Researcher develops
index for quake risk
BY DAVID F. SALISBURY
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.
Related
Information:
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. SR
|
