Regional earthquake loss
studies have been per
formed for several years to assist local and state governments, and emergency response planners in preparing and mitigating damage from future seismic events. Earthquake loss studies often include estimates of repair costs, deaths and casualties, functional loss to lifeline systems and emergency response facilities, and regional economic impacts on a short and long-term basis. A major limitation of many past regional loss studies is that they are stagnant as the inventory and geologic data collected, analysis done and the reports produced cannot be updated to reflect changes in the inventory, demographics or economy of the region.
One possible reason for this is that since the inventory data cannot be realistically included in the final reports, updating existing studies without undertaking a major data collection effort is virtually impossible. In addition, the effectiveness of the loss estimation results was limited in the sense that the analysis results have typically been summarized in a report or tabular format, making it cumbersome to quickly identify the geographic areas which are most likely to experience significant damage.
With the development and maturity of GIS technology, many of these limitations have been overcome and dynamic loss studies that present results in a more usable format can now be undertaken. A recent project, Regional Loss Model (RLM), undertaken by RMS has resulted in a GIS-based software package for performing regional earthquake loss estimates.
The implementation of integrated GIS technology provides an approach, which permits rapid evaluation of complex inventory databases under a variety of earthquake scenarios and allows the user to interactively view results almost immediately. The results can be summarized in tables of loss values as well as maps of damage estimates that can be overlaid to easily view which areas are likely to be most severely affected.
The power of GIS technology makes creating and modifying maps a simple task. For application in regional loss estimation, there are a variety of useful options that are available:
First, inventory collected and input into databases can be quickly and easily displayed on a map of the region. Using colors and symbols, many different attributes can be highlighted. For example, the number and location of seismically vulnerable structures in the region can easily be presented.
Second, existing maps, such as liquefaction potential maps available from the US Geological Survey, can be digitized and displayed on a map of the region. These can then be overlaid with maps of ground shaking and lifeline systems to qualitatively assess the most vulnerable lifeline components.
Third, using thematic maps of loss estimates, localities with high levels of damage can be quickly identified.
Fourth, the functionality of regional lifeline systems such as a water delivery system can be graphically displayed by mapping the system and using colors to identify those parts of the system that are most likely to be non-functional.
GIS technology provides emergency response planners and government officials with a powerful tool to visualize and understand the impact of earthquake on a region. A modern GIS is a system designed to store, analyze and display geographic information. The geographic information may be of an infinite variety of data sets, each associated with a spatial coordinate system. The spatial referencing of the information permits the data sets to be linked. This linkage provides a powerful tool for analysis and integration of data sets from ‘many different sources using the same GIS software.
Formally defined, a GIS is a computer-based system that provides for input, data storage and retrieval, manipulation and analysis, and output of geo-referenced data.
The data input component represents the resources required to convert data from existing formats to the one required for storage and analysis by the computer software and hardware. Original data sources may range from paper maps to tables to digital files, each of which may have their own format.
Conversion of these data to the required digital format can be simple and relatively automatic if the original data require only conversion from one digital format to another. In other cases, digitalization of maps and assignment of appropriate attributes can require major efforts. Data input is often the most costly and time-consuming component of a GIS project and may represent a cost several times that of the hardware and software. However, with the rapid expansion in usage and users, digital GIS databases are rapidly growing and many of these can be used for a variety of GIS projects. Further, with the advent of scanners to create digital images of paper maps the time and cost factors have come down.
Raster and vectors
The data stored in a GIS is comprises two sets of information or files. One is a graphic file, which specifies the location of points, lines and polygons. The second file specifies the characteristics or attributes of the graphic entites. The two files can serve to link and integrate a variety of information to a specific location. When data is the input in a GIS, it must be ascribed locations via point, line and area definitions and attributed characteristics via the attribute file. The user can operate on these files independently to retrieve, analyze and integrate the input data sets.
Two types of data structures are in primary use, one is termed raster and the other vector. The raster format consists of a regular grid of square or rectangular cells or pixels defined by row and column locations to which has been ascribed some value of the desired attribute. The vector format specifies the location or position of points, lines, and polygons for the data set of interest with an associated set of attributes for each.
With the specification of definitions for arcs, nodes and polygons and topological relations, the vector format provides a powerful tool for rapid manipulation and processing of large databases. However, vector format is complex. The simpler raster format is especially well suited for depicting rapidly varying geographic information and enhancement of digital images. For example, in remote sensing and satellite imagery. Most modern GISs use both data structures and include software to convert certain types of data sets from one format to the other.
Data manipulation and analysis
Modern relational database technology is used to store, retrieve and manipulate the graphic and attribute files of modern GISs. Analysis functions permit the integration and derivation of geographically linked data sets. They allow the overlay, query and development of new data layers. And permit the use of external function libraries and user-specified or programmed function software. Rapid advances in relational database technology, computational speed and storage capacities of hardware and GIS analysis software have led to dramatic improvements in the capabilities of GIS.
The output or display of derived GIS products may occur in a variety of formats, including paper maps, tabular files, CRT screen displays and digital computer files. The particular output format specified by the user determines the necessary software functions and hardware needed for each application. Convenient display of products at various stages of development often leads to new spatial insights and improved results. Rapid advances in computer networking and storage capacity of removable disks have greatly improved capabilities to exchange and integrate GIS datasets from various users. Arc View is one of the most popular GIS systems available today.
Seismic risk assessment
The magnitude of the damage and loss depend not only on the seismic hazards, but also upon the density of population, location and type of building exposure, the socio-economic makeup of the region and their spatial relationship to the hazard. If an earthquake occurs in a sparsely populated region, there will be little to no effect on regional infrastructure. The likelihood of loss of human life is also less. However, if the same earthquake were to occur near a large city, it could result in very high losses. Economically challenged regions with inexpensive non-engineered construction are more vulnerable and can expect heavy damages. In local economies which are dependent on one or two types of businesses or industries unemployment can be almost complete and recovery very slow.
Seismic risk assessment can be defined as a procedure to estimate the damage and loss to a seismic event by combining seismic hazards with the inventories of the built environment. The assessment procedure can be broken down into three simple steps:
Standardized Earthquake Loss Estimation Methodology
The earthquake loss estimation methodology will provide local, state and regional officials with the tools necessary to plan and stimulate efforts to reduce risk from earthquakes and to prepare for emergency response and recovery from an earthquake. The methodology will also provide the basis for assessment of nationwide risks of earthquake loss.
NATURE OF THE SEISMIC HAZARDS: Seismic hazards can consist of ground shaking demands, spectral response demands and ground failure from liquefaction, landsliding and surface fault ruptures.
QUANTIFY THE BUILT ENVIRONMENT INVENTORY OF THE AFFECTED REGION: Once the seismic hazard has been quantified, the next step is to create a spatial representation of the region’s structural, demographic and economic inventory. Inventories are the most expensive and time-consuming part of a seismic risk assessment with the final damage estimates highly dependent on the quality of the inventory data. GIS-based inventory collection systems have the flexibility to permit different levels of details in inventory collection as dictated by levels of funding. Data can be collected and stored as either site-specific information or can be aggregated on a regional basis and stored with the associated geographical unit.
COMBINE INVENTORY DATA WITH SEISMIC HAZARDS TO ASSESS THE IMPACT: The final step involves combining seismic hazard information with spatially distributed inventory data and then applying motion damage relationships to determine damage estimates. This procedure works for regional inventory aggregation models to a comprehensive site-specific analysis of every structure in the region. In addition to building damage, loss of function, casualties, monetary losses, shelter requirements and utility service outages are examples of impacts that can be evaluated using the same process.
A GIS-based software system creates the ideal framework to integrate the various components of seismic risk assessment. GIS technology provides a powerful tool for displaying outputs and permits users to ‘see’ the geographical distribution of impacts from different earthquake scenarios and assumptions. The interactive features of a GIS platform allows the user to overlay input and output data on thematically shaded maps of the region. The use of different display colors permits rapid visual identification of areas with potential for high damage and loss, areas that have both significant ground shaking and a large number of vulnerable buildings.
While GIS technology is optimal for spatial reasoning, interpretation of data in a spatial context, and for providing an intuitive graphical input/output mechanism to the user, it is a cumbersome environment for the execution of complex numerical algorithms. Alternatively, programming languages such as C++ can be applied to encode otherwise complex algorithmic and rule-based relationships.
Rather than accept the constraints associated with the different software environments, the current trend in software development is to create a system, which integrates each of the software strategies. To the user, the resulting system can have the look and feel of a GIS-based program. However, embedded within the GIS are the RDBMS technologies, C++ support libraries necessary to support the database management, and computational requirements of a regional risk assessment. Such a system is often referred to as Integrated Geographical Information System (IGIS).
The major problems with the non-GIS based frameworks are: