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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. - PowerPoint PPT Presentation
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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?
Strongly shaken areas MMI > VII for M 6
Assume that an earthquake of a certain size will strike in a certain time and cause
shaking within a certain area.
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 user-
hostile 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
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
Africa-Eurasia convergence rate varies smoothly
GSHAP
NUVEL-1Argus et al., 1989
SHORT RECORD OF SEISMICITY & HAZARD ESTIMATE
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
Africa-Eurasia convergence rate varies smoothly
GSHAP
NUVEL-1Argus et al., 1989
2004
2003
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
- Future earthquakes will be like past ones in location & timing
- Redefined from maximum acceleration predicted at10% 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
Lack of data; chose high model
Uncertainty in interpreting intensity data
Doesn’t consider space-time variability
Systematic
Measurement
- Large magnitude of 1811-12 and thus future large earthquakes
- High ground motion in large events
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)
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
Newman et al., 2001
PREDICTED HAZARD DEPENDS ON ASSUMED MAXIMUM
MAGNITUDE OF LARGEST EVENTS AND ASSUMED GROUND MOTION
MODEL
Frankel/Toro: St Louis 1.8 Memphis 1.3
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
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
NEW MADRID2% in 50 yr (1/2500 yr)
Memphis: TD at present is 64% of TI
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
Mw 7.7 (NMSZ)Mw 7.3 (Charleston)
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