Earthquake hazard isn’t a physical thing we
measure. It's something mapmakers define and
then use computer programs to predict. To decide
how much to believe a hazard map, we need to
know what the mapmakers assumed, and what the
effects of those choices were.
- Definition of hazard (political, not scientific)
- Where and when will earthquakes occur?
- If they occur, then
- how large?
- How large will ground motion be?
Assume that an earthquake of a certain size will strike in a
certain time and cause shaking within a certain area.
Strongly shaken areas MMI > VII for M 6
Include earthquakes of different magnitudes, assume some
areas more likely to have earthquakes, and have stronger
shaking close to the epicenter. Hazard at a given location is
described by the maximum shaking due to earthquakes that is
predicted to happen in a given period of time.
Two methods of predicting hazard
DHSA - deterministic seismic hazard assessment - chose
the biggest earthquake to worry about, decide where & how
big it will be, and how much shaking it will cause.
PSHA - probabilistic seismic hazard assessment - estimate
combined hazard from many different earthquakes. Use the
probabilities and uncertainties of factors like the
location and times of earthquakes and how much shaking
will result from an earthquake of a given magnitude.
DSHA makes society spend lots of money preparing for an event
that is very unlikely to happen during a structure's life.
PSHA defines hazard via a mathematical event rather than real
one, so results depend in complex ways on the probabilities and
uncertainties assumed. ”Simplicity is deeply veiled by userhostile notation, antonymous jargon, and proprietary
software"(Hanks and Cornell, 1994.
As probabilistic models cover longer time windows they become
about the same as deterministic ones, but emphasize extreme
cases even more
“Estimates of some
specific PSHA
studies are very
surprising,
particularly at small
exceedance rates.
High standard
deviations in ground
motion prediction
equations are a
leading candidate to
explain the surprising
hazard predictions.”
Anderson, 2010
SHORT
RECORD OF
SEISMICITY &
HAZARD
ESTIMATE
Africa-Eurasia
convergence
rate varies
smoothly
NUVEL-1
Argus et al., 1989
Predicted hazard from historic
seismicity is highly variable
Likely overestimated near
recent earthquakes,
underestimated elsewhere
More uniform hazard seems
more plausible - or opposite if
time dependence considered
Map changes after major
earthquakes
GSHAP
SHORT
RECORD OF
SEISMICITY &
HAZARD
ESTIMATE
Africa-Eurasia
convergence
rate varies
smoothly
NUVEL-1
Argus et al., 1989
Predicted hazard from historic
seismicity is highly variable
Likely overestimated near
recent earthquakes,
underestimated elsewhere
More uniform hazard seems
more plausible - or opposite if
time dependence considered
Map changes after major
earthquakes
2003
2004
GSHAP
Long record needed to see real hazard
Swafford & Stein, 2007
“Our glacial
loading model
suggests that
earthquakes
may occur
anywhere
along the rifted
margin which
has been
glaciated.”
Stein et al.,
1979
1985
2005
HIGH MODELED NMSZ HAZARD RESULTS FROM
HIGH-END ASSUMPTIONS
Systematic
past ones in location & timing
Doesn’t consider
space-time variability
- Redefined from maximum
acceleration predicted at
10% probability in 50 yr
to 2% in 50 yr (1/ 500 yr to 1/2500 yr)
Arbitrary choice on
policy grounds; no
cost/benefit analysis
- Future earthquakes will be like
Measurement
- Large magnitude of 1811-12
and thus future large
earthquakes
Uncertainty in
interpreting intensity
data
- High ground motion in large
events
Lack of data; chose
high model
Algermissen et al., 1982
Hazard redefined
from maximum
acceleration
predicted at
10% probability
in 50 yr
(1/ 500 yr )
to much higher
2% in 50 yr
(1/2500 yr)
Frankel et al., 1996
New Madrid hazard
higher than
California
results largely from
redefining hazard as
largest shaking
expected every
2500 yr:
Not so for 500 yr
500 yr
500 yr
2500 yr
Searer & Freeman, 2002
2500 yr
PREDICTED
HAZARD
DEPENDS ON
ASSUMED
MAXIMUM
MAGNITUDE OF
LARGEST
EVENTS AND
ASSUMED
GROUND
MOTION MODEL
Frankel/Toro:
St Louis 1.8
Memphis 1.3
Newman et
al., 2001
EFFECTS OF
ASSUMED
GROUND
MOTION MODEL
Effect as large as one
magnitude unit
Frankel model,
developed for maps,
predicts significantly
greater shaking for M >7
Frankel M 7 similar to
other models’ M 8
Frankel & Toro models
averaged in 1996 maps;
Atkinson & Boore not
used
Newman et al., 2001
ASSUMED HAZARD
DEPENDS ON
EARTHQUAKE
PROBABILITY
ASSUMPTION
Constant since last
event: time
independent
Small after last
event, then grows:
time dependent
Time dependent
lower until ~2/3 mean
recurrence
Results depend on
model & parameters
Hebden & Stein, 2008
RELATIVE PREDICTED HAZARD DEPENDS
ON POSITION IN EARTHQUAKE CYCLE
Time dependent
lower until ~2/3
mean
recurrence
Charleston &
New Madrid
early in their
cycles so time
dependent
predicts lower
hazard
Hebden & Stein, 2008
2% in 50 yr (1/2500 yr)
Memphis: TD at present is 64% of TI
NEW MADRID
Mw 7.7 (NMSZ)
Mw 7.3 (Charleston)
Time dependent
model for eastern
US predicts lower
New Madrid &
Charleston hazard
Effect larger than
lowering Mmax and
thus ground
motion model
Including GPS
makes effect much
greater
Hebden & Stein, 2008
Assume from GPS data no M7 on the way
Some hazard remains from earthquakes up to M ~ 6.7
Hazard ~ 1/10 that of USGS prediction
USGS, 2500 yr,
assumes M 7 coming
GPS, 500 yr, assumes
no M 7 coming
Hard to assess possible hazard of M7 on other faults
No evidence, but can’t exclude until we understand mechanics
CHARLESTON
2% in 50 yr (1/2500 yr)
Hebden & Stein, 2008