Transcript Document

USING WHAT WE KNOW AND DON’T IN EARTHQUAKE HAZARD
MITIGATION
Mitigating risks from earthquakes or other natural disasters involves
economic & policy issues as well as the scientific one of estimating the
hazard and the engineering one of designing safe structures.
$100M seismic
retrofit of Memphis
VA hospital,
removing nine
floors, bringing it
to California
standard
Does this make
sense?
How can we help
society decide?
THOUGHT EXPERIMENT: TRADEOFF
Your department is about to build a new building.
The more seismic safety you want, the more it will cost.
You have to decide how much of the construction budget to put into safety.
Spending more makes you better off in a future large earthquake. However,
you’re worse off in the intervening years, because that money isn't available
for office and lab space, equipment, etc.
Deciding what to do involves cost-benefit analysis. You try to estimate the
maximum shaking expected during the building's life, and the level of
damage you will accept.
You consider a range of scenarios involving different costs for safety and
different benefits in damage reduction.
You weigh these, accepting that your estimates for the future have
considerable uncertainties, and somehow decide on a
balance between cost and benefit.
THIS PROCESS, WHICH SOCIETY FACES IN PREPARING FOR
EARTHQUAKES, ILLUSTRATES TWO PRINCIPLES:
“There's no free lunch”
Resources used for one goal aren’t available for another, also
desirable, one. In the public sector there are direct tradeoffs.
Funds spent strengthening schools aren’t available to hire
teachers, upgrading hospitals may mean covering fewer
uninsured (~$1 K/yr), stronger bridges may result in hiring
fewer police and fire fighters (~$50 K/yr), etc...
“There's no such thing as other people's money”
Costs are ultimately borne by society as a whole. Imposing
costs on the private sector affects everyone via reduced
economic activity (a few % cost increase may decide whether a
building isn’t built or build elsewhere), job loss (or reduced
growth), and the resulting reduction in tax revenue and thus
social services.
SCIENCE/ENGINEERING AND ECONOMIC/POLICY ISSUES ARE
INTERRELATED: NO SHARP DIVISION BETWEEN THEM
e.g., definition of “hazard” (number of years/probability over which
to plan for maximum shaking)
is economic/policy based
Need to view these in holistic way to formulate sensible policy
Challenge is to develop sensible policies that balance costs and
benefits, given what we know and don't know about future
earthquakes and their effects.
Several approaches can help seismologists and engineers most
usefully contribute.
1) RECOGNIZE THAT THERE ARE NO UNIQUE OR CORRECT
STRATEGIES, SO SOCIETY HAS TO MAKE TOUGH CHOICES.
Use what we know about earthquake hazards and recurrence to
help society decide how much to accept in additional costs to
reduce both the direct and indirect costs of future earthquakes.
Need detailed analysis, which we don't have yet, of the costs and
benefits of various policies.
Strategy chosen shouldn't be a bureaucratic decision imposed
from above, but one made openly through the democratic process
on the community level - where costs and benefits of the policy
accrue.
SHOULD BUILDINGS IN MEMPHIS MEET CALIFORNIA
STANDARD?
New building code IBC 2000, urged by FEMA, would raise to California level
(~ UBC 4)
Essentially no analysis of costs & benefits of new code
Memphis Seismic
Hazard Abatement
40
'2000
Code Year
Code
Year
60
17.5
'94
110
25
20
'88
15
'84
10 (Not Req'd by code)
Pre 84 0
S.E. Shelby
Ctr Shelby
N.W. Shelby
0
20
40
60
80
100
120
Level of Hazard Abatement
in terms of PGA (%g)
J. Tomasello
INITIAL COST/BENEFIT ESTIMATES: MEMPHIS AREA
I: Present value: FEMA estimate of annual earthquake loss $17 million/yr,
part of which would be eliminated by new code, ~ 1% of annual construction
costs ($2 B).
II: Life-of-building: Use FEMA estimate to infer annual fractional loss in
building value from earthquakes. If loss halved by new code, than over 50 yr
code saves 1% of building value.
If seismic mitigation cost increase for new buildings with IBC 2000 >> 1%,
probably wouldn't make sense.
Similar results likely from sophisticated study including variations in
structures, increase in earthquake resistance with time as more structures
meet code, interest rates, retrofits, disruption costs, etc.
2) THOUGHTFULLY ADDRESS LIFE SAFETY
U.S. earthquake risk primarily to property; annualized losses estimated at
~$4 billion.
Also ~10 deaths/yr, averaged over larger numbers in major earthquakes.
Annual fatalities roughly constant since 1800, presumably in part because
population growth in hazardous areas offset by safer construction.
Situation could likely be maintained or improved by strengthening building
codes, so the issue is how to balance this benefit with alternative uses of
resources (flu shots, defibrillators, highway upgrades, etc.) that might save
more lives for less.
Estimated cost to save life (in U.S.) varies in other applications:
~$50 K highway improvements
~$100 K medical screening
~$5 M auto tire pressure sensors
Different strategies likely make sense in different areas within the U.S. and
elsewhere, depending on earthquake risk, current building codes, and
alternative demands for resources.
Hence seismic mitigation costs in Memphis area - $20-200 M/yr
(1-10% new construction cost) + any retrofits - could insure
20,000 - 200,000 people and save some lives that way
Tricky tradeoff here
3) EXPLICITLY DISCUSS UNCERTAINTIES
We know a lot less than we'd like about earthquake recurrence and hazards.
Although we hope to do better, we don't know if we can, given the complexity
shown by long earthquake records and the growing suspicion that earthquake
occurrence has a large random component.
We don't know whether to view earthquake recurrence as time-dependent or
independent, or even whether earthquakes are less likely in recently active
areas (Swafford et al, Thurs. AM). Hopefully on some time scale, perhaps a
few hundred years, we will have made and tested forecasts adequately to have
reasonable confidence in them.
Until then, we should explain what we know and what we don't.
There's no harm in discussing the limits of what we know. Individuals and
society make decisions given uncertainty: we buy life insurance and decide
how much to spend on safety features in cars. Business and political leaders
consider risks in deciding whether and how to invest. In fact, we help
ourselves by explaining what we don't know, since we want public funds to
learn more.
UNCERTAINTIES IN
NMSZ HAZARD MAPS
Areas of predicted significant
hazard differ significantly,
depending on poorly known
parameters.
Differences have major policy
implications (e.g. Memphis &
St. Louis).
Uncertainties won’t be
resolved for 100s-1000s
years
Uncertainties dominated by
systematic errors (epistemic)
and hence likely
underestimated
Newman et al., 2001
4) AVOID BIASING HAZARD ESTIMATES
Estimates biased toward high ("conservative") values distort policy
decisions by favoring seismic safety over other resource uses.
Don’t want poor education in earthquake-safe schools, or to turn away
patients from earthquake-safe hospitals
Need careful balance
An analogy might be the tendency during the Cold War to overestimate
Soviet military power, leaving the U.S. with enormous military strength but
diverting resources from health, education, and other societal goals.
5) HAZARD ASSESSMENTS AND MITIGATION POLICIES SHOULD
UNDERGO DISINTERESTED PEER REVIEW
Crucial economic and societal issues should fully explored before
a decision.
For example, arguments used to infer that the New Madrid zone is
as hazardous as California should have been carefully analyzed,
given their enormous cost implications.
These result largely from redefining the hazard from the largest
earthquake expected every 500 years to that expected every 2500
years.
EFFECT OF 2500 YEAR CRITERION - MUCH
LONGER THAN ORDINARY BUILDING LIFE
Considers maximum shaking at a geographic point over 500 or 2,500 yr (10%
or 2% in 50 yr) while neglecting much shorter life of ordinary structures.
Definition allows New Madrid hazard to be similar to that in California, although
annual California hazard is much lower.
By similar argument, in very long (three million hand) poker game. probability
of at least one pair and royal flush are comparable - although in one hand, the
probability of a pair is ~ 43%, whereas that of a royal flush is far less,
~ 1 / 100,000
Using this argument would lose money in ordinary duration game.
6) TAKE TIME TO GET THINGS RIGHT
Because major earthquakes in a given area are infrequent on human
timescale, we generally have time to formulate strategy carefully
(no need to rush to wrong answer)
Time can also help on both the cost and benefit sides.
As older buildings replaced by ones meeting newer standards, overall
earthquake resistance increases. Similarly, even where retrofitting isn't
cost-effective, higher standards for new ones may be.
Technological advances can make additional mitigation cheaper and
more cost-effective.
If understanding of earthquake probabilities becomes sufficient to
confidentially identify how probabilities vary with time, construction
standards could be adjusted accordingly where appropriate.
7) WE ARE NOT ALONE
There's increasing interest in making mitigation policy more rationally for
other hazards with considerable uncertainties.
“The direct costs of federal environmental, health, and safety regulations are
probably ~$200 billion annually, about the size of all federal domestic,
nondefense discretionary spending. The benefits of those regulations are even
less certain. Evidence suggests that some recent regulations would pass a
benefit-cost test while others would not.”
(Brookings Institution & American Enterprise Institute)
Viewing seismology and engineering as part of a holistic approach to
hazards mitigation will make our contributions more useful to society.
This utility will grow as we learn more about earthquakes and their effects
in different areas.