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Time-dependent seismic hazard maps for the New Madrid
seismic zoneand Charleston, South
Carolina areas
James Hebden Seth Stein
Department of Earth and Planetary
Sciences
Northwestern University
Underestimated uncertainty and
biasin measured
speed of light 1875-1960
Uncertainties are hard to assess and generally
underestimated
IS NEW MADRID IS AS HAZARDOUS AS CALIFORNIA?
Frankel et al., 1996
U.S. Geological Survey How robust is this model result?
HIGH MODELED HAZARD RESULTS FROM ASSUMPTIONS
- Redefined from maximum acceleration predicted at10% probability in 50 yr to 2% in 50 yr (1/ 500 yr to 1/2500 yr)
- Large magnitude of 1811-12 and thus future large earthquakes
-High ground motion in large events
- Time-independent recurrence of large events
Arbitrary choice on policy/economic grounds
Don’t understand how to best model recurrence
Lack of data
Uncertainty in interpreting intensity data
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 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
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
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
Time dependent model for
eastern US predicts lower New Madrid &
Charleston hazard
Effect larger than lowering
Mmax and thus ground motion
model
Mw 7.7 (NMSZ)Mw 7.3 (Charleston)
? ?
9k 7k 6k 4k12k 3k 1k Today
Portageville Cycle Reelfoot Cycle New Madrid Cycle
Slip
Cluster
Slip
Cluster
Slip
ClusterQuiescent Quiescent Quiescent
Holocene Punctuated SlipEarthquake history inferred from Mississippi river channels
Holbrook et al., 2006
NEW MADRID HAZARD WOULD BE EVEN LOWER IF RECENT EARTHQUAKE CLUSTER IS ENDING
Stein & Newman, 1994
GPS sites show little or no interseismic motion
Present seismicity may be 1811-12 aftershocks
Predicted seismic hazard depends on whether large earthquake recurrence modeled as time independent or time dependent
Time dependent model predicts lower New Madrid & Charleston hazards
Magnitude of effect depends on model type & parameters
Effect can be larger than Mmax or ground motion model
Significant contribution to uncertainty
SUMMARY