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GIS in earthquake hazard mapping, Wellington,
June Cahill ·· Ian R Brown Associates Ltd Associated reports prepared by Ian R Brown Associates include:
Reprint of article published in GIS Asia Pacific. Vol 2, No 5 View of Wellington urban area looking south along the Wellington urban motorway taken from a position near the upper right corner of Figure 6. The view is more or less along the strike of the Wellington Fault, a major source of damaging earthquakes. The railway yards in the middle of the photograph are located on reclaimed land, believed to have a high liquefaction potential.
Photograph taken by Lloyd Homer, Institute of Geological and
Nuclear Sciences Ltd,
Introduction
The final project in the earthquake hazard mapping program was carried out by Ian R Brown Associates Ltd, in conjunction with Kingston Morrison Ltd and Victoria University of Wellington. The project team included experts in GIS, geology, cartography, and geotechnical and structural engineering. This project itself involved the integration of all previous mapping studies to provide a combined earthquake hazard map series for the region. Mapping the Hazards The project was completed in five main stages:
The aim of the project was to produce technically sound, defensible, and innovative earthquake hazard maps for the five main urban parts of the Wellington region; Wellington, Porirua, Lower Hutt, Upper Hutt, and Kapiti (Figure 1). A comprehensive literature review found only a few examples of combining multiple earthquake hazards into one single map. The methods described in these papers were limited, and did not provide an adequate model for combining the Wellington hazard data. The single factor hazard mapping data (fault, tsunami, ground shaking, liquefaction, slope failure) were available in digital form from the Wellington Regional GIS. Examples of maps compiled with these data are given in Figures 2, 3, 4 and 5. With a subset of these data, we were able to develop and demonstrate a methodology that would enable meaningful combination of all hazards. The data were available in polygon format, with attributes attached to each polygon describing the effects of the related earthquake hazard. Because the effects of various earthquake hazards can be quite different, as well as affecting both the land and urban infrastructure in various ways, we had to find a sensible way to normalize, and then combine the data. This was accomplished by considering the "average" infrastructure in the region, and then assuming that this infrastructure existed in all parts of the region. We then investigated the effects of each earthquake hazard on each element of our "average" infrastructure using appropriate damage ratios or assessments. By calculating the damage for each of six infrastructure types, for each of the five earthquake hazards (and adjusting for return periods) we were able to sum the individual damage components and arrive at a total value that we called the "Combined Hazard Index", or CHI. GIS Approach The polygonal data for the individual earthquake hazards were converted to a format suitable for use in TECHBASE (MINEsoft, Ltd, Denver, USA) a relational database management system for 3-D GIS operations that we used on Sun Microsystems and Silicon Graphics hardware. We assigned the polygon attributes to a grid table in the relational database, where we had a column and row size of 10 m on the ground. The calculations we needed to carry out to arrive at our "combined hazard index", or CHI, were easily accomplished using the calculated field capability of TECHBASE. This also reduced the data storage requirements, as the calculation between data fields is stored in the field definition. We spent some time investigating the buffering of attribute information at polygon intersections, but it proved to be very difficult to formulate rules that would apply for automatic buffering. TECHBASE gridded data were then transferred to ER Mapper (Earth Resource Mapping, Perth, Australia) where the image processing capabilities of the system were used to derive a color scheme for expressing the variations of CHI value across each map. The presentation of CHI values was simplified by presenting a map key with color gradations ranging from low to high. An example of part of the Wellington combined hazard map is shown in Figure 6. Final maps were printed at a scale of 1:30,000, except for the Kapiti area which is at 1:40,000. During the map preparation phase of the project, we experimented with putting a SPOT panchromatic image under the combined seismic hazard map, but as a result we lost much of the subtle colour changes that were important for understanding the variations in seismic hazard. Using ER Mapper map composition tools, we were able to set out the other graphical elements, explanatory text, and vector based information on to an A0 size map. Draft maps were produced on an HP750C ink jet plotter. The Final Stage: Printing The final stage of the project was map printing. The choices for pre-print and printing in New Zealand were limited due to the A0 map size. This part of the work was carried out by Axiom Systems Ltd (digital pre-press), Terralink NZ Ltd (composite films, and colour proofs), and GP Print Ltd (offset printing). Andrew Vignaux of Axiom Systems Ltd devised software for taking ER Mapper Postscript files, and carrying out the color separation and conversion to Intergraph RLE (run-length encoded) format. The relatively compact RLE files were then copied by Terralink NZ Ltd to their Intergraph laser scanner film recorder. Without this software capability it is unlikely that we would have been able to print the maps using offset printing. When one of the maps was translated into a TIFF file with 400 dpi resolution, it required about 800Mb of disk storage, making it very difficult to process. International Applicability The combined earthquake hazard maps described in this article provide essential information to a range of users. The maps have been produced using the integration of two specialist GIS packages, TECHBASE and ER Mapper, and innovative techniques have been applied to complex spatial data. The methodology that was used is applicable to any other area that experiences or is threatened by seismic hazards, including Japan, Taiwan and Indonesia. The normalized damage ratios can also be used for other hazards as well, such as flooding, wind and fire, so that a map could be prepared for an area showing relative hazard index values covering all natural hazards.
Copies of the Wellington Regional Council combined earthquake hazard maps are freely available from: Hazard Analyst Further technical information on this project can be obtained by contacting: © 1996 Ian R Brown Associates Ltd |
Wellington is situated in one of the more tectonically active areas of
New Zealand, and has a record of destructive earthquakes since European
settlement from about 1840. Under the Resource Management Act (1991)
regional councils are required to control the use of the land in order
to avoid or mitigate the effects of natural hazards. In order to fulfill
this obligation, Wellington Regional Council (WRC) have pursued a programme
of earthquake hazard mapping within the Wellington region. Beginning in 1990
with the production of maps showing active faults, the council has since built
up a comprehensive earthquake hazard map library. Projects have been
carried out to map potential tsunami inundation areas around Wellington
Harbour, variations in ground response to strong shaking, liquefaction
potential, and earthquake induced slope failure. All of these data have
been digitised and forms part of the WRC GIS.