TOPIC 3: HOW WELL CAN WE PREDICT EARTHQUAKE HAZARDS? Predictions are important for hazard mitigation...
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Transcript of TOPIC 3: HOW WELL CAN WE PREDICT EARTHQUAKE HAZARDS? Predictions are important for hazard mitigation...
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TOPIC 3: HOW WELL CAN WE PREDICT EARTHQUAKE HAZARDS?
Predictions are important for hazard mitigation policy
How much should we believe them?
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HAZARD ASSESSMENT IS HARD
It has been described as "a game of chance of which we still don't know all
the rules”
Lomnitz, 1989
AND WHAT GOES INTO A MAP IS OFTEN NOT EXPLAINED OR EXPLAINED BADLY
Its "simplicity is deeply veiled by user-hostile notation, antonymous jargon, and proprietary software"
Hanks and Cornell, 1994
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How is the hazard defined?
Where do we expect earthquakes?
When do we expect earthquakes?
What will happen in those earthquakes?
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How is the hazard defined?
Hazard isn’t a physical thing we measure.
It’s something mapmakers define on policy grounds.
How they define hazard is the largest factor in determining
the hazard.
Different choices lead to different predicted hazards and thus favor different
policies.
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Frankel et al., 1996
Algermissen et al., 1982
Hazard redefined
from maximum acceleration predicted at10% probability in 50 yr (1/ 500 yr )
to much higher 2% in 50 yr (1/2500 yr)
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New Madrid hazard higher
than California results largely from redefining
hazard as largest shaking expected every
2500 yr:Not so for 500
yr500 yr 2500 yr
Searer & Freeman, 2002
500 yr
2500 yr
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ASSUMED HAZARD DEPENDS ON DEFINITION TIME WINDOW
Over 100 years, California site much more likely to be shaken strongly than NMSZ one
Over 1000 years, more NMSZ sites shaken strongly once; many in California shaken many times
Short time relevant for buildings with 50-100 yr life
Shaken areas MMI > VII
Random seismicity simulation including seismicity & ground motion differences
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Where do we expect earthquakes?
Can use
Earthquake history
Plate motions
Geology
GPS
On plate boundaries, these agree.
In other places, we have to chose which to use
Different choices lead to different predicted hazards
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Long record needed to see real hazard
Swafford & Stein, 2007
1933 M 7.3
1929 M 7.2
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“Our glacial loading model suggests that earthquakes may occur anywhere along the rifted margin which has been glaciated.”
Stein et al., 1979
1985
Concentrated hazard bull's-eyes at historic earthquake sites2005
Diffuse hazard along margin
GSC
Map depends greatly on assumptions & thus has
large uncertainty
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Peak Ground Acceleration
10% probability of exceedance in 50
years(once in 500 yr)
GSHAP (1999)GSHAP (1999)
Present StudyPresent Study HUNGARY: ALTERNATIVE HAZARD MAPS
Concentrated hazard inferred from historic seismicity alone
Diffuse hazard inferred incorporating geology
Toth et al., 2004
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When do we expect earthquakes?
When we have a long history, we can estimate the average recurrence time -
but there’s a lot of scatter
When we have a short history, we estimate the recurrence time of large earthquakes from small ones, but this
can be biased
In either case, we have to assume either that the probability of large earthquakes stays constant with time,
or that it changes
Different choices lead to different predicted hazards
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EARTHQUAKE RECURRENCE IS HIGHLY VARIABLE
M>7 mean 132 yr 105 yr Estimated
probability in 30 yrs 7-51%
Sieh et al., 1989
Extend earthquake history with paleoseismology
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When we have a long history, we can estimate the average recurrence time -
but there’s a lot of scatter
Mean 132 105
Mean 180 72
We can describe these using various distributions - Gaussian, log-normal, Poisson but it’s not clear that one is
better than another
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Gutenberg-Richter relationshiplog10 N = a -b MN = number of
earthquakes occurring ≥ M a = activity rate (y-intercept) b = slope M = Magnitude
When we have a short history, we estimate the recurrence time of large earthquakes from small ones, but this
can be biased
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POSSIBLE BIASES IN ESTIMATING THE MAGNITUDE AND RECURRENCE TIME OF LARGE EARTHQUAKES FROM
THE RATE OF SMALL ONES
Undersampling: record comparable to or shorter than mean recurrence - Usually find too-short recurrence time. Can also miss largest events Direct paleoseismic study: Magnitude overestimated, recurrence underestimatedEvents missed, recurrence overestimated
Earthquake
Rate
Stein & Newman, 2004
CHARACTERISTIC
UNCHARACTERISTIC
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SIMULATIONS
Short history: often miss largest earthquake or find a too-short
recurrence time
10,000 synthetic earthquake histories for G-R relation with slope b=1
Gaussian recurrence times for M> 5, 6, 7
Various history lengths given in terms of Tav, mean recurrence for M>7
Stein & Newman, 2004
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Long history: Can still find too-short or too-long recurrence time
Stein & Newman, 2004
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RESULTS VARY WITH AREA SAMPLED
Stein et al., 2005
Increasing area around main fault adds more small earthquakes
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ASSUMED HAZARD DEPENDS ON EARTHQUAKE PROBABILITY ASSUMPTION
Constant since last event: time independent (can’t be “overdue”)
Small after last event, then grows: time dependent
Time dependent lower until ~2/3 mean recurrence
Details depend on model & parameters
Hebden & Stein, 2008
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Time dependent lower until ~2/3 mean recurrence
Charleston & New Madrid early in their cycles so time dependent predicts lower hazard
RELATIVE PREDICTED HAZARD DEPENDS ON POSITION IN EARTHQUAKE CYCLE
Hebden & Stein, 2008
Southern San Andreas broke in 1857 M 7.7 Fort Tejon, late in cycle so time-dependent predicts higher hazard (“overdue”)
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California Time-
dependant probabilit
ies
Increased on
southern San
Andreas
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CHARLESTON
2% in 50 yr (1/2500 yr)Hebden & Stein, 2008
At present, time dependent predicts ~50% lower hazard
Still less in 2250
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What will happen in large earthquakes?
Major unknowns are magnitude of the earthquake and the
ground shaking it will produce
Tradeoff between these two parameters
Different choices lead to different predicted hazards
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EFFECTS OF ASSUMED GROUND MOTION MODEL
Effect as large as one magnitude unit
Frankel model predicts significantly greater shaking for M >7
Frankel M 7 similar to other models’ M 8
Newman et al., 2001
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Newman et al., 2001
Assumed maximum magnitude of largest events has largest effect near main fault
Assumed ground motion model has regional effect because it also applies
to small earthquakes off
main fault
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When we look at a hazard map, remember that it is just one of a large number of quite different and equally likely maps one could make,
depending on model assumptions
How is the hazard defined?
Where do we expect earthquakes?
When do we expect earthquakes?
What will happen in those earthquakes?
Often the last (Mmax, ground motion model) is discussed the most but the other assumptions are more
important
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Comparing maps made for different assumptions shows which features are best constrained (robust)
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We use these maps, but It’s hard to say how good they are
Won’t know for 100s or 1000s of years, when we have enough experience to see how good their predictions were.
Where the data are good, the assumptions and thus predictions are probably pretty good. Where the data
are poorer, the predictions are probably poorer.
Our best bet is probably to look at any given map, ask whether the prediction makes sense, and act
accordingly.
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2% in 50 yr (1/2500 yr)
154%
%106
Large uncertainty in maps
54% effect in Memphis
New Madrid: 200 years into hypothesized 500 year recurrence