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Regional Geologic Hazard Map 
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Fig.9 Regional geologic hazard map 
(scale 1:25,000) 
Geology 
The study area is mostly composed of banded 
biotite gneiss and granitic gneiss(Geological map 
of Dunjeon, 1982). The bedrock outcrops are 
severely weathered on the ground surface. No 
geologic event is recognized in recent years. 
Therefore, geologic features are divided into 
two: hard part(1) and soft part(2). 
Some R.Q.D(Rock Quality Designation) data 
were taken from the test borings performed 
in construction  sites(Soil investigation report, 
1990, Geological report for electrical railway, 
1989). R.Q.D allows to recognize condition of 
subsurface(R.Q.D(%) = 100 x length of full 
diameter rockcore in pieces > 0.1m / length 
of core run). R.Q.D of the area were 
classsified as followings: (3)0-20% (highly 
fractured), (2)20-50% (intermediate), (1)above 50% 
(low). They ranged from 0 to 30% at the 
depth of 20m from surface. 
Classification of Stability Rating 
  
After accomplishing the rating system of each 
factor, weights were developed from 
relationships between different factors(Table 2). 
When the rating and weights were determined, 
overlay process begin to create new maps(Fig. 
7). A formula developed by Environmental 
Protection Agency(Griner, 1989) is applied to 
calculate the stability rating index(SR) in our 
study: 
SR = Tw x Tr + Iw x Ly + Gw x Gr 
+ Vw x Vr + Rw x Rr ^ Sw x Sr + 
Qw x Qr + Cw x Cr + Bw x Br 
(SR; Stability rating index, w and r; weight 
and rating of the factors in Table 2) 
SR values in the study area ranged from 4 
to 46. A percentage cumulative curve(areas vs. 
SR values) was drawn. On this curve, three 
important break  points(15,20,25) were selected 
for classification of SR  values(Fig.8). The 
classification of geologic hazards(landslides) is 
presented as followings;  (1)stable(0-15), (2) 
potential unstable(15-20), (3)unstable(20-25), (4) 
very unstable(above 25). 
679 
ASSESSMENT OF 
REGIONAL GEOLOGIC HAZARD MAP 
A final map(Fig.9) was produced only on the 
area where nine envoronmental geologic data 
(Table 2) were available. ARC/INFO calculated 
the final SR values for composite polygons 
created by overlay  process(Fig.8). Although 
vegetations could not be sufficiently examined 
due to the limit of image processing technique 
and source data(images), the stability rating 
system was very efficient to examine landslide 
occurrence an hazard assessment. Natural 
hazards could be avoided, eliminated and 
reduced through this risk assessment. Most of 
the study areas are comprised in stable class 
(57%), but some places, particularly pediment 
areas which slope angles are less than 10 
degrees, are included in the second class(24%) 
due to soil texture(ML:3) and high 
groundwater level(rating:3). Unstable areas(third 
or forth class; 19%) are mostly located on 
slopes of higher than 30 degrees(or 20-30% 
of landslide frequency). 
CONCLUSIONS 
The regional geologic hazard map produced by 
GIS can be effectively applied to predict the 
landslide hazards. Unstable slopes(third or 
fourth class) in  Fig.9 should be carefully 
treated during construction according to geologic 
conditions, although most of the areas is 
assessed as  stable(first or second class). 
Consequently, the analysis of landslide activity 
by the hazard map can play an important role 
in optimal land use planning in the study 
area. The stability rating system adopted in the 
area may be changed in other regions due to 
different environmental characteristics, but slope, 
landslide frequency and groundwater level 
remain constant factors. 
The results illustrate that this approach is 
useful in providing information for preliminary 
planning and assessment of landslide hazards. 
Moreover, this technique can contribute to 
natural hazard reduction by recognition of 
landslide occurrences in the hazard map. The 
accuracy of the hazard map can be improved 
by application of more data layers through 
overlay process. This methodology can provide 
the better guide for environmental geologic 
study, and become a basis for construction of 
geological hazard information system. 
REFERENCES 
Anderson,J.R., et al 1976. A land use and 
land cover classification system for use with 
remote sensor data, U.S.G.S Geological Survey 
Professional Paper, 964. 
Brabb, Earl  E., 1987. Analyzing and 
portraying geologic and cartographic information 
for land-use planning, emergency response, and 
decision making in San Mateo County, 
Caifornia, Second Annual International 
Conference, Exhibits and Workshops on GIS, 
pp.362-374. 
Degraff,J. V., 1985. Using isopleth maps of 
landslide deposits as a tool in timber sale 
planning, Bulletin of the International 
Association of Engineering Geology, No.22, pp. 
445-453.