TOPIC 3: HOW WELL CAN WE PREDICT EARTHQUAKE HAZARDS?

Predictions are important for hazard mitigation policy

How much should we believe them?

HAZARD ASSESSMENT IS HARD

It has been described as "a game of chance of which we still don't know all

the rules”

Lomnitz, 1989

AND WHAT GOES INTO A MAP IS OFTEN NOT EXPLAINED OR EXPLAINED BADLY

Its "simplicity is deeply veiled by user-hostile notation, antonymous jargon, and proprietary software"

Hanks and Cornell, 1994

How is the hazard defined?

Where do we expect earthquakes?

When do we expect earthquakes?

What will happen in those earthquakes?

How is the hazard defined?

Hazard isn’t a physical thing we measure.

It’s something mapmakers define on policy grounds.

How they define hazard is the largest factor in determining

the hazard.

Different choices lead to different predicted hazards and thus favor different

policies.

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

ASSUMED HAZARD DEPENDS ON DEFINITION TIME WINDOW

Over 100 years, California site much more likely to be shaken strongly than NMSZ one

Over 1000 years, more NMSZ sites shaken strongly once; many in California shaken many times

Short time relevant for buildings with 50-100 yr life

Shaken areas MMI > VII

Random seismicity simulation including seismicity & ground motion differences

Where do we expect earthquakes?

Can use

Earthquake history

Plate motions

Geology

GPS

On plate boundaries, these agree.

In other places, we have to chose which to use

Different choices lead to different predicted hazards

Long record needed to see real hazard

Swafford & Stein, 2007

1933 M 7.3

1929 M 7.2

“Our glacial loading model suggests that earthquakes may occur anywhere along the rifted margin which has been glaciated.”

Stein et al., 1979

1985

Concentrated hazard bull's-eyes at historic earthquake sites2005

Diffuse hazard along margin

GSC

Map depends greatly on assumptions & thus has

large uncertainty

Peak Ground Acceleration

10% probability of exceedance in 50

years(once in 500 yr)

GSHAP (1999)GSHAP (1999)

Present StudyPresent Study HUNGARY: ALTERNATIVE HAZARD MAPS

Concentrated hazard inferred from historic seismicity alone

Diffuse hazard inferred incorporating geology

Toth et al., 2004

When do we expect earthquakes?

When we have a long history, we can estimate the average recurrence time -

but there’s a lot of scatter

When we have a short history, we estimate the recurrence time of large earthquakes from small ones, but this

can be biased

In either case, we have to assume either that the probability of large earthquakes stays constant with time,

or that it changes

Different choices lead to different predicted hazards

EARTHQUAKE RECURRENCE IS HIGHLY VARIABLE

M>7 mean 132 yr 105 yr Estimated

probability in 30 yrs 7-51%

Sieh et al., 1989

Extend earthquake history with paleoseismology

When we have a long history, we can estimate the average recurrence time -

but there’s a lot of scatter

Mean 132 105

Mean 180 72

We can describe these using various distributions - Gaussian, log-normal, Poisson but it’s not clear that one is

better than another

Gutenberg-Richter relationshiplog10 N = a -b MN = number of

earthquakes occurring ≥ M a = activity rate (y-intercept) b = slope M = Magnitude

When we have a short history, we estimate the recurrence time of large earthquakes from small ones, but this

can be biased

POSSIBLE BIASES IN ESTIMATING THE MAGNITUDE AND RECURRENCE TIME OF LARGE EARTHQUAKES FROM

THE RATE OF SMALL ONES

Undersampling: record comparable to or shorter than mean recurrence - Usually find too-short recurrence time. Can also miss largest events Direct paleoseismic study: Magnitude overestimated, recurrence underestimatedEvents missed, recurrence overestimated

Earthquake

Rate

Stein & Newman, 2004

CHARACTERISTIC

UNCHARACTERISTIC

SIMULATIONS

Short history: often miss largest earthquake or find a too-short

recurrence time

10,000 synthetic earthquake histories for G-R relation with slope b=1

Gaussian recurrence times for M> 5, 6, 7

Various history lengths given in terms of Tav, mean recurrence for M>7

Stein & Newman, 2004

Long history: Can still find too-short or too-long recurrence time

Stein & Newman, 2004

RESULTS VARY WITH AREA SAMPLED

Stein et al., 2005

Increasing area around main fault adds more small earthquakes

ASSUMED HAZARD DEPENDS ON EARTHQUAKE PROBABILITY ASSUMPTION

Constant since last event: time independent (can’t be “overdue”)

Small after last event, then grows: time dependent

Time dependent lower until ~2/3 mean recurrence

Details 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

Southern San Andreas broke in 1857 M 7.7 Fort Tejon, late in cycle so time-dependent predicts higher hazard (“overdue”)

California Time-

dependant probabilit

ies

Increased on

southern San

Andreas

CHARLESTON

2% in 50 yr (1/2500 yr)Hebden & Stein, 2008

At present, time dependent predicts ~50% lower hazard

Still less in 2250

What will happen in large earthquakes?

Major unknowns are magnitude of the earthquake and the

ground shaking it will produce

Tradeoff between these two parameters

Different choices lead to different predicted hazards

EFFECTS OF ASSUMED GROUND MOTION MODEL

Effect as large as one magnitude unit

Frankel model predicts significantly greater shaking for M >7

Frankel M 7 similar to other models’ M 8

Newman et al., 2001

Newman et al., 2001

Assumed maximum magnitude of largest events has largest effect near main fault

Assumed ground motion model has regional effect because it also applies

to small earthquakes off

main fault

When we look at a hazard map, remember that it is just one of a large number of quite different and equally likely maps one could make,

depending on model assumptions

How is the hazard defined?

Where do we expect earthquakes?

When do we expect earthquakes?

What will happen in those earthquakes?

Often the last (Mmax, ground motion model) is discussed the most but the other assumptions are more

important

Comparing maps made for different assumptions shows which features are best constrained (robust)

We use these maps, but It’s hard to say how good they are

Won’t know for 100s or 1000s of years, when we have enough experience to see how good their predictions were.

Where the data are good, the assumptions and thus predictions are probably pretty good. Where the data

are poorer, the predictions are probably poorer.

Our best bet is probably to look at any given map, ask whether the prediction makes sense, and act

accordingly.

2% in 50 yr (1/2500 yr)

154%

%106

Large uncertainty in maps

54% effect in Memphis

New Madrid: 200 years into hypothesized 500 year recurrence