A New Index Of Earthquake Risk Ranks Boston Equal To San Francisco

August 15, 1997

Boston 39; San Francisco 37; St. Louis 36; Jakarta 39; Mexico City 38; Tokyo 54.

No, they're not winter temperatures. These numbers represent the overall earthquake risk faced by residents of each city. The higher the number, the greater the risk.

The numbers were computed using a new Earthquake Disaster Risk Index, which aims to provide a simple and understandable measure of earthquake risk. It was developed by Rachel Davidson, a doctoral student in civil engineering at Stanford, and applied to 10 cities for her doctoral thesis completed last month.

"We've gained a lot of knowledge in the study of earthquake hazards, but there is a big gap in implementing it. We need a new tool to communicate what we know about earthquake risk," Davidson says.

According to the index, Bostonians face an overall earthquake risk comparable to San Franciscans, despite the lower frequency of major earthquakes in the Boston area. The reason: Boston has a much larger percentage of buildings constructed before 1975, when the city incorporated seismic safety measures into its building code.

Such an index can be useful for governments and international aid organizations as they allocate resources among various cities. Multinational companies might find it useful when deciding where to locate new factories. Insurance companies could use the index to help diversify their portfolios. If such an index became widely recognized, it might give cities an added incentive to reduce their ratings, Davidson argues.

"The Northridge earthquake of January 1994 and the Kobe earthquake of January 1995 have brought to our attention the unacceptable levels of risk our urban communities are facing," says Haresh C. Shah, professor emeritus of civil engineering and Davidson's thesis adviser. "It has become clear that the old paradigm of evaluating risk and developing mitigation strategies needs a fresh and innovative look. Rachel's work, for the first time, makes it possible for various decision makers to understand the risk of potential disaster that their communities face and how it compares with what other communities around the world are facing."

Gil Jamieson, the chief of risk assessment in the mitigation directorate of the Federal Emergency Management Agency, says that the index will be an important tool to support federal, state and local decision makers. It will aid in determining where mitigation resources should be targeted in relation to the greatest risk. The federal agency is currently promoting the idea of "disaster-resistant communities." The concept rests on the idea that through public and private partnerships and a system of incentives, risk can be reduced at the community level. Before this approach can be put into operation, however, officials need a method to characterize and quantify the risk that each community faces. Davidson's index could serve as the prototype for such a method, Jamieson says.

The index includes a number of factors in addition to the estimates of the size and frequency of earthquakes likely to strike a given city. Specifically, it takes four additional factors into account: * Exposure ­ the size of the city, number of inhabitants and the activities that it supports. * Vulnerability ­ how resistant a given area is to earthquake damage. * External context ­ how damage to the city affects people and activities in the surrounding area, the country and the world. * Emergency response and recovery ­ how well a city is organized to respond to emergencies of the magnitude expected and its consequent capability for reducing an earthquake's impact.

Each of these factors is further broken down into subcategories. Exposure, for example, consists of the size of the physical infrastructure, distribution of inhabitants, the size of the urban economy and the social-political system. Vulnerability is made up of the likelihood of physical damage; the odds that inhabitants will face death, injury or serious disruption of their lives; the expected economic costs from an earthquake; and the degree of disruption of a city's social-political system.

The next step in constructing the index was identifying indicators that accurately represent the different subcategories. For example, Davidson combined several indicators to come up with an overall hazard rating, including the largest earthquake likely to strike a town in the next 50 years, the intensity of ground shaking the quake is likely to produce and the percentage of the urbanized area of the city with soft soil. For exposure, she used population, number of housing units and size of the local economy. For vulnerability, she included the age of the seismic code in force in the city and the history of population growth. For emergency response and recovery, she chose an assessment of the quality of emergency planning and the number of hospital beds per 100,000 residents.

"It's important that the individual indicators accurately reflect the factors that they represent," Davidson says, "otherwise the EDRI will not be believable."

These individual indicators are then given different weights, depending on expert judgment of their relative importance, and are combined into indices for each factor. The individual factors are then similarly combined to provide an overall index.

The comparisons produced by the initial application of Davidson's index might come as something of a shock to the inhabitants of Boston. She calculates that Bostonians have about the same earthquake risk as San Franciscans. The hazard of a major earthquake is significantly higher in San Francisco than in Boston, but Boston, with street after street lined with buildings that were constructed without any seismic measures, is substantially more vulnerable to earthquake damage. St. Louis has about the same overall level of risk as well because of the greater vulnerability of its infrastructure.

Of the 10 cities that she indexed, Tokyo, at 54, was the riskiest by far. It has a hazard level equivalent to San Francisco, but its level of exposure is much higher and external factors ­ the adverse impact that disrupting the city would have on the entire country, for example ­ are also well above those of the other cities.

Davidson cautions against putting too much emphasis on her initial ratings. "The quality of the data, like that on emergency response planning and enforcement of building codes, is not as good as I would like," she says. Also, the weights that she has assigned to the different factors could use fine-tuning by surveying a number of experts in the field to get their opinions, she adds.

In addition to improving the quality of the index, Davidson would like to see the basic approach applied to other natural hazards, such as hurricanes, floods and tornadoes. If that was done, then the individual indices could be combined into an overall, multi-hazard index.

"I think that a multi-hazard index would be most useful to the government as a guide for how to deploy their emergency response efforts," she says.

If Davidson's approach is widely adopted, residents of the world's major cities will have a much better idea of how well, or how poorly, their governments are protecting them from natural hazards.

Stanford University

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