Transcript steinnepalx - Department of Earth and Planetary Sciences

Why aren't earthquake hazard maps better?
Seth Stein1, M. Liu2, Edward M. Brooks1,
Bruce D. Spencer3
GSHAP Hazard Map (1999)
1 Department
of Earth & Planetary
Sciences and Institute for Policy
Research, Northwestern
University
GPS C
10% in 50 yr
10% in 50 yr
2 Department
of Geological
Sciences, University of Missouri
3 Department
of Statistics and
Institute for Policy Research,
Northwestern University
Hazard Map (Chaulagain et al., 2015)
Society is playing a high-stakes game of chance
against nature
We want to
- assess the hazard - how often major earthquakes
happen and how bad they will be
- mitigate or reduce the risk - the resulting losses.
- Our limited knowledge about what the earth will do,
and limited resources, limit how well we can do either
-Society can still take sensible measures to reduce
damage in future events
Ideally, how much mitigation is enough?
Societally optimal level minimizes
total cost = sum of mitigation cost + expected loss
Expected loss = ∑ (loss in ith expected earthquake
x its assumed probability)
Expected loss depends on event & mitigation level
Compared to optimum
Less mitigation decreases
construction costs but
increases expected loss and
thus total cost
Stein & Stein, 2012
More mitigation gives less
expected loss but higher total
cost
Limitation1
(science):
Inaccurate hazard
and loss (damage)
estimates produce
nonoptimal
mitigation
Large uncertainties
remain despite
scientific &
engineering
advances
Stein & Stein, 2013
Limitation 2 (economics): Even without uncertainty, mitigation
usually isn’t optimal because of limited resources & other
needs
Still, undermitigation is better than no mitigation
Communities should do what they can
Hazard
assessment
failed
Geller
2011
2010 map predicts
probability of strong
shaking in next 30
years
But: 2011 M 9.1
Tohoku, 1995 Kobe M
7.3 & others in areas
mapped as low hazard
In contrast: map
assumed high hazard
in Tokai “gap”
Hazard model divided trench into segments
These were assumed to
break individually in future
earthquakes
Expected Earthquake Sources
50 to 150 km segments
M7.5 to 8.2
(Headquarters for Earthquake Research Promotion)
Giant earthquake broke many segments
Expected Earthquake Sources
50 to 150 km segments
M7.5 to 8.2
(Headquarters for Earthquake Research Promotion)
2011 Tohoku Earthquake
450 km long fault, M 9.1
(Aftershock map from USGS)
J. Mori
NY Times 3/21/11
Hazard maps are hard to get right: successfully
predicting future shaking depends on accuracy of
four assumptions over 500-2500 years
Where will large earthquakes occur?
When will they occur?
How large will they be?
How strong will their shaking be?
Uncertainty & poor performance can result
because these are often hard to assess
Slow plate
boundary
Africa-Eurasia
convergence rate
varies smoothly
(5 mm/yr)
NUVEL-1
Argus, Gordon, DeMets &
Stein, 1989
Swafford & Stein, 2007
GSHAP 1999
Slow plate
boundary
Africa-Eurasia
convergence rate
varies smoothly
(5 mm/yr)
NUVEL-1
Argus, Gordon, DeMets &
Stein, 1989
2003
2004
M 6.4
Swafford & Stein, 2007
M 6.3
GSHAP 1999
EARTHQUAKE RECURRENCE IS HIGHLY VARIABLE
Sieh et al., 1989
Extend earthquake history with
paleoseismology
M>7 mean 132 yr s 105 yr
Estimated probability in 30 yrs 7-51%
We don’t know whether to assume that probability of a
major earthquake is
- constant with time (time-independent) or
- small after a large earthquake and then increases
(time-dependent ).
Time
dependent
predicts lower
until ~2/3 mean
recurrence
Results depend
on both model
choice &
assumed mean
recurrence
Hebden & Stein, 2008
Have We Seen the Largest
Earthquakes in Eastern
North America?
• Largest known east of
Appalachians is 1886
Charleston SC, M ~7,
recurrence time ~ 500 yrs
• Could there be bigger?
• M 7.2 -7.4 occur offshore
Canadian east coast
• What do past event locations
tell us?
Simulations address issue of short record
record needed to see real hazard
Swafford & Stein, 2007
1933
1929
Given 300 years of data, what Mmax would we
observe if Mmax were really 7.0, 7.2, 7.4, 7.6?
Simulated seismicity – 10,000 catalogs
Generally miss largest events and so underestimate Mmax
Most likely to observe Mmax ~7 with recurrence time ~ sample length
Events larger than observed to date in ENA are possible
Locations of large events to date need not indicate higher hazard
Sensitivity
analysis predicted hazard
depends on
180%
- Assumed
magnitude of largest
future events
-Assumed ground
prediction motion
model
-Neither are well
known since large
earthquakes rare
Newman et al., 2001
275%
Seismic hazard uncertainty typically factor of 3-4
At best partially reducible because some key parameters
poorly known, unknown, or unknowable
Uncertainties not communicated to users
Stein
et al,
2012
Compare 510-year shaking record to Japanese National
Hazard (JNH) maps
Miyazawa and
Mori, 2009
The short time period since hazard maps began to be made is a
challenge for assessing how well they work. Hindcasts offer long
records, but are not true tests, as they compare maps to data that
were available when the map was made. Still, they give useful
insight.
Brooks et al., 2015
Geller (2011) argued that
“all of Japan is at risk from
earthquakes, and the
present state of
seismological science does
not allow us to reliably
differentiate the risk level
in particular geographic
areas,” so a map showing
uniform hazard would be
preferable.
Test: By the one measure
uniform and randomized
maps do better, but by
another the detailed JNH
maps perform better.
Uniform & random maps
JNH
worse
JNH
better
Brooks et al., 2015
Maps may be overparameterized (overfit)
A high order polynomial fits past data better than linear or quadratic
models, but this more detailed model predicts the future worse than
the simpler models.
Intermediate level of detail may be better for hazard maps.
NEPAL ILLUSTRATES
GSHAP Hazard Map (1999)
Because plausible alternative
parameter choices yield quite
different maps, maps have
significant uncertainties.
GP
10% in 50 yr
Shaking variations from past
earthquakes are useful for
characterizing site effects in
hazard maps.
Locations of past earthquakes
may not be useful for
assessing locations of future
ones.
10% in 50 yr
Hazard Map (Chaulagain et al., 2015)
At oceanic subduction zones, GPS data show variation in coupling that seem to
indicate locations of future earthquakes and likely reflect locations of past ones
Loveless
& Meade,
2011
Avouac et al., 2015
Nepal GPS data show no significant variation
in coupling between areas of recent large
earthquakes, or the 2015 earthquake
Moreover, earthquakes in past few hundred years have released less plate
motion than is accumulating.
Hence with present knowledge, the entire zone can be regarded as equally
hazardous and perhaps vulnerable to much larger earthquakes than those
currently known, with long recurrence times.
Although major scientific questions remain, they don’t
need to be solved to make sensible mitigation policies,
given available resources & other needs
Communities should do what they can