areas which are formed above clay and limestone, volcanic and geothermal area and of course an opened coastal areas surrounds by bay, which entirely, in
agreement with on going geomorphic processes which shape the Earth’s surface.
2.4.3 Geological Risk Map
The first step in the study of collective geological hazards is the plotting of specific information on maps at the same scale. A geological map, for example,
present the areal distribution of rock structure and type. The scale chosen and the emphasis on particularr features may be selected to optimize the use of
information for a particular need. In California, a new 1:750,000 scale geological map was produced in 1972
to give an over-view of the geological properties of the State with sufficient detail to be useful for preliminary land-use planning. Published in color, it emphasizes
recent volcanic rocks and volcanoes, earthquake fault and the major folds in the layered rocks. Maps with much more detail than feasible on the usual 1:250,000
to 1:1,000,000 scale maps are needed for specific hazard evaluations. For urban areas, specializied mapping for land-use planning and engineering design must
show considerable detail and even include geophysical and boreholes studies of local subsurface structure. The required scale may be of the order of 1:20,000.
Recent examples are slope maps produced by the U.S. Geological Survey with scale of 1:24,000. These maps indicaete the per cent of slope of hills and
mountains by means of color code so that assessment of hillside erosion and stability conditions can be made. Likewise, U.S.G.S and Corps of Engineers flood
hazard maps at about this scale show the elevations attained by major historical floods and floods of a specific frequency of occurences Bolt et al 1975: 288.
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There are several unsatisfactory features of the usual geological map published in most countries. First, these maps often emphasize the formations
igneous, basin deposits, etc. rather than the rock types involved. Alluvium consists of fine- and coarse-grained material may have depth and horizontal facies
changes that lead to major seismic response consequences. Again, it is not sufficient to say that a given formation consists largely of sandstone and shale
without mapping bed boundaries. The Geological Survey of New South Wales in Australia has tried to solve the problem by indicating overburden and underlying
rock units by appropriate symbols. In this way, the map color defines the underlying rock, while the map symbol tags the type of overburden. In New
Zealand, the Soil Bureau of the Department of Scientific and Industrial Researche produces maps of soil type that may be read in conjunction with standard
geological maps. In the New England States, USA, one series of maps delineates bedrock and another the superficial glacial deposits Bolt et al 1975: 288.
Another weakness is lack of detail when mapping the weathered conditions of the rock types. The depth of weathering may be of considerable importance in
estimating the response of the ground to strong earthquake motion. In the same way, locations of unobscured bedrock exposures deserve plotting on the basic
geological maps so that when detailed investigations are needed these outcrops can be revisited quickly Alluvial deposits often require sub-division, appropriate
to the scale used e.g. 1:250,000 showing flood plains, lake deposits, colluvial, residual soils, and so on. In this way, parts of a particular surficial deposit,
consisting of fine-grained material with braided stream channels of coarser material, could be identified from the map.
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In many country and also in Indonesia, a recent imaginative development is the use of computers to calculate and draw predictive hazard maps. Once the
controling parameters of the hazard are known these can be combined into mathematical form and programmed once and for all.
The differences between this research compared with another that mentioned above, principally in geological and geomorphological interpretation
point of view. This research thoroughly used GIS and Remote Sensing Technology for determining geological hazard sensitive area through integrating
remote sensing capability especially principal component analysis PCA procedures to obtain common picture of present rocks and minerals distribution
which indicating past as well as endogenetic and exogenetic processes.
2.5. Geology of Research Area
