Department of Civil Engineering

Indian Institute of Science, Bangalore

INTEGRATION OF HAZARD MAPS ON GIS PLATFORM

Geographical Information System (GIS) provides a perfect environment for accomplishing comprehensive regional information including seismic damage assessment. GIS has the capability to store, manipulate, analyze and display a large amount of required spatial and tabular data. One of the most important features of a geographic information system is the data analyses of both spatial (graphic) and tabular (non-graphic) data. A final hazard index map for BMP area is developed using Analytic Hierarchy Process (AHP) on GIS platform. AHP is a multi-criteria decision method that uses hierarchical structures to represent a problem and then develop priorities for the alternatives based on the judgment of the user (Saaty, 1980).Application of GIS for microzonation mapping is amply demonstrated by many researchers all over the world. Nath (2006) used GIS as integration tool to map seismic ground motion hazard for Sikkim Himalaya in India. In this study, similar approach of Nath (2006) is used to develop a hazard index map where in the seismic hazard parameters are integrated and coupled with ground information. The hazard index maps are prepared using both deterministic and probabilistic approaches.

Saaty's Analytical Hierarchy process constructs a matrix of pair-wise comparisons (ratios) between the factors of earthquake hazard parameters (EHP). The constructed matrix shows the relative importance of the EHP based on their weights. If 9 earthquake hazard parameters are scaled as 1 to 9, 1 meaning that the two factors are equally important, and 9 indicating that one factor is more important than the other. Reciprocals of 1 to 9 (i.e., 1/1 to 1/9) show that one is less important than others. The allocation of weights for the identical EHP depends on the relative importance of factors and participatory group of decision makers. Then the individual normalized weights of each EHP are derived from the matrix developed by pair-wise comparisons between the factors of EHP. This operation is performed by calculating the principal Eigen vector of the matrix. The results are in the range of 0 to 1 and their sum adds up to '1' in each column. The weights for each attribute can be calculated by averaging the values in each row of the matrix. These weights will also sum to '1' and can be used in deriving the weighted sums of rating or scores for each region of cells or polygon of the mapped layers.

Since EHP vary significantly and depends on several factors, they need to be classified into various ranges or types, which are known as the features of a layer. Hence each EHP features are rated or scored within EHP and then this rate is normalized to ensure that no layer exerts an influence beyond its determined weight. Therefore, a raw rating for each feature of EHP is allocated initially on a standard scale such as 1 to 10 and then normalized using the relation,

Where, Ri is the rating assigned for features with single EHP, Rmin and Rmax is minimum and maximum rate of particular EHP.

Earthquake hazard parameters

Seismic microzonation is subdividing a region into smaller areas having different potential for hazardous earthquake effects. The earthquake effects depend on ground geomorphological attributes consisting of geological, geomorphology and geotechnical information. The parameters of geology and geomorphology, soil coverage/thickness, and rock outcrop/depth are some of the important geomorphological attributes. Other attributes are the earthquake parameters, which are estimated by hazard analysis and effects of local soil for a hazard. The Peak Ground Acceleration (PGA) [from deterministic or probabilistic approach], amplification/ site response, predominant frequency, liquefaction and land slide due to earthquakes are some of the important seismological attributes. Weight of the attributes depends on the region and decision maker, for example flat terrain has weight of 0 value for land slide and deep soil terrain has highest weight for site response or liquefaction. Different attributes considered for Bangalore microzonation are presented below;

Geomorphological Attributes

The geomorphological attributes (here after called as themes) considered in this study are the geology and geomorphology (GG), rock depth/ soil thickness (RD/ST), soil type and strength (represented in terms of average shear wave velocity) (SS), drainage pattern (DP) and elevation level (EL).

Seismological Attributes

The seismological thematic maps have been generated based on detailed studies of seismic hazard analysis, site response studies and liquefaction analysis. From these studies different earthquake hazard parameters are mapped. But for final Index map preparation and GIS integration only selected maps presented earlier are considered as themes:

Peak ground acceleration (PGA) at rock level based on synthetic ground motions from MCE based on DSHA.

PGA at rock level at 10 % probability in 50 years exceedance based on PSHA.

Amplification factor based on ground response analysis using SHAKE2000.

Predominant frequency based on site response and experimental studies.

Factor of safety against Liquefaction potential.

INTEGRATION OF DIFFERENT LAYERS (THEMES)

For seismic microzonation and hazard delineation the different themes as presented above, considering both geomorphological and seismological are integrated to generate seismic microzonation maps. The final microzonation maps can be represented in three forms, 1) hazard map, 2) vulnerability map, and 3) risk map. Because earthquake loss not only depends on the hazard caused by earthquakes, but also on exposure (social wealth) and its vulnerability. Usually hazard map gives the hazard index (HI) based on hazard calculation and site conditions. Vulnerability map gives us the expected degree of losses within a defined area resulting from the occurrence of earthquakes and often expressed on a scale from 0 (no damage) to 1 (full damage). Vulnerability study includes all the exposure such as man-made facilities that may be impacted in an earthquake. It includes all residential, commercial, and industrial buildings, schools, hospitals, roads and railroads, bridges, pipelines, power plants, communication systems, and so on. Risk map will be combination of hazard classes and vulnerability classes an output risk classes. At present only hazard maps have been prepared and presented for BMP area.

Hazard index is the integrated factor, depends on weights and ranks of the seismological and geomorphological themes. Theme weight can be assigned based on their contribution to the seismic hazard. Rank can be assigned with in theme based on their values closer to hazards. Usually higher rank will be assigned to values, which is more hazardous in nature, for example larger PGA will have the higher rank. The contributing themes and their weights are listed below in Table 3. Once the identical weights are assigned then normalized weights can be calculated based on the pair-wise comparison matrix. Some of the attributes (like PGA and Vs) has two values for the same theme, hence both are given same weights with different percentage. The normalized weights are calculated using Saaty's Analytical Hierarchy Process (Nath, 2004).

Table 3: Themes and its weights for GIS integration

 

 

 

 

 

 

 

 

 

 

 

 

In this method, a matrix of pair-wise comparisons (ratio) between the factors is built, which is used to derive the individual normalized weights of each factor. The pair-wise comparison is performed by calculating the principal Eigen vector of the matrix and the elements of the matrix are in the range of 0 to 1 summing to '1' in each column. The weights for each theme can be calculated by averaging the values in each row of the matrix. These weights will also sum to '1' and can be used in deriving the weighted sum of rating or scores of each region of cells or polygon of the mapped layers. Since the values within each thematic map/layer vary significantly, those are classified into various ranges or types known as the features of a layer. Table 4 shows the pair-wise comparison matrix of themes and the calculated of normalized weights. With in individual theme a grouping has been made according to their values. Then rank is assigned based on the values. Usually these ranks varies from 1 to 10, highest rank is assigned for values more hazard in nature. These rank are normalized to 0 -1 using the equation 9. The assigned ranks with normalized values are given in Table 5. Based on above attributes, two types of hazard index map are generated. One is deterministic seismic microzonation map (DSM), which is basically deterministic hazard index map using PGA from deterministic approach and other themes. Another map is the probabilistic seismic microzonation map (PSM). Probabilistic hazard index are calculated similar to DSM but PGA is obtained from probabilistic seismic hazard analysis.

 

Table 4: Pair-wise comparison matrix of Themes and their normalized weights

 

 

 

 

 

 

 

 

 

 

 

 

Table 5: Normalized ranks of the themes

 

 

Webpage is maintained by

Dr P Anbazhagan

Lecturer

Department of Civil Engg

Indian Institute of Science

Bangalore, India 560012

Suggestions and queries may be directed to Dr P Anbazhagan

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This page was last updated on August 25, 2011

Index

Themes

Weights

PGA

Rock level PGA using DSHA-DPGA

9

 

Rock level PGA using PSHA-PPGA

9

AF

Amplification factor

8

ST

Soil Thickness using borehole

7

SS

Equivalent Shear wave velocity for Soil

6

FS

Factor of safety against liquefaction

5

PF

Predominant period / frequency

4

EL

Elevation levels

3

DR

Drainage pattern

2

GG

Geology and geomorphology

1

Theme

PGA

AF

ST

Vs

FS

PF

EL

DR

GG

Weights

PGA

1

9/8

9/7

9/6

9/5

9/4

9/3

9/2

9/1

0.200

AF

8/9

1

8/7

8/6

8/5

8/4

8/3

8/2

8/1

0.178

ST

7/9

7/8

1

7/6

7/5

7/4

7/3

7/2

7/1

0.156

Vs

6/9

6/8

6/7

1

6/5

6/4

63

6/2

6/1

0.133

FS

5/9

5/8

5/7

5/6

1

5/4

5/3

5/2

5/1

0.111

PF

4/9

4/8

4/7

4/6

4/5

1

4/3

4/2

4/1

0.089

EL

3/9

3/8

3/7

3/6

3/5

3/4

1

3/2

3/1

0.067

DR

2/9

2/8

2/7

2/6

2/5

2/4

2/3

1

2/1

0.044

GG

1/9

1/8

1/7

1/6

1/5

1/4

1/3

1/2

1

0.022

Themes

Values

Weight

Ranks

Normalized

Ranks

SOT (m)

≤ 5.0

0.2857

1

0

5 - ≤10

2

0.25

10- ≤ 15

3

0.5

15- ≤ 20

4

0.75

>20

5

1

EVS (m/s)

≤ 100

0.2381

4

1

100- ≤ 200

3

0.66

200- ≤ 300

2

0.33

>300

1

0

FSL

< 1

 

0.1905

3

1

1 - ≤ 2

2

0.5

> 2

1

0

DPGA (g)

≤ 0.120

0.1429

1

0

0.12 - ≤ 0.13

2

0.25

0.13 - ≤0.14

3

0.5

0.14- ≤0.15

4

0.75

>0.15

5

1

SRAF

1- ≤ 2

0.0952

1

0

2- ≤ 3

2

0.66

3- ≤ 4

3

0.33

>4

4

1

SPF (Hz)

≤ 3.5

0.0476

5

1

3.5- ≤ 5.0

4

0.75

5- ≤ 7.5

3

0.5

7.5- ≤9.5

2

0.25

9.5- ≤11

1

1

PPGA (g)

≤ 0.20

0.1429

1

0

0.2- ≤ 0.22

2

0.66

0.22- ≤ 0.24

3

0.33

0.24- ≤ 0.26

4

1