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