the most signifi- Fig. 3 Distribution of Chi-chi earthquake-induced landslides and PGA of East-West component (Rectangle whole area: SPOT images, "A" area: color aerial photos, "B" area: panchromatic aerial photos) cant damage have occurred in western Taiwan. The buildings truncated by the earthquake fault were destroyed without exception. Those in tens of meters wide belt along the fault-scarp's edge on hanging-wall were badly damaged, resulting from well-developed compression cracks. Slightly apart from the belt toward the east, the damaged structures were suddenly decreased. Little damage was found on the foothills, even in front of the fault scarp, indicating that structural damages were mainly caused by ground deformations rather than ground motion (George C. and Lee C., 2000). In contrast to the structure damages, landslides were dispersed in the study area with various landslide densities, showing that the seismic forces and the local geologic and topographic conditions had an influence on site response. The annual precipitation ranges from 1,600-2,900 mm; more than 70% of the annual precipitation is concentrated in the months from May through September. This kind of precipitation pattern easily induces secondary hazards. 3.2 Ground acceleration During the Chi-chi earthquake on September 21, 20 J. of the Jpn. Landslide Soc., Vol. 38, No (2002)

4 Wang, W-N. et al. : Distributions of landslides triggered by the Chi-chi Earthquake in Central Taiwan on September 21, , the strong ground motions were recorded at many seismic stations. The peak ground acceleration (PGA) of more than 0.5g and 0.6g was found near the Chelungpu fault, and the PGA approached 1.0g at Hsinchei elementary school and Sun-moon Lake in Nantou County. Ten stations near the Chelungpu fault show the typical pulse-like velocity waveform near-fault ground motions. of the PGA contours (showing in Fig. 3) of over 300 gal cover the region east of the earthquake fault; a belt of the dense PGA contours appears along the earthquake fault zone, and few PGAs exceeding 300 gal were found in the foothills, indicating a result of westward thrusting. More than 10,000 aftershocks were recorded. Almost the large aftershocks occurred in the area between the Chelungpu fault and the Lishan fault, implying that the earthquake other faults in the area (Fig. 2). faulting might have activated plains and hilly area of about 245,000 ha ("A" area in Fig. 3). The panchromatic aerial photos taken from November 1999 through January 2000 are utilized to examine mountainous area interpreted areas ("B" area in Fig.3). The total with aerial photos is about 375,000 ha. The smallest landslide identified from aerial photos is about 10m2. Although most of earthquake-induced landslides can be easily identified through the light-toned scars in aerial photos, the reactivated landslides with a little displacement are hardly detectable in the photos. It is therefore necessary to examine them by means of a field check. Field study is used to supplement and verify the accuracy of the information gathered from aerial photos, and to complete the preliminary study. The identified landslide area based on SPOT images and aerial photos includes the outcropped area caused by failed mass sliding. 4. Applications of remote sensing The SPOT image and aerial photo interpretation save much time in investigating earthquake-induced landslides, especially in the region with difficult access can and frequent aftershocks. The SPOT image easily identifies landslides with a difference in the NDVI (Normalized Difference Vegetation Index) retrieved from the SPOT image taken before and after the earthquake. This study employed primarily by SPOT images and then used aerial photo interpretations with subsequent field surveys. The SPOT images can show an overall view of the distribution of the earthquake-induced landslides and regional geology. The images used in this study were taken on 26 and 31 December 1998, 27 September 1999, and 12 October The NDVI varies with the vegetation covers. Comparing pre-with post-chi-chi earthquake images, the differences in the NDVI usually signify the presence of landslides, excluding creep-type landslides and smaller ones. The smallest landslide identified by the SPOT images is about 300 m2. Stereoscopic examination of adjacent pairs of aerial photos can detect detailed slope features and even creep-type landslides and smaller ones. Aerial photos can provide a surprising amount of information regarding an inaccessible and useful for mitigating secondary area quickly. It is very important hazards. The aerial color photos used in this study are on a scale of 1/17,000 and were taken on September 22-26, 1999; the area covered by these aerial photos is mainly 5. Results 5.1 Ground acceleration and distribution of landslides Initially, SPOT images identified about 10,100 earthquake-induced landslides with a total area of about 14,300 ha in a 946,855 ha interpreted area (Fig. 3, Fig. 4 Distribution of Chi-chi earthquake-induced landslides and PGA of Vertical component J. of the Jpn. Landslide Soc., Vol.38, No (2002) 21

5 Fig. 5 Variation in landslide amount and its rate with PGA magnitude and 4). The landslides are distributed almost throughout the region between the Chelungpu fault and the Lishan fault, but nearly disappear in east of the Lishan fault. To the west of the Chelungpu fault, the Taichung basin, few earthquake-induced landslides were found except in the hills at the southern end of the basin. Figure 5 shows the variation in landslide rate and landslide amount with PGA. Over 90% of the landslides are distributed in the areas with PGA over 300gal in EW component or over 200gal in vertical component. The landslide rate and landslide amount initially increase to some extents with PGA, then decrease abruptly with PGA. The areas with PGA of gal in EW component have a high landslide rate, ranging in %, and contain 9,412 landslides, 93% of the total landslide amount identified with SPOT images ; the highest amount (4,607) appears in the PGA gal area. The number of landslides in the PGA gal area is much less landslide than that in the gal area, but few differences between the landslide rates in these two areas because of the greater value in PGA in the narrower covering area. A similar trend is shown in the case of the vertical component (Fig. 5). It appears that the vertical ground motion-damaged slopes is limited to a lesser degree and a narrower range of shaking ( gal) than is the horizontal ground motion ( gal). Whether the vertical or the horizontal ground motion has a decisive influence on slope stability is difficult to determine. Figure 6 illustrates that the Opl formation is most widely exposed in each range of the PGA in the EW component (Appendix A). Owing to indurate Oligocene rocks, the Opl has a rate ranging from 3-5%. No landslide rate over 5% can be found in any formation un- Fig. 6 PGA magnitude (EW component) vs. geological formation area and variation in landslide rate in each area 22 J. of the Jpn. Landslide Soc., Vol. 38, No (2002)

6 less the PGA is more than 400gal. The Pcl formation has the greatest landslide rate, 9.2%, in a gal area. The Pcl is always expressed in cuesta and hog- bedding slide, most of the landslides in this formation are steep and shallow landslides. The Ptk2 (Houoyen- 20% in a gal area in contrast to that in gal, and few slopes failed over 600 gal or below 400 gal, indicating that the ground motion of gal in the EW component has a close relation to the widespread shallow landslides at Chiu-chiu-feng. The PGA at or near some gigantic landslides such as the Chiu-fen-erh- 5.2 Landslides in each geologic region Based on aerial photo interpretation, about 26,000 earthquake-induced landslides were identified in the 375,000 ha studied area (Appendix B), with the landslide rate defined as a percentage landslide area to the identified of the sum of the area. Figure 7 shows that the landslide rate in the Taichung basin, the outer foothill zone, the inner foothill zone, and the Husehshan range is 1.6%, 4.0%, 7.0%, and 3.6%, respectively. The number of landslides equal or less in size than one hectare reach 21,000 (80% of the total amount), indicating that most of the landslides are steep and shallow landslides, debris slides, and rock falls. Four gigantic landslides were also identified by aerial photo interpretation. Some landslides that could not be recognized the aerial photos were found during field surveys. Earthquake-induced landslides can be ascribed to topographic and/or geologic factors such as steep gradient, surface faulting, well-developed discontinuity, and reactivating landslide. (1) Taichung basin About 240 landslides with a 67 ha total landslide area (Appendix B) were found on terrace scarps in the main part of the basin (Qte and Qlt), and 1,200 landslides with 279 ha of landslide area on the hills (Ptkl and Ptk2). Therefore, in the landslide rate is % in the former, and % in the latter. The average landslide area is less than 0.2 ha/landslide, indicating that the landslides are mainly steep and shallow landslides. (2) Outer foothill zone An amount of 6,713 landslides were found in this region, resulting in an average landslide rate of 3.4% and an average area of 0.4 ha/landslide (Appendix B). In Fig. 7 Variation in landslide amount and its rate in each eological region the western part of this region, the landslide rate ranges from %, and the landslide types are mainly steep and shallow landslides and rock falls from the escarpments of cuestas and hogbacks. A few landslides occurred along dip slopes or at the earthquake cated about 10 km east of Taichung City and is the largest landslide reactivated by the Chelungpu faulting. It is m in length by m wide. The rock exposed in the I-chiang-chiao landslide is Pliocene shale (Pcs). Many steep and shallow landslides of tens of centimeters in depth were concentrated on the upper part of the Chiu-chiu-feng hilly area, expressing a widespread bad-land topography. The total area of these landslides exceeds 1,140 ha; the landslide rate of this area is 23.3% (Appendix B, Ptk2). It could be ascribed to the weathered and loose conglomerates on the land surface and a strong ground motion (PGA: 415 gal in vertical, 639 gal in NS, and 517gal in EW components) near the Chiu-chiu-feng area. (3) Inner foothill zone About 40% of the earthquake-induced landslides occurred in the inner foothill zone. Of the four geological regions, the inner foothill zone has the greatest value in average landslide rate 6.8%, in landslide amount, 10,476, and in average landslide area, 0.9 ha/landslide (Appendix B). Because of well-developed discontinuities such as bedding planes and joints, the landslides in this area were mainly rock sliding along the bedding plane, or steep and shallow landslides from the escarpments of dip slopes. There were two gigantic rockslides, the Chiu-fen-erh-shan region. slide and the Tsaoling slide, in this J. of the Jpn. Landslide Soc., Vol. 38, No (2002) 23

7 many rock-falls and debris slides scattered over the mountainous area. The landslide materials have accumulated on a steep slope or its foot, where is near a populated area. This constitutes a very high potential secondary hazardous area during heavy rains. Several basins appear in the Husehshan terrace range. The deposits distribute in the uplands (high-leveled terraces) bordering the bigger basins such as the Puli The Chiu-fen-erh-shan slide is located in about 10 km from the northeast of epicenter; the width and length were both about 1,100m. Its total area is about 180 ha. The failed materials are mainly composed of thickbedded sandstone with thin shale (Msl), resulting in a landslide rate and an average landslide area of 11.9% and 1.0ha/ landslide, respectively, in this formation. These materials average 50m thick and comprise 70 ~ 106 m3. The slide moved along a bedding plane with a strike/dip of N30-50 E/SE The PGA near this slide during the Chi-chi earthquake was gal in EW component, 370.5gal in NS component, and gal in vertical component. The Tsaoling slide is located 32km south of the epicenter and about 5km east of the southern extension of the Chelungpu fault. The PGA near this area was about 450 gal in EW and 200 gal in vertical components (Fig. 3, and 4) during the earthquake. This strong ground motion caused a 620ha rock slide; a rock mass of 120 ~106 m3 moved along the bedding plane with a strike/dip of N60-70 W/SW10-15 and dammed up mainly Pliocene sandstone (Pcl), resulting in a landslide rate of 16.8% and an average landslide area of 1.9 ha/ landslide in this formation. Several old landslides reactivated by the Chi-chi earthquake were also found in the inner foothill zone. The Hungtsaiping landslide is located in 3km southwest of the Chiu-fen-erh-shan slide. It occupies the southern 200 ha of an old 500 ha landslide. The Hungtsaiping landslide is mainly composed of Miocene weathered sandstone with shale. (4) Husehshan range The average landslide rate and area are 3.6% and 0.5 ha/landslide, respectively, in this region (Appendix B). Though the rocks in the Husehshan range are hard, the rugged topography and fractured rocks result in 5.3 Relationship between landslide and distance from earthquake fault/epicenter Some researchers have proposed that the distribution of earthquake-induced landslides has a close relation to the distance from an earthquake fault; the nearer to the earthquake fault, the more landslides and /or the greater the landslide rates appear (Okunishi, 1996; Lang and Nakamura, 1997; Tomatsu and Nakano, 1999). By dividing the studied area into six 5-km-wide belts paralleling the Chelungpu fault (Fig.8), the greatest value of the landslide amount and landslide rate are not found in the belt beside the fault (0-5 km), but are found in the 5-10 km belt and the km belt, respectively. Beyond the km belt, both of the amount and rate decrease with distance from the Chelungpu fault. This discrepancy could result from the geological variations of the area as shown in Fig. 2. In the case of Kueichulin formation, the landslide rates decrease regularly with distances from the earthquake fault as shown in Fig.9. In these circumstances, under similar geologic conditions, the occurrence of earthquake-induced landslides has a close relation to the distance from an earthquake fault. Landslide rate generally decreases with the distance from the epicenter, but the exact outer boundary of slope instabilities can not be clearly defined (Technical Committee for Earthquake Geotechnical Engineering, 1993). The studied area can be concentrically divided into eleven 5-km-wide belts encircling The landslide rate, a percentage encircled area, generally decreases the epicenter. of landslide area to with epicentral distance (Fig. 10). The landslide rate in the km belt is 6.5%, 1.8% more than that in the km belt, but the landslide amount in the former is less than that in the latter, assuming that this is a result of the appearance of many larger landslides in the west southern 24 J. of the Jpn. Landslide Soc., Vol. 38, No (2002)

8 Fig. 10 Variation in landslide amount and its rate with distance from the epicenter area from the epicenter including the Tsaoling landslide, the largest landslide in the study area. 6. Conclusions The relationship between regional geology and the distribution of landslides is clearly identified in the SPOT image covering a wide area, but landslides of less than 300m2 and reactivated landslides are difficult to identify due to the limited resolution of the image. The detailed features of an individual landslide, even as small as 10m2, can be clearly delineated with stereoscopic interpretation of pairs of aerial photos. The reactivated landslides, which remained unclear using SPOT image interpretation, were easily detected on aerial photos. Though aerial photo interpretation can manually acquire data on landslides distributed over a wide region, this method is more time-consuming and expensive than SPOT image interpretation. Studies of areas where earthquake-induced hazards are of concern should start with very high-altitude images and succeed with low-level aerial photos, thereby providing information on regional features down to site-specific features. Some reactivated landslides with little displacement have been found during ground truth checks. SPOT image and aerial photo interpretation is not a substitute for field surveys, but is a most useful preliminary. Owing to westward thrusting, the Chi-chi earthquake-induced landslides distributed mostly in the hanging wall of the Chelungpu fault, the earth- quake fault, and nearly disappear in the Lushan formation (Ml) just east of the Lishan fault. Almost all the aftershocks also occurred in the region between Chelungpu fault and the Lisan fault, indicating that the distribution region of earthquake-induced landslides is consistent with the aftershock region (Kawabe, 2000) About 26,000 earthquake-induced landslides were identified in a 375,000 ha area studied by aerial photo interpretation and field checks. The landslide types were mainly shallow landslides, debris slides, and rock falls. Ascribing to well-developed geological discontinuities, the inner foothill zone has a higher average landslide rate and landslide amount than other geological regions in the study area. Though the PGA contours are not concentric lines, the landslide rate generally decreases with epicentral distance. More than 90% of the landslides are distributed in the area with horizontal PGAs of greater than 300 gal. The ground motion of gal PGA in the EW component caused a widespread shallow landslide in the Chiu-chiu-feng area. In general, the distribution of landslides is related to distance from the earthquake fault. This study has shown that, under similar geological conditions, the landslide rate decreases with distance from the earthquake fault. Acknowledgement This work was supported by a grant from the Soil & Water Conservation Bur., COA, Taiwan, ROC. We are indebted to Prof. HAYASHI Setsuo of Mie Univ., Prof. FURUYA Takahiko of Chiba Univ., Prof. SATO Osamu of Niigata Univ., and Prof. KAMAI Toshitaka and Prof. CHIGIRA Masahiko of Kyoto Univ. for their continuing interest. The technical assistance of colleagues of Energy & Resources Lab., Industrial Technology Research Institute, Taiwan is greatly acknowledged. References George C. and Lee C. L. (2000): The Chi-Chi, Taiwan Earthquake of September 21, 1999: Reconnaissance Report, MCEER, University at Buffalo, State University of New York, pp Ho C. S. and Tan L. P. (1960): Geology and coal deposits of the area between Tungshih, Taichung and Kuohsing, Nantou, J. of the Jpn. Landslide Soc., Vol. 38, No (2002) 25

9 Taiwan, Bulletin of the Geological Survey of Taiwan, No. 12, pp Ho C. S. (1976): Foothill tectonics of Taiwan, Bulletin of the Geological Survey of Taiwan, No. 25, pp Ho C. S. (1982): Tectonic evolution of Taiwan, The explanatory text of the tectonic map of Taiwan, MOEA, Taiwan, and ROC, p Kawabe H. (2000): The earthquake and earthquake motion, Jishin-Sabo, Kokon Shoin Co., p. 13. (in Japanese) Lang, Yu-Hua and Nakamura, H. (1997): Characteristics of earthquake induced landslides and theoretical analysis on model slopes, Journal of the Japan Landslide Society, 34-3, pp (in Japanese) Okunishi K. (1996): Characteristics of slope failure in Rokko mountain area, Southern Hyogo Prefecture Earthquake and geomorphologic hazards, Kokon Shoin Co., p (in Japanese) Technical Committee for Earthquake Geotechnical Engineering (1993): Manual for Zoning on Seismic Geotechnical Hazards, The Japanese Society of Soil Mechanics and Foundation Engineering, pp Tomatsu, O. and Nakano, Y. (1999): Study on the slope failure triggered by the earthquakes in Gifu Prefecture, general report of a study on instability and hazard in mountainous area by earthquake (Project manager : Prof. Hayashi S.), pp (in Japanese) Appendix-A Landslide rate and amount in different geological formation and PGA (EW component) range Formation Appendix-B Earthquake-induced landslides in various rock formations and geological regions Formation 26 J. of the Jpn. Landslide Soc., Vol. 38, No (2002)