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

No, they're not winter temperatures. These numbers represent the overall earthquake risk faced byresidents 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 asimple and understandable measure of earthquake risk. It was developed by Rachel Davidson, adoctoral student in civil engineering at Stanford, and applied to 10 cities for her doctoral thesiscompleted last month.

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

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 muchlarger percentage of buildings constructed before 1975, when the city incorporated seismic safetymeasures into its building code.

Such an index can be useful for governments and international aid organizations as they allocateresources among various cities. Multinational companies might find it useful when deciding where tolocate new factories. Insurance companies could use the index to help diversify their portfolios. If suchan 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 broughtto 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 thatthe old paradigm of evaluating risk and developing mitigation strategies needs a fresh and innovativelook. Rachel's work, for the first time, makes it possible for various decision makers to understand therisk of potential disaster that their communities face and how it compares with what other communitiesaround the world are facing."

Gil Jamieson, the chief of risk assessment in the mitigation directorate of the Federal EmergencyManagement Agency, says that the index will be an important tool to support federal, state and localdecision makers. It will aid in determining where mitigation resources should be targeted in relation tothe 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'sindex 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 ofearthquakes 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 thesize of the physical infrastructure, distribution of inhabitants, the size of the urban economy and thesocial-political system. Vulnerability is made up of the likelihood of physical damage; the odds thatinhabitants will face death, injury or serious disruption of their lives; the expected economic costs froman 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 differentsubcategories. For example, Davidson combined several indicators to come up with an overall hazardrating, including the largest earthquake likely to strike a town in the next 50 years, the intensity ofground shaking the quake is likely to produce and the percentage of the urbanized area of the city withsoft 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 ofpopulation growth. For emergency response and recovery, she chose an assessment of the quality ofemergency 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," Davidsonsays, "otherwise the EDRI will not be believable."

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

The comparisons produced by the initial application of Davidson's index might come as something of ashock to the inhabitants of Boston. She calculates that Bostonians have about the same earthquake riskas San Franciscans. The hazard of a major earthquake is significantly higher in San Francisco than inBoston, but Boston, with street after street lined with buildings that were constructed without anyseismic measures, is substantially more vulnerable to earthquake damage. St. Louis has about the sameoverall 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 equivalentto San Francisco, but its level of exposure is much higher and external factors ­ the adverse impact thatdisrupting the city would have on the entire country, for example ­ are also well above those of theother cities.

Davidson cautions against putting too much emphasis on her initial ratings. "The quality of the data, likethat on emergency response planning and enforcement of building codes, is not as good as I wouldlike," she says. Also, the weights that she has assigned to the different factors could use fine-tuning bysurveying 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 appliedto other natural hazards, such as hurricanes, floods and tornadoes. If that was done, then the individualindices 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 deploytheir emergency response efforts," she says.

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

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By David F. Salisbury