Time-dependent seismic hazard maps for the New Madrid seismic zone and Charleston, South Carolina areas James Hebden Seth Stein Department of Earth and Planetary Sciences Northwestern University

Underestimated uncertainty and bias in measured speed of light 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 at 10% probability in 50 yr to 2% in 50 yr (1/ 500 yr to 1/2500 yr) - Large magnitude of 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 yr2500 yr Searer & Freeman, 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

CHARLESTON 2% in 50 yr (1/2500 yr)

NEW MADRID 2% in 50 yr (1/2500 yr)

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)

?? 9k7k6k4k12k3k1kToday Portageville CycleReelfoot CycleNew Madrid Cycle Slip Cluster Slip Cluster Slip Cluster Quiescent Holocene Punctuated Slip Earthquake 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 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